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UBC Theses and Dissertations

Magnesium-lime process for decolourization of kraft mill effluents Rush, Richard John 1976

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MAGNESIUM-LIME PROCESS FOR DECOLOUR!ZA7TON OF KRAFT MILL EFFLUENTS by RICHARD JOHN RUSH B.A.Sc. U n i v e r s i t y o f W a t e r l o o , 1974 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n t h e Department o f C IVIL ENGINEERING We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY A p r i l OF'BRITISH COLUMBIA 1976 I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e H e a d o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f C i v i l E ngineering The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 D a t e iA^w'tP S O ) /QlQ> i ABSTRACT A magnesium-plus-lime c o a g u l a t i o n p r o c e s s , i n c o r p o r a t i n g mag-nesium r e c o v e r y by s l u d g e c a r b o n a t i o n , r e c e n t l y d e v e l o p e d f o r water t r e a t m e n t has been t e s t e d f o r i t s a p p l i c a t i o n t o the d e c o l o u r i z a t i p n o f k r a f t m i l l e f f l u e n t s . T o t a l b l e a c h e d k r a f t m i l l e f f l u e n t (BKME), a f t e r b i o l o g i c a l t r e a t m e n t and e f f l u e n t from the f i r s t c a u s t i c e x t r a c t i o n s t a g e o f a c o n v e n t i o n a l b l e a c h e r y (E-j o r T-20 e f f l u e n t ) were examined i n t h e l a b o r a t o r y . I t was shown t h a t a c o m b i n a t i o n o f low magnesium and low l i m e dosages can a c h i e v e b e t t e r d e c o l o u r i z a t i o n o f k r a f t m i l l e f f l u e n t s than 3-5 times as much l i m e a l o n e , ( i . e . : 30-60 mg/1 M g + + p l u s 375-475 mg/1 l i m e (as CaO) a t pH -11.1 f o r BKME; and 150-300 mg/1 Mig + + p l u s 1.875-3750 mg/1 l i m e (as CaO) a t pH -12.1 f o r T-20 e f f l u e n t . ) C o l o u r removals o f 90-95% were a c h i e v e d u s i n g e i t h e r f r e s h o r r e c y c l e d magnesium. R e s u l t s o f the s t u d y showed t h a t g r e a t e r than 90% magnesium r e c o v e r y can be a t t a i n e d by o p e r a t i n g the s l u d g e c a r b o n a t o r t o a f i n a l pH -7.5, w i t h complete m i x i n g and a good C 0 2 d i f f u s e r s y s t e m p r o v i d e d . (Key words: magnesium, l i m e , k r a f t m i l l e f f l u e n t , d e c o l o u r i z a t i o n , c o l o u r r e m o v a l , Magnesiurn C a r b o n a t e P r o c e s s . ) i i TABLE OF CONTENTS Page ABSTRACT i TABLE OF CONTENTS i i LIST OF TABLES i v LIST OF FIGURES v ACKNOWLEDGEMENT v i CHAPTER 1 INTRODUCTION 1 2 SUMMARY 6 3 BACKGROUND ON THE COLOUR PROBLEM 9 3.1 Na t u r e o f C o l o u r i n P u l p M i l l E f f l u e n t s 9 3.2 E f f e c t s o f C o l o u r on R e c e i v i n g Waters 17 3.3 Summary o f C o l o u r Removal T e c h n o l o g y 20 4 REVIEW OF MAGNESIUM COAGULATION PROCESSES 25 4.1 H i s t o r i c a l Development 25 4.2 T h e o r e t i c a l C o n s i d e r a t i o n s 33 4.3 P r a c t i c a l C o n s i d e r a t i o n s 45 5 EXPERIMENTAL MATERIALS AND METHODS 48 5.1 S e l e c t i o n o f E f f l u e n t 48 5.2 Sample S t o r a g e 51 5.3 Chemical P r e p a r a t i o n s 51 5.4 A n a l y t i c a l T e c h n i q u e s 52 6 RESULTS AND DISCUSSION 57 6.1 R e s u l t s o f T e s t i n g w i t h F r e s h C h e m i c a l s 57 6.1.1 J J a r T e s t R e s u l t s 57 I l l Page 6.1.2 B a t c h D e c o l o u r i z a t i o n and Magnesium Recovery R e s u l t s 55 6.2 R e s u l t s o f T e s t i n g w i t h R e c y c l e d Magnesium 98 6.2.1 J a r T e s t R e s u l t s gg 6.2.2 Batc h D e c o l o u r i z a t i o n and Magnesium Recovery T08 R e s u l t s 7 CONCLUSIONS AND RECOMMENDATIONS 1 2 3 7.1 C o n c l u s i o n s 123 7.2 Recommendations f o r F u t u r e R e s e a r c h 123 7.2.1 Bench S c a l e 123 7.2.2 P i l o t S c a l e 124 7.2.3 Concept f o r F u l l S c a l e I m p l e m e n t a t i o n 126 BIBLIOGRAPHY 128 APPENDICES 135 A Development o f T e s t P r o c e d u r e s 135 B S e t t l e a b i l i t y o f T - 2 0 , _ E f f l u e n t and Polymer S e l e c t i o n ' 149 C Example C a l c u l a t i o n t o D e r i v e t h e Sl u d g e and S u p e r n a t a n t Volume R e s u l t i n g from S l u d g e D e w a t e r i n g to 5% and 60% S o l i d s 153 D S l u d g e C a r b o n a t i o n M o n i t o r i n g Data 155 i v LIST OF TABLES TABLE DESCRIPTION PAGE 1 C o l o u r L e v e l s o f V a r i o u s P r o c e s s Sewers and Com-b i n e d O u t f a l l s f o r Nine B.C. B l e a c h e d K r a f t M i l l s 10 2 S o l u b i l i t y o f Mg(0H) 2 a t 25°C 34 3 T y p i c a l C h a r a c t e r i s t i c s o f C r e s t b r o o k P u l p and Pap e r L t d . T o t a l M i l l E f f l u e n t 49 4 Average R e s u l t s o f S e v e r a l I n - P l a n t C o l o u r B a l a n c e s a t C r e s t b r o o k P u l p and Paper L t d . 50 5 J a r T e s t s w i t h F r e s h C h e m i c a l s - BKME 59 6 J a r T e s t s w i t h F r e s h C h e m i c a l s - BKME 59 7 J a r T e s t s w i t h F r e s h C h e m i c a l s - BKME 60 8 J a r T e s t s w i t h F r e s h C h e m i c a l s - BKME 60 9 J a r T e s t s w i t h F r e s h C h e m i c a l s - T-20 E f f l u e n t 63 10 J a r T e s t s w i t h F r e s h C h e m i c a l s - T-20 E f f l u e n t 64 11 J a r T e s t s w i t h F r e s h C h e m i c a l s - T-20 E f f l u e n t 65 12 B a t c h D e c o l o u r i z a t i o n I n i t i a l C o n d i t i o n s and R e s u l t s 67 13 Summary o f D e c o l o u r i z a t i o n Loop Mass B a l a n c e R e s u l t s 72 14 Summary o f C a r b o n a t i o n Loop Mass B a l a n c e R e s u l t s 79 15 C a r b o n a t i o n S u p e r n a t a n t C h a r a c t e r i s t i c s 81 16 Summary o f B a t c h T e s t Volume B a l a n c e R e s u l t s 82 17 C 0 2 S o l u b i l i t y o f V a r i o u s Temperatures 87 18 J a r T e s t s o f T-20 E f f l u e n t w i t h Lime a l o n e 100 19 J a r T e s t s o f T-20 E f f l u e n t w i t h F r e s h MgS0 4 101 20 J a r T e s t s w i t h R e c y c l e d M g + + - T-20 E f f l u e n t 102 21 J a r T e s t s w i t h R e c y c l e d M g + + - T-20 E f f l u e n t 103 22 J a r T e s t s w i t h R e c y c l e d M g + + - BKME 104 V LIST OF FIGURES FIGURE DESCRIPTION PAGE 1 Magnesium Ca r b o n a t e P r o c e s s Diagram 3 2 T y p i c a l M o l e c u l a r Weight D i s t r i b u t i o n o f A l k a l i L i g n i n o f S i m u l a t e d K r a f t P u l p M i l l Wastewater 14 3 C o l o u r vs. pH f o r T o t a l M i l l E f f l u e n t 15 4 S o l u b i l i t y Diagram f o r Magnesium i n W a t e r ^ a t Atmos-p h e r i c C o n d i t i o n s ! T o t a l Carbonate = 10 M 35 5 Temperature I n f l u e n c e on Magnesium S o l u b i l i t y 36 6 Comparison o f T h e o r e t i c a l S o l u b i l i t y o f Mg(0H) ? With Observed J a r T e s t and P i l o t P l a n t V a l u e s 38 7 S o l u b i l i t y o f MgCO^'XrlpO as a F u n c t i o n o f Time For The I n d i c a t e d H y d r a t e Forms 39 8 Lime R e q u i r e d t o R a i s e t h e pH t o 11 as a F u n c t i o n o f t h e Raw Wastewater A l k a l i n i t y 39 9 Lime v s . P e r c e n t C o l o u r Removal a t Three Magnesium L e v e l s 61 10 S c h e m a t i c o f M a t e r i a l S o u r c e s and S i n k s 70 11 pH v s . C o l o u r , M g + + and C a + + i n t h e S u p e r n a t a n t From a T y p i c a l BKME S l u d g e C a r b o n a t i o n U s i n g F r e s h C h e m i c a l s 91 12 pH v s . C o l o u r , M g + + and C a + + i n t h e S u p e r n a t a n t From a T y p i c a l T-20 Sl u d g e C a r b o n a t i o n Run U s i n g F r e s h C h e m i c a l s 92 13 pH vs. C o l o u r , - M g + + and. C a + + i n the 'Supernatant From a BKME Sl u d g e Carbon a t-ioifr Runuusing R e c y c l e d Magnesi urn 120 14 pH v s . Colour^';Mg + + and C a + + i n the S u p e r n a t a n t From a T-20 S l u d g e C a r b o n a t i o n Run U s i n g R e c y c l e d Magnesium 121 15 S c h e m a t i c o f the EPS Magnesium C o a g u l a t i o n P i l o t P l a n t 125 v i ACKNOWLEDGEMENT I wi s h t o thank Dr. W. K. Oldham and Dr. R. M. R. B r a n i o n f o r t h e i r s u p p o r t and g u i d a n c e t h r o u g h o u t the c o u r s e o f t h i s s t u d y . I would a l s o l i k e t o e x p r e s s my thanks t o Mrs. E l i z a b e t h MacDonald f o r h e r v a l u a b l e a s s i s t a n c e and p a t i e n c e w i t h me i n the C i v i l E n g i n e e r i n g Water P o l l u t i o n C o n t r o l L a b o r a t o r y . A l s o acknowledged a r e the p e r s o n n e l a t the Skookumchuck M i l l o f C r e s t b r o o k P u l p and Paper L t d . f o r a s s i s t a n c e i n o b t a i n i n g m i l l e f f l u e n t samples and the E n v i r o n m e n t a l P r o t e c t i o n S e r v i c e a t B u r l i n g t o n , O n t a r i o f o r p a r t i a l f u n d i n g . Thanks a r e a l s o due t o my w i f e , L i n d a , f o r p r o v i d i n g e n c o u r a g e -ment when I needed i t , t o my f e l l o w g r a d u a t e s t u d e n t s f o r b e a r i n g w i t h me, and to Mrs. I r e n e Green f o r d o i n g most o f the t y p i n g . 1 CHAPTER 1 INTRODUCTION The p u l p and p a per i n d u s t r y i s n o t o r i o u s f o r i t s h i g h w a t e r consumption. T h i s h i g h w a ter use i n e v i t a b l y l e a d s to the d i s c h a r g e o f l a r g e volumes o f wastewaters t h a t g e n e r a l l y c o n t a i n s i g n i f i c a n t l e v e l s o f BOD, COD, suspended s o l i d s , t o x i c i t y and c o l o u r . T r a d i t i o n a l l y , w astewater t r e a t m e n t has been aimed a t r e d u c i n g BOD, suspended s o l i d s , and i n Canada, t o x i c i t y , w i t h l i t t l e o r no emphasis on c o l o u r removal. The l a c k o f emphasis on c o l o u r removal a p p e a r s t o have been f o r two r e a s o n s . F i r s t , c o l o u r i s e x t r e m e l y d i f f i c u l t and e x p e n s i v e t o remove and, s e c o n d l y , no u r g e n t r e a s o n s t o remove c o l o u r have been i d e n t i f i e d . However, i n r e c e n t y e a r s a c o n s i d e r a b l e amount o f e f f o r t has gone i n t o development o f c o l o u r removal t e c h n i q u e s , prompted by a t r e n d i n the i n d u s t r y toward w a t e r c o n s e r v a t i o n t h r o u g h reuse and r e c y c l i n g o f p r o c e s s w a t e r s . In a d d i t i o n , t h e r e has been an i n c r e a s i n g p u b l i c demand f o r s u r f a c e w a t e r s t h a t n o t o n l y a r e f r e e o f h a r m f u l s u b s t a n c e s , b u t a l s o l o o k c l e a n . W h i l e many t e c h n i c a l l y f e a s i b l e c o l o u r removal p r o c e s s e s have been d e v e l o p e d and some ( n o t e a b l y l i m e t r e a t m e n t i n the U.S.A.) have been de m o n s t r a t e d on a f u l l - s c a l e b a s i s , no p a r t i c u l a r method o f c o l o u r r e -d u c t i o n has emerged ajg the ' p r a c t i c a b l e t e c h n o l o g y ' . T h i s i s n o t l i k e l y t o happen. Because o f the wide range o f e f f l u e n t s and v a r i e t y o f c i r c u m -s t a n c e s w i t h i n the i n d u s t r y , no one method i s l i k e l y t o be a p p l i c a b l e i n a l l s i t u a t i o n s . A l t h o u g h Canada has no f u l l - s c a l e c o l o u r removal f a c i l i t i e s a t t h i s t i m e , r e s e a r c h and development work i s b e i n g c o n d u c t e d by govern-mental , i n d u s t r i a l and u n i v e r s i t y s e c t o r s . D u r i n g the p a s t y e a r the a u t h o r 2 r e v i e w e d o n g o i n g r e s e a r c h and a l s o the l i t e r a t u r e p e r t a i n i n g t o c o l o u r removal t e c h n o l o g y . The f i n a l r e p o r t on t h i s s t u d y by Rush and Shannon (1975) i d e n t i f i e d s e v e r a l s p e c i f i c a r e a s s u i t a b l e f o r f u t u r e r e s e a r c h e f f o r t s by t h e E n v i r o n m e n t a l P r o t e c t i o n S e r v i c e (EPS) and Wastewater T e c h n o l o g y C e n t r e (WTC) a t the Canada C e n t r e f o r I n l a n d Waters (CCIW) i n B u r l i n g t o n , O n t a r i o . One p r o m i s i n g method o f c o l o u r r e d u c t i o n i d e n t i f i e d , was a magnesium c o a g u l a t i o n p r o c e s s d e v e l o p e d and t e s t e d by Thompson e t a l (1972) f o r the t r e a t m e n t o f water s u p p l i e s . The s y s t e m has proven t o be f u l l y as e f f e c t i v e as alum f o r t h e removal o f o r g a n i c c o l o u r and t u r b i d i t y from n a t u r a l w a t e r s . A t the same t i m e , i t e l i m i n a t e s s l u d g e d i s p o s a l problems common t o a l l o t h e r c h e m i c a l t r e a t m e n t p r o c e s s e s . The b a s i c o p e r a t i o n a l sequence f o r such a system, termed a Magnesium Carbonate (MgCOg) P r o c e s s i s shown i n F i g u r e 1. By c a r b o n a t i n g t h e s l u d g e g e n e r a t e d f r o m the magnesium and l i m e c o a g u l a t i o n , s o l u b l e magnesium b i c a r b o n a t e (MgfHCOg)^) i s r e c o v e r e d i n the f i l t r a t e f o r r e c y c l e and l i m e i s r e -c a l c i n e d f o r r e u s e . CO^ from t h e l i m e k i l n can be used f o r both s l u d g e and e f f l u e n t c a r b o n a t i o n . So, t h e o r e t i c a l l y a l l t h e components o f t h i s  s y s t e m can be r e c o v e r e d and r e u s e d . Very p r e l i m i n a r y s t u d i e s by Thompson and h i s c o l l e a g u e s and by V i n c e n t (1974) a t Domtar R e s e a r c h i n Quebec, i n d i c a t e t h a t magnesium h y d r o x i d e , p r e c i p i t a t e d ' i n s i t u 1 by l i m e a d d i t i o n , t o a c h i e v e a pH o f 11.0 o r g r e a t e r , would have s e v e r a l advantages o v e r the l i m e a l o n e systems c u r r e n t l y i n use f o r c o l o u r removal. Most i m p o r t a n t o f t h e s e i s a lower l i m e r e q u i r e m e n t , meaning a l o w e r pH i n t h e t r e a t e d waste and l i k e l y l o w e r o v e r a l l t r e a t m e n t c o s t s , due t o the a p p a r e n t r e c o v e r a b i l i t y o f a l l the c h e m i c a l components. M g C 0 3 - 3 H 2 0 M A K E - U P UNTREATED WATER OR f WASTEWATER A M g ( H C 0 3 ) 2 RECYCLE LIME (TO pH>1l) PRIMARY CLARIFIER SLUDGE ( C a C 0 3 + Mg(OH) 2 + IMPURITIES) C 0 2 SLUDGE CARBONATION TREATED EFFLUENT RECARBONATION CO 2 C a C 0 3 VACUUM FILTER ROTARY LIME KILN CaO FOR RECYCLE FIGURE 1. MAGNESIUM CARBONATE PROCESS DIAGRAM ( T H O M P S O N E T A L . 1 9 7 2 ) CO 4 The main c o n c e r n i n t h i s p r o c e s s does n o t appear t o be i n t h e d e c o l o u r i z a t i o n , but r a t h e r i n t h e s l u d g e c a r b o n a t i o n f o r magnesium r e -c o v e r y . The c r i t i c a l f a c t o r s have been i d e n t i f i e d as (1) s e p a r a t i o n o f magnesium from t h e p r i m a r y ' c o l o u r s l u d g e 1 w i t h o u t s i g n i f i c a n t c o l o u r r e l e a s e and (2) good o v e r a l l r e c o v e r y o f magnesium so t h a t i t does not accumulate i n t h e li m e r e c a l c i n a t i o n s t e p s and s e r i o u s l y a f f e c t t h e r e -a c t i v i t y o f the l i m e . P i l o t p l a n t t e s t i n g o f t h i s MgCO^ P r o c e s s f o r the t r e a t m e n t o f h i g h l y c o l o u r e d k r a f t m i l l e f f l u e n t s was p l a n n e d a t the Wastewater Tech-n o l o g y C e n t r e , f o r l a t e 1975. In o r d e r t o p r o v i d e t e c h n i c a l i n f o r m a t i o n f o r the o p e r a t i o n o f t h i s p i l o t p l a n t , t h e a u t h o r was c o n t r a c t e d i n May 1975 t o cond u c t a l a b o r a t o r y s t u d y o f k r a f t m i l l e f f l u e n t d e c o l o u r i z a t i o n by t h e Magnesium Carbonate P r o c e s s ( a l s o r e f e r r e d t o h e r e i n as magnesium-p l u s - l i m e t r e a t m e n t and magnesium c o a g u l a t i o n ) . A p o r t i o n o f the t o t a l a n t i c i p a t e d r e s e a r c h package was s e p a r a t e d o u t , and i s r e p o r t e d h e r e i n . The s p e c i f i c o b j e c t i v e s o f t h i s r e s e a r c h i n c l u d e d : 1. p r o v i d i n g background i n f o r m a t i o n on the c o l o u r problem c r e a t e d by p u l p and paper m i l l s ; 2. r e v i e w i n g magnesium c o a g u l a t i o n p r o c e s s e s ; 3. d e v e l o p i n g methods and m a t e r i a l s f o r t h e d e c o l o u r i z a t i o n o f s e l e c t e d c o l o u r e d waste streams from a b l e a c h e d k r a f t mi 11; 4. g i v i n g a b r i e f j a r t e s t e v a l u a t i o n o f the a b i l i t y o f mag-n e s i u m - p l u s - l i m e t o d e c o l o u r i z e ; a. t o t a l b l e a c h e d k r a f t m i l l e f f l u e n t (BKME) a f t e r b i o l o g i -c a l t r e a t m e n t ; b. e f f l u e n t f r o m the f i r s t c a u s t i c e x t r a c t i o n s t a g e o f •k-k a CE-jDE 2D b l e a c h e r y (E-| o r T-20 e f f l u e n t ) , and s e l e c t t e n t a t i v e optimum c o n d i t i o n s f o r subsequent b a t c h t e s t i n g o f t h e s e e f f l u e n t s ; c o n d u c t i n g s e v e r a l b a t c h d e c o l o u r i z a t i o n - s l u d g e c a r b o n a -t i o n t e s t s o f both BKME and T-20 e f f l u e n t t o d e t e r m i n e the e x t e n t o f c o l o u r r e l e a s e and magnesium r e c o v e r y d u r i n g s l u d g e c a r b o n a t i o n ; e v a l u a t i n g t h e e f f i c i e n c y o f r e c y c l e d supernatant.; as a s o u r c e o f magnesium i o n s ( M g + + ) and d e t e r m i n i n g whether the use o f t h i s m a t e r i a l g i v e s s i g n i f i c a n t l y d i f f e r e n t mass b a l a n c e r e s u l t s than b a t c h t e s t s w i t h f r e s h c h e m i c a l s ; d i s c u s s i n g the r e s u l t s and making c o n c l u s i o n s and recommen-d a t i o n s f o r f u t u r e r e s e a r c h . These were the main o b j e c t i v e s o f t h i s s t u d y . C = C h l o r i n e E ==Sodium h y d r o x i d e D = C h l o r i n e d i o x i d e 6 CHAPTER 2 SUMMARY T h i s r e s e a r c h s t u d y was composed o f e s s e n t i a l l y t h r e e main p a r t s . 1. Development o f t e s t p r o c e d u r e s . 2. T e s t i n g w i t h f r e s h magnesium and f r e s h l i m e . 3. T e s t i n g w i t h r e c y c l e d magnesium and f r e s h l i m e . Methods and m a t e r i a l s were d e v e l o p e d f o r i n v e s t i g a t i n g d e c o l o u r -i z a t i o n o f c a u s t i c e x t r a c t i o n (T-20) e f f l u e n t and t o t a l b l e a c h e d k r a f t m i l l b i o l o g i c a l l y t r e a t e d e f f l u e n t (BKME). In a d d i t i o n , a d e t a i l e d p r o -c e d u r e f o r s t u d y i n g magnesium r e c o v e r y f r o m a s l u d g e c a r b o n a t i o n p r o c e s s was d e v e l o p e d . Based on p r e l i m i n a r y j a r t e s t i n g o f the t o t a l m i l l e f f l u e n t , a dosage o f 60 mg/1 M g + + (added as a s o l u t i o n o f MgSO^) p r e c i p i t a t e d by 500 mg/1 Ca(0H)2 (375 mg/1 CaO) was s e l e c t e d f o r use i n b a t c h magnesium r e c o v e r y s t u d i e s . A t t h i s dosage o f f r e s h c h e m i c a l s , > 90% o f the i n i t i a l c o l o u r was removed a t a c o a g u l a t i o n pH o f 11.1-11.2. S i m i l a r j a r t e s t i n g o f c a u s t i c e x t r a c t i o n e f f l u e n t w i t h f r e s h c h e m i c a l s l e a d t o the s e l e c t i o n o f 300 mg/1 M g + + , i n c o n j u n c t i o n w i t h 2500 mg/1 C a ( 0 H ) 2 (1875 mg/1 CaO) and 2 mg/1 o f an a n i o n i c polymer (Dow A- 2 3 ) , as t h e c h e m i c a l dosages f o r use i n b a t c h magnesium r e c o v e r y s t u d i e s . Under t h e s e c o n d i t i o n s , 90-95% o f the i n i t i a l c o l o u r was removed a t a f i n a l pH o f 12.1-12.2. C o l o u r r e l e a s e i n b a t c h s l u d g e c a r b o n a t i o n t e s t s o f BKME w i t h f r e s h c h e m i c a l s ranged f r o m 25.0 t o 38.1%. Mass b a l a n c e r e s u l t s o f t h e s e t e s t s showed t h a t 75.6-85.9% o f the magnesium s e n t t o t h e c a r b o n a t o r was r e c o v e r e d i f the f i n a l s l u d g e was 5% s o l i d s o r 90.0-91.5% i f the f i n a l s l u d g e was f i r s t dewatered t o 60% s o l i d s . The magnesium l o s t i n the f i n a l 7 s l u d g e made up 2.9-7.1% o f the i n c i n e r a t e d f i n a l s l u d g e s o l i d s mass (as MgO). C a l c i u m l o s s e s i n t o the c a r b o n a t i o n s u p e r n a t a n t v a r i e d f r o m 2.0 t o 10.0% o f the t o t a l C a + + mass i n the system. In t h e T-20 e f f l u e n t runs w i t h f r e s h c h e m i c a l s , c o l o u r r e l e a s e was l e s s , r a n g i n g f r o m 17.0 t o 26.9%. From 68.0 t o 79.1% o f the M g + + s e n t t o c a r b o n a t i o n was r e c o v e r e d , assuming t h e f i n a l s l u d g e was 5% s o l i d s , o r 89.3-90.7% i f 60% s o l i d s was a c h i e v e d . In t h e s e t e s t s , t h e i n c i n e r a t e d f i n a l s l u d g e s o l i d s ( r e c a l c i n e d l i m e ) c o n t a i n e d 2.9-8.9% MgO. La r g e C a + + l o s s e s i n t o t h e s u p e r n a t a n t , r a n g i n g f r o m 11.5 t o 35.1% o f t h e t o t a l , were a l s o i n c u r r e d i n t h e s e r u n s , i n d i c a t i n g t h a t l o c a l i z e d a r e a s o f low pH were p r e s e n t i n the c a r b o n a t i o n column d u r i n g t h e s e t e s t s . E x p e r i m e n t s , u s i n g r e c y c l e d s u p e r n a t a n t as a s o u r c e o f M g + + , showed t h a t t h i s m a t e r i a l was an e f f e c t i v e s o u r c e o f magnesium i o n s f o r subsequent p r e c i p i t a t i o n by l i m e . However, i n o r d e r t o r e a c h a h i g h enough pH f o r > 9 0 % c o l o u r r e m o v a l , i n c r e a s e d l i m e dosages were r e q u i r e d t o overcome t he h i g h e r a l k a l i n i t y o f the s u p e r n a t a n t p l u s e f f l u e n t m i x t u r e . Dosages s e l e c t e d f r o m p r e l i m i n a r y j a r t e s t i n g o f BKME w i t h r e c y c l e d s u p e r n a t a n t were 60 mg/1 M g + + (80% r e c y c l e d + 20% makeup MgSO^) i n c o n j u n c t i o n w i t h 625 mg/1 C a ( 0 H ) 2 (475 mg/1 CaO). F o r the T-20 e f f l u e n t , dosages s e l e c t e d were 300 mg/1 M g + + (80% r e c y c l e d + 20% makeup MgS0 4) i n c o n j u n c t i o n w i t h 5000 mg/1 C a ( 0 H ) 2 (3750 mg/1 CaO) and 2 mg/1 o f an a n i o n i c polymer (Dow A-23). In the t e s t s w i t h r e c y c l e d M g + + the more c o n c e n t r a t e d s l u d g e e x t r a c t e d f r o m t h e b a t c h r e a c t o r appeared t o a f f e c t C 0 2 t r a n s f e r e f f i c i e n c y i n the c a r b o n a t i o n column, and l i k e l y r e s u l t e d i n a n o n u n i f o r m pH d i s t r i -b u t i o n ( a p p a r e n t l y p o c k e t s o f h i g h pH), t h e r e i n . T h i s r e s u l t e d i n s l i g h t l y 8 l e s s c o l o u r r e l e a s e , a r e d u c t i o n i n C a + + l o s s e s to the r e c y c l e d s u p e r -n a t a n t , and a l s o somewhat lower magnesium r e c o v e r y i n t h e s e t e s t s than w i t h f r e s h magnesium ( i . e . , 60.0 t o 82.3%). I t i s , t h e r e f o r e , v e r y i m p o r t a n t to i n s u r e t h a t complete m i x i n g and e f f i c i e n t C 0 2 d i f f u s i o n systems a r e p r o v i d e d i n f u t u r e s l u d g e c a r -b o n a t i o n t e s t s . However, i t s h o u l d be n o t e d t h a t the MgO c o n t e n t o f the r e c a l c i n e d l i m e d e c r e a s e d and o v e r a l l q u a l i t y o f t h e l i m e improved c o n s i d -e r a b l y o v e r the c o r r e s p o n d i n g runs u s i n g f r e s h c h e m i c a l s , because o f the b e t t e r C a + + r e t e n t i o n i n the s l u d g e . Hydrogen i o n a c t i v i t y p r o v e d t o be t h e main f a c t o r a f f e c t i n g the outcome o f t h e s l u d g e c a r b o n a t i o n t e s t s , and c o n t i n u o u s measurement o f pH i n the s l u d g e c a r b o n a t o r , t o a f i n a l pH o f a p p r o x i m a t e l y 7.5, was j u d g e d t o be t h e most e f f e c t i v e method o f c o n t r o l l i n g the b a t c h c a r b o n -a t i o n . F i n a l l y , t h e r e s u l t s o f t h i s s t u d y showed t h a t t h e s o - c a l l e d "Magnesium Carbonate P r o c e s s " , i n c o r p o r a t i n g magnesium r e c o v e r y by s l u d g e c a r b o n a t i o n , which was d e v e l o p e d f o r w a t e r t r e a t m e n t , can be a p p l i e d t o the d e c o l o u r i z a t i o n o f h i g h l y c o l o u r e d k r a f t m i l l e f f l u e n t s . F u r t h e r r e -s e a r c h i s w a r r a n t e d . CHAPTER 3 BACKGROUND ON THE COLOUR PROBLEM 3.1 N a t u r e o f C o l o u r i n Pu l p M i l l E f f l u e n t s O r g a n i c c o l o u r i s a v e r y complex component o f n a t u r a l waters and some wa s t e w a t e r s . F o r de c a d e s , c o l o u r removal has been a c o n c e r n i n w a t e r t r e a t m e n t . B l a c k and C h r i s t m a n ( 1 9 6 3 ) , H a l l (1970) and o t h e r s have r e v i e w e d much o f the s i g n i f i c a n t r e s e a r c h i n t h i s a r e a . The c o l o u r i n p u l p and pa p e r m i l l e f f l u e n t s can be e x p e c t e d t o have u n i q u e p r o p e r t i e s o f i t s own, r e s u l t i n g f r o m t h e c h e m i c a l r e -a c t i o n s d u r i n g p u l p i n g and b l e a c h i n g . However, many o f the o b s e r v a t i o n s and r e s e a r c h t e c h n i q u e s o f w a t e r c h e m i s t s p r o v i d e e x c e l l e n t background i n f o r m a t i o n f o r i n v e s t i g a t i o n s o f p u l p and p a p e r m i l l c o l o u r c h a r a c t e r -i s t i c s . C o l o u r v a l u e s e n c o u n t e r e d i n m i l l wastes a re g e n e r a l l y s e v e r a l o r d e r s o f magnitude h i g h e r than s u r f a c e w a t e r v a l u e s . T y p i c a l c o l o u r v a l u e s f o r some p u l p and p a p e r e f f l u e n t s and o t h e r s u b s t a n c e s o f g e n e r a l i n t e r e s t have been g i v e n by T y l e r and F i t z -g e r a l d (1972) as f o l l o w s : B l e a c h e d k r a f t m i l l e f f l u e n t 2,000 APHA U n i t s C a u s t i c E x t r a c t 20,000 APHA U n i t s Unbleached k r a f t m i l l e f f l u e n t 700 APHA U n i t s Magnesium base s u l p h i t e e f f l u e n t 5,000 APHA U n i t s C o f f e e 10,000 -15,000 APHA U n i t s C o c a - C o l a 8,500 APHA U n i t s D r a f t b e e r 1 ,000 APHA U n i t s N o r t h e r n Canadian w a t e r s 100-200 APHA U n i t s 10 R e s u l t s o f a s t u d y by B.C. Research (1973) o f c o l o u r l e v e l s i n n i n e B.C. k r a f t m i l l s (shown i n T a b l e 1) more o r l e s s c o n f i r m t h e s e g e n e r a l i z a t i o n s and a l s o show the l a r g e f l u c t u a t i o n i n c o l o u r o f the same e f f l u e n t s t r e a m . TABLE 1 COLOUR LEVELS* OF VARIOUS PROCESS SEWERS AND COMBINED OUTFALLS FOR NINE B.C. BLEACHED KRAFT MILLS (B.C. RESEARCH, 1973) COLOUR IN EFFLUENT STREAM IN APHA.UNITS M i l l U n b l eached C a u s t i c A c i d Whole M i l l White Water B l e a c h B l e a c h O u t f a l l A 1248 + 530 11430 + 1747 646 + 148 3664 + 1293 B 3504 + 1565 9472 + 2380 854 + 185 2544 + 708 C 1464 + 601 5484 + 3125 617 + 105 3668 + 886 D 3724 + 1101 4448 + 1197 1121 + 449 2600 + 670 E 782 + 319 7376 + 1857 678 + 249 2308 + 420 F 1359 + 1119 9254 + 1236 1121 + 449 1992 + 299 G 966 + 504 9420 + 2692 868 + 365 H 2664 + 943 16964 + 3301 2341 + 1210 2160 + 872 I 1707 + 636 22780 + 5925 1600 + 693 2222 + 511 * V a l u e s e x p r e s s e d as means and s t a n d a r d d e v i a t i o n s , r e p r e s e n t i n g 10 s a m p l i n g d a t e s . S i m i l a r i n v e n t o r i e s o f m i l l p r o c e s s c o l o u r l o a d s and s o u r c e s s h o u l d be c o n d u c t e d a t o t h e r m i l l s to get a c l e a r e r p i c t u r e o f the c o l o u r c o n t r i b u t i o n o f v a r i o u s e f f l u e n t s . I t i s g e n e r a l l y a c c e p t e d t h a t a l m o s t a l l t he c o l o u r o f t h e s e e f f l u e n t s i s due t o l i g n i n and/or l i g n i n d e r i v a t i v e s ( H a r t l e r and N o r r s t r o m , 1969 and Swanson e t a l , 1973). However, the p a r t i c u l a r method o f wood p u l p i n g and p u l p b l e a c h i n g employed w i l l d e t e r m i n e t h e c h a r a c t e r i s t i c s 11 o f t h e e f f l u e n t and t h e n a t u r e o f the chromophores ( c o l o u r p r o d u c i n g s t r u c t u r e s ) p r e s e n t . Fundamental r e s e a r c h has been g o i n g on i n the f i e l d o f l i g n i n c h e m i s t r y f o r more than 100 y e a r s b u t o n l y r e c e n t l y has the s t r u c t u r e o f th e l i g n i n m o l e c u l e been f a i r l y w e l l e l u c i d a t e d . A l t h o u g h a p r e c i s e d e f -i n i t i o n o f l i g n i n i s d i f f i c u l t , i t can be t e n t a t i v e l y d e s c r i b e d as a h i g h l y b r a n c h e d o r c r o s s - l i n k e d , h i g h l y r e a c t i v e polymer w i t h a s u b u n i t o f m o l e c u l a r w e i g h t o f about 840. S e v e r a l reviews o f l i g n i n c h e m i s t r y have been c o m p i l e d , e.g., Sarkanen and Ludwig (1971) and Marton (196 6 ) , and p a r t s o f t h e s e two r e f e r e n c e s a r e d e v o t e d t o the r e a c t i o n s i n v o l v e d i n c h e m i c a l p u l p i n g and b l e a c h i n g p r o c e s s e s . In a d d i t i o n , works by G i e r e r (1970), G i e r e r e t a l (1973 ) , and Bodenheimer and Smith (1966) p r o v i d e d f u r t h e r d e t a i l s on p u l p i n g and b l e a c h i n g r e a c t i o n s . D u r i n g p u l p i n g , n a t u r a l l i g n i n o f wood i s d i s s o l v e d by a p p r o p r i a t e c h e m i c a l s i n p r e p a r a t i o n f o r su b s e q u e n t s e p a r a t i o n f r o m t he wood f i b r e s d u r i n g p u l p b l e a c h i n g . Important r e a c t i o n s i n t h e s e p r o c e s s e s i n c l u d e s u l p h o n a t i o n , s u l p h i d i z a t i o n , and h y d r o l y s i s i n p u l p i n g and h a l o g e n a t i o n and o x i d a t i o n i n b l e a c h i n g . These r e a c t i o n s r e s u l t i n v a r i o u s s t r u c t u r a l u n i t s , w hich g i v e r i s e t o e f f l u e n t c o l o u r . A l t h o u g h a g r e a t d e a l o f e f f o r t has been aimed a t i d e n t i f y i n g t h e s p e c i f i c c h r o m o p h o r i c groups r e s -p o n s i b l e f o r c o l o u r , t h e m a t t e r has n o t been c o m p l e t e l y r e s o l v e d . Most o f t h e r e s e a r c h t o date has d e a l t w i t h k r a f t e f f l u e n t s , s i n c e t h i s i s the predominant p u l p i n g p r o c e s s i n use. K r a f t p u l p i n g g i v e s r i s e t o t h i o l i g n i n and a l k a l i l i g n i n whereas s u l p h i t e p u l p i n g r e s u l t s i n 1 i g n o s u l p h p n i c a c i d o r p r e c i p i t a t e d l i g n o s u l p h o n a t e . I t i s r e c o g n i z e d t h a t r e a c t i o n s i n t h e b l e a c h p l a n t f u r t h e r de-grade t h e p r o d u c t s o f the p u l p i n g o p e r a t i o n , so t h a t t he b l e a c h p l a n t e f f l u e n t s c o n t a i n a c o m b i n a t i o n o f p r e - e x i s t i n g chromophores and some 12 a d d i t i o n a l p o s s i b l y d i f f e r e n t , c h r o m o p h o r i c groups c r e a t e d by the b l e a c h i n g p r o c e s s i t s e l f . F a l k e h a g e t a l (1966) have summarized the major chromo-p h o r i c systems i n k r a f t l i g n i n as CH = CH double bonds c o n j u g a t e d w i t h the a r o m a t i c r i n g and q u i n o n e m e t h i d e s and q u i n o n e s which may a l s o s e r v e as o x i d a t i v e s p e c i e s c r e a t i n g f u r t h e r c h r o m o p h o r i c s t r u c t u r e s . M i n o r systems i d e n t i f i e d i n c l u d e d c h a l c o n e s t r u c t u r e s , f r e e r a d i c a l s and metal complexes w i t h c a t e c h o l s t r u c t u r e s . I t i s assumed t h a t a c o m p o s i t e s e r i e s o f chromophore systems a r e p r e s e n t i n t h e s e e f f l u e n t s . Subsequent s t u d i e s by the N a t i o n a l C o u n c i l f o r A i r and Stream Improvement (NCASI) B u l l e t i n #239 and #242, (1970 ) , M e s h i t s u k a and Nakano (1973) and o t h e r s have p r o v i d e d more s p e c i f i c i n f o r m a t i o n which c o n f i r m s t h e s e g e n e r a l i z a t i o n s . In p a r t i c u l a r , o- and p- benzoquinone a r e f o u n d to be i m p o r t a n t i n t e r m e d i a t e c o l o u r e d s t r u c t u r e s formed i n b l e a c h i n g and the p r e s e n c e o f m e t a l s , p a r t i c u l a r l y i r o n , has been shown by the l a t t e r a u t h o r s to c o n t r i b u t e t o the c o l o u r o f l i g n i n d e g r a d a t i o n p r o d u c t s . Complexing o f i r o n w i t h o r g a n i c m o l e c u l e s was a l s o r e p o r t e d i n NCASI B u l l e t i n #273 (1974) but i n s u f f i c i e n t work has been done on t h i s t o d e t e r -mine the r o l e o f m e t a l s i n c o l o u r f o r m a t i o n . S e v e r a l p r o p e r t i e s o f the v a r i o u s chromophores a r e i m p o r t a n t when c o n s i d e r i n g c o l o u r removal t e c h n i q u e s . The more note w o r t h y o f t h e s e i n c l u d e i ) p o l y d i s p e r s i t y i i ) s o l u b i l i t y i i i ) pH dependence i v ) e l e c t r i c a l c h a r g e . One o f the most i m p o r t a n t c h a r a c t e r i s t i c s o f l i g n i n s and t h e i r d e g r a d a t i o n p r o d u c t s i s t h e i r p o l y d i s p e r s e n a t u r e ( e x i s t o v e r a wide range o f m o l e c u l a r w e i g h t s ) . M o l e c u l a r w e i g h t d i s t r i b u t i o n s o f o r g a n i c s i n p u l p m i l l e f f l u e n t s have been s t u d i e d u s i n g gel f i l t r a t i o n t e c h n i q u e s as r e v i e w e d and d e s c r i b e d by Obiaga and G a n c z a r c z y k (1972) and o t h e r s . An example o f a m o l e c u l a r w e i g h t d i s t r i b u t i o n , f o r a l k a l i l i g n i n , i s shown i n 13 F i g u r e 2. V a r y i n g d i s t r i b u t i o n s have been f o u n d f o r d i f f e r e n t e f f l u e n t s ; however, a l l have d e m o n s t r a t e d p o l y d i s p e r s i t y . In g e n e r a l , m o l e c u l a r w e i g h t s i n k r a f t p u l p m i l l e f f l u e n t s seem t o be lower than i n s u l p h i t e l i q u o r s , and both p u l p i n g e f f l u e n t s c o n t a i n more h i g h m o l e c u l a r w e i g h t c o l o u r e d compounds than do b l e a c h i n g e f f l u e n t s because o f the f u r t h e r d e g r a d a t i o n w hich o c c u r s d u r i n g b l e a c h i n g . T y p i c a l l y a combined m i l l e f f l u e n t may c o n t a i n m a t e r i a l s o f M.W. 5 400 t o £ 1 5 0 , 0 0 0 . F u r t h e r i n f o r m a t i o n on the p o l y d i s p e r s e n a t u r e i s p r o v i d e d i n p a p e r s by S h o t t o n e t a l ( 1 9 7 2 ) , McNaughton e t a l ( 1 9 6 7 ) , Pew and Connors (1971) C o l l i n s e t a l (1967) and ( 1 9 7 1 ) , and S o u n d a r a r j a n and Wayman (1970). The f a c t t h a t the c o l o u r e d m a t e r i a l s e x i s t o v e r a b r o a d m o l e c u l a r w e i g h t range compounds t h e p r o b l e m o f c o l o u r removal and c o m p l i c a t e s the s t u d y o f removal mechanisms. F o r example, t h e r e i s c o n s i d e r a b l e e v i d e n c e t h a t v a r i o u s s i z e f r a c t i o n s o f the l i g n i n compounds are r e s p o n s i b l e f o r d i f f e r e n t amounts o f the t o t a l c o l o u r . Rankin and Benedek (1973) have shown t h a t i n t e r m e d i a t e m o l e c u l a r w e i g h t m a t e r i a l ( a v e r a g e m.w. 5,600) i s r e s p o n s i b l e f o r t h e l a r g e s t p o r t i o n o f t h e c o l o u r o f a l k a l i n e I n d u l i n (a commercial k r a f t l i g n i n ) w h i l e h i g h m o l e c u l a r w e i g h t s p e c i e s (^150,000 m.w.) e x e r t v e r y l i t t l e c o l o u r and s m a l l e r ( 1 4 , 0 0 0 m.w.) c o n s t i t u e n t s c o n t r i b u t e some c o l o u r . The e f f e c t o f pH on o r g a n i c c o l o u r e d m a t e r i a l i s w e l l known from e a r l i e r work i n the water t r e a t m e n t f i e l d . A s i m i l a r pH - c o l o u r i n t e n -s i t y r e l a t i o n e x i s t s i n m i l l e f f l u e n t s . T h a t i s , as pH goes up, c o l o u r goes up. The c o n v e r s e o c c u r s when t h e pH i s l o w e r e d . The i n t e r a c t i o n i s r e v e r s i b l e but n o t l i n e a r as d e m o n s t r a t e d by a c u r v e o b t a i n e d by H e r s c h -m i l l e r (1972) i n F i g u r e 3. T h i s r e l a t i o n s h i p i s a t t r i b u t e d to changes i n t h e q u i n o i d s t r u c t u r e p r e s e n t i n t h e s e w a s t e s. Somewhat s i m i l a r c u r v e s have been f o u n d by o t h e r s . SO 50 20 5 0.5 xlO 3  i i l I i M.W. ELUTION VOLUME, mi FIGURE 2.TYPICAL MOLECULAR WEIGHT DISTRIBUTION OF ALKALI LIGNIN OF SIMULATED KRAFT PULP MILL WASTEWATER. ( G A N C Z A R C Z Y K and O B I A G A . 1 9 7 4 ) 4 * 2600 CO § 2200 o O i Q. CC Z) O _i O o 1800-1400-1000 FIGURE3.COLOUR Vs. pH FOR TOTAL MILL EFFLUENT (HERSCHMILLER 1972) 16 Marton (1971) d i s c u s s e s the pH r e l a t e d b e h a v i o u r o f l i g n i n s . In aqueous s o l u t i o n , l i g n i n s behave as h y d r o c o l l o i d s ; t h e y are s o l u b l e a t low c o n c e n t r a t i o n s i n the absence o f d i s s o l v e d s a l t s ; as the pH i s l o w e r e d n u c l e i s t a r t to form and a t pH 3 p r e c i p i t a t i o n commences. O t h e r s t u d i e s , i n c l u d i n g NCASI B u l l e t i n # 273, ( 1 9 7 3 ) , have n o t e d p r e c i p i t a t e f o r m a t i o n and c o n c u r r e n t c o l o u r d e c r e a s e when pH i s a d j u s t e d t o = 2.5 o r l e s s . Whether o r n o t l i g n i n d e r i v a t i v e s i n p u l p and p a per m i l l e f f l u e n t s are s o l u b l e depends on many f a c t o r s , i n c l u d i n g pH and m o l e c u l a r w e i g h t d i s t r i b u t i o n . I t i s l i k e l y t h a t , i n most c a s e s , a combined sys t e m o f s o l u b l e and c o l l o i d a l p a r t i c l e s w i l l be p r e s e n t . However, t h o s e i n a l k a l i n e k r a f t m i l l e f f l u e n t a r e g e n e r a l l y c o n s i d e r e d t o be s o l u b l e , w h i l e i n s u l p h i t e l i q u o r , t h e r e appears t o be two f r a c t i o n s o f l i g n o s u l p h -o n a t e s . These are termed <* and ^ f r a c t i o n s where t h e <* f r a c t i o n has h i g h m o l e c u l a r w e i g h t s g e n e r a l l y c o n s i d e r e d t o be c o l l o i d a l and fi has s m a l l mole-c u l a r w e i g h t m a t e r i a l u s u a l l y t h o u g h t o f as t r u l y s o l u b l e , ( P i l i n s k a y a e t a l , 1973). A n o t h e r f a c t o r which c o u l d have p a r t i c u l a r i n t e r e s t i n c o l o u r removal i s the e l e c t r i c a l charge on the c o l o u r p a r t i c l e s . A c c o r d i n g t o Swanson e t a l (1973), a l l f r a c t i o n s o f u n b l e a c h e d k r a f t m i l l e f f l u e n t c o l o u r have been f o u n d t o c a r r y a r e l a t i v e l y h i g h n e g a t i v e c h a r g e . Re-f e r e n c e s to e l e c t r o p h o r e t i c m o b i l i t y ( o r z e t a p o t e n t i a l ) o f c o l o u r i n o t h e r waste streams a r e s c a r c e ; however, i t i s g e n e r a l l y a c c e p t e d t h a t the m a j o r i t y o f t h e s e c o l o u r b o d i e s a r e n e g a t i v e l y c h a r g e d . In summary, i t can be s a i d t h a t b a s i c knowledge and u n d e r s t a n d -i n g o f t h e s e chromophore systems i s n o t w e l l d e v e l o p e d and c o n s i d e r a b l y more work i s n e c e s s a r y t o a c h i e v e t h i s . 17 3.2 E f f e c t s o f C o l o u r on R e c e i v i n g Waters A l t h o u g h s e v e r a l r e g u l a t o r y a g e n c i e s now e n f o r c e c o l o u r removal r e q u i r e m e n t s , and many methods o f d e c o l o u r i z a t i o n have been d e v e l o p e d , s u r p r i s i n g l y few s t u d i e s have been aimed a t q u a n t i f y i n g the e f f e c t s o f c o l o u r on the r e c e i v i n g e n v i r o n m e n t . The r e a s o n u s u a l l y g i v e n f o r r e q u i r i n g d e c o l o u r i z a t i o n o f p u l p m i l l e f f l u e n t s i s t h a t r e c e i v i n g w a t e r c o l o u r a t i o n i s a e s t h e t i c a l l y o b j e c -t i o n a b l e . However, i t i s n o t y e t known what magnitudes o f c o l o u r change can be d e t e c t e d o r which i n t e n s i t y o r hue p e o p l e c o n s i d e r u n p l e a s a n t o r i d e n t i f y w i t h p o l l u t i o n . The r e s u l t s o f a s t u d y by NCASI i n t h e U.S.A., d i s c u s s e d by Gellman and B e r g e r (1974) s h o u l d p r o v i d e some b a s i s f o r s e t t i n g c o l o u r removal g u i d e l i n e s f o r a e s t h e t i c r e a s o n s . In t h i s s t u d y , c o l o u r e d d i s c h a r g e s from s e v e r a l m i l l s i t e s were c a r e f u l l y v a r i e d o v e r a b r o a d range o f c o l o u r i n t e n s i t i e s . F or each c o l o u r l e v e l , the p e r c e p -t i o n s o f a s c r e e n e d p a n e l o f s t u d e n t s were s t u d i i e d a t a number o f v i e w i n g a n g l e s and under v a r y i n g shade c o n d i t i o n s . The f i n a l NASCI r e p o r t s h o u l d be a v a i l a b l e i n t h e n e a r f u t u r e . A n o t h e r much c i t e d e f f e c t o f c o l o u r i s t h a t i t causes problems a t downstream water t r e a t m e n t p l a n t s . Bouveng and L u n d s t e d t (1966) and (1968) and Hedburg e t a l (1968) have s t u d i e d the c o a g u l a n t r e q u i r e m e n t s f o r p r o d u c i n g p o t a b l e water from s u r f a c e water a f f e c t e d by k r a f t and s u l p h i t e m i l l e f f l u e n t s . They found t h a t both e f f l u e n t s caused i n c r e a s e d c o a g u l a n t (alum and a c t i v a t e d s i l i c a ) r e q u i r e m e n t s , w i t h the s u l p h i t e e f f e c t s b e i n g more pronounced than k r a f t . An e x t e n s i v e s t u d y by P r i h a (1971) o f 1 i g n o s u l p h o n a t e b u i l d u p i n a l a r g e F i n n i s h l a k e c o n f i r m e d t h e s e f i n d i n g s . The r e s u l t s o f t h i s s t u d y show t h a t , when l i g n o s u l p h o n a t e c o n t e n t i s moderate (3-5 mg/1), 18 l a k e w a t e r can be p u r i f i e d i n t o s a t i s f a c t o r y d r i n k i n g w a t e r w i t h conven-t i o n a l methods. When t h e c o n c e n t r a t i o n i n c r e a s e s , the n e c e s s a r y amount o f c h e m i c a l s a l s o i n c r e a s e s , and a p o i n t i s f i n a l l y r e a c h e d when s a t i s -f a c t o r y d r i n k i n g w a t e r cannot be o b t a i n e d even w i t h h i g h c h e m i c a l dosage. I t i s r e p e a t e d t h r o u g h o u t t h e l i t e r a t u r e t h a t l i g n i n - d e r i v e d c o l o u r b o d i e s a r e b i o l o g i c a l l y s t a b l e and a r e t h e r e f o r e u n a f f e c t e d by c o n v e n t i o n a l b i o l o g i c a l t r e a t m e n t systems. T h i s g e n e r a l i z a t i o n r e q u i r e s c l a r i f i c a t i o n , f o r t h e r e a r e r e s e a r c h e r s who f e e l t h a t m o d i f i e d b i o -l o g i c a l systems a r e c a p a b l e o f a c h i e v i n g a s i g n i f i c a n t degree o f c o l o u r removal ( e . g . , G a n c z a r c y k and O b i a g a (1974) and NSRF ( 1 9 7 4 ) ) . Some work has been c a r r i e d o u t to determine t h e e x t e n t o f l i g n i n d e g r a d a t i o n and the l o n g - t e r m BOD e x e r t e d on a r e c e i v i n g stream. Both K r o n e r and Moore (1954) and Raabe (1968) d i s c u s s e d l i g n i n d e g r a d a t i o n and c o n c l u d e d t h a t (i*:) t h e r e are m i c r o - o r g a n i s m s which a r e c a p a b l e o f d e g r a d i n g l i g n i n and ( i i ) de-g r a d a t i o n i s slow and u s u a l l y i n c o m p l e t e . K r o n e r and Moore (1954) s t u d i e d the p e r s i s t e n c e o f k r a f t l i g n i n i n water under v a r i o u s c o n d i t i o n s . They found t h a t 41-46% o f the l i g n i n r emained a f t e r 20 days o f a e r o b i c , d a y l i g h t d e g r a d a t i o n c o n d i t i o n s . They a l s o d e t e r m i n e d t h a t exposure t o s u n l i g h t and the p r e s e n c e o f a l g a e enhanced d e c o m p o s i t i o n . Raabe (1968) p r o v i d e d e v i d e n c e t h a t l i g n i n d e g r a d a t i o n does o c c u r i n r i v e r waters and t h i s d e g r a d a t i o n does e x e r t a l o n g - t e r m oxygen demand. The d e g r a d a t i o n o f l i g n i n o v e r a 100 day p e r i o d was s t u d i e d . R e s u l t s s u g g e s t e d t h a t 5-20 day BOD was r e l a t e d t o c a r b o h y d r a t e c o n t e n t , whereas l o n g term BOD was l a r g e l y due t o l i g n i n d e c o m p o s i t i o n . More r e c e n t d a t a from a s t u d y by Bouveng and Solyom (1973) c o n f i r m e d Raabes' (1968) work, ( i . e . , t h a t p u l p m i l l e f f l u e n t s c o n t a i n a r e a d i l y d e g r a d a b l e o r g a n i c f r a c -t i o n and a s l o w l y d e g r a d a b l e f r a c t i o n ) . They f o u n d t h a t u n b l e a c h e d k r a f t 19 m i l l e f f l u e n t s c o n t a i n a h i g h e r p o r t i o n o f the r e a d i l y d e g r a d a b l e f r a c t i o n than b l e a c h e d k r a f t e f f l u e n t s . P r i h a (1971) p o i n t s o u t t h a t d e g r a d a t i o n o f l i g n o s u l p h o n a t e s i s e x t r e m e l y s l o w , and even when c o u p l e d w i t h n a t u r a l f l o w t h r o u g h a w a ter system, i n p u t o f t e n exceeds o u t p u t . He o b s e r v e d a b u i l d - u p o f t h e s e p o l l u t a n t s i n a l a k e where t h e y e x h i b i t e d s t r a f i f i c a t i o n and s e a s o n a l c o n c e n t r a t i o n v a r i a t i o n . A n o t h e r more o b v i o u s c o n c e r n i s the p o s s i b i l i t y o f d e c r e a s e d l i g h t p e n e t r a t i o n , c a u s i n g a r e d u c t i o n o f p r i m a r y p r o d u c t i v i t y , the b a s i s o f t h e e n t i r e a q u a t i c f o o d c h a i n . A number o f p a p e r s i n c l u d i n g t h o s e by Edmondson (1956) and Jones (1966) d e s c r i b e t h e r e l a t i o n between l i g h t i n t e n s i t y and growth r a t e o f a l g a e . However, s u r p r i s i n g l y l i t t l e work has been done t o d e t e r m i n e What e f f e c t , i f any, p u l p m i l l c o l o u r has on p h o t o s y n t h e s i s by p h y t o p l a n k t o n . P a r k e r and S i b e r t (1973) s t u d i e d the impact o f a p u l p m i l l e f f l u e n t upon t h e A l b e r n i I n l e t i n B r i t i s h Columbia. They c o n c l u d e d t h a t i n c r e a s e d a b s o r p t i o n o f t h e l i g h t by the e f f l u e n t c o l o u r s i g n i f i c a n t l y r e d u c e d p h o t o s y n t h e s i s such t h a t i t was p a r t i a l l y r e s p o n s i b l e f o r the oxygen d e f i c i e n t c o n d i t o n i n the mixed zone and around the h a l o c l i n e . A more r e c e n t s t u d y o f the e f f e c t s o f p u l p m i l l c o l o u r on p h y t o -p l a n k t o n p r o d u c t i o n i n B. C. c o a s t a l w a t e r s , c o n d u c t e d by S t o c k n e r e t a l (1975) i n d i c a t e s t h a t c o l o u r may be a more s e r i o u s p r o b l e m than p r e v i o u s l y s u s p e c t e d . These r e s e a r c h e r s f o u n d t h a t c o l o u r i s one o f the c r i t i c a l f a c t o r s d e p r e s s i n g p r i m a r y p r o d u c t i v i t y around m i l l s and showed a l i n e a r r e l a t i o n s h i p between l i g h t a t t e n u a t i o n and c o l o u r . They a l s o found t h a t t h e r e was some t o x i c i t y a s s o c i a t e d w i t h t h e c o l o u r , but t h i s was l i m i t e d t o about a 150 meter r a d i u s around t h e m i l l o u t f a l l . F u r t h e r s t u d i e s a r e 20 underway to show more c o n c l u s i v e l y any t o x i c e f f e c t s o f c o l o u r - c a u s i n g compounds. A n o t h e r s i m i l a r s t u d y c o n d u c t e d by B. C. Research (Walden, 1975), u s i n g f r e s h w ater l a b o r a t o r y c u l t u r e s , has a l s o found p r i m a r y p r o d u c t i o n t o be s i g n i f i c a n t l y r e d u c e d by p u l p m i l l e f f l u e n t c o l o u r . T e s t i n g one s p e c i f i c green a l g a e a t normal daytime l i g h t i n t e n s i t i e s , p r i m a r y p r o -d u c t i o n was r e d u c e d by 80% a t a depth o f 1 metre i n a c o l o u r as low as 15 C. II. When a mixed c u l t u r e o f a l g a e from l o c a l B. C. l a k e s was t e s t e d under the same c o n d i t o n s , about 40% r e d u c t i o n i n p r i m a r y p r o d u c t i v i t y r e s u l t e d . F u r t h e r s t u d i e s a r e a l s o p l a n n e d by t h i s r e s e a r c h e r . 3.3 Summary o f C o l o u r Removal T e c h n o l o g y 3.3.1 G e n e r a l S e v e r a l comprehensive l i t e r a t u r e r e v i e w s o f c o l o u r removal t e c h -n o l o g y are a v a i l a b l e . The more r e c e n t o f t h e s e i n c l u d e r e p o r t s by T y l e r and F i t z g e r a l d ( 1 9 7 2 ) , V i n c e n t ( 1 9 7 4 ) , Gellman and B e r g e r (1974) and Rush and Shannon (1975). I t i s n o t p o s s i b l e i n the c o n t e x t o f t h i s r e p o r t t o p r o v i d e an i n - d e p t h l i t e r a t u r e r e v i e w . However, a b r i e f summary o f a l t e r -n a t i v e d e c o l o u r i z a t i o n methods i s p r e s e n t e d h e r e , w i t h more d e t a i l s on the e x i s t i n g f u l l - s c a l e l i m e t r e a t m e n t f a c i l i t i e s i n t h e U.S.A. C o l o u r r e d u c t i o n can be a c h i e v e d by e i t h e r t e c h n i c a l changes w i t h i n t h e m i l l o r by e f f l u e n t t r e a t m e n t systems. I n - p l a n t m o d i f i c a t i o n can range from s i m p l e i n e x p e n s i v e measures, such as p u l p washing and b e t t e r ' h o u s e k e e p i n g 1 p r a c t i c e s , t o complex t e c h n o l o g i c a l changes such as i n c r e m e n t a t i o n o f oxygen b l e a c h i n g . While i n - p l a n t changes s h o u l d be con-s i d e r e d and implemented as a f i r s t p r e f e r e n c e , e x t e r n a l t r e a t m e n t may o f t e n s t i l l be r e q u i r e d . T h e r e a r e a l a r g e number o f e x t e r n a l t r e a t m e n t methods which a r e 21 known to be t e c h n i c a l l y f e a s i b l e f o r c o l o u r removal from p u l p m i l l 2+ e f f l u e n t s . Chemical t r e a t m e n t methods, based on p r e c i p i t a t i o n w i t h Ca , 3+ 3+ Al , o r Fe , have advanced t o the s t a g e o f f u l l s c a l e a p p l i c a t i o n , as has t h e r e s i n s e p a r a t i o n method o f Uddeholm-Kamyr. The Rohm and Haas r e s i n p r o c e s s , membrane p r o c e s s e s ( b o t h u l t r a f i l t r a t i o n and r e v e r s e osmosis) and a c t i v a t e d carbon a d s o r p t i o n have been t e s t e d on a p i l o t s c a l e , w h i l e c e r t a i n foam s e p a r a t i o n t e c h n i q u e s , o z o n a t i o n , magnesium c o a g u l a t i o n and e x t r a c t i o n w i t h amines, have been examined on a s m a l l e r l a b o r a t o r y s c a l e . These are methods known to be t e c h n i c a l l y f e a s i b l e . Economics i s a n o t h e r q u e s t i o n t h a t remains t o be answered by f u r t h e r p i l o t and f u l l s c a l e t e s t i n g . S e v e r a l o t h e r methods have been and a r e b e i n g i n v e s t i g a t e d . These i n c l u d e i r r a d i a t i o n t r e a t m e n t , a d s o r p t i o n on f l y ash and o t h e r waste m a t e r i a l s , a d s o r p t i o n on a c t i v a t e d a l u m i n a , l a n d d i s p o s a l , e l e c t r o l y s i s and b i o l o g i c a l t r e a t m e n t . I t i s w e l l e s t a b l i s h e d t h a t c o n v e n t i o n a l b i o l o g i c a l t r e a t m e n t systems are i n e f f e c t i v e i n t h e removal o f c o l o u r from p u l p m i l l e f f l u e n t s . In f a c t , a c o l o u r i n c r e a s e a c r o s s waste s t a b i l i z a t i o n ponds i s n o t uncommon. Thus, some form o f advanced o r t e r t i a r y t r e a t m e n t i s n e c e s s a r y f o r c o l o u r removal. Chemical c o a g u l a t i o n t e c h n i q u e s have been d e v e l o p e d t o the g r e a t -e s t e x t e n t , w i t h t h e use o f aluminum and i r o n s a l t s most p o p u l a r i n E u r o p e , S c a n d i n a v i a and J a p a n , and l i m e t r e a t m e n t d o m i n a t i n g i n the U.S.A. F u l l s c a l e d e m o n s t r a t i o n o f c h e m i c a l t r e a t m e n t f a c i l i t i e s has shown t h a t 85-90% removal o f t h e t o t a l m i l l c o l o u r can be a c h i e v e d w i t h a l l t h r e e o f t h e s e p o p u l a r c o a g u l a n t s . G e n e r a l l y , t h i s has meant t h a t the r e s i d u a l c o l o u r i n t h e t r e a t e d e f f l u e n t has ranged from 100-200 C U . 3.3.2 C u r r e n t Lime Tr e a t m e n t T e c h n o l o g y i n the U n i t e d S t a t e s Because l i m e use and r e c o v e r y i s a l r e a d y an i n t e g r a l p a r t o f 22 k r a f t p u l p p r o d u c t i o n , l i m e t r e a t m e n t became t h e e a r l y f a v o u r i t e f o r e f f l u e n t d e c o l o u r i z a t i o n f o r economic r e a s o n s . In the l a s t few y e a r s s e v e r a l v a r i a t i o n s o f l i m e t r e a t m e n t methods have been p a t e n t e d and a t l e a s t f i v e o f t h e s e have been d e m o n s t r a t e d on a f u l l - s c a l e b a s i s . Re-s u l t s o f t h e s e a r e d i s c u s s e d v e r y b r i e f l y h e r e , t o g i v e some background on the l i m e dosages and c o l o u r removal e f f i c i e n c i e s a c h i e v e d by c u r r e n t l i m e p r a c t i c e s . 3.3.2.1 M a s s i v e Lime P r o c e s s A t the I n t e r n a t i o n a l Paper Company's b l e a c h e d k r a f t m i l l i n S p r i n g h i l l , L o u i s i a n a , dosages o f 10,000-30,000 mg/1 l i m e were used t o t r e a t c a u s t i c e x t r a c t i o n s t a g e and u n b l e a c h e d d e c k e r e f f l u e n t . A c c o r d i n g t o Oswalt and Land ( 1 9 7 3 ) , t h i s t r e a t m e n t r e s u l t e d i n about 72% r e d u c t i o n i n t he t o t a l m i l l e f f l u e n t c o l o u r . T r e a t e d e f f l u e n t had a pH o f about 12.2 and was r e c a r b o n a t e d b e f o r e d i s c h a r g e t o a waste s t a b i l i z a t i o n pond. In t h i s p r o c e s s , the l i m e - o r g a n i c s l u d g e i s i n t e g r a t e d i n t o the k r a f t r e c o v e r y system. 3.3.2.2 M o d i f i e d Lime (Lime Mud) P r o c e s s A l s o t e s t e d a t S p r i n g h i l l was a s y s t e m t h a t r e p l a c e s a p o r t i o n o f the l i m e dosage w i t h l i m e mud (CaCO^) from t h e r e c a r b o n a t i o n c l a r i f i e r . O s walt (1974) r e p o r t e d t h a t c o m b i n a t i o n s such as 3000 mg/1 o f f r e s h CaO p l u s 8,000 mg/1 o f l i m e mud worked as w e l l as the m a s s i v e l i m e p r o c e s s , f o r r e d u c i n g t h e c o l o u r o f the f i r s t c a u s t i c e x t r a c t i o n s t a g e e f f l u e n t . Foaming problems were n o t as s e r i o u s as w i t h massive l i m e t r e a t m e n t . 3.3.2.3 Minimum Lime P r o c e s s e s A t l e a s t t h r e e p a t e n t e d v e r s i o n s o f t h e minimum l i m e p r o c e s s have been, o r are b e i n g t e s t e d . These are based on a d d i t i o n o f a p p r o x i -m a t e l y s t o i c h i o m e t r i c q u a n t i t i e s ( w i t h r e s p e c t t o t h e average m o l e c u l a r w e i g h t o f the c o l o u r b o d i e s ) o f l i m e t o t h e c o l o u r e d e f f l u e n t . I n t e r s t a t e 23 Paper Company's v e r s i o n , a t t h e i r l i n e r b o a r d m i l l i n R i c e b o r o , G e o r g i a , was the f i r s t f u l l - s c a l e l i m e d e c o l o u r i z a t i o n f a c i l i t y i n the U.S.A. The most r e c e n t r e s u l t s , p u b l i s h e d by EPA ( 1 9 7 4 ) , s t a t e t h a t about 50% o f t h e t o t a l m i l l e f f l u e n t ( ~1200 C U . ) i s combined w i t h a l i m e dosage o f a p p r o x i m a t e l y 1000 mg/1 as C a ( 0 H ) 2 . A f t e r f l o c c u l a t i o n and c l a r i f i c a t i o n , the e f f l u e n t has a r e s i d u a l c o l o u r o f about 125 C U . and c o n t a i n s 700-750 mg/1 o f C a ( 0 H ) 2 a t a pH o f 12.2. The t h i n l i m e s l u d g e ( 2 % s o l i d s ) f r o m t h e c l a r i f i e r i s t h i c k e n e d to about 6% and d i s p o s e d o f i n a s l u d g e pond. A t the p r e s e n t t i m e , the h i g h l y - a l k a l i n e , d e c o l o u r i z e d e f f l u e n t undergoes n a t u r a l r e c a r b o n a t i o n i n a l a r g e s t a b i l i z a t i o n pond. F i n a l pH i s c:10.2 and a l l but 35 ppm o f t h e c a r r y - o v e r l i m e i s p r e c i p i -t a t e d i n the pond. E f f l u e n t r e c a r b o n a t i o n i s p l a n n e d f o r t h e n e a r f u t u r e a t t h i s mi 11. The G e o r g i a P a c i f i c Company's p a t e n t e d p r o c e s s was d e v e l o p e d a t the C r o s s e t t , Arkansas m i l l . Gould (1973) r e p o r t e d 90% c o l o u r removal on a b l e a c h e r y e f f l u e n t s t r e a m a t the Woodland Maine M i l l . However, a t C r o s s e t t , 2000-3000 mg/1, as CaO added t o t h e e f f l u e n t from the f i r s t c a u s t i c e x t r a c t i o n s t a g e o f the b l e a c h e r y , t y p i c a l l y , reduce c o l o u r v a l u e s o f 12,000 C U . down to 4000-5000 C U . Lime s l u d g e i s combined w i t h l i m e mud b e f o r e d e w a t e r i n g t o 60% s o l i d s and r e c o v e r y i n a k i l n . A t the C o n t i n e n t a l Can Company's m i l l i n Hodge, L o u i s i a n a , dosages o f a p p r o x i m a t e l y 1000 mg/1 o f GaO are added t o t h e e n t i r e e f f l u e n t o f an u n b l e a c h e d k r a f t - N e u t r a l S u l p h i t e Semi-Chemical (NSSC) m i l l , as r e p o r t e d by S p r u i l l (1973 and 1974). The s y s t e m t y p i c a l l y reduces c o l o u r from an a v e r a g e o f 1200 C U . t o 300 t o 400 C.U., w i t h i n d i v i d u a l removal e f f i c i e n c i e s b e i n g q u i t e dependent on NSSC p r o d u c t i o n ; t h i s a c c o u n t s f o r 60% o f t h e c o l o u r , a l t h o u g h i t i s l e s s than 25% o f t o t a l p r o d u c t i o n . Here, 24 e f f l u e n t i s r e c a r b o n a t e d and s l u d g e i s dewatered t o 35% s o l i d s and r e c o v -e r e d i n a k i l n . In g e n e r a l , c u r r e n t l i m e t r e a t m e n t t e c h n o l o g y has proven t o be f a i r l y r e l i a b l e and e f f e c t i v e . The main drawbacks appear t o be the l a r g e amount o f s l u d g e r e s u l t i n g from t he h i g h l i m e dosages, and a l s o the need f o r more e f f i c i e n t e f f l u e n t r e c a r b o n a t i o n systems. 25 CHAPTER 4 REVIEW OF-MAGNESIUM COAGULATION PROCESSES 4.1 H i s t o r i c a l Development T r a d i t i o n a l l y , l i m e , f e r r i c c h l o r i d e and alum have been the p r e -f e r r e d c o a g u l a n t s f o r water and wastewater c l a r i f i c a t i o n . U n t i l r e c e n t l y , the huge volumes o f c h e m i c a l s l u d g e s were viewed as more o f a n u i s a n c e than as a s e r i o u s p o l l u t i o n problem. But w i t h t h e i n c r e a s i n g t r e n d toward t e r t i a r y ( c h e m i c a l ) t r e a t m e n t o f e f f l u e n t s and the l a c k o f s a t i s f a c t o r y s l u d g e d i s p o s a l o r r e c o v e r y a l t e r n a t i v e s ( e s p e c i a l l y f o r the huge volumes o f alum s l u d g e b e i n g g e n e r a t e d ) , the p r o b l e m has been compounded. D e s p i t e p r o g r e s s b e i n g made i n the a r e a o f s l u d g e d i s p o s a l and r e c o v e r y , the r e p l a c e m e n t o f t h e s e t r a d i t i o n a l c h e m i c a l s , as p r i m a r y co-a g u l a n t s , now appears q u i t e p o s s i b l e . A p r o c e s s - r e c e n t l y r e p o r t e d by Thompson, S i n g l e y , and B l a c k (1972) uses magnesium c a r b o n a t e as a c o a g u l a n t . T h i s new t e c h n o l o g y stems d i r e c t l y from B l a c k ' s e a r l i e r work on e l i m i n a t i n g p o l l u t i o n from l i m e - s o d a water s o f t e n i n g s l u d g e s a t Dayton, Ohio. H e r e , B l a c k had been f a c e d w i t h t h e t a s k o f s e p a r a t i n g the h i g h l y h y d r a t e d Mg(0H) 2 f r o m the f i n e , c r y s t a l l i n e C aC0 3 o f the s l u d g e . T h i s was n e c e s s a r y f o r two r e a s o n s . 1. The Mg(0H) 2 has g e l a t i n o u s p r o p e r t i e s s i m i l a r to A U O H ) ^ , making the s l u d g e more d i f f i c u l t t o dewater t h a n i f CaCO^ were p r e s e n t a l o n e . 2. In cases where l i m e r e c o v e r y i s t o be p r a c t i s e d , a b u i l d u p o f i n s o l u b l e magnesium o x i d e w i l l r e s u l t i f t h e magnesium h y d r o x i d e i s n o t removed, b e f o r e c a l c i n a t i o n . Up t o t h a t t i m e , s e v e r a l p h y s i c a l methods o f Mg(0H) 2 s e p a r a t i o n , i n c l u d i n g s e l e c t i v e s o f t e n i n g and the use o f a c e n t r i f u g e t o s e l e c t i v e l y c l a s s i f y M g(0H) 2 i n t o the c e n t r a t e , had been attempted but were c o n s i d e r e d u n s u i t a b l e . 26 In 1957, B l a c k and E i d s n e s s d e m o n s t r a t e d t h a t Mg(0H) 2 c o u l d be s e l e c t i v e l y d i s s o l v e d from CaCO^ by u s i n g carbon d i o x i d e gas. T h i s has a l l o w e d s u b s e q u e n t c a l c i n a t i o n o f CaCO^ t o h i g h q u a l i t y CaO, a t Dayton, s i n c e 1958. F o r a t i m e , the s u p e r n a t a n t , c o n t a i n i n g d i s s o l v e d magnesium b i c a r b o n a t e , was d i s c h a r g e d t o a r e c e i v i n g stream. However, by 1970 t h e r e was c o n s i d e r a b l e p r e s s u r e f r o m l o c a l a u t h o r i t i e s t o remove t h i s d i s s o l v e d m a t e r i a l . T h i s l e d B l a c k and h i s a s s o c i a t e s t o the d i s c o v e r y o f a r e l a t i v e l y s i m p l e and i n e x p e n s i v e s y s t e m t o r e c o v e r the magnesium as MgC0 3-3H 20 from t h i s e f f l u e n t ( d e s c r i b e d by B l a c k , Shuey and F l e m i n g ( 1 9 7 1 ) ) . T h i s w i l l be d i s c u s s e d i n d e t a i l i n a l a t e r s e c t i o n on magnesium r e c o v e r y . D u r i n g the same p e r i o d , B l a c k d i s c o v e r e d t h a t f r o t h f l o t a t i o n p r o v i d e d a h i g h l y s e l e c t i v e method o f s e p a r a t i n g r e l a t i v e l y pure c a l c i u m c a r b o n a t e from c l a y , s i l t and o t h e r c o n t a m i n a n t s . T h i s was t r u e o n l y i f t h e magnesium had been removed p r i o r t o f l o t a t i o n . U n t i l t h i s t i m e , l i t t l e a t t e n t i o n had been g i v e n t o magnesium as a c o a g u l a n t , a l t h o u g h magnesium h y d r o x i d e had l o n g been r e c o g n i z e d as an e f f e c t i v e c o a g u l a n t w i t h p r o p e r t i e s s i m i l a r t o t h e h y d r o l y s i s p r o d u c t s o f aluminum and f e r r i c i o n s . F l e n t j e ' s (1927) work a t the Oklahoma C i t y water t r e a t m e n t p l a n t i s an e a r l y example o f magnesium enhanced c o a g u l a t i o n . He a t t r i b u t e d improved c l a r i f i c a t i o n o f water w i t h i n c r e a s i n g l i m e dosage t o p r e c i p i t a t i o n o f magnesium h y d r o x i d e f r o m t h e h a r d w a t e r . F u r t h e r s t u d i e s p r o v e d magnesium c h l o r i d e to be an e f f e c t i v e c o a g u l a n t as w e l l . In o r d e r t o use t h e magnesium b i c a r b o n a t e n a t u r a l l y p r e s e n t i n the w a t e r , F l e n t j e o p e r a t e d the p l a n t u s i n g e x c e s s l i m e t r e a t m e n t i n c o n j u n c t i o n w i t h f e r r i c s u l p h a t e t o t r e a t h a r d , t u r b i d , r i v e r water. He r e p o r t e d no d e c r e a s e i n f i l t e r r u n s , l e s s a l g a e i n t h e s e t t l i n g b a s i n s and g r e a t e r b a c t e r i a l removal. No c o a g u l a n t r e c y c l e was attempted. 27 Much l a t e r Lecompte (1966) r e p o r t e d and p a t e n t e d a method o f r e c l a i m i n g p a p e r m i l l wastewaters by c o a g u l a t i o n w i t h magnesium c a r b o n a t e , which i s formed i n s i t u by a d d i n g f i n e magnesium o x i d e t o water c o n t a i n i n g c a l c i u m b i c a r b o n a t e . T h i s i s i l l u s t r a t e d by the f o l l o w i n g r e a c t i o n : MgO + Ca ( H C 0 3 ) 2 ^ ; MgC0 3 + C a C 0 3 + H 20 Then, when t h i s 'make up' water i s combined w i t h t h e i n f l u e n t waste s t r e a m and a s u f f i c i e n t l i m e dosage, a l l t h e M g C 0 3 i s p r e c i p i t a t e d as Mg(0H) 2. T h i s g e l a t i n o u s f l o e removes suspended m a t e r i a l and c o l o u r b o d i e s as i t s e t t l e s . As i n F l e n t j e ' s work, no attempt a t magnesium r e c o v e r y was men-t i o n e d , a l t h o u g h Lecompte d i d r e c o g n i z e t h a t r e c a r b o n a t i o n o f t h e e f f l u e n t would d i s s o l v e any c a r r y o v e r o f magnesium h y d r o x i d e f l o e i n t h e form o f magnesium b i c a r b o n a t e . O t h e r r e p o r t e d a p p l i c a t i o n s o f magnesium c o a g u l a t i o n i n c l u d e t h e use o f magnesium o x i d e as a f l u o r i d e a d j u s t m e n t agent by the AWWA (1971) and a p r o c e s s f o r i r o n removal from groundwater u s i n g magnesium o x i d e r e -p o r t e d by O'Connor and Benson (1970). In the f o r m e r , MgO i s u s u a l l y added i n t h e form o f a l i m e t h a t i s h i g h i n magnesium c o n t e n t . Both the f l u o r i d e s and the p r e c i p i t a t e d magnesium h y d r o x i d e a r e removed by s e t t l i n g and then d i s c a r d e d . The l a t t e r p r o c e s s i n v o l v e s t h e r a p i d m i x i n g o f s m a l l q u a n t i t i e s o f MgO i n t o a e r a t e d w a t e r , g i v i n g a l m o s t complete i r o n r e m o v a l . However, i n t h i s p r o c e s s , MgO i s d i s s o l v e d w i t h o u t the f o r m a t i o n o f Mg(0H) 2, so the i r o n removal may n o t be s t r i c t l y by c o a g u l a t i o n . O f t e n magnesium h y d r o x i d e forms i n a l k a l i n e s y s t e m s , a i d i n g s o l i d s removal as i t s e t t l e s , ( C u l p and Culp ( 1 9 7 1 ) ) . I t i s l i k e l y t h a t , i n t h i s way, many o f t h e t r e a t m e n t systems u s i n g l i m e a re enhanced c o n s i d -e r a b l y by Mg(0H) 2 p r e c i p i t a t i o n . A l t h o u g h an i n - d e p t h r e v i e w o f the l i t e r a t u r e would u n d o u b t e d l y 28 r e v e a l o t h e r s p e c i f i c a p p l i c a t i o n s o f magnesium f o r c o a g u l a t i o n i t i s s a f e t o say t h a t i t s ' use has n o t been w i d e s p r e a d . In g e n e r a l , i t can be s a i d t h a t , a l t h o u g h magnesium compounds a r e e f f e c t i v e c o a g u l a n t s , t h e y have not been used e x t e n s i v e l y because o f t h e i r h i g h c o s t s . Both magnesium s u l p h a t e and magnesium c h l o r i d e are more e x p e n s i v e than alum. So c a l l e d " b a s i c magnesium c a r b o n a t e " (4MgC0 3.Mg(0H) 2.5H 20) i s e x t r e m e l y e x p e n s i v e , and the t r i h y d r a t e form, MgCOy 3H 20, i s c u r r e n t l y u n a v a i l a b l e . T h i s was the s i t u a t i o n around 1970. C l e a r l y , what was needed was a cheap s o u r c e o f magnesium i o n s . E a r l i e r , Monis (1956) had l i s t e d f o u r major s o u r c e s o f magnesium c a r b o n a t e . 1. From d o l o m i t i c l i m e s t o n e 2. From deepwell b r i n e s 3. From b i t t e r n s o r "mother" l i q u o r s o b t a i n e d from s o l a r e v a p o r a t i o n o f s e a w a t e r f o r s a l t 4. From sea w a t e r w i t h o u t e v a p o r a t i o n , u s i n g l i m e t o p r e c i p i -t a t e o u t magnesium The l a t t e r method has been r e c e n t l y i n v e s t i g a t e d a t Nova S c o t i a Research F o u n d a t i o n (NSRF) ( r e p o r t e d by Rapson e t a l (1973)) as t h e b a s i s f o r c l a r i f i c a t i o n o f p u l p m i l l w a s t e s. T h i s p r o c e s s may p r o v i d e c o a s t a l f a c i l i t i e s w i t h cheap magnesium h y d r o x i d e formed i n s i t u and may even produce some e x c e s s magnesium s a l t s f o r s a l e . But such a p r o c e s s , even i f proven f e a s i b l e , w i l l have l i m i t e d a p p l i c a t i o n because o f i t s l o c a t i o n r e q u i r e m e n t s . However, a c o m b i n a t i o n o f t h i n g s now i n d i c a t e t h a t magnesium s h o u l d be g i v e n f u r t h e r c o n s i d e r a t i o n as a c o a g u l a n t . 1. Alum i s i n a p o s i t i o n o f i n c r e a s i n g d i s f a v o u r because o f s l u d g e d i s p o s a l problems. 2. W a t e r - s o f t e n i n g s l u d g e e l i m i n a t i o n i s now mandatory i n many a r e a s . 29 3. The most p r a c t i c a l d i s p o s a l o f w a t e r - s o f t e n i n g s l u d g e s l e a d s t o the r e c o v e r y o f l a r g e q u a n t i t i e s o f i n e x p e n s i v e magnesium c a r b o n a t e as (MgCOg.3^0). T h i s means a^  new l o s t c o s t s o u r c e  o f magnesium. U s i n g t h e s e t e r m s . o f r e f e r e n c e and B l a c k ' s f o u r b a s i c d i s c o v e r i e s : (1) s e p a r a t i o n o f magnesium h y d r o x i d e from c a l c i u m c a r b o n a t e ; (2) f l o t a t i o n o f c a l c i u m c a r b o n a t e from raw w a t e r i m p u r i t i e s ; (3) t h e use o f magnesium as a r e c y c l e d c o a g u l a n t ; and (4) t h e p r o d u c t i o n o f magnesium c a r b o n a t e f r o m s l u d g e s o f w a t e r s h i g h i n magnesium c o n c e n t r a t i o n , a new t e c h n o l o g y f o r w a t e r t r e a t m e n t has been d e v e l o p e d and d e s c r i b e d by Thompson, S i n g l e y and B l a c k ( 1 9 7 2 ) ; i t i s f u r t h e r o u t l i n e d i n an EPA r e p o r t (1971). The b a s i s o f t h i s new s y s t e m can be d e s c r i b e d b r i e f l y as f o l l o w s , and w i l l be d i s c u s s e d i n more d e t a i l l a t e r . Lime i s added t o a w a t e r which c o n t a i n s magnesium b i c a r b o n a t e o r has had magnesium c a r b o n a t e added. T h i s p r e c i p i t a t e s magnesium h y d r o x i d e and c a l c i u m c a r b o n a t e a t pH > 1 1 . The r e s u l t i n g s l u d g e i s c a r b o n a t e d t o s o l u b i l i z e t h e magnesium as magnesium b i c a r b o n a t e , which can be r e c o v e r e d by vacuum f i l t r a t i o n , the f i l t r a t e b e i n g r e c y c l e d and r e u s e d . The f i l t e r cake ( c o n t a i n i n g CaCC^ and suspended s o l i d s ) * c a n be e a s i l y h a n d l e d and d i s p o s e d o f as l a n d f i l l o r r e c a l c i n e d , as economics d i c t a t e . A l t h o u g h o r i g i n a l l y d e v e l o p e d as a w a t e r t r e a t m e n t p r o c e s s , t h e tremendous p o t e n t i a l o f such a r e c y c l e d c o a g u l a n t i s immed-i a t e l y o b v i o u s . To d a t e , t h e r e have been t h r e e EPA funded p r o j e c t s r e l a t e d to the Magnesium Carbonate P r o c e s s . The f i r s t was a l a b o r a t o r y s t u d y a t G a i n e s v i l l e , F l o r i d a by EPA (1971). The s e c o n d c o n s i s t e d o f f u l l - s c a l e d e m o n s t r a t i o n p r o j e c t s a t Montgomery, Alabama and M e l b o u r n e , F l o r i d a r e -p o r t e d by B l a c k , Crow and E i d s n e s s I n c . (1974) and t h e t h i r d p r o j e c t was f o r t h e t r e a t m e n t o f m u n i c i p a l and i n d u s t r i a l wastes (DuBose 1973). 30 From the w a t e r t r e a t m e n t s t u d i e s , a number o f s i g n i f i c a n t advan-tages o v e r the alum p r o c e s s have been i d e n t i f i e d . The p r i m a r y advantage i s t h a t t h e e x i s t i n g p r o b l e m o f s l u d g e d i s p o s a l can be e l i m i n a t e d o r a t l e a s t g r e a t l y r e d u c e d ('.depending on whether l i m e r e c a l c i n a t i o n i s economi-c a l ) . A l l w a t e r i s r e c y c l e d w i t h i n t h e p r o c e s s , a l o n g w i t h the t h r e e b a s i c w a t e r t r e a t m e n t c h e m i c a l s ; l i m e ; magnesium b i c a r b o n a t e ; and c a r b o n d i o x i d e . O t h e r advantages found were; (1) i n c r e a s e d f l o e s e t t l i n g r a t e s ; (2) s i m p l i c i t y o f o p e r a t i o n and c o n t r o l ; (3) reduced c o s t s when s l u d g e t r e a t m e n t and d i s p o s a l c o s t s a r e c o n s i d e r e d ; (4) more complete d i s i n f e c t i o n ; and (5) r e d u c e d c o r r o s i o n r a t e . With r e s p e c t to advanced t r e a t m e n t o f m u n i c i p a l w a s t e s , magnesium i s known t o a f f e c t phosphorus removal by l i m e p r e c i p i t a t i o n . Ferguson and M c C a r t y (1971) f o u n d t h a t i f t h e pH o f c o a g u l a t i o n i s below 9.0, t h e p r e s e n c e o f magnesium d e c r e a s e s phosphorus removal but a t h i g h pH, r e -moval was enhanced by magnesium. V i n c e n t (1974) found phosphorus removals o f 90-95% from p u l p m i l l e f f l u e n t c o n t a i n i n g i n i t i a l c o n c e n t r a t i o n s r a n g i n g f r o m .7 t o 31.5 mg/1 (as P ) . DuBose (1973) c a r r i e d o u t p i l o t p l a n t t r e a t m e n t o f raw m u n i c i p a l wastes o f v a r i o u s s t r e n g t h s , r a n g i n g from t o t a l COD < 400 t o t o t a l COD > 8 0 0 . H i s d a t a showed t h a t 10% t o 30% more t o t a l COD and much g r e a t e r r e d u c t i o n i n p h o s p h a t e , suspended s o l i d s and c o l o u r c o u l d be removed by c o a g u l a t i o n w i t h r e c y c l e d magnesium b i c a r b o n a t e and l i m e than c o u l d be a t t a i n e d by l i m e t r e a t m e n t a l o n e . T h i s s t u d y a l s o d e m o n s t r a t e d t h a t .the s u p e r i o r i t y o f magnesium p l u s l i m e o v e r l i m e a l o n e was even more pronounced as the s t r e n g t h o f the waste i n c r e a s e d and recommended f u r t h e r s t u d i e s on i n d u s -t r i a l wastes o r d i g e s t e r s u p e r n a t a n t . A c c o r d i n g t o B l a c k , Crow and E i d s n e s s ( 1 9 7 4 ) , t h e U.S.A. EPA has completed some a n a l y s i s o f t r a c e m e t a l s i n both alum and magnesium t r e a t e d 31 w a t e r s . The magnesium t r e a t e d w a t e r showed metal c o n c e n t r a t i o n s a t l e a s t 50% l o w e r than t h o s e f o u n d i n the alum t r e a t e d w a t e r s . There were no s i g n i f i c a n t c o n c e n t r a t i o n s o f m e t a l s f o u n d i n e i t h e r , however. As an example, c o p p e r was 17 p a r t s p e r b i l l i o n (ppb) i n the alum t r e a t e d w a ter and 9 ppb i n the magnesium t r e a t e d w a t e r . Z i n c was 190 ppb i n alum t r e a t e d w a t e r and 76 ppb i n the magnesium t r e a t e d water. These r e s u l t s a r e merely an i n d i c a t i o n o f the p o t e n t i a l o f magnesium c o a g u l a t i o n f o r removal o f t r a c e m e t a l s , whether from w a t e r , m u n i c i p a l sewage o r i n d u s t r i a l wastewater. T h i s s h o u l d be i n v e s t i g a t e d i n more d e t a i l . To date t he o n l y major i n d u s t r i a l a p p l i c a t i o n s o f magnesium c o-a g u l a t i o n r e p o r t e d have been i n p p u l p and paper m i l l c l a r i f i c a t i o n . Lecompte's work, mentioned p r e v i o u s l y , has formed the b a s i s o f a lime-magnesium s y s t e m f o r r e n o v a t i o n o f paper m i l l wastewaters a t K i m b e r l e y - C l a r k ' s m i l l i n Hunts-v i l l e , O n t a r i o , (Gropp and Montgomery, 1972). 80-100% o f the p r o c e s s water i s t r e a t e d w i t h dosages as low as 5-10 mg/1 M g + + and 150 mg/1 CaO and r e c y c l e d t o t h e p a p e r machine. P r e l i m i n a r y i n v e s t i g a t i o n s o f k r a f t p u l p m i l l d e c o l o u r i z a t i o n by B l a c k , Crow and E i d s n e s s (1974) and V i n c e n t (1974) i n d i c a t e t h a t a com-b i n a t i o n o f l i m e and magnesium would p r o v i d e s e v e r a l advantages o v e r con-v e n t i o n a l l i m e t r e a t m e n t . The f o r m e r r e s e a r c h e r s f o u n d t h a t l i m e dosages o f about 400 mg/1 as C a ( 0 H ) 2 a l o n g w i t h 60-90 mg/1 o f M g + + r e s u l t e d i n 90% c o l o u r r e m o v a l , and 72% COD removal f r o m t o t a l k r a f t m i l l e f f l u e n t c o n t a i n -i n g 530 C U . o f c o l o u r and 592 mg/1 COD. P r e l i m i n a r y l a b o r a t o r y s t u d i e s by V i n c e n t i n Quebec i n d i c a t e t h a t 500 mg/1 o f l i m e and 30-60 mg/1 magnesium have about t he same d e c o l o u r i z i n g e f f e c t on k r a f t m i l l e f f l u e n t s as 1000 mg/1 o f l i m e a l o n e and r e s u l t s i n s m a l l e r volumes o f s l u d g e t o h a n d l e . With a l l t y p e s o f e f f l u e n t examined, the use o f magnesium h y d r o x i d e formed " i n s i t u " a l l o w e d much l o w e r l e v e l s o f l i m e t o be used than i f l i m e were used a l o n e . 32 From t h e s e v e r y p r e l i m i n a r y s t u d i e s , i t a p p e a r s t h a t magnesium enhanced l i m e t r e a t m e n t has s e v e r a l advantages o v e r c o n v e n t i o n a l l i m e de-c o l o u r i z a t i o n p r o c e s s e s . These i n c l u d e , (1) e a s i e r s l u d g e h a n d l i n g , (2) l o w e r c o a g u l a t i o n pH o f about 11.3, thus r e s u l t i n g i n a l o w e r c a l c i u m l o s s i n the d e c o l o u r i z e d e f f l u e n t and l e s s time r e q u i r e d f o r n a t u r a l pH s t a b i l i z a t i o n a f t e r d i s c h a r g e ; (3) c h e m i c a l c o s t s b e i n g s u b s t a n t i a l l y l e s s , (4) p o s s i b l y b e t t e r removal e f f i c i e n c i e s f o r c o l o u r and o t h e r p a r a m e t e r s ; and (5) h o p e f u l l y , t o x i c i t y b e i n g e l i m i n a t e d , a l t h o u g h t h i s remains t o be shown. As p r e v i o u s l y - m e n t i o n e d , MgCO^.SH^O i s n o t a v a i l a b l e i n q u a n t i t i e s a t t h i s ' t i m e ; however, a c c o r d i n g t o B l a c k (1975) i t soon w i l l be. I f the pr.iice i s as low (5<£/lb) as a n t i c i p a t e d , t h i s c h e a p e r s o u r c e o f magnesium i o n s would make c o n s i d e r a b l e improvements i n the c o s t p i c t u r e o f any mag-nesium c o a g u l a t i o n system. T h i s appears t o be an added i n c e n t i v e f o r c u r r e n t and f u t u r e r e s e a r c h , a l t h o u g h use o f o t h e r magnesium s o u r c e s ( p a r t i c u l a r l y MgO o r d o l o m i t i c l i m e ) c o u l d be j u s t i f i e d i n some c a s e s . I t s h o u l d be e v i d e n t t h a t t h e r e s e a r c h t o date has m e r e l y s c r a t c h e d the s u r f a c e o f p o s s i b l e a p p l i c a t i o n s o f magnesium c o a g u l a t i o n . The p o t e n t i a l f o r p u l p and p a p e r waste t r e a t m e n t i s o n l y one p o s s i b l e a p p l i c a t i o n and s h o u l d be s t u d i e d i n d e t a i l . I t seems l i k e l y t h a t s i m i l a r b e n e f i t s would a r i s e i n t r e a t m e n t o f o t h e r c o l o u r e d e f f l u e n t s , such as t e x t i l e w a s t e s , as w e l l as i n t r e a t m e n t o f f l u o r i d e w a s t e s , removal o f t r a c e m e t a l s from m u n i c i p a l and i n d u s t r i a l w a s t e s , and s i l i c a removal t o meet i n d u s t r i a l w a t e r t r e a t m e n t r e q u i r e m e n t s , t o mention a few. I t i s , t h e r e f o r e , i m p o r t a n t t h a t t h i s new t e c h n o l o g y be e v a l u a t e d o v e r a wide range o f a p p l i c a t i o n s and be a p p l i e d where b e n e f i c i a l . 33 4.2 T h e o r e t i c a l C o n s i d e r a t i o n s o f Magnesium Treatment 4.2.1 Magnesium S o l u b i l i t y The water-works p r o f e s s i o n has l o n g been i n t e r e s t e d i n magnesium s o l u b i l i t y i n w a t e r because o f i t s c o n t r i b u t i o n t o water h a r d n e s s . Mag-nesium i s p r e s e n t t o some e x t e n t i n a l m o s t a l l n a t u r a l w a t e r s . I t i s found p r e d o m i n a n t l y i n c o n j u n c t i o n w i t h a n i o n s such as c a r b o n a t e , b i c a r -b o n a t e , h y d r o x i d e , c h l o r i d e , s u l p h a t e and n i t r a t e , d epending on the p a r -t i c u l a r e n v i r o n m e n t a l c o n d i t i o n s . The s o l u b i l i t y o f magnesium i s known t o depend on pH, t e m p e r a t u r e , a l k a l i n i t y and t o t a l i o n i c s t r e n g t h o f the s o l u t i o n . Stumm and Morgan (1970) s t u d i e d the e q u i l i b r i u m o f a s y s t e m c o n t a i n i n g o n l y magnesium, carbon d i o x i d e , and water. In such a s y s t e m , f o u r s o l i d phases o f magnesium can be f o u n d i n v a r i o u s c o n c e n t r a t i o n s , depending m a i n l y on pH. These a r e M g ( 0 H ) 2, ( B r u c i t e ) ; MgC0 3 ( M a g n e s i t e ) ; MgC0 3.3H 20 ( N e s g u e h o n i t e ) ; and M g 4 ( C 0 3 ) ( 0 H ) 2 . 3 H 2 0 ( H y d r o m a g n e s i t e ) . F i g -ure 4 shows the p H - s o l u b i l i t y r e l a t i o n s h i p o f t h i s s y s t e m f o r a t o t a l c a r -bonate c o n c e n t r a t i o n o f 10 M. MgC0 3.3H 20 i s l e s s s o l u b l e t h a n MgC0 3 f o r a l l pH v a l u e s ; t h u s , MgC0 3 i s n o t shown. I t can be seen t h a t f o r pH v a l u e s up t o 7.5, MgC0 33H 20 i s the l e a s t s o l u b l e . Between pH 7.5-pH 9.0 M g 4 ( C 0 3 ) 3 ( O H ) 2 - 3 H 2 0 . control's, s o l u b i l i t y and a t pH above 9.0, M g ( 0 H ) 2 i s by f a r the l e a s t s o l u b l e . Because l i m e - s o d a w a t e r s o f t e n i n g e f f i c i e n c y i s d i r e c t l y r e l a t e d t o t h e s o l u b i l i t y o f magnesium h y d r o x i d e , c o n s i d e r a b l e e f f o r t has gone i n t o d e t e r m i n i n g the s o l u b i l i t y p r o d u c t c o n s t a n t f o r Mg(0H) 2- The EPA (1971) s t u d y r e v i e w e d the l i t e r a t u r e and f o u n d many v a l u e s o f p k g p f o r M g ( 0 H ) 2 , r a n g i n g f r o m 10.31 to 11.38 a t t e m p e r a t u r e s around 25-30°C. T a b l e 2, from B l a c k ( 1 9 6 6 ) , shows c a l c u l a t e d v a l u e s f o r the t h e o r e t i c a l s o l u b i l i t y o f Mg(0H) 2 a t 25°C based on a k g p = 2.5 x l o - 1 1 . L a r s o n e t a a l (1959) used the s o l u b i l i t y p r o d u c t c o n s t a n t o f I r a v e r s and Nouvel (1929) t o i l l u s t r a t e 34 TABLE 2 SOLUBILITY OF Mg(0H) 2 AT 25°C •11 ks = 2.5 x l o S o l u b i l i t y pH mg/1 moles/1 gm/l As As.+ C a C 0 3 Mg 10.56 1.84 x l O " 4 1.07 X l O " 2 18.4 4.4 10.70 1.0 x l O " 4 5.8 X 1 0 " 3 10.0 2.4 10.80 6.3 x 1 0 " 5 3.6 X 1 0 " 3 6.3 1.5 10.90 4.0 x 1 0 " 5 2.3 X 1 0 " 3 4.0 1.0 11.00 2.5 X l O " 5 1.4 X 1 0 " 3 2.5 0.6 11.10 1.6 X 1 0 " 5 9.1 X l O " 4 1.6 0.4 11.20 , 2.8 X 1 0 " 5 1.6 X l O " 4 0.3 0.07 o B l a c k (1966) the i n f l u e n c e o f t e m p e r a t u r e and pH on the s o l u b i l i t y o f magnesium i n a w a t e r o f 50 mg/1 a l k a l i n i t y as shown i n F i g u r e 5. I t has been p o i n t e d o u t t h a t f o r e f f i c i e n t p r e c i p i t a t i o n , a h i g h e r pH i s needed w i t h c o l d w a t e r t h a n w i t h warm water. The w e l l known s e n s i t i v i t y o f pH t o t e m p e r a t u r e i s a l s o i n d i c a t e d b ut the e f f e c t s o f a l k a l i n i t y and t o t a l d i s s o l v e d s o l i d s a r e not shown. I f the o t t a l a l k a l i n i t y were 25 mg/1, t h e pH a t 167° would be .14 l o w e r ; w i t h a t o t a l a l k a l i n i t y o f 100 mg/1, the pH a t 167° would be .16 pH u n i t s h i g h e r , a c c o r d i n g t o the work o f L a n g l i e r (1946). A c c o r d -i n g t o the d a t a o f Naesaenen ( 1 9 4 1 ) , i f the t o t a l d i s s o l v e d m i n e r a l s was i n c r e a s e d from 200 mg/1 t o 600 mg/1, t h e s o l u b i l i t y would i n c r e a s e by about 30 p e r c e n t . In summary i t can be s a i d that: 1. An i n c r e a s e i n pH means a d e c r e a s e i n Mg(0H) 9 s o l u b i l i t y FIGURE 4. SOLUBILITY DIAGRAM FOR MAGNESIUM IN WATER AT ATMOSPHERIC CONDITIONS. TOTAL CARBONATE = 1 0 " 3 M. (STUMM AND MORGAN , 1970) 36 12.0 11.0 10.0 X Q . 9.0 8.01 30 v N SJ -10 -20 -40 -100 -200 • N 50 70 90 0 130 150 170 TEMPERATURE (°F) 190 210 MAGNESIUM SOLUBILITY (as parts per million C a C 0 3 ) BASED ON THE SOLUBILITY PRODUCT CONSTANTS OF TRAVERS AND NOUVEL . (1929) pH VARIATION FIGURE 5. TEMPERATURE INFLUENCE ON MAGNESIUM SOLUBILITY ( L A R S O N ET A L , 1959) 37 2. An i n c r e a s e i n a l k a l i n i t y means a d e c r e a s e i n Mg(0H)2 s o l u b i l i t y 3. An i n c r e a s e i n t e m p e r a t u r e means a d e c r e a s e i n MgCOH)^ s o l u b i l i t y 4. An i n c r e a s e i n t o t a l d i s s o l v e d s o l i d s means an i n c r e a s e i n Mg(0H)2 s o l u b i l i t y . In p r a c t i c e , t h e amount o f Mg(0H)2 s o l u b l e under g i v e n con-d i t i o n s has been found t o be h i g h e r than p r e d i c t e d by t h e o r y . T h i s i s i l l u s t r a t e d i n F i g u r e 6, t a k e n from r e c e n t s t u d i e s a t Montgomery, Alabama by B l a c k , Crow and E i d s n e s s (1974). T h i s d i f f e r e n c e a r i s e s from t he f a c t t h a t c o n d i t i o n s p r e v a i l i n g i n c o n t r o l l e d l a b o r a t o r y s t u d i e s a re seldom found i n l a r g e s c a l e o p e r a t i o n . D e t e n t i o n time i n p r a c t i c e i s o f t e n l o n g e r than i n j a r t e s t s g i v i n g more time f o r MgCOg.Sr^O t o r e s o l u b i l i z e . The t i m e - s o l u b i l i t y r e l a t i o n s h i p o f MgCO^-SH^O i s shown i n F i g u r e 7. In add-i t i o n , Stumm and Morgan (1970) s t a t e t h a t c o m p l e x a t i o n o f the magnesium, w i t h both i n o r g a n i c and o r g a n i c l i g a n d s , i n c r e a s e s t h e s o l u b i l i t y as does the f o r m a t i o n o f i o n p a i r s . ( In i o n p a i r s , the metal i o n and base are s e p a r a t e d by one o r more w a t e r m o l e c u l e s ; i n a complex, the l i g a n d i s i m m e d i a t e l y a d j a c e n t t o the metal c a t i o n ) . These a u t h o r s r e p o r t t h a t , w h i l e complex formers p r e s e n t i n a s o l u t i o n may o f t e n have l i t t l e o r no e f f e c t on t h e s o l u b i l i t y o f s o l i d s , t h e y may a f f e c t t he k i n e t i c s o f n u c l e a -t i o n and o f growth and d i s s o l u t i o n o f c r y s t a l s . In p r a c t i c e t h e s o l u -b i l i t y r e l a t i o n s h i p i,s used t o d e t e r m i n e r e c o v e r y e f f i c i e n c i e s and c o s t e s t i m a t e s f o r v a r i o u s t r e a t m e n t c o n d i t i o n s . S a t i s f a c t o r y t r e a t m e n t o f a w a t e r r e q u i r e s a s p e c i f i c amount o f magnesium h y d r o x i d e . The s o l u b i l i t y r e l a t i o n s h i p d e v e l o p e d f o r t h a t w a t er o r wastewater d e t e r m i n e s the amount o f magnesium s a l t t h a t must be added t o p r e c i p i t a t e t h i s amount a t v a r i o u s c o a g u l a t i o n pH's. JAR TESTS PILOT PLANT 1 1 i [ 1 1 1 i 1 1 1 10.50 10.60 10.70 10.80 10.90 11.00 11.10 11.20 11.30 11.40 11.50 COAGULATION pH FIGURE 6.COMPARISON OF THEORETICAL SOLUBILITY OF Mg(OH) 2 WITH OBSERVED JAR TEST AND PILOT PLANT VALUES. TBLACK enow & EIDSNESS 1 9 7 4 ) 39 24 H T3 CD ' Cd O T3 C W 03 o I X c? o cn 2CH 16 A O o •1— X cn 12 4 T o Heated to 103 C for 1 hr. as M g C 0 3 - 3 H 2 0 o 200°C for 2 hrs. as M g C 0 3 3 H 2 0 a 200°C for 2 hrs. as M g C 0 3 2 H 2 0 • Air dried as MgC0 3 - 3H 2O — i — 4 i 8 10 — i — 12 14 — i — 16 18 T I M E (min.) F IGURE 7 . S O L U B I L I T Y O F M g C 0 3 - X H 2 0 A S A F U N C T I O N O F T I M E F O R T H E I N D I C A T E D H Y D R A T E F O R M S ( E P A , 1971) A L K A L I N I T Y (mg/i a s C a C 0 3 ) F IGURE 8. L I M E R E Q U I R E D T O R A I S E T H E p H T O 11 A S A F U N C T I O N O F T H E R A W W A S T E W A T E R A L K A L I N I T Y (TCHOBANOGLOUS ,1970) 40 4.2.2 Chemical R e a c t i o n s The c h e m i s t r y o f magnesium c o a g u l a t i o n i s a r e l a t i v e l y s i m p l e c o m b i n a t i o n o f water s o f t e n i n g and c o n v e n t i o n a l c o a g u l a t i o n . Lime i s added t o r a i s e the pH t o about 11.1-11.3, so t h a t any magnesium p r e s e n t i s p r e c i p i t a t e d as magnesium h y d r o x i d e . Remembering t h a t a l l C 0 2 and HCOg must be c o n v e r t e d to C 0 3 b e f o r e t h e OH" c o n c e n t r a t i o n can i n c r e a s e , the well-known s o f t e n i n g r e a c t i o n s a r e summarized below. C 0 2 + C a " ( 0 H ) 2 ~ * C a C 0 3 f + H 20 (1) C a ( H C 0 3 ) 2 + Ca(0H) 2->- 2 C a C O ^ + 2H 20 (2) With f u r t h e r a d d i t i o n o f l i m e , magnesium b i c a r b o n a t e i s c o n v e r t e d t o mag-nesium h y d r o x i d e by the f o l l o w i n g two r e a c t i o n s . M g ( H C 0 3 ) 2 + C a ( 0 H ) 2 - > MgC0 3 + CaCOgf + 2H 20 (3) MgC0 3 + C a ( 0 H ) 2 - * - M g ( 0 H ) 2 | C a C 0 3 | (4) When magnesium i s p r e s e n t as MgCT,, o r MgSO^ ( i . e . , non c a r b o n a t e h a r d n e s s ) a r e a c t i o n such as (5) o c c u r s . MgS0 4 + C a ( 0 H ) 2 Mg(0H) 2i+ C a S 0 4 (5) I t s h o u l d be p o i n t e d o u t t h a t , when magnesium c a r b o n a t e i s used as a c o a g u l a n t , no i n c r e a s e i n t o t a l d i s s o l v e d s o l i d s r e s u l t a c c o r d i n g t o r e a c t i o n ( 4 ) . Whereas a d d i t i o n o f s a l t s o t h e r than magnesium c a r b o n a t e , would r e s u l t i n an i n c r e a s e i n n o n c a r b o n a t e hardness (and TDS) p r o p o r t i o n a l to the amount o f make-up s a l t added as shown i n r e a c t i o n ( 5 ) . From t h e s e r e a c t i o n s , i t would seem t h a t t h e amount o f l i m e r e q u i r e d f o r t h e removal o f f r e e c arbon d i o x i d e , b i c a r b o n a t e ( u s u a l l y t h e t o t a l ) a l k a l i n i t y and magnesium ( n o n c a r b o n a t e ) hardness from a p a r t i c u l a r w a t e r o r waste c o u l d be c a l c u l a t e d s t o i c h i o m e t r i c a l l y . However, i n p r a c t i c e i t has been found t h a t such c a l c u l a t i o n s cannot be used t o d e t e r m i n e t h e l i m e r e q u i r e m e n t s f o r magnesium p r e c i p i t a t i o n . S e v e r a l e m p e r i c a l r e l a t i o n -s h i p s , such as t h e graph, shown i n F i g u r e 8, have been used to e s t i m a t e 41 the l i m e r e q u i r e d t o re a c h a g i v e n pH v a l u e under v a r i o u s a l k a l i n i t y con-d i t i o n s . In magnesium c o a g u l a t i o n s t u d i e s , Thompson e t a l (1972) i l l u s -t r a t e d the n o n - s t o i c h i o m e t r i c r e q u i r e m e n t s o f l i m e by t h e f o l l o w i n g example. "A w a t e r , c o n t a i n i n g both o r g a n i c c o l o u r and t u r b i d i t y , has an a l k a l i n i t y o f 40 ppm and c o n t a i n s 30 ppm magnesium as CaCOo. J a r t e s t s show t h a t t h e minimum e f f e c t i v e amount o f Mg(0H)2 t o be p r e c i p i t a t e d w i l l be p r o v i d e d by a dosage o f 60 mg/1 MgC03.3rL0. The most e c o n o m i c a l t r e a t -ment o f t h i s w a ter r e q u i r e s t h a t t h e e n t i r e dosage o f 60 mg/1 MgC03.3rL0 be added and t h a t v e r y l i t t l e o f t h e 30 ppm o f magnesium p r e s e n t i n t h e w a t e r be used. Tfois r e s u l t s f r o m f o u r r e a s o n s : (a) The dosage o f l i m e r e q u i r e d f o r t h e p r e c i p i t a t i o n o f t h e r e q u i r e d amount o f Mg(0H) 2 from 90 mg/1 o f MgC0 3.3rL0 i s s i g n i f i c a n t l y l e s s than t h a t r e q u i r e d t o p r e c i p i t a t e a l l o f t n e Mg(0H) 2 from t h e 30 mg/1 o f MgCO, p r e s e n t p l u s 30 mg/1 added, and the pH i s s i g n i f i c a n t l y l o w e r . (b; The amount o f C0p r e q u i r e d t o r e d i s s o l v e t h e Mg(0H) 2 from the s l u d g e w i l l be the same i n e i t h e r c a s e , b u t , the amount o f (XL r e q u i r e d t o r r e e a r b o n a t e t he h i g h pH w a t e r o f the s e c o n d p r o c e d u r e w i l l be much g r e a t e r than t h e amount r e q u i r e d from t he lower pH w a t e r o f the f i r s t p r o c e d u r e . ( c ) A f t e r a few c y c l e s , a pH v a l u e i s i d e n t i f i e d a t which com-p l e t e r e c o v e r y and r e c y c l i n g o f c o a g u l a n t i s a c h i e v e d , a f t e r which no new c o a g u l a n t i s added. (d) The unused 30 ppm o f magnesium r e m a i n i n g i n t h e w a t e r r e p -r e s e n t s r e s e r v e c o a g u l a n t which can be used i f n e c e s s a r y by i n c r e a s e d l i m e a d d i t i o n " . F o r any p a r t i c u l a r t r e a t m e n t s y s t e m an economic a n a l y s i s i s r e q u i r e d t o i d e n t i f y optimum o p e r a t i n g c o n d i t i o n s . I t seems t h a t more e x p e r i e n c e i s r e q u i r e d , o v e r a wide range o f c o n d i t i o n s , to d e v e l o p a good method f o r d e t e r m i n i n g minimum l i m e dosages r e q u i r e d . A n o t h e r dosage t h a t must be d e t e r m i n e d i s the amount o f magnesium r e q u i r e d t o p r o v i d e s u f f i c i e n t Mg(0H)2 f l o e f o r t r e a t m e n t . U s u a l l y , j a r t e s t s a r e used t o i d e n t i f y c o a g u l a n t dosage r e q u i r e m e n t s . However, Thompson e t a l (1972) have d e v e l o p e d an e q u a t i o n f o r e s t i m a t i n g t he r e q u i r e d magnesium dosage f o r w a t e r t r e a t m e n t , u s i n g s t e p w i s e l i n e a r r e g r e s s i o n t e c h n i q u e s . Minimum magnesium dosage was dependent on o r g a n i c c o l o u r and t o a l e s s e r e x t e n t on t u r b i d i t y , a l k a l i n i t y and hardn e s s o f t h e w a t e r as shown by the f o l l o w i n g e q u a t i o n : 42 Minimum magnesium dosage (mg/1 o f MgCO-j.Sr^O) = 8.33 + 0.03 ( t u r b i d i t y i n JTU) + 0.46 ( o r g a n i c c o l o u r i n C U . ) - 0.03 ( t o t a l a l k a l i n i t y as mg/1 CaC0 3) + 0.14 ( t o t a l h ardness as mg/1 CaC0 3) I t may be p o s s i b l e t o d e v e l o p s i m i l a r e q u a t i o n s f o r any g i v e n w a t e r o r w a s t e w a t e r t o be t r e a t e d . In any c a s e , e x t e n s i v e j a r t e s t i n g i s r e q u i r e d t o s i m u l t a n e o u s l y o p t i m i z e the two c h e m i c a l dosage r e q u i r e m e n t s ( i . e . , l i m e and magnesium). 4.2.3 Magnesium Recovery A t p r e s e n t t h e r e a r e f o u r p o s s i b l e methods b e i n g c o n s i d e r e d f o r r e c o v e r i n g magnesium v a l u e s . These a r e : a. S e l e c t i v e s o f t e n i n g b. C e n t r i f u g a t i o n c. L e a c h i n g o f c a l c i n e d s l u d g e d. S l u d g e c a r b o n a t i o n a. S e l e c t i v e s o f t e n i n g can be employed t o s e p a r a t e magnesium hy-d r o x i d e from l i m e - s o d a w a t e r s o f t e n i n g s l u d g e s but i t i s a cumbersone method t h a t r e q u i r e s major s o f t e n i n g p l a n t changes and more c o m p l i c a t e d o p e r a t i n g c o n d i t i o n s . T h i s would be l i m i t e d to magnesium r e c o v e r y from w a t e r s o f t e n i n g s l u d g e s and would n o t be a p p l i c a b l e f o r g e n e r a l use i n c o a g u l a t i o n systems. b. P h y s i c a l s e p a r a t i o n o f s l u d g e i n t o i t s components by c e n t r i -f u g a t i o n has been i n v e s t i g a t e d . E a r l y s t u d i e s y i e l d e d n e g a t i v e r e s u l t s . But w i t h improved equipment and t e c h n i q u e s , r e c e n t s t u d i e s by D o r r - O l i v e r r e p o r t s u c c e s s f u l s e p a r a t i o n o f C a C 0 3 from Mg(0H) 2 and i n o r g a n i c s , :(Phundstein e t a l 1974). Lime s l u d g e s f r o m t e r t i a r y t r e a t m e n t f a c i l i t i e s as w e l l as s l u d g e s p r o d u c e d from w a t e r s o f t e n i n g p r o c e s s e s have been 43 c l a s s i f i e d u s i n g a combined a p p l i c a t i o n o f the d i s c n o z z l e , and the s o l i d bowl c e n t r i f u g e . The use o f t h e d i s c n o z z l e c e n t r i f u g e p r o v i d e s w ashing . o f t h e s l u d g e to c l a s s i f y magnesium h y d r o x i d e , h y d r o x y a p a t i t e and o r g a n i c m a t e r i a l t o produce r e l a t i v e l y c l e a n C a C 0 3 f o r c a l c i n a t i o n . S l u d g e i s a l s o t h i c k e n e d i n t h i s s t a g e , to a l l o w optimum f e e d to the s o l i d bowl c e n t r i f u g e , where f u r t h e r c l a s s i f i c a t i o n and d e w a t e r i n g o c c u r s . High r e j e c t i o n s o f Mg(0H)2 and i n e r t s a r e r e p o r t e d a l o n g w i t h h i g h r e c o v e r y o f CaC0 3. I f t h i s equipment s t a n d s up t o f u r t h e r f i e l d t e s t i n g , then c e n t r i f u g a l s e p a r a t i o n o f magnesium h y d r o x i d e from c a l c i u m c a r b o n a t e may p r o v i d e a v i a b l e method f o r magnesium r e c o v e r y . c. L e a c h i n g o f c a l c i u m c a r b o n a t e f r o m i n c i n e r a t e d s l u d g e , c o n t a i n -i n g CaO and MgO, has been p r o p o s e d by Rapson e t a l (1973) a t t h e Nova S c o t i a R e s e a r c h F o u n d a t i o n (NSRF) as a method o f r e c o v e r i n g magnesium v a l u e s from t h e s e a w a t e r - l i m e t r e a t m e n t system. The m i x t u r e o f c a l c i u m and magnesium o x i d e s from t h e k i l n i s t o be t r e a t e d w i t h s e a w a t e r , t o d i s s o l v e t h e l i m e and t o p r e c i p i t a t e more magnesium h y d r o x i d e . The l i m e s o l u t i o n i s p a r t i a l l y n e u t r a l i z e d w i t h a c i d and r e t u r n e d t o t h e f l o c c u l a -t i o n t a nk w h i l e t h e magnesium-hydroxide can be c a l c i n e d t o produce a h i g h q u a l i t y ( ? 98% MgOj p r o d u c t f o r s a l e . I t s h o u l d be p o i n t e d o u t t h a t t h i s method has n o t y e t been proven e f f e c t i v e . However, s t u d i e s a r e p r e s e n t l y underway by NSRF t o e v a l u a t e i t s f e a s i b i l i t y . d. The main magnesium r e c o v e r y p r o c e s s , which p r o m i s e s to;;.make co-a g u l a t i o n w i t h magnesium an e c o n o m i c a l l y f e a s i b l e p r o c e s s , i s t h a t o f s l u d g e c a r b o n a t i o n ( d i s c o v e r e d by B l a c k and E i d s n e s s (1957) and b r i e f l y d i s c u s s e d e a r l i e r ) . T h i s method can be used f o r e i t h e r o f two p u r p o s e s ; 1) t o produce magnesium c a r b o n a t e t r i h y d r a t e (MgC0 3-3H20) from l i m e - s o d a w a t e r s o f t e n i n g s l u d g e s f o r use as a c o a g u l a n t , and; 2) to r e c o v e r 44 magnesium from c o a g u l a t i o n systems u s i n g such s a l t s , thus p r o v i d i n g a r e c y c l e d M g ( H C 0 3 ) 2 t h a t can be r e u s e d i n the p r o c e s s . P r o d u c t i o n o f magnesium c a r b o n a t e f r o m t h e c a r b o n a t e d s l u d g e i n v o l v e s : ( i ) the s e l e c t i v e removal o f the Mg(0H) 2 from t h e p r i m a r y s l u d g e , by k i l n s t a c k gas c o n t a i n i n g 19-21 p e r c e n t C 0 2 o r by pure C 0 2 , and; ( i i ) removal o f t h e s o l i d - p h a s e CaCO^ by e i t h e r s e t t l i n g o r f i l t r a t i o n and the r e c o v e r y o f p r o d u c t magnesium c a r b o n a t e from the c l a r i f i e d l i q u o r by h e a t i n g and a e r a t i o n . The f o l l o w i n g r e a c t i o n s a p p l y . S l u d g e C a r b o n a t i o n : M g(0H) 2 + 2 C 0 2 M g ( H C 0 3 ) 2 (1) Mg(0H 2) + C 0 2 + 2H 20 MgC0 3.3H 20 (2a) MgC0 3.3H 20 + C 0 2 M g ( H C 0 3 ) 2 + 2H 20 (2b) Mg(0H) 2 + M g ( H C 0 3 ) 2 + H 2 0 - > 2 MgC0 3.3H 20 (3) P r o d u c t R e c o v e r y : M g ( H C 0 3 ) 2 + 2H 20 35 - 45°C ->- MgC0 3.3H 20 + C 0 2 (4) I t can be seen t h a t the f i r s t t h r e e r e a c t i o n s a r e r e s p o n s i b l e f o r c o n v e r -s i o n o f i n s o l u b l e h y d r o x i d e t o s o l u b l e b i c a r b o n a t e . R e a c t i o n (3) shows t h a t as time goes on, t h e r e i s an i n c r e a s i n g tendency f o r i n s o l u b l e MgC0 3.3H 20 t o form and be l o s t w i t h CaCOy I t i s t h e r e f o r e n e c e s s a r y t o c o n t r o l the f e e d o f s l u d g e so t h a t a s u p e r s a t u r a t e d s o l u t i o n o f Mg(HC0) 3 i s n o t pr o d u c e d t o r e a c t w i t h e x c e s s Mg(0H) 2. F o r p r o d u c t r e c o v e r y , t h e magnesium b i c a r b o n a t e s o l u t i o n , c l a r i -f i e d u s u a l l y by f i l t r a t i o n , p a s s e s t o a heat exchange u n i t where i t i s warmed to 35-45°C a f t e r which i t i s a e r a t e d by compressed a i r i n a mechan-i c a l l y mixed b a s i n . The MgC0 3.3H 20 p r e c i p i t a t e s r a p i d l y ( - 9 0 min) and the r e s u l t i n g p r o d u c t MgC0 3.3H 20 i s vacuum f i l t e r e d , d r i e d and bagged f o r shipm e n t , h o p e f u l l y t o o t h e r i n s t a l l a t i o n s where i t may be used as a c o a g u l a n t . 45 In a magnesium c o a g u l a t i o n s y s t e m , the M g + + would be added and the r e s u l t i n g s l u d g e c o u l d be c a r b o n a t e d s i m i l a r l y . However, i n t h i s c a s e , magnesium r e c o v e r y would s i m p l y r e q u i r e r e c y c l i n g the f i l t r a t e , c o n t a i n i n g magnesium b i c a r b o n a t e , t o the head o f t h e system. The c h e m i c a l r e a c t i o n s d e s c r i b i n g t h e c o a g u l a t i o n p r o c e s s r e s u l t i n g from the r e c y c l e d magnesium b i c a r b o n a t e a r e : Mg(HC0^) 2 + C a ( 0 H ) 2 - > M g C 0 3 + C a C 0 3 + 2H 20 MgC0 3 + C a ( 0 H ) 2 - » - Mg(0H) 2 + CaC0 3 T h i s , i n f a c t , i s t h e b a s i s f o r t h e 'magnesium r e c o v e r y ' method i n v e s t i -g a t e d i n t h e p r e s e n t r e s e a r c h s t u d y . 4.3 P r a c t i c a l C o n s i d e r a t i o n s From the p i l o t and f u l l - s c a l e s t u d i e s , as w e l l as l a b o r a t o r y s t u d i e s c o n d u c t e d t o d a t e , s e v e r a l g e n e r a l o p e r a t i n g and c o n t r o l p r a c t i c e s have e v o l v e d , which s h o u l d be c o n s i d e r e d d u r i n g f u t u r e i n v e s t i g a t i o n s o r a p p l i c a t i o n s o f magnesium c o a g u l a t i o n p r o c e s s e s . These a r e summarized as f o l l o w s : 1. Magnesium s h o u l d be added as a s o l u b l e s a l t p r i o r to a d d i t i o n o f a l i m e s l u r r y , so t h a t magnesium h y d r o x i d e forms i n s i t u . 2. (a) V i n c e n t (1974) found t h a t l i m e s l u r r y i n wa t e r appeared to f u n c t i o n b e t t e r than a s l u r r y i n a p o r t i o n o f the e f f l u e n t o r a d r y powder when c o n s i d e r i n g r e a c t i o n r a t e (pH s t a b i l i -z a t i o n ) . (b) L o o k i n g a t t h e t y p e o f l i m e used r e l a t i v e t o sp e e d o f d e c o l o u r i z a t i o n : ( i ) Commercial q u i c k l i m e , was, by f a r , the b e s t when added as a w a t e r s l u r r y . ( i i ) When a s l u r r y i n e f f l u e n t o r a dry power was used, c a l -cium h y d r o x i d e was b e t t e r than the o x i d e . 46 These r e s u l t s a r e v e r y p r e l i m i n a r y i n n a t u r e and cannot be g e n e r a l i z e d t o systems t r e a t i n g o t h e r t y p e s o f wastes. 3. C o n t r o l l i n g the pH i s t h e key t o s u c c e s s f u l o p e r a t i o n o f magnesium c o a g u l a t i o n systems. (a) pH c o n t r o l i n the f l o c c u l a t o r (~pH 11.0-11.3) p r o -v i d e s optimum c o n d i t i o n s f o r magnesium h y d r o x i d e f o r m a t i o n . (b) pH c o n t r o l i n the s l u d g e c a r b o n a t o r (-pH 7.3) p r o v i d e s optimum r e c o v e r y o f magnesium b i c a r b o n a t e . (c) pH s t a b i l i z a t i o n o f t r e a t e d e f f l u e n t t o below pH 8.6-8.8 i s n e c e s s a r y t o p r e v e n t c a l c i u m c a r b o n a t e b u i l d i n g up i n t h e f i l t e r s . S t a n d a r d f i l t e r s and, c o v e r e d w i t h 4 i n c h e s o f 1.2 mm a n t h -r a c i t e , p r o v i d e d maximum f i l t e r runs f o r water t r e a t m e n t . Diatomateous e a r t h f i l t e r s a r e a l s o a c c e p t a b l e . 5. R e c y c l e d s l u d g e s h o u l d be p r o v i d e d i n a l a r g e s c a l e system because: (a) R e c y c l e d c a l c i u m c a r b o n a t e i n c r e a s e s both r a t e and amount o f magnesium p r e c i p i t a t i o n . (b) Some o f the magnesium h y d r o x i d e i n the r e c y c l e d s l u d g e r e s o l u b l i z e s t o magnesium c a r b o n a t e , which can be p r e c i p i -t a t e d a g a i n . A l t h o u g h some con t a m i n a n t s may a l s o be r e l e a s e d i n t h e s e r e a c t i o n s , t he o v e r a l l e f f e c t i s c o n s i d e r e d t o be o f some v a l u e . ( c ) C r y s t a l l i n e c a l c i u m c a r b o n a t e , a l r e a d y p r e s e n t , a c t s as a s e e d o r n u c l e u s f o r p r e c i p i t a t i o n , p r e v e n t i n g a b u i l d - u p . on equipment. (d) S l i g h t l y r e d u c e d l i m e r e q u i r e m e n t s r e s u l t because o f the c a u s t i c i ' t y i n the h i g h pH (11.2) s l u d g e water. S m a l l , 47 dense f l o e p a r t i c l e s form r a p i d l y and s e t t l e v e r y w e l l . 6. Pure carbon d i o x i d e s h o u l d be c o n s i d e r e d f o r use i n s m a l l p l a n t s n o t r e c o v e r y i n g l i m e f o r s e v e r a l r e a s o n s . (a) The r a t e o f s o l u b i l i z a t i o n i s more r a p i d . (b) The amount o f magnesium b i c a r b o n a t e d i s s o l v e d by pure C 0 2 i s c o n s i d e r a b l y more than t h a t d i s s o l v e d by l o w e r p e r -centage C 0 2 p r o d u c e d by on s i t e g e n e r a t i o n . ( c ) Feed o f l i q u i d C 0 2 i s s i m p l e r , more f l e x i b l e and e a s i e r to automate. (d) Foaming problems a r e more s e v e r e u s i n g low p e r c e n t a g e co2. 48 CHAPTER 5 EXPERIMENTAL MATERIAL AND METHODS 5.1 S e l e c t i o n o f E f f l u e n t The m i l l chosen as a s o u r c e o f wastewater f o r t h i s work was t h e C r e s t b r o o k P u l p and Paper L t d . M i l l a t Skookumchuck, B.C. T h i s i s a 400-ton p e r day f a c i l i t y , p r o d u c i n g e s s e n t i a l l y 100% b l e a c h e d k r a f t p u l p f o r e x p o r t , a l t h o u g h some u n b l e a c h e d p u l p i s p r o d u c e d d u r i n g b l e a c h p l a n t i n t e r r u p t i o n s . Wood f u r n i s h i s e n t i r e l y c h i p s (45% p i n e , 35% s p r u c e , 13% f i r and l a r c h , and 7% o t h e r ) . C h i p age i s f o u r t o s i x months. P u l p i n g i s c a r r i e d o u t i n a c o n t i n u o u s Kamyr d i g e s t e r and b l e a c h i n g i s by a c o n v e n t i o n a l CEDED sequence. Water usage i s a p p r o x i m a t e l y 10,000,000 Imp. g a l / d a y (.53 m 3/sec) o r 25,000 Imp gal/AD t o n (114 m3/AD t o n ) . I n t a k e i s from the Skookumchuck R i v e r and d i s c h a r g e i s t o the Upper Kootenay R i v e r , which has a mean monthly f l o w v a r y i n g f r o m 22,000 c f s (623 m 3/sec) d u r i n g e a r l y summer down t o 650 c f s (18 m 3/sec) i n the w i n t e r months. E x t e r n a l t r e a t m e n t i n c l u d e s two p r i m a r y s e t t l i n g ponds o f 2,500,000 Imp. g a l . (11365 m 3) c a p a c i t y f o l l o w e d by a seven-day r e t e n t i o n a e r a t i o n l a g o o n . T y p i c a l u n t r e a t e d and t r e a t e d t o t a l m i l l e f f l u e n t c h a r a c t e r i s t i c s a r e shown i n T a b l e 3. The r e a s o n s f o r s e l e c t i n g t h i s m i l l as a wastewater s o u r c e i n c l u d e d : 1) i t i s one m i l l i n B.C. t h a t has a c o l o u r p r o b l e m and i s s e e k i n g a v i a b l e d e c o l o u r i z a t i o n method; 2) m i l l p e r s o n n e l were e x t r e m e l y c o - o p e r a t i v e and r e c e p t i v e t o the arrangement; 3) a l a r g e amount o f background work on c o l o u r removal has been done a t t h i s m i l l , so a comparison o f t h e p r a c t i c a b i l i t y o f t h i s new t e c h n o l o g y t o o t h e r methods may be p o s s i b l e ; and 4) i n - p l a n t c o l o u r b a l a n c e s have 49 been c a r r i e d o u t by m i l l p e r s o n n e l and were a v a i l a b l e , so t h a t a p p r o -p r i a t e waste streams c o u l d be chosen f o r t r e a t m e n t . TABLE 3 TYPICAL CHARACTERISTICS OF CRESTBROOK PULP AND PAPER LTD. - TOTAL MILL EFFLUENT Parameter U n t r e a t e d T r e a t e d Suspended S o l i d s (mg/1) 300 60 B0D 5 (mg/1) 225 60 pH 6.2 7.2 Temperature (°C) 28 18 C o l o u r ( C U . ) 2000 2500 Morgan (1975) Waste streams c o n t r i b u t i n g f l o w t o the t h r e e main sewers a r e as f o l l o w s : A c i d Sewer: T-10 T h i c k e n e r E f f l u e n t T-10 Washer E f f l u e n t T-30 E f f l u e n t C a u s t i c Sewer: T-20 E f f l u e n t Steam P l a n t R e c a u s t i c i z e r Chemical P r e p a r a t i o n G e n e r a l Sewer: P u l p i n g Group Unbleached White Water Hot Water Tank O v e r f l o w Machine Room B l e a c h Tower D r a i n s 50 The a v e r a g e r e s u l t s o f s e v e r a l i n - p l a n t c o l o u r b a l a n c e s a r e summarized i n T a b l e 4. TABLE 4 AVERAGE RESULTS OF SEVERAL IN-PLANT COLOUR BALANCES AT CRESTBROOK PULP AND PAPER LTD. Major C o n t r i b u t i o n % o f T o t a l S o u r c e t o C o l o u r i n Sewers M i l l C o l o u r T-20 E f f l u e n t 98.9% o f C a u s t i c Sewer 76.0% O t h e r 1.1% o f C a u s t i c Sewer 0.9% T o t a l C a u s t i c Sewer 100% 76.9% T-10 Washer 46% o f A c i d Sewer 5.9% T-10 T h i c k e n e r 27.2% o f A c i d Sewer 3.5% T-30 E f f l u e n t 26.8% o f A c i d Sewer 3.5% T o t a l A c i d Sewer 100% 12.9% U n b l . White Water 90.2% o f G e n e r a l 9.2% O t h e r 9.8% o f G e n e r a l 1.0% T o t a l G e n e r a l Sewer 100% 10.2% C r e s t b r o o k P e r s o n n e l (1975) These r e s u l t s i n d i c a t e t h a t e f f l u e n t from t he f i r s t c a u s t i c e x t r a c t i o n tower o f t h e b l e a c h p l a n t (T-20 e f f l u e n t ) i s r e s p o n s i b l e f o r 70 t o 80% o f the t o t a l m i l l c o l o u r . However, the c o l o u r b a l a n c e s were o b t a i n e d d u r i n g p e r i o d s o f s t e a d y m i l l o p e r a t i o n when t h e r e were no s p i l l s , so t h e l o n g t erm r e l a t i v e c o n t r i b u t i o n o f T-20 e f f l u e n t , i n r e a l i t y , i s l e s s t h a n shown by t h e s e t e s t s . In f a c t , e x t e r n a l measure-ment o f t o t a l m i l l e f f l u e n t d u r i n g e x t e n d e d p e r i o d s o f b l e a c h p l a n t shutdown have shown t h a t o n l y a 50 t o 60% c o l o u r r e d u c t i o n can be a c h i e v e d by c o m p l e t e l y s h u t t i n g down the b l e a c h p l a n t . T h e r e f o r e , i t would seem t h a t t r e a t m e n t o f the t o t a l m i l l e f f l u e n t would be n e c e s s a r y 51 t o a c h i e v e a t o t a l c o l o u r r e d u c t i o n i n t h e o r d e r o f 90% a t t h i s m i l l . However, by m o n i t o r i n g and c o n t a i n i n g l i q u o r s p i l l s and i n t e n s i f y i n g o t h e r h o u s e k e e p i n g programs, i t may be p o s s i b l e t o a c h i e v e an a c c e p t a b l e t o t a l m i l l e f f l u e n t c o l o u r by t r e a t i n g o n l y T-20 e f f l u e n t . In any c a s e , i t was d e c i d e d t o t e s t t h i s new t e c h n o l o g y on both t o t a l m i l l b i o l o g i c a l l y t r e a t e d e f f l u e n t (BKME) (-2000 C U . ) and h i g h l y c o l o u r e d c a u s t i c e x t r a c t i o n e f f l u e n t (-16000 C.U.), from T-20 o f t h e b l e a c h p l a n t . 5.2 Sample S t o r a g e A l l samples were s t o r e d a t 4°C. P r i o r t o t e s t i n g t h e samples were a l l o w e d t o r e a c h room t e m p e r a t u r e . 5.3 Chemical P r e p a r a t i o n s 5.3.1 Lime Reagent grade C a ( 0 H ) 2 was chosen f o r t h i s s t u d y as the p r e f e r r e d method o f l i m e a d d i t i o n . In a l l c a s e s , whether i t was added d r y o r as a s l u r r y , t h e C a ( 0 H ) 2 powder was weighed a c c u r a t e l y on a S a r t o r i u s Model 2442, f o u r p l a c e b a l a n c e , i m m e d i a t e l y p r i o r t o usage. S e l e c t i o n o f a method f o r l i m e a d d i t i o n i s d i s c u s s e d i n more d e t a i l i n Appendix A. 5.3.2 Magnesium A s i d e from e a r l y t e s t s , which were c o n d u c t e d u s i n g s t o c k MgCl 2.6H 20< (1 ml = 60 M g + + ) , a l l c o a g u l a t i o n s were p e r f o r m e d w i t h s t o c k MgS0it.7H 20 s o l u t i o n (1 ml = 30 mg M g + + ) . Both s t o c k s o l u t i o n s were p r e p a r e d by d i s s o l v i n g an amount o f r e a g e n t grade c h e m i c a l i n d i s t i l l e d w a t e r t o g i v e t h e d e s i r e d s t o c k s o l u t i o n c o n c e n t r a t i o n o f M g + + i o n s . A more d e t a i l e d d i s c u s s i o n o f the s e l e c t i o n o f a magnesium s o u r c e i s i n c l u d e d i n Appendix A. 52 5.3.3 S e t t l i n g A i d An a n i o n i c o r g a n i c polymer, Dow P u r i f l o c A-23, p r o v e d t o be an e f f e c t i v e s e t t l i n g a i d f o r T-20 c o l o u r s l u d g e . A w o r k i n g s o l u t i o n o f t h i s c h e m i c a l (.02%) was p r e p a r e d d a i l y from a s t o c k s o l u t i o n (.2%) a c c o r d i n g t o t h e m a n u f a c t u r e r ' s recommendations, Dow (1967). R e s u l t s o f T-20 s l u d g e s e t t l i n g t e s t s and polymer s e l e c t i o n t r i a l s a r e shown i n Appendix B. 5.3.-4 P l a t i n u m - C o b a l t (Pt-Go) S t a n d a r d A 500 c o l o u r u n i t ( C U . ) s t a n d a r d was p r e p a r e d a c c o r d i n g t o S t a n d a r d Methods (APHA e t a l , 1971). 5.3.5 Lanthanum C h l o r i d e L a C l 2 s t o c k s o l u t i o n f o r magnesium and c a l c i u m d e t e r m i n a t i o n by a t o m i c a b s o r p t i o n was p r e p a r e d a c c o r d i n g t o APHA e t a l (1971). 5.3.6 Magnesium and C a l c i u m S t a n d a r d s ++ ++ A l l s t a n d a r d s f o r Mg and Ca a n a l y s i s were p r e p a r e d from 10,000 mg/1 c e r t i f i e d a t o m i c a b s o r p t i o n s t a n d a r d s . The d e s i r e d con-c e n t r a t i o n s were o b t a i n e d by d i l u t i o n w i t h d i s t i l l e d w a t er and 1% L a C l 2 was added as 10 mis o f 10% s t o c k s o l u t i o n i n 100 ml v o l u m e t r i c f l a s k s . 5.3.7 pH A d j u s t m e n t C h e m i c a l s A wide range o f s t a n d a r d s was n e c e s s a r y t o m i n i m i z e the volume change d u r i n g pH a d j u s t m e n t f o r c o l o u r measurement. The d i l u t i o n s o f HCl used f o r pH r e d u c t i o n were c o n c e n t r a t e d H C l , 3N, IN, .5N, .IN, .02N, .01N and .001N.. 5.4 A n a l y t i c a l T e c h n i q u e s 5.4.1 C o l o u r Measurement A f t e r a c o n s i d e r a b l e l i t e r a t u r e s e a r c h , d i s c u s s i o n s w i t h 53 s e v e r a l o t h e r r e s e a r c h e r s and p r e l i m i n a r y t e s t i n g i n the l a b o r a t o r y , t h e c o l o u r measurement t e c h n i q u e chosen was the one recommended by the NCASI (1971) i n the U.S. and t h e CPPA (1973) i n Canada. A d e t a i l e d d e s c r i p t i o n o f t h e method has been g i v e n by t h e CPPA (1973) and t h i s p r o c e d u r e was a d o p t e d w i t h o u t s i g n i f i c a n t m o d i f i c a t i o n . B r i e f l y , t h i s i n v o l v e d sample d i l u t i o n t o the p r o p e r range f o r a b s o r b a n c e measurement, i f n e c e s s a r y ; sample pH a d j u s t m e n t t o pH 7.6 ± .1; f i l t r a t i o n t h r o u g h .8 u membrane pap e r ; r e a d j u s t m e n t o f pH t o 7.6 ± .1; measurement o f a b s o r b a n c e on a Bausch and Lomb S p e c t r o n i c 600 U.V. S p e c t r o p h o t o m e t e r a t a w a v e l e n g t h o f 465 nm; and comparison o f a b s o r b a n c e t o a p l a t i n u m -c o b a l t s t a n d a r d a c c o r d i n g t o APHA e t a l (1971). C o l o u r u n i t s were r e c o r d e d as C U . ( c o l o u r u n i t s ) . APHA u n i t s , Hazen u n i t s , Pt-Co u n i t s a r e e q u i v a l e n t terms. For mass b a l a n c e c a l c u l a t i o n s , c o l o u r masses r e f e r r e d t o a r e as mg o f P t . 5.4.2 C a l c i u m and Magnesium A f t e r comparing r e s u l t s o f samples r e c e i v i n g no d i g e s t i o n and samples p r e p a r e d by s e v e r a l d i g e s t i o n methods p r i o r t o a t o m i c a b s o r p t i o n (A.A.) a n a l y s i s , a d r y a s h i n g t e c h n i q u e , d e s c r i b e d below, was s e l e c t e d t o d e s t r o y o r g a n i c compounds t h a t appeared t o be t y i n g up Mg and Ca . A l l ashed samples were d i l u t e d t o the p r o p e r range w i t h d i s t i l l e d w a t e r , and 1% L a C l 2 , p r i o r t o a n a l y s i s on a P e r k i n Elmer Model 500 A t o m i c A b s o r p t i o n S p e c t r o p h o t o m e t e r , a c c o r d i n g t o APHA e t a l (1971). A i r and a c e t y l e n e were used as o x i d a n t and f u e l , r e s p e c t i v e l y , t h r o u g h o u t t h i s r e s e a r c h . 54 5.4.3 Dry A s h i n g T e c h n i q u e The d e t a i l s o f t h e d r y a s h i n g method used a r e p r e s e n t e d below: 1. The p r e v i o u s l y d r i e d sample was p l a c e d i n a c o l d m u f f l e f u r n a c e and h e a t e d g r a d u a l l y t o 700°C. 2. The upper t e m p e r a t u r e was m a i n t a i n e d f o r 8 t o 12 h o u r s , o r u n t i l t h e specimen was a g r e y i s h w h i t e a s h . 3. The ash was c o o l e d , removed and m o i s t e n e d w i t h 1 t o 2 mis d i s t i l l e d w a t e r . I n i t i a l l y , d r o p s o f c o n c e n t r a t e d HCl and HN0 3 were then added and g e n t l e h e a t a p p l i e d t o remove e x c e s s m o i s t u r e . L a t e r i t was found t h a t t h e HN0 3 and h e a t i n g was n o t n e c e s s a r y on most samples. 4. When the ash was c o m p l e t e l y decomposed, t h e sample was d i l u t e d w i t h d i s t i l l e d w a t er and 1% L aC l 2 , f o r sub-sequent A.A. a n a l y s i s . F i l t e r p a p e r s were ashed d i r e c t l y s i n c e t h e y were the " a s h l e s s " t y p e . U n f i l t e r e d samples o f u n t r e a t e d e f f l u e n t , t r e a t e d e f f l u e n t , c o l o u r s l u d g e , and s u p e r n a t a n t were e v a p o r a t e d t o d r y n e s s i n a 103°C d r y i n g oven p r i o r t o a s h i n g . In. o r d e r t o a v o i d e v a p o r a t i n g and i n c i n e r a t i n g t h e l a r g e q u a n t i t y o f s l u d g e water i n t h e u n t h i c k e n e d f i n a l s l u d g e , t h e s l u d g e sample was f i l t e r e d t h r o u g h a 12.5 cm d i a m e t e r #40 Whatman paper on a Buchner f u n n e l . The paper and e v a p o r a t i o n d i s h were preweighed so t h a t s l u d g e d r y w e i g h t and ash w e i g h t c o u l d be d e t e r m i n e d . ++ ++ The v a l u e s o b t a i n e d f o r Mg and Ca on d i l u t i o n o f t h i s ash were t a k e n as M g + + and C a + + c o n t e n t o f t h e d r y f i n a l s l u d g e s o l i d s , r e s p e c t i v e l y . 55 5.4.4 pH Measurement A l l pH measurements were p e r f o r m e d on a F i s h e r Aceumet Model 210 pH meter. B u f f e r s o l u t i o n s o f pH 7.00 and 10.00 were used t o c a l i b r a t e t h e i n s t r u m e n t a t l e a s t once a day. 5.4.5 T o t a l A l k a l i n i t y P r e l i m i n a r y t e s t s by the a u t h o r showed t h e i n f l e c t i o n p o i n t o f t h e t i t r a t i o n c u r v e f o r both t o t a l BKME and T-20 e f f l u e n t t o be a t pH = 4.5. In a l l c a s e s , t o t a l a l k a l i n i t y was d e t e r m i n e d on 20 ml samples, p r e f i l t e r e d t h r o u g h #40 Whatman pa p e r , t i t r a t e d t o a pH o f 4.5 w i t h H2SO4 o f a p p r o p r i a t e n o r m a l i t y . i . e . T o t a l BKME ( I n & Out) - .02 N H 2 S 0 4 T-20 E f f l u e n t ( I n & Out) - l . N HzS0h S u p e r n a t a n t (BKME) - . I N tt2S0h S u p e r n a t a n t (T-20) - .2506 N H2SO4 T o t a l A l k a l i n i t y = N o r m a l i t y j 50,000 x ml t i t r e 5.4.6 C o n d u c t i v i t y and Temperature A Radiometer C o n d u c t i v i t y M e t e r , Model No. CDM3, was used t o p e r f o r m c o n d u c t i v i t y measurements i n a l l c a s e s , a c c o r d i n g t o t h e m a n u f a c t u r e r ' s i n s t r u c t i o n s . A l l c o n d u c t i v i t y v a l u e s were r e c o r d e d i n m i c r o Siemens ( y s ) , e q u i v a l e n t t o m i c r o Mhos. A te m p e r a t u r e com-p e n s a t i o n probe was a v a i l a b l e , but s i n c e a l l t e s t s were c o n d u c t e d between 20 t o 25°C, no compen s a t i o n was r e q u i r e d . However, t e m p e r a t u r e was measured m a n u a l l y and r e c o r d e d . 5.4.7 P e r c e n t S o l i d s o f S l u d g e s In a l l c a s e s , r e p r e s e n t a t i v e samples o f s l u d g e s were t r a n s -f e r r e d by wide mouth p i p e t t e t o preweighed gooch c r u c i b l e s , and 56 f i l t e r e d under s l i g h t vacuum t h r o u g h Reeve A n g e l , g l a s s - f i b r e , f i l t e r p a p e r. Reweighing o f t h e c r u c i b l e , a f t e r d r y i n g a t 103°C f o r a t l e a s t one hour and a c o o l i n g f o r a t l e a s t 15 minutes i n a d e s i c c a t o r , gave s o l i d s c o n c e n t r a t i o n , a f t e r a p p l y i n g t h e a p p r o p r i a t e d i l u t i o n f a c t o r . 1 so l id s = m g / 1 s o l i d s lo s o l i d s 1 0 j 0 0 0 x s g < assuming s.g. -1.0 then y s o l i d s = mg/1 d r y s o l i d s  /o s o 1 1 d s 10,000 57 CHAPTER 6 RESULTS AND DISCUSSION 6.1 R e s u l t s o f T e s t i n g w i t h F r e s h C h e m i c a l s 6.1.1 J a r T e s t R e s u l t s 6.1.1.1 G e n e r a l . To e v a l u a t e the c o l o u r removal e f f i c i e n c y o f v a r i o u s l i m e and magnesium d o s a g e s , i t was n e c e s s a r y t o d e f i n e an o b j e c t i v e f o r r e s i d u a l c o l o u r i n t h e t r e a t e d w a s t e s . I t was f e l t t h a t t h i s t e c h n o l o g y s h o u l d p r o v i d e a l e v e l o f d e c o l o u r i z a t i o n e q u a l t o o r b e t t e r than e x i s t i n g l i m e t r e a t m e n t f a c i l i t i e s a t a s i m i l a r o r l e s s e r c o s t . With t h i s i n mind, a c o l o u r r e d u c t i o n o b j e c t i v e o f 90 t o 95% was s e l e c t e d . In most k r a f t m i l l s , t h i s would mean a r e s i d u a l c o l o u r o f 500 t o 1 ,000 C U . f o r c a u s t i c e x t r a c t i o n e f f l u e n t and 100 t o 200 C U . f o r t o t a l m i l l e f f l u e n t . These v a l u e s c o r r e s p o n d e d t o r e s i d u a l s o b t a i n e d w i t h l i m e t r e a t m e n t i n the p u b l i s h e d l i t e r a t u r e and seemed l i k e r e a s o n a b l e c r i t e r i a f o r t h e s e l e c t i o n o f t e n t a t i v e optimum dosage l e v e l s o f l i m e and magnesium f o r use d u r i n g the s u b s e q u e n t b a t c h t e s t s i n t h i s s t u d y . Two o t h e r c r i t e r i a c o n s i d e r e d i n t h e f i n a l s e l e c t i o n o f c h e m i c a l dosages were: i ) t h e d e c o l o u r i z e d e f f l u e n t pH s h o u l d be k e p t as low as p o s s i b l e t o m i n i m i z e r e c a r b o n a t i o n c o s t s ; i i ) a c u r s o r y l o o k a t c h e m i c a l c o s t s i n d i c a t e d t h a t a minimum usage o f l i m e would be d e s i r e d . I t s h o u l d be emphasized t h a t t h e r e s u l t s o f t h e s e j a r t e s t s a r e p r e l i m i n a r y . A more d e t a i l e d e v a l u a t i o n s h o u l d be c a r r i e d o u t o v e r a w i d e r range o f raw waste c h a r a c t e r i s t i c s t o d e t e r m i n e t h e most economic dosages f o r any l a r g e s c a l e use. 58 6.1.1.2 BKME. R e s u l t s o f p r e l i m i n a r y 100 ml c o a g u l a t i o n t e s t s o f th e t o t a l m i l l b i o l o g i c a l l y t r e a t e d e f f l u e n t a r e shown i n T a b l e s 5, 6, 7 and 8. The d a t a i n T a b l e 5 f o r Time a d d i t i o n o n l y , and i n T a b l e s 6 and 7 f o r i n c r e a s i n g l i m e dosages a t t h e c o n s t a n t M g + + l e v e l s o f 30 mg/1 and 60 mg/1, r e s p e c t i v e l y , were used t o c o n s t r u c t t h e graphs i n F i g u r e 9. From t h e s e r e s u l t s , i t can be seen t h a t 91 t o 92% c o l o u r removal was a c c o m p l i s h e d by 2,000 mg/1 C a ( 0 H ) 2 (1,500 mg/1 CaO) a l o n e o r by u s i n g h a l f as much l i m e (750 mg/1 CaO) i n c o n j u n c t i o n w i t h 30 mg/1 M g + + . A M g + + a d d i t i o n o f 60 mg/1, p r e c i p i t a t e d by o n l y 500 mg/1 Ca(0H ) 2 (375 mg/1 CaO) a l s o p r o v i d e d 92% improvement. F i g u r e 9 i l l u s t r a t e s t h a t a c o m b i n a t i o n o f low magnesium and l i m e dosages a c h i e v e d g r e a t e r c o l o u r removal e f f i c i e n c i e s than d i d a much h i g h e r l i m e dosage by i t s e l f . The e f f e c t o f i n c r e a s i n g t h e M g + + dosage w h i l e h o l d i n g t h e l i m e dosage a t 375 mg/1 as CaO i s shown i n T a b l e 8. With no magnesium, c o l o u r removal was an u n a c c e p t a b l e , 73.3%, w h i l e a d d i t i o n o f 30 and 60 mg/1 o f M g + + y i e l d e d 86.7 and 91.9% c o l o u r r e m o v a l s , r e s p e c t i v e l y . A t t h i s low l i m e l e v e l , f u r t h e r a d d i t i o n s o f the a c i d i c magnesium s u l p h a t e s o l u t i o n gave o n l y m a r g i n a l improvement, p r o b a b l y because o f t h e d e c r e a s e i n pH o u t o f t h e optimum range f o r Mg(0H) 2 p r e c i p i t a t i o n . T h i s a n t a g o n i s t i c e f f e c t on pH caused by t h e a c i d i c magnesium s o l u t i o n and t h e c a u s t i c l i m e i s an i m p o r t a n t f a c t o r when t r y i n g t o a r r i v e a t an optimum s e t o f c o n d i t i o n s f o r c o a g u l a t i o n o f a p a r t i c u l a r waste. As n o t e d e a r l i e r , Thompson e t a l (1972) i d e n t i f i e d a pH o f T l . l t o 11.4 as t h e optimum c o a g u l a t i o n pH f o r t r e a t i n g s u r f a c e w a t e r s , based on a d e t a i l e d economic e v a l u a t i o n . C g l g u r % Removal 06-01-1 0 500 375 11.62 600 71.4 06-01-2 0 750 560 11.84 400 80.9 06-01-3 0 1000 750 11.94 320 84.8 06-01-4 0 1500. 1125 12.12 225 89.3 06-01-5 0 2000 1500 . 12.21 180 91.4 06-01-6 0 3000 2250 12.33 150 92.9 TABLE 6 JAR TESTS WITH FRESH CHEMICALS BKME I n i t i a l C o l o u r = 2100 I n i t i a l pH = 7 . 1 0 J a r No. Magnesium mg/1 as M g + + Lime mg/1 as C a ( 0 H ) 2 CaO pH C o l o u r % Removal 06-02-7 30 100 75 9.10 N.A. Very Poor 06-02-8 30 250 190 10.62 N.A. Poor 06-02-9 30 400 300 11.10 410 80.5 06-02-10 30 500 375 11.47 315 85.0 06-02-11 30 750 560 11.72 215 89.8 06-02-12 30 1000 750 11.90 170 91.9 TABLE 5 JAR TESTS WITH FRESH CHEMICALS EFFLUENT TYPE - BKME I n i t i a l C o l o u r = 2100 I n i t i a l pH = 7.12 Magnesium Lime mg/1 as mg/1 as J a r No. M g + + C a ( 0 H ) 2 CaO pH 60 TABLE 7 JAR TESTS WITH FRESH CHEMICALS BKME I n i t i a l C o l o u r = 2200 I n i t i a l pH = 7.10 Magnesium mg/1 as Lime mg/1 as J a r No. .. ++ Mg C a ( 0 H ) 2 CaO PH Cglflur. % Removal 06-05-1 60 100 75 9.35 N.A. V e r y P o o r 06-05-2 60 200 150 10.00 N.A. V e r y Poor 06-05-3 60 300 225 10.68 810 63.2 06-05-4 60 400 300 10.85 300 86.4 06-05-5 60 500 375 11.21 175 92.0 06-05-6 60 600 450 . 11.45 140 93.6 06-05-7 60 750 560 11.71 115 94.7 06-05-8 60 1000 750 11.91 105 95.2 J a r No. 06-06-1 06-06-2 06-06-3 06-06-4 06-06-5 06-06-6 06-06-7 06-06-8 * Note: TABLE 8 JAR TESTS WITH FRESH CHEMICALS BKME I n i t i a l C o l o u r .= 2285* Magnesium mg/1 as Mg I n i t i a l pH = 7.30 Lime mg/1 as C a ( 0 H ) 9 CaO PH C o l o u r c. u. %. Removal 0 500 375 11.60 610 73.3 15 500 375 11.50 356 84.4 30 500 375 11.42 303 86.7 45 500 375 11.25 285 87.5 60 500 375 11.10 184 91.9 75 500 375 11.00 165 92.8 90 500 375 10.90 154 93.3 120 500 375 10.64 161 93.0 T h i s was t h e same sample as i n t h e t h r e e p r e v i o u s t a b l e s . Note t h a t t h e c o l o u r o f t h i s sample i n c r e a s e d o v e r t he s i x days. T h i s was because the sample was s t o r e d a t room t e m p e r a t u r e d u r i n g t h i s e a r l y p e r i o d o f t h e s t u d y . A f t e r t h i s samples were s t o r e d a t 4°C. o LU cr rr ZD O _J o o h -~Z. LU o cc LU 100 n 80 6 0 -4 0 -2 0 -0 0 A--o— A •o LIME ALONE LIME + 60 mg/i M g + + LIME + 30 mg/i Mg++ T 500 1000 — i 2000 3000 Ca (0H)2 mg// FIGURE 9. LIME Vs PERCENT COLOUR REMOVAL AT THREE MAGNESIUM LEVELS-BKME CXi 62 R e s u l t s o f t h e s e p r e l i m i n a r y j a r t e s t s i n d i c a t e d t h a t t h e r e l a t i o n s h i p between M g + + dosage, l i m e d osage, pH and c o l o u r removal e f f i c i e n c y i n k r a f t m i l l e f f l u e n t s i s not a s i m p l e one. F o r example, a l t h o u g h 30 mg/1 M g + + p l u s 300 mg/1 CaO y i e l d e d a pH o f 11.1, o n l y 80.5% c o l o u r r e d u c t i o n was a c h i e v e d . A t t h i s low magnesium l e v e l , 750 mg/1 CaO was needed t o p r o v i d e an a c c e p t a b l e e f f l u e n t (91.9% r e m o v a l ) , but t h e r e s u l t i n g pH o f 11.90 was q u i t e h i g h . A t 60 mg/1 o f M g + + and 375 mg/1 CaO a pH o f 11.21 and a c o l o u r r e d u c t i o n o f 92.0% r e s u l t e d . I t a p p e a r e d t h a t i t was n o t the r e s u l t i n g pH t h a t was so i m p o r t a n t , p e r se. As l o n g as enough M g + + was p r e s e n t , i t formed a good f l o e above pH 10.8, but the l i m e was i m p o r t a n t a l s o , s i n c e i n a l l c a s e s , i n c r e a s i n g l i m e dosage c o n t i n u e d t o improve t he t r e a t e d e f f l u e n t c o l o u r . The l i m e s e r v e d as more th a n a s i m p l e pH a d j u s t e r , w i t h f l o e f o r m a t i o n and s l u d g e s e t t l e a b i 1 i t y i m p r o v i n g as the lime:magnesium r a t i o i n c r e a s e d . A more e x t e n s i v e s e t o f j a r t e s t s , i n c l u d i n g measurement o f M g + + and l i m e l o s s e s i n the d e c o l o u r i z e d e f f l u e n t , i n c o n j u n c t i o n w i t h d e t a i l e d economic c o n s i d e r a t i o n s , would be n e c e s s a r y b e f o r e optimum c o n d i t i o n s c o u l d be s e l e c t e d f o r l a r g e s c a l e a p p l i c a t i o n . T h i s was beyond t he scope o f t h e p r e s e n t s t u d y . Remembering t he p r e v i o u s l y s t a t e d c r i t e r i a and c a r e f u l l y e x a m i n i n g t h e t e s t r e s u l t s , a dosage l e v e l o f 60 mg/1 M g + + p l u s 500 mg/1 C a ( 0 H ) 2 (375 mg/1 CaO) was chosen f o r use i n the d e t a i l e d b a t c h t e s t s . 6.1.1.3 T-20 E f f l u e n t . T y p i c a l r e s u l t s o f p r e l i m i n a r y (100 ml) c o a g u l a t i o n t e s t s o f T-20 e f f l u e n t a r e g i v e n i n T a b l e s 9, 10 and 11. Ve r y poor d e c o l o u r i z a t i o n r e s u l t e d f r o m low l i m e and magnesium l e v e l s ^ i n the range s e l e c t e d f o r BKME t r e a t m e n t . 63 TABLE 9 JAR TESTS WITH FRESH CHEMICALS T-20 E f f l u e n t I n i t i a l C o l o u r = 12,400 I n i t i a l pH = 11.5 Magnesium Lime mg/1 as mg/1 as J^ r N o- M g + + C a ( 0 H ) ? CaO pH 05-28-10 •Colour % Removal c u . 05-28-1 u l U U U 750 12.15 N.A. V e r y P o o r 05-28-2 0 2500 1875 12.40 N.A. V e r y P o o r 05-29-3 0 5000 3750 12.50 2280 81.6 05-28-4 60 500 375 11.75 N.A. V e r y P o o r 05-28-5 60 1000 750 12.05 N.A. V e r y P o o r 05-28-6 60 2500 1875 12.35 2320 81.3 05-28-7 180 2500 1875 12.20 1060 91.5 05-28-8 300 2500 1875 12.10 740 94.0 05-28-9 600 iQ7c n ->o 0 1000 50 2.15 875 2.40 2.50 0 00 75 1.75 0 00  50 2.05 0 875 2.35 80 875 12.20 00 875 12.10 00 2500 18 5 11.78 120 5000 3750 12.40 300 5000 3750 12.35 600 5000 3750 12.25 6000 0 0 9.80 510 95.9 u ^ - « . o - i u \cu UUUU 2.40 1020 91.7 05-28-11 0 5000 3750 2.35 580 95.3 05-28-12 0 5000 3750 2.25 410 96.7 05-28-13 6000 .80 N.A. V e r y P o o r For example, 5,000 mg/1 C a ( 0 H ) 2 (3,750 mg/1 CaO) a l o n e gave o n l y 81.6% removal and 2,500 mg/1 C a ( 0 H ) 2 (1,875 mg/1 CaO) p l u s 60 mg/1 M g + + gave 81.3% r e m o v a l . However, a t the h i g h e r magnesium l e v e l s shown i n T a b l e 10, c o l o u r removal was improved s u b s t a n t i a l l y a t t h e s e l i m e l e v e l s . A co m b i n a t i o n o f 300 mg/1 M g + + and 1,875 mg/1 CaO gave 94.0% c o l o u r r e d u c t i o n a t a c o a g u l a t i o n pH o f 12.10. Twice t h i s M g + + dosage gave m a r g i n a l l y b e t t e r removal a t a pH o f 11.78, as d i d t w i c e t h i s l i m e dosage a t a pH o f 12.35. 64 TABLE 10 JAR TESTS WITH FRESH CHEMICALS T-20 E f f l u e n t I n i t i a l C o l o u r = 12,900 I n i t i a l pH = 11.45 J a r No. Magnesium mg/1 as Mg Lime mg/1 as C a ( 0 H ) 2 CaO pH C o l o u r c. u. % Removal 05-29-1 180 500 375 11.22 N.A. Very Poor 05-29-2 180 1500 1125 12.00 1760 86.4 05-29-3 180 3000 2250 12.30 1090 91.6 05-29-4 300 500 375 10.70 N.A. Ver y P o o r 05-29-5 300 1500 1125 11.75 1410 81.1 05-29-6 300 3000 2250 12.20 760 94.1 05-29-7 600 500 375 10.40 4020 68.8 05-29-8 600 1500 1125 10.63 1110 91.4 05-29-9 600 3000 2250 11.90 480 96.3 As t h e r e s u l t s i n T a b l e 11 i n d i c a t e , a t a c o n s t a n t M g T T l e v e l , i n c r e a s e d l i m e dosages y i e l d e d o n l y m a r g i n a l improvements above a CaO v a l u e o f about 2,000 mg/1. There appeared t o be l i t t l e b e n e f i t i n a d d i n g more than 2,000 mg/1 CaO. However, as mentioned b e f o r e , a more e x t e n s i v e j a r t e s t i n g s t u d y would be n e c e s s a r y t o d e t e r m i n e o p t i m a l c h e m i c a l dosages f o r any p a r t i c u l a r e f f l u e n t . I t i s i n t e r e s t i n g t o note t h a t even t h e h i g h l e v e l o f 15,000 mg/1 CaO a l o n e gave o n l y 89.1% c o l o u r r e m o v a l . 65 TABLE 11 JAR TESTS WITH FRESH CHEMICALS T-20 E f f l u e n t I n i t i a l C o l o u r = 16,680 I n i t i a l pH = 11.20 J a r No. Magnesium mg/1 as Mg Lime mg/1 as C a ( 0 H ) 2 CaO pH C o l o u r c u . % Remc 06-16-1 0 10000 7500 12.55 2160 87.1 06-16-2 0 20000 15000 12.60 1820 89.1 06-16-3 300 2500 1875 12.15 940 .94.4 06-16-4 300 3750 2815 12.30 700 95.8 06-16-5 300 5000 3750 12.35 640 96.2 06-16-6 300 6250 4690 12.42 645 96.1 06-16-7 300 7500" 5625 12.45 550 96.7 06-16-8 450 2500 1875 11.96 675 95.9 06-16-9 450 3750 2815 12.20 505 97.0 06-16-10 450 5000 3750 12.31 425 97.5 06-16-11 450 6250 4690 12.40 430 97.4 06-16-12 450 7500 5625 12.40 380 97.7 From t h e s e d a t a , and k e e p i n g i n mind t he p r e v i o u s l y d i s c u s s e d c r i t e r i a , a c o m b i n a t i o n o f 300 mg/1 M g + + i n c o n j u n c t i o n w i t h 2,500 mg/1 C a ( 0 H ) 2 (1,875 mg/1 CaO) was s e l e c t e d as the t e n t a t i v e optimum dosage l e v e l s f o r use d u r i n g t h e d e t a i l e d b a t c h t e s t s on T-20 e f f l u e n t . A f t e r t h e f i r s t b a t c h run o f T-20 e f f l u e n t , i t became more a p p a r e n t t h a t a s e t t l i n g a i d was needed. Subsequent t e s t i n g , d i s c u s s e d i n Appendix B, showed t h a t 1 t o 2 mg/1 o f an a n i o n i c polymer (Dow A-23) s e r v e d t h i s p u r p o s e . I t was, t h e r e f o r e , used i n the r e m a i n i n g T-20 r u n s . 66 6.1.2 Bat c h D e c o l o u r i z a t i o n and Magnesium Re c o v e r y R e s u l t s 6.1.2.1 G e n e r a l . B e f o r e e x t e n s i v e d a t a c o l l e c t i o n was u n d e r t a k e n , a few b a t c h runs were c o n d u c t e d t o g a i n e x p e r i e n c e and d e v e l o p p r o -c e d u r e s f o r t h e s e e x p e r i m e n t s . Both BKME and T-20 e f f l u e n t were used d u r i n g t h e s e r u n s , w i t h d a t a c o l l e c t i o n becoming more e f f i c i e n t as equipment and p r o c e d u r a l m o d i f i c a t i o n s were made. As a l r e a d y m e n t i o n e d , t he major o b j e c t i v e o f t h e s e b a t c h t e s t s was t o c a r r y o u t complete c o l o u r , magnesium ( M g + + ) and c a l c i u m ( C a + + ) b a l a n c e s around t h e two l o o p s o f t h e p r o c e s s ( i . e . , t h e d e c o l o u r -i z a t i o n l o o p and t h e s l u d g e c a r b o n a t i o n l o o p ) . These d a t a were i n -te n d e d t o p r o v i d e t h e answers t o t h e f o l l o w i n g key q u e s t i o n s : 1. What i s t h e e x t e n t o f t h e c o l o u r r e l e a s e d u r i n g s l u d g e c a r b o n a t i o n ? 2. How much magnesium can be r e c o v e r e d i n t h e s l u d g e s u p e r n a t a n t f o r r e c y c l e and how much remains i n t h e f i n a l s l u d g e a f t e r c a r b o n a t i o n ? The o t h e r major o b j e c t i v e o f t h e s e b a t c h s t u d i e s was t o m o n i t o r s l u d g e pH and c o n d u c t i v i t y i n c o n j u n c t i o n w i t h measurement o f s l u d g e f i l t r a t e pH, a l k a l i n i t y , c o l o u r , magnesium and c a l c i u m a t i n t e r v a l s d u r i n g s l u d g e c a r b o n a t i o n . These d a t a p r o v i d e d an i n s i g h t i n t o t h e f a c t o r s a f f e c t i n g c o l o u r r e l e a s e and magnesium r e c o v e r y , as w e l l as i d e n t i f y i n g a method o f c o n t r o l l i n g t h e s l u d g e c a r b o n a t i o n i n a l a r g e s c a l e p r o c e s s . Three runs each f o r BKME and T-20 e f f l u e n t were c o n d u c t e d u s i n g f r e s h MgSO.* and C a ( 0 H ) 2 as c o a g u l a n t s . The e f f l u e n t d e s c r i p t i o n s and c h e m i c a l dosages used f o r each o f t h e runs a r e summarized i n T a b l e 12. TABLE 12 BATCH DECOLOURIZATION INITIAL CONDITIONS AND RESULTS UNTREATED EFFLUENT DESCRIPTION CHEMICAL DOSAGES TREATED EFFLUENT DESCRIPTION jn # Type PH T o t a l A l k a l . mg/1 Spec. Cond. yS C o l o u r c. u. Mg mg/1 C a+ + mg/1 Mg mg/1 Lime Ca(OH) 9 mg/1 L as CaO mg/1 Polymer pH mg/1 T o t a l A l k a l . mg/1 Spec. Cond. vS C o l o u r c. u. Mg mg/1 Ca' mg, 1 BKME 8.3 240 14000 2250 8.30 65 60 500 375 0 11.21 155 18000 220 10.50 78 2 BKME 7.4 240 16000 2370 12.00 96 60 500 375 0 11.10 135 19400 240 5.60 156 7 BKME 7.5 175 13000 1500 4.5 40 60 500 375 0 11.40 180 17500 70 4.25 175 8 BKME 7.4 180 13000 1500 5.4 40 60 625 475 0 11.40 155 15000 100 4.75 93 3 T-20 10.9 1325 49000 16680 5.76 27 1.50 2500 1875 0 12.30 1700 84000 1450 3.64 344 4 T-20 10.4 1125 52400 17270 4.40 32 300 2500 1875 1 12.10 1250 74000 810 4.70 510 5 T-20 11.8 1825 68000 16000 5.80 34 300 2500 1875 2 12.25 1600 80000 900 2.60 390 6 T-20 11.6 1750 68000 16000 4.6 35 300 5000 3750 2 12.35 2075 80000 940 3.60 313 Note: Mg added as a f r e s h s o l u t i o n o f MgS0 4.7H20, e x c e p t i n Runs #8 and #6 where i t was added 80% as r e c y c l e d magnesium from the p r e v i o u s r u n , p l u s 20% as a s o l u t i o n . o f f r e s h MgS0 A.7H o0. 68 The s c h e m a t i c d i a g r a m , F i g u r e 10, shows the g e n e r a l s o u r c e s and s i n k s measured f o r the m a t e r i a l s b a l a n c e c a l c u l a t i o n s , u s i n g f r e s h c h e m i c a l s . Where: IN was the mass measured i n the u n t r e a t e d wastewater. DOSE was t h e mass o f c h e m i c a l s added. OUT was the mass measured i n the t r e a t e d e f f l u e n t . C S . was t h e mass measured i n the p r i m a r y c o l o u r s l u d g e . OTHER LOSSES (O.L.) were t h e measured c h e m i c a l l o s s e s on equipment, a f t e r d e c o l o u r i z a t i o n . The mass u n a c c o u n t e d f o r i n the d e c o l o u r i z a t i o n l o o p (ERRORi) was the d i f f e r e n c e between t h e mass c a l c u l a t e d i n the C S . minus t h e amount measured i n the C S . A n e g a t i v e r e s u l t meant a n e t g a i n o f m a t e r i a l . Between t h e two l o o p s , a sample o f c o l o u r s l u d g e was t a k e n f o r a n a l y s i s ( C S . SAMPLE). C S . t o CARB r e f e r s t o the mass o f a g i v e n m a t e r i a l i n the c o l o u r s l u d g e e n t e r i n g t h e c a r b o n a t o r . S l u d g e samples were ta k e n a t i n t e r v a l s d u r i n g c a r b o n a t i o n and f i l t e r e d . SAMPLES r e f e r s t o the mass measured i n the f i l t r a t e and FILTERS r e f e r s t o t h e mass measured on t h e f i l t e r p a p e r s . SUPER i n d i c a t e s t he mass measured i n t h e s u p e r n a t a n t a f t e r c a r b o n a t i o n and f i n a l s l u d g e s e t t l i n g . F.S. i n d i c a t e s t he mass measured i n the f i n a l s l u d g e a f t e r c a r b o n a t i o n and s e t t l i n g . The mass un a c c o u n t e d f o r i n the c a r b o n a t i o n l o o p (ERR0R 2) r e f e r s t o t h e d i f f e r e n c e between t h e masses c a l c u l a t e d and measured i n t h e f i n a l s l u d g e . To c a l c u l a t e t he mass o f m a t e r i a l s t h a t would be r e c y c l e d , t he f o l l o w i n g a s sumptions were made: 69 1. C o l o u r r e c y c l e d = (SUPER + SAMPLES). 2. Magnesium r e c y c l e d = (SUPER + FILTERS'+ SAMPLES). 3. C a l c i u m r e c y c l e d = (SUPER + SAMPLES). 4. Magnesium i n the F.S. S o l i d s = ( F . S . ) . 5. C a l c i u m i n t h e F.S. S o l i d s = ( F . S . + F I L T E R S ) . O b v i o u s l y , t h e volume o f f i n a l s l u d g e and s u p e r n a t a n t r e c y c l e i s a f u n c t i o n o f t h e f i n a l s l u d g e s o l i d s c o n c e n t r a t i o n a c h i e v e d . T h e r e f o r e , t h e f i n a l s l u d g e h a n d l i n g method used i n p r a c t i c e would have a s i g n i f i c a n t e f f e c t on the mass b a l a n c e s i n t h e s l u d g e c a r -b o n a t i o n l o o p . F o r p u r p o s e s o f o b t a i n i n g r e a l i s t i c magnesium r e c o v e r y v a l u e s and comparing t h e r e s u l t s o f t h e s e t e s t s , two h y p o t h e t i c a l s l u d g e h a n d l i n g methods were e v a l u a t e d : 1. S u p p o s i n g t h e f i n a l s l u d g e (F.S.) had r e a c h e d 5% s o l i d s . In t h i s c a s e , more s u p e r n a t a n t would be wasted w i t h t h e d i l u t e s l u d g e t h a t would go t o some k i n d o f d i s p o s a l . 2. S u p p o s i n g t h e f i n a l s l u d g e was dewatered t o 60% s o l i d s f o r l i m e r e c o v e r y i n a k i l n . Both t h e s e c a s e s were though t o be r e a l i s t i c a l t e r n a t i v e s f o r f i n a l s l u d g e h a n d l i n g . However, mass b a l a n c e s f o r o t h e r c a s e s can be worked o u t f r o m t h e raw d a t a . An example o f t h e c a l c u l a t i o n s used t o d e r i v e s l u d g e and r e c y c l e volumes f o r s l u d g e o f a g i v e n s o l i d s c o n c e n t r a t i o n i s shown i n Appendix C. S i n c e c o l o u r s l u d g e from t h e d e c o l o u r i z a t i o n system would n o t n o r m a l l y be t h i c k e n e d i n p r a c t i c e , mass b a l a n c e s i n t h e d e c o l o u r -i z a t i o n l o o p were c a l c u l a t e d o n l y u s i n g t he a c t u a l volumes o b t a i n e d d u r i n g t h e s e t e s t s . R e s u l t s o f t h e b a t c h d e c o l o u r i z a t i o n and b a t c h s l u d g e c a r -b o n a t i o n s t u d i e s a r e d i s c u s s e d s e p a r a t e l y i n t h e f o l l o w i n g s e c t i o n s . 70 r D E C O L O U R I Z A T I O N / L O O P \ O U T O T H E R L O S S E S (O.L . ) — T E R R O R C O L O U R S L U D G E ( C S . ) r C A R B O N A T I O N L O O P C S . S A M P L E C S . TO C A R B . S A M P L E S - > - E R R O R 2 F I N A L S L U D G E ( F .S. ) FIGURE 10. SCHEMATIC OF MATERIAL SOURCES AND SINKS 71 6.1.2.2 Batc h D e c o l o u r i z a t i o n R e s u l t s . A. Mass b a l a n c e s : i ) BKME T a b l e 12 summarizes t he i n i t i a l c o n d i t i o n s and r e s u l t s o f a l l t h e b a t c h d e c o l o u r i z a t i o n r u n s . These b a t c h c o a g u l a t i o n s were e s s e n t i a l l y l a r g e (10 t o 30-l i t r e ) j a r t e s t s , s o , as e x p e c t e d , c o l o u r r e m o v a l s were e q u i v a l e n t t o th o s e o b t a i n e d i n the p r e l i m i n a r y j a r t e s t s . The p e r c e n t c o l o u r r e m o vals a t t a i n e d and the magnesium and c a l c i u m l o s s e s o c c u r r i n g i n th e d e c o l o u r i z e d e f f l u e n t d u r i n g a l l t h e r u n s , a r e shown i n T a b l e 13. I t s h o u l d be n o t e d t h a t p e r c e n t c o l o u r removal i n a l l t h e b a t c h runs was c a l c u l a t e d on a mass b a s i s ( i . e . % removal = M a s s jjjjj ~ ^ s s 0 U T x 100%); whereas, i n the j a r t e s t s , i t was c a l c u l a t e d s i m p l y on a c o n c e n t r a t i o n b a s i s (% removal = C o n c' ^ " C ° j | c - 0 U T x 100%) w i t h no r e g a r d t o t h e volume o f s l u d g e wasted. In any c a s e , g r e a t e r than 90% c o l o u r r e d u c t i o n s were a c h i e v e d i n a l l r u n s . The r e s i d u a l c o l o u r i n t h e BKME t e s t s ranged from 7 0.to 240 C U . Because t he p r o c e s s water used a t the C r e s t b r o o k m i l l was from v e r y s o f t s u r f a c e r u n o f f , t h e l e v e l s o f M g + + and C a + + i n the raw waste were q u i t e low compared t o the dosages employed. The M g + + c o n c e n t r a t i o n i n the t o t a l m i l l samples ranged f r o m 5.76 t o 12.0 mg/1, r e p r e s e n t i n g 7.9 t o 16.7% o f the t o t a l magnesium used i n t r e a t i n g t h i s e f f l u e n t . In t h e s e r u n s , e x c l u d i n g t h e r e s u l t s o f Run #1 (because t h e a n a l y t i c a l method used d u r i n g t h a t run d i d n o t i n c l u d e t h e a d d i t i o n o f L a C l 2 ) , 4.25 t o 5.6 mg/1 M g + + o r 6.2 t o 7.4% remained i n t h e d e c o l o u r -i z e d e f f l u e n t . 72 TABLE 13 SUMMARY OF DECOLOURIZATION LOOP MASS BALANCE RESULTS P e r c e n t P e r c e n t o f t h e T o t a l Mass L o s t Run C o l o u r i n t h e D e c o ^ o u r i z e d E f f l u e n t No. Removal Magnesium C a l c i u m 1 90.8 14.3 21.8 2 90.4 7.4 40.0 7 95.5 6.2 .53.3 8 94.8 7.3 23.2 3 93.5 1.7 18.5 4 97.0 -9 2 2- 3 5 95.1 -6 21.1 6 94.8 -9 8.5 The C a T T c o n c e n t r a t i o n i n t h e u n t r e a t e d e f f l u e n t r a n g e d f r o m 40 t o 96 mg/1. The wide v a r i a t i o n was c a u s e d by t h e CaC0 3 used f o r pH a d j u s t m e n t , p r i o r t o b i o l o g i c a l t r e a t m e n t a t t h i s m i l l . The measured c o n c e n t r a t i o n s a c c o u n t e d f o r 12.4 t o 26.2% o f t h e t o t a l C a + + i n t h e BKME r u n s . A g a i n , e x c l u d i n g t h e r e s u l t s o f Run #1 f o r t h e same r e a s o n , t h e t r e a t e d e f f l u e n t C a + + c o n t e n t r a n g e d f r o m 156 t o 175 mg/1 o r 218 t o 245 mg/1 as CaO. T h i s r e p r e s e n t e d a l o s s o f 40.0 t o 53.3% t o t h e d e c o l o u r i z e d e f f l u e n t . i i ) T-20 E f f l u e n t In t h e s e t e s t s , c o l o u r r e d u c t i o n s o f 93.5 t o 97.0% were a t t a i n e d , w i t h r e s i d u a l c o l o u r s r a n g i n g f r o m 810 t o 1 ,450 C U . The h i g h e r r e s i d u a l c o l o u r was a d i r e c t r e s u l t o f a l o w e r - t h a n - i n t e n d e d 73 Mg dosage used i n Run #3. T h i s a l s o a f f e c t e d t h e Mg mass b a l a n c e s t o a c e r t a i n e x t e n t . The magnesium and c a l c i u m c o n t r i b u t i o n s o f t h e u n t r e a t e d e f f l e u n t were even more i n s i g n i f i c a n t f o r t h i s wastewater because o f t h e l a r g e r c h e m i c a l dosages used. Magnesium c o n c e n t r a t i o n i n t h e raw T-20 e f f l u e n t was between 4.5 t o 5.8 mg/1 o r o n l y 1.1 t o 3.7% o f the t o t a l . C o n c e n t r a t i o n s i n t h e d e c o l o u r i z e d T-20 e f f l u e n t were even l e s s than i n BKME, v a r y i n g f r o m 2.6 t o 4.7 mg/1. E x c l u d i n g t h e Run #3 r e s u l t s , t h i s r e p r e s e n t e d o n l y .6 t o .9% o f t h e t o t a l magnesium i n t h e system. The C a + + c o n c e n t r a t i o n i n the raw T-20 e f f l u e n t was co n -s i s t e n t l y a b o u t 30 mg/1. A t t h e h i g h l i m e dosages u s e d , t h i s r e p -r e s e n t e d o n l y 1.8 t o 2.2% o f t h e t o t a l C a + + p r e s e n t . In t h e s e r u n s , t h e C a + + c o n t e n t o f t h e d e c o l o u r i z e d e f f l u e n t was between 344 t o 510 mg/1 o r 482 t o 714 mg/1 as CaO. T h i s r e p r e s e n t e d a l o s s o f 18.5 t o 21.1% o f t h e C a + + t o t h e d e c o l o u r i z e d e f f l u e n t . Measured c h e m i c a l l o s s e s on equipment d u r i n g both BKME and T-20 e f f l u e n t b a t c h d e c o l o u r i z a t i o n t e s t s were n o r m a l l y about 1%. Most o f t h e s e l o s s e s were r e c o v e r e d from t h e w a l l s o f t h e v e s s e l used f o r t h e c o a g u l a t i o n and c l a r i f i c a t i o n , p o i n t i n g t o t h e p o s s i b i l i t y o f s c a l e b u i l d u p on equipment i n a l a r g e s c a l e a p p l i c a t i o n o f the p r o -c e s s . The e r r o r s i n t h e d e c o l o u r i z a t i o n l o o p mass b a l a n c e s showed a p e r c e n t a g e v a r i a t i o n between .6 and 6.7%. T h i s was w i t h i n t h e a c c u r a c y o f t h e volume measurements and a n a l y t i c a l p r o c e d u r e s used. B. F a c t o r s a f f e c t i n g t h e mass b a l a n c e s : i ) G e n e r a l In t h e c o l o u r removal s t e p , t h e aim was t o m i n i m i z e t h e 74 c o l o u r , magnesium, and c a l c i u m c o n c e n t r a t i o n s i n t h e d e c o l o u r i z e d e f f l u e n t . The f i r s t v a r i a b l e i s a measure o f t h e t r e a t m e n t e f f i c i e n c y , w h i l e t h e l a t t e r two r e p r e s e n t c h e m i c a l l o s s e s from t h e system , a l t h o u g h ++ Ca can be r e c o v e r e d a t a c o s t by r e c a r b o n a t i n g t h e d e c o l o u r i z e d e f f l u e n t . The main f a c t o r s which a f f e c t e d t h e s e c o n c e n t r a t i o n s were the u n t r e a t e d e f f l u e n t c h a r a c t e r i s t i c s , and the dosages o f t h e two c h e m i c a l s used i n t h e t r e a t m e n t . T h e s e , i n t u r n , d e t e r m i n e d t h e f i n a l c o a g u l a t i o n pH r e a c h e d . O t h e r c o n s i d e r a t i o n s d i s c u s s e d here i n c l u d e r a p i d m i x i n g and f l o c c u l a t i n g t i m e s and sp e e d s , s e t t l i n g t i m e , use o f s e t t l i n g a i d s , and c o l o u r s l u d g e s o l i d s c o n c e n t r a t i o n , i i ) C o a g u l a t i o n pH The f i n a l pH a c h i e v e d d u r i n g c o a g u l a t i o n has been i d e n t i f i e d by Thompson e t a l (1972) as the key v a r i a b l e i n c o l o u r removal w i t h M g + + and l i m e . However, i t must be remembered t h a t t h i s pH i s a f u n c t i o n o f t h e l i m e and magnesium dosages u s e d , as w e l l as the i n i t i a l pH and a l k a l i n i t y o f t h e raw waste. D u r i n g t h e s e b a t c h r u n s , t h e dosages used t o t r e a t BKME y i e l d e d a pH between 11.1 and 11.4 g i v i n g a c o l o u r r e d u c t i o n o f >90%. A l t h o u g h s m a l l d a r k f l o w p a r t i c l e s were v i s i b l e a f t e r M g + + a d d i t i o n t o T-20 e f f l u e n t , w i t h i n i t i a l pH j u s t above 11, no a p p r e c i a b l e c o l o u r removal r e s u l t e d u n t i l enough l i m e had been added t o a c h i e v e a f i n a l pH above 12. F u r t h e r i n v e s t i g a t i o n o f t h e optimum pH f o r c o a g u l a t i o n i s r e q u i r e d t o i d e n t i f y t h e mechanism(s) o f c o l o u r removal by t h e Mg(0H) 2 + CaC0 3 p r e c i p i t a t e formed d u r i n g c o a g u l a t i o n o f t h e s e e f f l u e n t s . The i n f l u e n c e o f the u n t r e a t e d e f f l u e n t a l k a l i n i t y on the amount o f l i m e r e q u i r e d t o r e a c h a g i v e n pH l e v e l can be seen from t h e 75 l i m i t e d d a t a i n T a b l e 12. The lo w e r i n i t i a l a l k a l i n i t y i n Run #7 (180 mg/1), r e s u l t e d i n a h i g h e r pH (11.4) than i n Runs #1 and 2 ( a l k a l i n i t y = 240 mg/1) where t h e f i n a l . pH was 11.21 and 11.1, r e s p e c t i v e l y . How-e v e r , i t i s d i f f i c u l t t o say what e f f e c t t h i s had on t h e p e r c e n t c o l o u r removal because t h e i n i t i a l c o l o u r i n Run #7 was o n l y 1 ,500 C U . com-p a r e d t o about 2,300 C U . i n Runs #1 and 2. The r e s u l t s o f Runs #4 and 5 a r e an a p p a r e n t anomaly t o t h i s a l k a l i n i t y - l i m e r e q u i r e m e n t r e l a t i o n s h i p . Run #4 had an i n i t i a l a l k a -l i n i t y o f 1,125 mg/1, b u t the f i n a l pH was o n l y 12.1 compared t o 12.25 i n Run #5 which had an i n i t i a l a l k a l i n i t y o f 1,825 mg/1. The i n c r e a s e d polymer dosage i n Run #5 may p a r t i a l l y e x p l a i n t h i s d i f f e r e n c e . The e f f e c t o f a l k a l i n i t y on l i m e r e q u i r e m e n t s became more a p p a r e n t d u r i n g t e s t i n g w i t h r e c y c l e d magnesium and i s d i s c u s s e d f u r t h e r i n t h a t s e c t i o n . The pH - magnesium s o l u b i l i t y r e l a t i o n s h i p has been d i s -c u s s e d e a r l i e r . A comparison o f t h e r e s u l t s o f the runs w i t h BKME a t a pH o f 11.1 t o 11.4, t o t h o s e w i t h T-20 e f f l u e n t a t a pH o f 12.1 -12.25, i l l u s t r a t e s t h i s pH e f f e c t on magnesium l o s s e s . A t the lower pH, 4.25 to 5.6 mg/1 o f M g + + remained i n t h e d e c o l o u r i z e d BKME e f f l u e n t whereas o n l y 2.6 to 4.7 mg/1 M g + + remained i n the t r e a t e d T-20 e f f l u e n t . C a l c i u m l o s s e s a r e more d i f f i c u l t t o e x p l a i n on t h e b a s i s o f s o l u b i l i t y a l o n e because o t h e r f a c t o r s p l a y i m p o r t a n t r o l e s as w e l l . However, t he l o s s e s a t pH 12.1 t o 12.25 were a p p r o x i m a t e l y t w i c e t h o s e a t pH 11.1 t o 11.4. i i i ) M i x i n g Times and Speeds These v a r i a b l e s were m o n i t o r e d d u r i n g most o f t h e b a t c h d e c o l o u r i z a t i o n r u n s , t o e n s u r e t h a t pH s t a b i l i z a t i o n had o c c u r r e d b e f o r e a l l o w i n g t h e s l u d g e t o s e t t l e . The pH u s u a l l y s t a b i l i z e d 76 w i t h i n f i v e m i n u t e s , as i t d i d i n the p r e l i m i n a r y j a r t e s t s o f t h e two e f f l u e n t t y p e s . No improvement i n c o l o u r removal e f f i c i e n c y r e s u l t e d f r o m c o n t i n u i n g f l o c c u l a t i o n f o r more than 5 t o 10 m i n u t e s . The e f f e c t s o f r a p i d m i x i n g and f l o c c u l a t i o n on M g + + and C a + + l o s s e s were n o t s t u d i e d e x t e n s i v e l y d u r i n g t h i s work. However, i n d i c a t i o n s were t h a t e x t e n d e d m i x i n g t i m e s (beyond 15 m i n u t e s ) and f a s t e r m i x i n g speeds o n l y cause f l o e d e s t r u c t i o n . T h i s would i n t u r n cause p o o r e r s e t t l i n g and i n c r e a s e d M g + + and C a + + l o s s e s i n f l o e c a r r y o v e r . i v ) S l u d g e S e t t l e a b i T i t y and S o l i d s C o n c e n t r a t i o n S e t t l i n g t i m e s i n t h e s e t e s t s v a r i e d from two hours t o o v e r n i g h t , i n some c a s e s , so no d i r e c t c o mparison can be made between t e s t s . In g e n e r a l , s l u d g e from BKME s e t t l e d w e l l and y i e l d e d a f a i r l y c l e a r e f f l u e n t i n l e s s t h a n an hour. On t h e o t h e r hand, T-20 e f f l u e n t s l u d g e s e t t l e d p o o r l y , r e q u i r i n g s e v e r a l hours t o a c h i e v e a f i n a l s l u d g e volume o f 25 t o 40% o f t h e o r i g i n a l volume. T h i s p oor s e t t l e -a b i l i t y would have r e s u l t e d i n l a r g e amounts o f f l o e c a r r y o v e r i n the two hours a l l o t t e d f o r s e t t l i n g . However, t h e r e s u l t s o f p r e l i m i n a r y e v a l u a t i o n o f s e t t l i n g a i d s (Appendix B) showed t h a t s e t t l i n g t i m e s c o u l d be r e d u c e d t o l e s s than two hours by a d d i t i o n o f an a n i o n i c polymer. I t was e x p e c t e d t h a t polymer use would d e c r e a s e t h e l o s s e s ++ ++ o f Mg and Ca t o t h e d e c o l o u r i z e d e f f l u e n t by l i m i t i n g f l o e c a r r y o v e r . T h i s was n o t the c a s e . The C a + + l o s s i n Run #5 (2 mg/1 Dow A-23) was h i g h e r than i n Run #3 (no polymer) and Run #4 (1 mg/1 Dow A-23) had t h e h i g h e s t M g + + and C a + + l o s s e s o f a l l t h e T-20 e f f l u e n t r u n s . From t he r e s u l t s o f Run #4, i t appeared t h a t t o o l i t t l e polymer 77 was worse t h a n none a t a l l because s m a l l l i g h t f l o e a g g l o m e r a t i o n s formed, but some f a i l e d t o s e t t l e w i t h the f l o e b l a n k e t . As i n t h e p r e l i m i n a r y j a r t e s t s , c o l o u r removal was n o t improved s i g n i f i c a n t l y by polymer usage. So, w h i l e the polymer dosage o f 2 mg/1 r e s u l t e d i n good s l u d g e s e t t l i n g , no o t h e r b e n e f i t s c o u l d be seen from t h e s e l i m i t e d d a t a . The s o l i d s c o n t e n t o f t h e c o l o u r s l u d g e f r o m BKME t r e a t m e n t ranged f r o m 1.20 t o 1.60% and from 1.20 t o 2.04% i n T-20 e f f l u e n t t r e a t m e n t . However, t h e method o f l i q u i d - s o l i d s s e p a r a t i o n i n t h e s e b a t c h t e s t s was n o t v e r y e f f i c i e n t . With t i m e d s l u d g e w a s t i n g , i n a l a r g e s c a l e system a c o l o u r s l u d g e c o n c e n t r a t i o n a p p r o a c h i n g 5% s o l i d s c o u l d l i k e l y be a t t a i n e d . T h i s would mean a s l i g h t l y l a r g e r volume o f d e c o l o u r i z e d e f f l u e n t and hence s l i g h t l y h i g h e r c o l o u r , Mg and Ca l o s s e s i n t h a t e f f l u e n t . However, as d i s c u s s e d i n t h e n e x t s e c t i o n , t h e o v e r a l l e f f e c t o f an i n c r e a s e d s o l i d s c o n c e n t r a t i o n i n t h e c o l o u r s l u d g e may be a p o s i t i v e one. 6.1.2.3 B a t c h S l u d g e C a r b o n a t i o n R e s u l t s A. Mass b a l a n c e s : i ) G e n e r a l P e r t i n e n t s l u d g e c a r b o n a t i o n r e s u l t s f r o m each run have been summarized i n T a b l e 14. P a r t 1 o f t h i s t a b l e g i v e s the r e s u l t s as a p e r c e n t o f t h e t o t a l mass i n the c o l o u r removal system, so t h a t o v e r a l l m a t e r i a l r e c o v e r i e s and l o s s e s can be seen f o r the two assumed s l u d g e d i s p o s a l methods. P a r t 2 g i v e s the r e s u l t s as a p e r c e n t o f t h e mass  o f m a t e r i a l s c a r b o n a t e d , a l l o w i n g a more d i r e c t c o m p a r i s o n o f t h e s l u d g e c a r b o n a t i o n e f f i c i e n c i e s f r o m t h e v a r i o u s r u n s . Not shown i n t h i s t a b l e a r e t h e measured l o s s e s on the equipment which were <1% i n 78 most c a s e s . The e r r o r s i n the c a r b o n a t i o n l o o p Mg and Ca mass b a l a n c e s were ± 3 t o 6% i n most c a s e s . Run #1 had l a r g e r e r r o r s (20 t o 30%) because the a n a l y t i c a l methods were not f u l l y d e v e l o p e d a t t h a t t i m e . O t h e r w i s e , e r r o r s ranged f r o m z e r o i n Run #7 f o r C a + + t o 10% i n Run #5 f o r M g + + . I n f o r m a t i o n on t h e s u p e r n a t a n t r e c y c l e q u a l i t y , o b t a i n e d i n each o f t h e r u n s , i s g i v e n i n T a b l e 15, f o r comparison w i t h t h e c o r r e s p o n d i n g u n t r e a t e d and t r e a t e d e f f l u e n t v a l u e s shown i n T a b l e 12. i i ) C o l o u r 1. BKME. In the t e s t s o f t o t a l m i l l e f f l u e n t w i t h f r e s h c h e m i c a l s , 22.7 t o 36.3% o f the t o t a l c o l o u r mass i n the system was c o n t a i n e d i n the s u p e r n a t a n t r e c y c l e . T h i s r e p r e s e n t e d a c o l o u r r e l e a s e f r o m t h e c o l o u r s l u d g e d u r i n g c a r b o n a t i o n o f 25.0 t o 38.1%. With t h i s amount o f c o l o u r b e i n g r e c y c l e d , t h e r e s u l t a n t c o l o u r mass d i s p o s e d o f i n t h e f i n a l s l u d g e o f t h e b a t c h s y s t e m v a r i e d f r o m 56.4 t o 68.1% o f t h e t o t a l , b u t t h i s would be h i g h e r when f l o w t h r o u g h e q u i l i b r i u m c o n d i t i o n s were e s t a b l i s h e d i n a c o n t i n u o u s system. The r e c y c l e d c o l o u r was c o n c e n t r a t e d i n o n l y 4.9 t o 6.4% o f t h e o r i g i n a l volume (as shown i n T a b l e 1 6 ) , meaning t h a t , i n a l l c a s e s , t he c o l o u r c o n c e n t r a t i o n o f the s u p e r n a t a n t was h i g h e r than i n t h e o r i g i n a l u n t r e a t e d e f f l u e n t ( i . e . , 5,575 t o 9,400 C U . compared t o 1 ,500 t o 2,300 C U . i n t h e u n t r e a t e d w a s t e ) . The e f f e c t o f r e c y c l i n g t h i s h i g h l y c o l o u r e d s u p e r n a t a n t i s d i s c u s s e d i n t h e s e c t i o n on the use o f r e c y c l e d magnesium. 2. T-20 e f f l u e n t . C o r r e s p o n d i n g r e s u l t s f o r t h e c a u s t i c e x t r a c t i o n e f f l u e n t t e s t s w i t h f r e s h c h e m i c a l s showed, g e n e r a l l y , a lo w e r p e r c e n t o f t h e c o l o u r i n the r e c y c l e . The s u p e r n a t a n t , a f t e r TABLE 14 ( P a r t 1) SUMMARY OF CARBONATION LOOP MASS BALANCE RESULTS Run No. A c t u a l P e r c e n t S o l i d s P e r c e n t o f t h e T o t a l Mass In The Supern a t a n t R e c y c l e In The F i n a l Sludae i t F.S. = 5% S o l i d s * I f F.S. = 60% S o l i d s I f F.S. = 5% S o l i d s I f F.S. = 60% S o l i d s C.S. t o Carb. F.S. C o l o u r M g + + C a + + C o l o u r M g + + C a + + C o l o u r M g + + C a + + C o l o u r M g + + C a + + 99 3.65 22.7 67.7 2.3 26.1 78.4 2.7 68.1 34.0 51.7 64.7 23.1 51.5 1. 46 2.10 32.5 73.7 9.5 34.0 77.2 10.0 57.9 8.8 39.9 56.4 5.5 39.2 26 3.19 29.9 74.1 2.0 36.3 89.3 2.5 65.6 26.2 44.8 59.2 11.5 44.3 2. 24 6.33 25.4 61.0 1.0 34.4 80.2 1.3 69.4 34.4 71.3 60.4 15.2 70.9 1. 70 3.83 16.0 71.3 24.5 19.5 87.3 30.2 77.5 29.7 53.6 74.0 13.2 47.8 1. 20 3.45 25.1 82.4 30.7 28.8 94.5 35.1 71.9 27.0 50.5 68.2 14.9 46.0 2. 04 2.60 18.7 71.0 11.5 24.8 93.3 15.5 76.4 42.7 57.9 70.3 20.4 53.8 4. 03 31.10 16.2 58.3 5.2 17.2 61.9 5.5 78.6 39.8 80.0 77.6 36.4 79.7 * Note: Run #8 F.S. = 6.33% Run #6 F.S. = 31. 1% ^ 1 <JD TABLE 14 ( P a r t 2) SUMMARY OF CARBONATION LOOP MASS BALANCE RESULTS P e r c e n t o f t h e Mass C a r b o n a t e d P e r c e n t o f t h e I n c i n e r a t e d F.S. ^ f\ 1 T A c Ma c c In t h e S u p e r n a t a n t R e c y c l e In t h e F i n a l S l u d g e Run No. I f F.S. = 5% S o l i d s * I f FS = 60% S o l i d s I f FS = 5% S o l i d s * I f FS = 60% S o l i d s C o l o u r M g + + C a + + C o l o u r Mg + +- C a + + ++ ++ C o l o u r Mg Ca C o l o u r M g + + • C a + + OU 1 1 Ub r lobb MgO CaO O t h e r 1 25.0 79.1 13.0 28.8 91.5 3.5 75.0 39.7 65.8 71.2 27.0 65.4 7.1 64.8 28.1 2 35.9 85.9 17.9 37.6 90.0 18.8 64.1 10.3 74.5 62.4 6.4 73.6 2.9 88.4 8.7 7 31.4 75.6 4.4 38.1 91.1 5.0 68.6 26.7 97.0 59.2 11.7 96.0 5.2 73.9 20.9 8 26.8 62.6 1.3 36.2 82.3 1.8 73.2 35.3 91.7 63.8 15.6 91.2 3.7 80.0 16.3 3 17.0 72.9 33.0 20.8 89.3 40.8 83.0 30.4 72.2 79.2 13.4 64.4 2.9 74.9 20.9 4 26.9 79.1 39.3 29.7 90.7 45.0 73.1 25.9 64.8 70.3 14.3 59.1 6.8 81.4 11.9 5 19.6 68.0 15.8 26.1 89.3 21.5 80.4 40.9 79.8 73.9 19.5 74.1 8.9 79.9 11.2 6 17.0 56. 7 6.2 18.2 .60.2 6.7 83.0 38.8 96.7 81.9 35.3 96.3 4.8 82.2 13.0 * Note: Run #8 F.S. = 6.33% Run #6 F.S. = 31.1% 81 TABLE 15 CARBONATION SUPERNATANT CHARACTERISTICS T o t a l S p e c i f i c Run U n f i l t e r e d F i l t e r e d A l k a l i n i t y Conductance C o l o u r Mg Ca No. pH pH mg/1 C a C 0 3 yS C U . mg/1 mg/1 1 N.A. 7.3 2250 N.A. 5575 525 93 2 N.A. 7.0 1400 28000 3790 263 192 7 7.7 7.8 3650 43500 9400 960 135 8 7.6 7.8 4375 43000 13500 980 110 3 7.2 7.5 6578 90000 12825 552 1750 4 7.3 7.3 N.A. 61000 11810 734 1240 5 7.7 7.9 6830 91000 16000 1250 1050 6 N.A. 7.7 5765 80000 12550 900 865 s l u d g e c a r b o n a t i o n , was found t o c o n t a i n 16,0 t o 28.J 3% o f the t o t a l c o l o u r mass. T h i s r e p r e s e n t e d a c o l o u r r e l e a s e from t h e c o l o u r s l u d g e o f 17.0 t o 29.7 %. With t h e s e p e r c e n t a g e s o f c o l o u r b e i n g r e c y c l e d , t h e r e s u l t a n t c o l o u r mass i n the f i n a l s l u d g e was h i g h e r than f o r BKME run s ( i . e . , 68.2 t o 78.6% o f t h e t o t a l mass). S i n c e a much l a r g e r c o l o u r s l u d g e volume was g e n e r a t e d i n t h e s e r u n s t h a n i n BKME t e s t s , t h e r e c y c l e volumes were a l s o h i g h e r (16.9 t o 39.1% o f t h e o r i g i n a l v o lume). The r e s u l t a n t c o l o u r con-c e n t r a t i o n s were g e n e r a l l y about t he same as t h e i n f l u e n t c o l o u r , as seen by comparing T a b l e s 12 and 15. TABLE 16 SUMMARY OF THE BATCH TEST VOLUME BALANCE RESULTS Run No. T o t a l Vol ume ( l i t r e s ) In Dose Out C S . P e r c e n t o f t h e T o t a l Volume I f F.S. = 5% S o l i d s * R e c y c l e F.S. I f F.S. = R e c v c l e 60% S o l i F S 1 10 99.8 .2 93.8 6.2 5.3 .9 6.1 I • o .1 2 10 99.8 .2 93.5 6.5 5.9 .6 6.4 .1 7 30 96.3 3,7 94.0 6.0 4.9 1.1 5.9 .1 8 26 91.3 8.7 94.8 5.2 3.8 1.4 5.05 .15 3 10 99.5 .5 74.2 25.8 20.9 4.9 25.9 .4 4 10 93.0 7.0 60.4 39.6 34.1 5.8 39.1 .5 5 10 88.0 12.0 77.2 22.8 16.9 5.9 22.3 .5 6 6.52 69.9 30.1 78.2 21.8 19.3 2.5 20.5 1.3 * Note: Run #8 F.S. = 6.33% Run #6 F.S. = 31.1% ds CO ro 83 i i i ) Magnesium 1. BKME. O v e r a l l magnesium r e c o v e r y , i n BKME t e s t s w i t h f r e s h c h e m i c a l s , ranged from 67.7 t o 89.3% o f t h e t o t a l mass i n t h e system. T h i s was 75.6 t o 91.5 % o f the magnesium p r e s e n t i n t h e s l u d g e e n t e r i n g t h e c a r b o n a t i o n s t a g e . Recovery o f t h i s magnitude was c o u p l e d w i t h measured magnesium l o s s e s i n t h e f i n a l s l u d g e , v a r y i n g f r o m a low o f 5.5% t o a h i g h o f 34.0% o f t h e t o t a l mass i n the system o r 6.4 t o 39.7% o f t h e mass c a r b o n a t e d . T h i s amount o f magnesium i n t h e f i n a l s l u d g e a c c o u n t e d f o r 2.9 t o 7.1% o f the i n c i n e r a t e d f i n a l s l u d g e s o l i d s mass, c a l c u l a t e d as MgO. Magnesium c o n c e n t r a t i o n i n t h e c a r b o n a t i o n r e c y c l e v a r i e d f r o m 263 t o 980 mg/1, depe n d i n g on the s u p e r n a t a n t volume. 2. T-20 e f f l u e n t . In c a r b o n a t i o n o f t h e T-20 c o l o u r s l u d g e , s l i g h t l y b e t t e r o v e r a l l magnesium r e c o v e r y was a c h i e v e d i n most c a s e s because l o s s e s i n the d e c o l o u r i z e d e f f l u e n t became l e s s s i g n i f i c a n t a t t h e h i g h e r magnesium dosage l e v e l used. With f r e s h c h e m i c a l s , 71.0 t o 94.5% o f t h e t o t a l mass was r e c o v e r e d i n t h e s u p e r n a t a n t . However, the 68.0 t o 90.7% r e c o v e r e d f r o m t h e mass c a r b o n a t e d was j u s t e q u i v a l e n t o r s l i g h t l y l e s s than r e c o v e r y i n BKME r u n s . Measured magnesium l o s s e s t o the f i n a l s l u d g e were 13.2 t o 42.7% o f the t o t a l mass o r 13.4 t o 40.9% o f t h e c a r b o n a t e d mass. As MgO, t h i s a c c o u n t e d f o r 2.9 t o 8.9% o f the i n c i n e r a t e d f i n a l s l u d g e s o l i d s mass. Magnesium c o n c e n t r a t i o n i n the r e c y c l e from t h e s e runs a l s o v a r i e d g r e a t l y , d e p e n d i n g on d i l u t i o n , and dosage l e v e l , r a n g i n g f r o m 552 mg/1 i n Run #3 t o 1,250 mg/1 i n Run #5. 84 i v) C a l c i u m 1. BKME. While magnesium i n the s u p e r n a t a n t r e c y c l e was a r e c o v e r y , c a l c i u m i n t h a t s t r e a m was c o n s i d e r e d a l o s s . A wide v a r i a t i o n i n C a + + l o s s e s t o the r e c y c l e was o b s e r v e d d u r i n g t h e s e t e s t s , d e p e n d i n g on the f a c t o r s d i s c u s s e d i n the n e x t s e c t i o n . In BKME runs w i t h f r e s h c h e m i c a l s , 2.0 t o 10.0% o f t h e t o t a l c a l c i u m i n the s y s t e m appeared i n the c a r b o n a t i o n s u p e r n a t a n t . T h i s was 3.0 t o 18.8% o f t h e mass i n t h e c a r b o n a t e d c o l o u r s l u d g e . On t h e o t h e r hand, o v e r a l l C a + + r e c o v e r y i n the f i n a l s l u d g e ranged from o n l y 39.2 t o 51.7% o f t h e t o t a l mass i n t h e system. T h i s , however, r e p -r e s e n t e d 68.4 t o 97.0% o f t h e C a + + s e n t t o the c a r b o n a t o r . T h i s amount o f c a l c i u m i n the f i n a l s l u d g e a c c o u n t e d f o r 64.8 t o 88.4% o f t h e i n c i n e r a t e d f i n a l s l u d g e s o l i d s mass as CaO, l e a v i n g 8.7 t o 28.1% unaccounted f o r by MgO o r CaO i n t h e s e t e s t s . Unburned o r g a n i c s i n t h e e a r l y runs a t lower t e m p e r a t u r e s a l o n g w i t h s u l p h a t e , c h l o r i d e and o t h e r i n o r g a n i c o x i d a t i o n m a t e r i a l s c o u l d make up t h e d i f f e r e n c e . F u r t h e r i n v e s t i g a t i o n i s r e q u i r e d t o a c c o u n t f o r t h e e n t i r e mass o f t h e r e c a l c i n a t e d l i m e . C a l c i u m c o n c e n t r a t i o n i n t h e s u p e r n a t a n t v a r i e d between 93 and 192 mg/1 d u r i n g t h e s e t e s t s . 2. T-20 e f f l u e n t . O v e r a l l c a l c i u m l o s s e s i n t h e c a r -b o n a t i o n s u p e r n a t a n t d u r i n g T-20 e f f l u e n t runs w i t h f r e s h c h e m i c a l s were much h i g h e r than i n BKME runs because i n c o m p l e t e m i x i n g and C 0 2 t r a n s f e r was a c h i e v e d ; 11.5 t o 35.1% o f the t o t a l mass was f o u n d i n t h a t s t r eam. T h i s amounted t o 15.8 t o 45.0% o f t h e c a l c i u m i n i t i a l l y p r e s e n t i n the c a r b o n a t e d c o l o u r s l u d g e . These l a r g e l o s s e s i n the r e c y c l e d s u p e r n a t a n t , c o u p l e d w i t h l o s s e s i n t h e d e c o l o u r i z e d e f f l u e n t , r e s u l t e d i n poor o v e r a l l l i m e r e c o v e r y i n t h e f i n a l s l u d g e . O n l y 46.0 85 t o 57.9% o f the t o t a l C a + + i n the system ended up i n t h e f i n a l s l u d g e . L o o k i n g a t the c a r b o n a t i o n l o o p a l o n e , t h e r e c o v e r y from t h e mass e n t e r i n g t h e c a r b o n a t o r was a l s o lower t h a n i n BKME t r e a t m e n t ( i . e . , 59.1 t o 79.8%). T h i s amount o f c a l c i u m (as CaO) i n t h e f i n a l s l u d g e r e p r e s e n t e d 74.9 t o 81.4% o f the i n c i n e r a t e d f i n a l s l u d g e s o l i d s mass l e a v i n g 11.2 t o 20.9% unaccounted f o r by t h e sum o f MgO and CaO. These r e s u l t s were s i m i l a r t o t h o s e o b t a i n e d i n the BKME r u n s . C a l c i u m c o n c e n t r a t i o n i n the s u p e r n a t a n t r e c y c l e ranged from 1,050 t o 1,750 mg/1 i n t h e s e t e s t s w i t h f r e s h c h e m i c a l s . B. F a c t o r s a f f e c t i n g t h e mass b a l a n c e s : i ) G e n e r a l S i m p l y s t a t e d , t h e aim o f c a r b o n a t i n g t h e p r i m a r y c o l o u r s l u d g e i s t o r e c o v e r t h e maximum amount o f s o l u b l e M g + + i n the s u p e r -n a t a n t , w h i l e m i n i m i z i n g t h e f i n a l c o l o u r and C a + + r e l e a s e i n t o t h e same r e c y c l e stream. As o u t l i n e d p r e v i o u s l y , t h i s M g + + r e c o v e r y method r e l i e s on the pH - magnesium s o l u b i l i t y r e l a t i o n s h i p . R educing the pH o f the c o l o u r s l u d g e by C 0 2 gas a d d i t i o n , f r o m t h e i n i t i a l pH o f 11 o r 12, t o a f i n a l pH near 7.5 r e s u l t s i n e s s e n t i a l l y complete s o l u b i l i z a t i o n o f Mg(0H) 2 w h i l e c a u s i n g v e r y l i t t l e d i s s o l u t i o n o f the CaC03 p r e c i p i t a t e i n the time a l l o w e d . The r e a c t i o n s and r a t e con-s t a n t s c o n t r o l l i n g C 0 2 n e u t r a l i z a t i o n have been g i v e n by C o t t o n and W i l k i n s o n (1972). The n e u t r a l i z a t i o n o f C 0 2 o c c u r s by two pa t h ways. A t pH >10, t h e predominant r e a c t i o n i s t h e d i r e c t r e a c t i o n o f C 0 2 •+ 0H~: C0 2 + OH" = HC0 3" (slow) (1) the n H C O 3 " + OH" = C 0 3 2 " + H 20 ( I n s t a n t a n e o u s ) (2) 86 where the r a t e law i s - d [ C 0 2 ] — ^ = k Q H - [OH -] [ C 0 2 ] (3) and k Q H - = 8500 s e c " 1 ( m o l / 1 ) " 1 F o r pH <8 t h e p r i n c i p a l mechanism i s d i r e c t h y d r a t i o n o f C 0 2 : C 0 2 + H 20 = H 2 C 0 3 (slow) (4) th e n H 2 C 0 3 + OH" = HC0 3" + H 20 ( i n s t a n t a n e o u s ) (5) where the r a t e law i s pseudo - 1 s t o r d e r - d [ C 0 2 ] —nt~ = k C 0 2 [ C 0 2 ] (6) a n d K C Q 2 = 0.03 s e c " 1 In t h e pH range 8 t o 10, both mechanisms a r e i m p o r t a n t and f o r each h y d r a t i o n r e a t i o n (1) and ( 4 ) , t h e r e i s a c o r r e s p o n d i n g d e h y d r a t i o n r e a c t i o n . H 2 C 0 3 H 20 + C 0 2 (7) where k^QQ = 20 s e c " 1 and HG0 3" -»• C 0 2 + OH" (8) where k u r n - _ 0 „ n n - t - i HC0 3 = 2 x 10 s e c From the f i r s t s e t o f e q u a t i o n s , i t can be seen t h a t , i n i t i a l l y , t h e r a t e o f d i s s o l u t i o n o f Mg(0H) 2 w i l l be governed by a l k a l i n i t y and C 0 2 s o l u b i l i t y , which i n t u r n i s a f f e c t e d by t e m p e r a t u r e , as shown i n T a b l e 17. As the pH d r o p s below 8, o n l y C 0 2 s o l u b i l i t y c o n t r o l s . I t i s w e l l known t h a t H 2 C 0 3 + CaC0 3 can r e a c t t o g i v e s o l u b l e C a ( H C 0 3 ) 2 , so t h a t a f t e r r e a c t i o n (4) has t a k e n p l a c e , some CaC0 3 d i s s o l u t i o n c o u l d be a n t i c i p a t e d a t pH below 8, a l t h o u g h t h i s r e a c t i o n w i l l have t o compete w i t h t he d e h y d r a t i o n r e a c t i o n ( 7 ) . TABLE 17 87 Temp °C Carbon D i o x i d e a * q 0 1.713 0.3346 1 1.646 0.3213 2 1.584 0.3091 3 1.527 0.2978 4 1.473 0.2871 5 1.424 0.2774 6 1.377 0.2681 7 1.331 0.2589 8 1.282 0.2492 9 1.237 0.2403 10 1.194 0.2318 11 1.154 0.2239 12 1.117 0.2165 13 1.083 0.2098 14 1.050 0.2032 15 1.091 0.1970 16 0.985 0.1903 17 0.956 0.1845 18 0.928 0.1789 19 0.902 0.1737 20 0.878 0.1688 21 0.854 0.1640 22 0.829 0.1590 23 0.804 0.1540 24 0.781 0.1493 25 0.759 0.1449 26 0.738 0.1405 27 0.718 0.1366 28 0.699 0.1327 29 0.682 0.1292 30 0.665 0.1257 35 0.592 0.1105 40 0.530 0.0973 45 0.479 0.0860 50 0.436 0.0761 60 0.359 0.0578 (Lange & F o r k e r , 1967) The column headed " a " g i v e s t h e volume o f gas r e d u c e d t o s t a n d a r d c o n d i t i o n s (0°C and 760 riim) d i s s o l v e d i n one volume o f w a t e r when t h e p r e s s u r e o f t h e gas ( w i t h o u t t h e aqueous t e n s i o n ) i s 760 mm; t h i s v a l u e " a " i s t h e a b s o r p t i o n c o -e f f i c i e n t " . The column headed "q" g i v e s t h e w e i g h t o f gas i n grams d i s s o l v e d i n 100 grams o f w a t e r a t a t o t a l p r e s s u r e ( p a r t i a l p r e s s u r e o f t h e gas p l u s t h e aqueous t e n s i o n a t t h e s t a t e d t e m p e r a t u r e ) o f 760 mm. 88 B l a c k and E i d s n e s s ( 1 9 5 7 ) , c a r b o n a t i n g a s l u d g e c o n t a i n i n g Mg(0H) 2 and 36 g/1 o f CaC0 3 w i t h 11% C 0 2 , f o u n d t h a t o n l y 80 mg/1 o f CaC0 3 was d i s s o l v e d a f t e r 30 m i n u t e s ' c a r b o n a t i o n , a t a gas f l o w f i v e t i m e s t h a t r e q u i r e d t o d i s s o l v e a l l o f the Mg(0H) 2 p r e s e n t . A c c o r d i n g t o J o h n s t o n ( 1 9 1 5 ) , who s t u d i e d the s o l u b i l i t y o f c a l c i u m and magnesium c a r b o n a t e s i n n a t u r a l w a t e r s , the e q u i l i b r i u m r a t i o a t 16°C i s [ M g + + ] [Ca + +}> = 14,000, when the p a r t i a l p r e s s u r e o f C 0 2 i n the atmosphere i s g r e a t enough t o p r e v e n t p r e c i p i t a t i o n o f Mg(0H) 2. A n o t h e r e x p l a n a t i o n f o r t h e s e phenomena i s t h a t a s a t u r a t e d s o l u t i o n o f Ca(HC0 3)2 has a lower pH t h a n a s a t u r a t e d s o l u t i o n o f M g ( H C 0 3 ) 2 . As c a r b o n a t i o n p r o c e e d s , the pH i s b u f f e r e d a t a pH o f a p p r o x i m a t e l y 7.5, due t o the M g ( H C 0 3 ) 2 , a l l o w i n g l i t t l e o f t h e c a l c i u m t o d i s s o l v e . From r e c e n t l a r g e s c a l e magnesium r e c o v e r y t e s t s on w a t e r t r e a t m e n t s l u d g e s , upper l i m i t s o f 20,000 t o 25,000 mg/1 a l k a l i n i t y a t 20 t o 25°C have been found t o be the p r a c t i c a l v a l u e s t o l e r a b l e i n s l u d g e c a r b o n a t i o n , a t the p a r t i a l p r e s s u r e s when u s i n g pure C 0 2 ; s o l u b i l i z a t i o n has n o t been s i g n i f i c a n t u n t i l t he pH d r o p s below 6.6 (Thompson, 1975). However, t h e s e p r a c t i c a l v a l u e s were unknown t o the a u t h o r a t t h e t i m e o f t e s t i n g , and c l o s e m o n i t o r i n g d u r i n g s l u d g e c a r b o n a t i o n was c a r r i e d o u t t o g a i n a b e t t e r u n d e r s t a n d i n g o f t h e p r o c e s s and i d e n t i f y p r a c t i c a l c o n t r o l methods f o r use i n f u t u r e p i l o t p l a n t o p e r a t i o n . T a b l e s 1 t o 8 i n Appendix D show the r e s u l t s o f a l l the s l u d g e c a r b o n a t i o n m o n i t o r i n g r u n s . These d a t a , a l o n g w i t h the r e s u l t s p r e s e n t e d i n T a b l e 14, p r o v i d e d much u s e f u l i n f o r m a t i o n on the f a c t o r s a f f e c t i n g t h e s l u d g e c a r b o n a t i o n p r o c e s s . 89 In g e n e r a l , t h e r e was l i t t l e d i f f e r e n c e i n the r e s u l t s o b t a i n e d i n the t r e a t m e n t o f the two e f f l u e n t t y p e s w i t h f r e s h c h e m i c a l s . The f a c t o r s f o u n d t o a f f e c t t h e outcome o f the s l u d g e c a r b o n a t i o n were e s s e n t i a l l y t h e same f o r t o t a l m i l l e f f l u e n t as f o r c a u s t i c e x t r a c t i o n e f f l u e n t . The para m e t e r s i d e n t i f i e d as h a v i n g a s i g n i f i c a n t i n f l u e n c e on t h e mass b a l a n c e r e s u l t s were: f i n a l s l u d g e volume; f i n a l c a r b o n a t i o n pH; t i m e ; c o l o u r s l u d g e and f i n a l s l u d g e s o l i d s c o n c e n t r a t i o n ; and m i x i n g . D i s c u s s i o n o f each f o l l o w s under the a p p r o p r i a t e h e a d i n g s , i i ) F i n a l s l u d g e volume The r e a s o n i n g and methodology f o r c a l c u l a t i n g t h e c a r b o n a t i o n l o o p mass b a l a n c e s , on t h e b a s i s o f assumed v a l u e s o f 5% and 60% s o l i d s i n the f i n a l s l u d g e , has been d i s c u s s e d e a r l i e r . I f c a l c u l a t i o n s were made on the b a s i s o f 5% s o l i d s i n the f i n a l s l u d g e , t h e v a l u e s o f c o l o u r , magnesium and c a l c i u m i n the r e c y c l e were lower than on the b a s i s o f 60% s o l i d s because o f the l o s s e s i n the l i q u i d p o r t i o n o f t h e d i l u t e (5%) s l u d g e . F o r example, r e s u l t s o f BKME t e s t s w i t h f r e s h c h e m i c a l s showed t h a t o v e r 90% o f the magnesium s e n t t o the c a r b o n a t o r would be r e c o v e r e d i f the f i n a l s l u d g e was dewatered t o 60% s o l i d s , whereas o n l y 75 t o 85% r e c o v e r y would be o b t a i n e d i f t h e f i n a l s l u d g e was o n l y 5% s o l i d s . On t h e o t h e r hand, about 3% more c o l o u r and 1% more c a l c i u m were r e c y c l e d a t the h i g h e r s o l i d s c o n t e n t . R e s u l t s o f T-20 t e s t s show more o f a d i f f e r e n c e i n mass b a l a n c e r e s u l t s because o f t h e l a r g e r volume o f s l u d g e p r o d u c e d . A t 60% s o l i d s , 90% magnesium r e c o v e r y r e s u l t e d , but a t 5% s o l i d s o n l y 68 t o 79% o f t h e magnesium i n t h e c a r b o n a t o r was r e c o v e r e d i n the r e c y c l e . In t h e s e t e s t s , 3 t o 5% more c o l o u r and about 6% more c a l c i u m was r e c y c l e d a t the h i g h e r s l u d g e c o n c e n t r a t i o n . 90 O b v i o u s l y , t h e magnesium l o s s e s and c a l c i u m r e c o v e r i e s i n the f i n a l s l u d g e a r e a f f e c t e d i n a s i m i l a r manner. S e v e r a l f i n a l h a n d l i n g methods a r e a v a i l a b l e and the one chosen f o r a l a r g e s c a l e a p p l i c a t i o n w i l l have s i g n i f i c a n t e f f e c t s on t h e c h e m i c a l r e c o v e r i e s a c h i e v e d . i i i ) C a r b o n a t i o n pH U n d o u t e d l y , pH was the most i m p o r t a n t f a c t o r a f f e c t i n g t h e outcome o f t h e s l u d g e c a r b o n a t i o n p r o c e s s . A f i n a l c a r b o n a t i o n pH below 7.5 was s e l e c t e d f o r t h e f i r s t runs w i t h s a m p l i n g a t v a r i o u s time i n t e r v a l s d u r i n g c a r b o n a t i o n . S l u d g e pH i n t h e c a r b o n a t o r and s l u d g e f i l t r a t e pH were both measured. The f i l t r a t e pH was c o n s i s -t e n t l y h i g h e r (.1 t o 1 pH u n i t ) than t h e s l u d g e pH i n the c a r b o n a t o r and i t soon became a p p a r e n t t h a t c o n t i n u o u s pH measurement i n the c a r b o n a t i o n c y l i n d e r would p r o v i d e t h e b e s t i n d i c a t i o n o f t h e c a r -b o n a t i o n p r o g r e s s . A l l f u r t h e r runs were m o n i t o r e d i n t h i s manner, w i t h CO2 gas f l o w b e i n g v a r i e d m a n u a l l y and s h u t o f f when a s l u d g e pH o f 7.5 was r e a c h e d . The r e s u l t s o f a l l c a r b o n a t i o n m o n i t o r i n g runs a r e p r e s e n t e d i n T a b l e s 1 t o 8 o f Appendix D. T y p i c a l pH v s . c o l o u r , M g + + and C a + + i n t h e s u p e r n a t a n t f o r BKME and T-20 e f f l u e n t c o l o u r s l u d g e c a r b o n a t i o n w i t h f r e s h c h e m i c a l s a r e shown g r a p h i c a l l y i n F i g u r e s 11 and 12. S e v e r a l g e n e r a l o b s e r -v a t i o n s can be made from t h e s e r e s u l t s . C o l o u r and magnesium r e l e a s e were v e r y c l o s e l y r e l a t e d d u r i n g a l l t e s t s . L i t t l e r e l e a s e o f t h e s e m a t e r i a l s o c c u r r e d above pH 10.5, w i t h t h e b u l k o f t h e c o l o u r and M g + + b e i n g r e l e a s e d between pH 10.0 and pH 8.5. In a l l r u n s , a d e f i n i t e . l e v e l l i n g o f f t r e n d i n c o l o u r and M g + + c o n c e n t r a t i o n was n o t e d as the pH d e c r e a s e d below 8.5. 91 11 10 9 8 7 PH IGURF11 p H V s C O L O U R , M g + + and C a + + I N THE SUPERNATANT F R O M A TYPICAL B K M E S L U D G E CARBONATION USING F R E S H C H E M I C A L S . *- C H A N G E IN C O N C E N T R A T I O N A N D p H D U R I N G S L U D G E S E T T L I N G 92 1 2 0 C H 1000 800 400 2 0 0 r 16000 14000 rIGURE 12 pH Vs COLOUR ,Mg++,Ca + + . IN THE SUPERNATANT FROM A TYPICAL T-20 SLUDGE CARBONATION USING FRESH CHEMICALS. * C H A N G E IN C O N C E N T R A T I O N A N D p H D U R I N G S L U D G E S E T T L I N G 93 In some c a s e s the measured c o l o u r o f the s u p e r n a t a n t even d e c r e a s e d s l i g h t l y a t pH's below 8.0. T h i s was t h o u g h t t o be due t o some c a l c i u m c a r b o n a t e coming o u t o f s o l u t i o n and p o s s i b l e r e p r e c i p i t a t i n g c o l o u r m o l e c u l e s p r i o r t o the c o l o u r d e t e r m i n a t i o n . Magnesium, on the o t h e r hand, c o n t i n u e d t o i n c r e a s e s l i g h t l y as the pH approached 7.5. Mass d a t a from Runs #3 and 4 showed t h a t magnesium r e c o v e r y c o u l d be improved s l i g h t l y by c a r b o n a t i n g t o a pH near 7.3. The c r i t i c a l p a r ameter g o v e r n i n g the a c c e p t a b i l i t y o f t h e magnesium r e c o v e r y p r o c e s s i s t h o u g h t t o be the p e r c e n t MgO i n the i n c i n e r a t e d f i n a l s l u d g e s o l i d s . A s m a l l amount o f MgO i n t h e r e c a l -c i n e d l i m e i s d e s i r e d t o a v o i d problems o f magnesium b u i l d u p i n the r e c o v e r y s y s t e m o f t h e m i l l , because o f i t s a d v e r s e e f f e c t on l i m e r e a c t i v i t y as w e l l as on s e d i m e n t a t i o n and f i l t r a t i o n r a t e s . Average v a l u e s o f MgO i n commercial h i g h c a l c i u m q u i c k l i m e and r e b u r n e d l i m e have been g i v e n by J o h n s t o n e t a l (1968) as .3 t o 2.5% and .5 t o 3.0%, r e s p e c t i v e l y , whereas d o l o m i t i c l i m e may c o n t a i n 37 t o 41% MgO a c c o r d i n g t o the N a t i o n a l Lime A s s o c i a t i o n (1951). In t h e s e runs u s i n g f r e s h c h e m i c a l s , 2.9 t o 8.9% o f the i n c i n e r a t e d s l u d g e s o l i d s mass was a c c o u n t e d f o r by MgO. The maximum p e r m i s s i b l e l e v e l o f MgO i n a l i m e f o r r e u s e i n a k r a f t m i l l w i l l v a r y f r o m m i l l t o m i l l , but i s w e l l above t h i s r a n g e , a c c o r d i n g t o Smook (1975); i n any c a s e , a c c o r d i n g t o Thompson ( 1 9 7 5 ) , w i t h o p t i m i z a t i o n o f the p r o c e s s , much b e t t e r l i m e q u a l i t y s h o u l d be a c h i e v a b l e . However, i n a f u l l s c a l e o p e r a t i o n , i f the magnesium c o n t e n t o f the f i n a l s l u d g e i s h i g h e r than the p e r m i s s i b l e l e v e l , t h i s would d i c t a t e c a r b o n a t i n g t o a lower pH. Such a c o u r s e o f a c t i o n c o u l d s e r i o u s l y a f f e c t the C a + + mass b a l a n c e r e s u l t s . The c o n c e n t r a t i o n 94 p r o f i l e f o r C a + + , as the pH d e c r e a s e d , was q u i t e d i f f e r e n t from t h o s e o f c o l o u r and magnesium, r e f l e c t i n g t h e e f f e c t o f pH on CaC0 3 s o l u b i l i t y . In a l l c a s e s , C a + + c o n c e n t r a t i o n dropped from the upper pH t o a minimum v a l u e (around 60 mg/1) between pH 10.5 and 9.5, t h e n i n c r e a s e d g r e a t l y as t h e pH was l o w e r e d . L a r g e r i n c r e m e n t a l i n c r e a s e s i n C a + + o c c u r r e d as the pH d e c r e a s e d below 8.0 and l a r g e C a + + l o s s e s r e s u l t e d i n runs where c a r b o n a t i o n was n o t s t o p p e d as soon as the measured s l u d g e pH r e a c h e d 7.5. However, as d i s c u s s e d l a t e r , m i x i n g c o n d i t i o n s were such t h a t a n o n - u n i f o r m pH d i s t r i b u t i o n a c t u a l l y e x i s t e d i n t h e c a r b o n a t o r , p o s s i b l y c a u s i n g t h e e x c e s s i v e C a + + d i s s o l u t i o n . The pH e f f e c t on C a + + c o n c e n t r a t i o n i n the s u p e r n a t a n t was more n o t i c e a b l e i n t h e T-20 e f f l u e n t t e s t s than i n BKME r u n s , w i t h as much as 40 and 45% C a + + r e l e a s e i n Runs #3 and 4, r e s p e c t i v e l y . Not o n l y d i d t h e s e C a + + l o s s e s r e p r e s e n t a d i r e c t r e d u c t i o n i n r e c o v e r a b l e l i m e i n the f i n a l s l u d g e , but i n the form o f c a l c i u m b i c a r b o n a t e a l k a l i n i t y , t h e y a l s o a d v e r s e l y a f f e c t e d the outcome o f t h e n e x t c y c l e . T h i s i s d i s c u s s e d l a t e r i n the s e c t i o n on t h e use o f r e c y c l e d magnesium. T h e r e f o r e , as i n many e n g i n e e r i n g p r o b l e m s , a t r a d e o f f s i t u a t i o n e x i s t s . While c o l o u r r e l e a s e was e s s e n t i a l l y t h e same between pH 8.0 and 7.5, magnesium r e c o v e r y c o n t i n u e d t o improve s l i g h t l y and c a l c i u m l o s s e s i n t o the r e c y c l e r o s e m a r k e d l y o v e r the same pH range. From t h e s e r e s u l t s , i t can be seen t h a t an o p e r a t i n g pH near 7.5 m a i n t a i n e d by e f f i c i e n t m i x i n g and C 0 2 d i f f u s i o n , would be a good t a r g e t f o r any f u t u r e i n v e s t i g a t i o n s . However, more e x t e n s i v e t e s t i n g on a c o n t i n u o u s b a s i s , t a k i n g i n t o a c c o u n t v a r i o u s economic and t e c h n i c a l c o n s i d e r a t i o n s , i s n e c e s s a r y t o e s t a b l i s h an e x a c t optimum s l u d g e c a r b o n a t i o n pH. 95 i v ) Time Time, i n the s l u d g e c a r b o n a t i o n s t e p o f t h i s bench s c a l e work, was not an e f f e c t i v e c o n t r o l v a r i a b l e , because o f the non-u n i f o r m f l o w o f CO2 gas a l l o w e d d u r i n g the t e s t s . However, time d i d have some a f f e c t on the f i n a l s u p e r n a t a n t q u a l i t y a c h i e v e d . As i n d i c a t e d i n F i g u r e s 11 and 12, the c o l o u r , M g + + and C a + + c o n c e n -t r a t i o n s i n t h e s u p e r n a t a n t u s u a l l y changed d u r i n g t h e s l u d g e s e t t l i n g p e r i o d i n t h e s e t e s t s . C o l o u r u s u a l l y d e c r e a s e d s l i g h t l y o r remained about t h e same. Magnesium, on the o t h e r hand, u s u a l l y i n c r e a s e d s l i g h t l y o r s t a y e d c o n s t a n t . The most n o t a b l e e f f e c t o f " t i m e " , a f t e r the C 0 2 gas had been s h u t o f f , was on the c a l c i u m c o n c e n t r a t i o n i n the s u p e r n a t a n t . In a l l c a s e s , C a + + i n c r e a s e d a p p r e c i a b l y d u r i n g t h e s l u d g e s e t t l i n g p e r i o d , even though t h e measured pH r o s e s l i g h t l y i n most c a s e s . T h i s p o i n t s t o t h e time dependency o f t h e C0 2 + H 20 -> H2CO3 r e a c t i o n , which p r e c e d e s d i s s o l u t i o n t o C a ( H C 0 3 ) 2 as d i s c u s s e d p r e v i o u s l y . Thus, i n o r d e r t o reduce C a + + l o s s e s , i t i s i m p o r t a n t t o a l l o w o n l y enough time f o r M g + + t o s o l u b i l i z e , then s e p a r a t e t h e s o l i d and l i q u i d p o r t i o n s q u i c k l y , so t h a t a minimum o f C a ( H C 0 3 ) 2 s o l u b i l i z e s i n t h e s u p e r n a t a n t r e c y c l e . A l t h o u g h i t was d i f f i c u l t t o s e p a r a t e t h e e f f e c t s o f time and pH d u r i n g t h e s e t e s t s , t h e r e s u l t s o f c a r b o n a t i o n m o n i t o r i n g i n d i c a t e d t h a t , i f t h e c a r -b o n a t i o n c o n t i n u e d f o r more than 60 m i n u t e s , s u b s t a n t i a l amounts o f C a + + were measured i n the s u p e r n a t a n t . On the o t h e r hand, when c a r b o n a t i o n d u r i n g Run #4 was c a r r i e d o u t i n o n l y 25 m i n u t e s , t h e f i n a l pH o f 7.3 r e s u l t e d i n both the h i g h e s t r e c o v e r y o f M g + + and t h e h i g h e s t l o s s o f C a + + f o r any o f t h e T-20 e f f l u e n t r u n s ; t h i s p o i n t s t o the r e l a t i v e p o s i t i v e i m p o r t a n c e o f pH, as compared t o t h e n e g a t i v e i m p o r t a n c e o f t i m e . 96 v) C o l o u r S l u d g e S o l i d s C o n c e n t r a t i o n The s o l i d s c o n c e n t r a t i o n s o f the c o l o u r s l u d g e s e n t t o c a r b o n a t i o n i n each run a r e shown i n T a b l e 14, a l o n g w i t h t h e f i n a l s l u d g e s o l i d s c o n c e n t r a t i o n s a c h i e v e d and the mass b a l a n c e r e s u l t s . A g a i n , i t was d i f f i c u l t t o s i n g l e o u t the e f f e c t o f t h i s one v a r i a b l e , s i n c e the f i n a l pH and o t h e r v a r i a b l e s were not c o n s t a n t i n a l l t h e r u n s . However, e a r l y e v i d e n c e o f t h i s d i l u t i o n e f f e c t was n o t i c e d a f t e r Runs #1 and 2 u s i n g BKME. In t h e s e t e s t s , the volume o f e f f l u e n t t r e a t e d was 10 l i t r e s . T h i s p r o v i d e d o n l y 620 t o 650 mis o f c o l o u r s l u d g e f o r c a r b o n a t i o n . (Subsequent runs s t a r t e d w i t h more e f f l u e n t t o a v o i d s l u d g e d i l u t i o n . ) In o r d e r t o have enough s l u d g e f o r s a m p l i n g d u r i n g c a r b o n a t i o n , the s l u d g e was d i l u t e d t o one l i t r e i n Run #1 and t o two l i t r e s i n Run #2, r e s u l t i n g i n p e r c e n t s o l i d s c o n c e n t r a t i o n o f .99% and .46%, r e s p e c t i v e l y . Comparing t h e mass b a l a n c e r e s u l t s o f t h e s e and o t h e r c a r b o n a t i o n runs i n P a r t 2 o f T a b l e 14, a s i g n i f i c a n t l y h i g h e r c o l o u r r e l e a s e o c c u r r e d i n t h e more d i l u t e s y s t e m s . C a + + l o s s e s i n the s u p e r n a t a n t were a l s o h i g h e r i n the more d i l u t e s l u d g e systems. Both o f t h e s e f a c t o r s p o i n t t o the advantage o f c a r b o n a t i n g a more c o n c e n t r a t e d c o l o u r s l u d g e . U n f o r t u n a t e l y , i n t h e s e t e s t s , magnesium r e c o v e r y a l s o d e c r e a s e d s l i g h t l y when more c o n c e n t r a t e d s l u d g e was c a r b o n a t e d . I t may be a n t i c i p a t e d from r e s e a r c h o f Swanson e t a l ( 1 9 7 3 ) , and o t h e r s i n v e s t i g a t i n g the mechanisms o f c o l o u r removal by c h e m i c a l p r e c i p i t a t i o n , t h a t c h e m i c a l r e a c t i o n s as w e l l as p h y s i c a l i n t e r -a c t i o n s o c c u r d u r i n g c o a g u l a t i o n . D u r i n g the p r e s e n t s t u d y , i t was not p o s s i b l e t o c o n c l u d e whether the r e l e a s e s e n c o u n t e r e d d u r i n g c a r b o n a t i o n were caused by the 97 r e v e r s i o n o f a c h e m i c a l r e a c t i o n between c o l o u r and Mg , as t h e Mg(0H)2 r e s o l u b i l i z e d , o r whether i t was a s i m p l e p h y s i c a l r e l e a s e o f e n t r a p p e d c o l o u r from the f l o e , which was enhanced by d i l u t i o n and m i x i n g . E x p e r i m e n t s c o n t r o l l i n g d i l u t i o n , m i x i n g , pH and time s h o u l d be h e l p f u l i n e l u c i d a t i n g t h e mechanism(s) i n v o l v e d i n magnesium c o a g u l a t i o n . v i ) F i n a l S l u d g e S o l i d s C o n c e n t r a t i o n The a c t u a l s l u d g e s o l i d s c o n c e n t r a t i o n s a c h i e v e d i n each r u n , a f t e r c a r b o n a t i n g t h e c o l o u r s l u d g e , a r e shown a l o n g w i t h the c o l o u r s l u d g e s o l i d s c o n c e n t r a t i o n s and mass b a l a n c e r e s u l t s i n T a b l e 14. S i n c e d i l u t e c o l o u r s l u d g e u s u a l l y meant d i l u t e f i n a l s l u d g e , t h e d i s c u s s i o n o f the e f f e c t s o f c o l o u r s l u d g e s o l i d s c o n c e n t r a t i o n i s a p p l i c a b l e here as w e l l . In g e n e r a l , a more h i g h l y c o n c e n t r a t e d , b e t t e r s e t t l i n g f i n a l s l u d g e r e s u l t e d i n lower c o l o u r r e l e a s e , lower Ca l o s s e s i n t o the s u p e r n a t a n t , and lower Mg r e c o v e r y . These r e s u l t s may p a r t i a l l y r e f l e c t t he d e c r e a s e d m i x i n g e f f i c i e n c y i n t h e d e n s e r systems. v i i ) M i x i n g M i x i n g i n t h e s e b a t c h c a r b o n a t i o n t e s t s was n o t as e f f i c i e n t as i t s h o u l d have been. M i x i n g speed was v a r i e d t h r o u g h o u t t h e c a r -b o n a t i o n t o t r y t o a c h i e v e good C 0 2 d i f f u s i o n t h r o u g h t h e s l u d g e w i t h o u t h i n d e r i n g t h e f o r m a t i o n o f t h e f i n a l s l u d g e f l o e . T h i s was d i f f i c u l t t o do i n a s i n g l e v e s s e l and s h o u l d not be a t t e m p t e d i n f u t u r e t e s t s . CO2 t r a n s f e r and s l u d g e s e t t l i n g would l i k e l y be a c c o m p l i s h e d b e t t e r i n a two-stage system c o n s i s t i n g o f a p r o p e r l y d e s i g n e d c a r b o n a t o r w i t h a good d i f f u s e r system and complete m i x i n g p r o v i d e d , f o l l o w e d by a s e p a r a t e s l u d g e t h i c k e n e r . 98 Poor m i x i n g , i n some o f the runs w i t h t h i c k e r s l u d g e , p r o b a b l y c a u s e d v a r i a b l e s l u d g e pH and a r e s u l t a n t r e d u c t i o n i n M g + + r e c o v e r y e f f i c i e n c y ( p o c k e t s o f h i g h e r pH) and e x c e s s i v e CaC0 3 d i s s o l u t i o n ( p o c k e t s o f lower pH). In a l a r g e s c a l e a p p l i c a t i o n , e s p e c i a l l y i f a combined s l u d g e c a r b o n a t o r - t h i c k e n e r i s u s e d , m i x i n g would become a c r i t i c a l f a c t o r i n the outcome o f the s l u d g e c a r b o n a t i o n s t e p . 6.2 R e s u l t s o f T e s t i n g w i t h R e c y c l e d Magnesium 6.2.1 J a r T e s t R e s u l t s 6.2.1.1 G e n e r a l . To t h e a u t h o r ' s knowledge, no p r e v i o u s c o l o u r removal work has been done on k r a f t m i l l e f f l u e n t s u s i n g r e c y c l e d s u p e r n a t a n t as a s o u r c e o f M g + + . Thompson e t a l (1972) r e p o r t e d t h a t r e c y c l e d M g + + was as e f f e c t i v e as f r e s h M g + + i n t r e a t i n g c o l o u r e d s u r f a c e w a t e r s . However, i t was n o t known whether t o e x p e c t s i m i l a r r e s u l t s i n t h e s e t e s t s because o f the much more i n t e n s l y c o l o u r e d , h i g h l y a l k a l i n e , s u p e r n a t a n t r e c y c l e o b t a i n e d i n the t r e a t m e n t o f BKME and T-20 e f f l u e n t s . In g e n e r a l , i t was n e c e s s a r y t o a s s e s s t h e c o l o u r removal e f f i c i e n c y o f r e c y c l e d s u p e r n a t a n t a l o n e and i n c o m b i n a t i o n w i t h v a r i o u s amounts o f makeup MgSO^, t o compare w i t h r e s u l t s o f t e s t s u s i n g f r e s h c h e m i c a l s . The same b a s i c c r i t e r i a were used t o s e l e c t dosage l e v e l s o f l i m e and magnesium f o r use d u r i n g b a t c h t e s t s , t o d e t e r m i n e any s i g n i f i c a n t e f f e c t s o f u s i n g M g + + r e c y c l e on t h e o v e r a l l d e c o l o u r i z a t i o n - r e c o v e r y mass b a l a n c e s . I n i t i a l work w i t h M g + + r e c y c l e was done on T-20 e f f l u e n t samples, so r e s u l t s o f t h e s e t e s t s a r e r e p o r t e d f i r s t . 99 6.2.1.2 T-20 E f f l u e n t . S i n c e i t was n o t known whether t h e M g + + measured i n the s u p e r n a t a n t would a l l be a v a i l a b l e f o r a second p r e -c i p i t a t i o n , o r whether t he r e l e a s e d c o l o u r m o l e c u l e s may i n t e r f e r e i n some way, t h r e e s e t s o f j a r t e s t s were d e s i g n e d t o s e p a r a t e t h e r o l e s o f C a ( 0 H ) 2 , f r e s h MgS0 4 and M g + + r e c y c l e , i n c o l o u r r e m o v a l . The c o l o u r removal e f f i c i e n c y o f l i m e a l o n e on t h i s e f f l u e n t ( i n i t i a l c o l o u r = 16,000 C U . ) i s shown i n T a b l e 18. I t can be seen t h a t o v e r 20,000 mg/1 C a ( 0 H ) 2 (15,000 mg/1 CaO) were n e c e s s a r y t o a c h i e v e 90% c o l o u r r e d u c t i o n . Treatment o f the same waste w i t h l i m e a t 2,500 mg/1 C a ( 0 H ) 2 and v a r i o u s dosages o f f r e s h MgSCt a r e shown i n T a b l e 19. In t h e s e t e s t s , 300 mg/1 M g + + p r e c i p i t a t e d by o n l y 2,500 mg/1 C a ( 0 H ) 2 (1,875 mg/1 CaO) p r o v i d e d 92.6% c o l o u r r e d u c t i o n . H i g h e r M g + + dosages a t t h i s l i m e l e v e l p r o v i d e d o n l y s l i g h t l y b e t t e r r e m o v a l s . These r e s u l t s c o n f i r m t h o s e found i n the p r e v i o u s l y d i s c u s s e d j a r t e s t s u s i n g f r e s h c h e m i c a l s and can be compared d i r e c t l y t o the r e s u l t s i n T a b l e s 20 and 21. The s u p e r n a t a n t from c a r b o n a t i o n Run #5, u s i n g t h e same T-20 e f f l u e n t , was c o l l e c t e d and used f o r j a r t e s t e v a l u a t i o n o f e f f l u e n t c o l o u r removal u s i n g r e c y c l e d M g + + . The c h a r a c t e r i s t i c s o f t h i s e f f l u e n t and t h e r e s u l t i n g s u p e r n a t a n t a r e g i v e n i n T a b l e s 12 and 15, r e s p e c t i v e l y . The M g + + c o n c e n t r a t i o n i n the r e c y c l e was 1,250 mg/1. The s u p e r n a t a n t c o l o u r was the same as the i n i t i a l e f f l u e n t c o l o u r (16,000 C.U.), so the i n i t i a l c o l o u r c o n c e n t r a t i o n o f t h e s e samples was n o t changed by r e c y c l e a d d i t i o n . The major e f f e c t o f t h e s u p e r -n a t a n t a d d i t i o n was t o i n c r e a s e t h e u n t r e a t e d sample a l k a l i n i t y because o f t h e h i g h (6,800 mg/1) a l k a l i n i t y i n t h e c a r b o n a t i o n s u p e r n a t a n t r e l a t i v e t o the 1800 mg/1 i n the u n t r e a t e d e f f l u e n t . TABLE 18 JAR TESTS OF T-20 EFFLUENT WITH LIME ALONE I n i t i a l c o l o u r = 16,000 c u . I n i t i a l pH = 11.75 Lime „ C o l o u r J a r No. Magnesium + + mg/1 as A23 ( C U . ) (Mass I n (Mass Out (mg/1 as Mg ) C a ( 0 H ) 2 CaO (mg/1) pH ^ g as P t ) jTimg as P t ) % Removal 07-21-1 0 ' ., 1250 950 12.25 9800 1440 980 31.9 07-21-2 .. 0 2500 1875 12.40 9000 1440 900 37.5 07-21-3 ,0 •, 5000 3750 12.46 3600 1440 360 75.0 02-21-4 0 10000 7500 12.52 1950 1440 195 86.0 07-21-5 00 20000 15000 12.53 1450 1440 145 89.9 07-21-6 b' 40000 30000 12.55 1000 • 1440 100 93.1 o o TABLE 19 Magnesium ++ J a r No. (mg/1 Mg 07-21-7 30 07-21-8 60 07-21-9 90 07-21-10 120 07-21-11 150 07-21-12 300 07-21-13 600 • 07-21-14 1200 ) JAR TESTS T-20 EFFLUENT WITH FRESH MgS0 4 I n i t i a l C o l o u r = 16,000 ' I n i t i a l pH = 11.75 Lime mg/1 as as C a ( 0 H ) 2 CaO 2500 1875 2500 1875 2500 1875 2900 1875 2500 1875 2500 1875 2500 1875 2500 1875 A23 mg/1 2 2 2 2 2 2 2 2 PH C o l o u r ( C U . ) (Massil.'n (Mass o u t rnra as P t ) , fn g: as P t ) 12.40 6350 1420 12.35 3400 1420 12.32 3200 1420 12.34 2600 1420 12.30 1900 1420 12.15 1050 1410 11.80 650 1390 10.48 880 1390 635 55.3 340 76.1 320 77.5 260 81.7 190 .86.6 105 92.6 65 95.3 88 93.5 TABLE 20 JAR TESTS WITH RECYCLED M g + + - T-20 EFFLUENT I n i t i a l C o l o u r = 16,000 c u . I n i t i a l pH = 1 1 . 7 5 S u p e r n a t a n t pH = 7.85 S u p e r n a t a n t C o l o u r = 16,000 c u . J a r No. ** R e c y c l e % Magnesium R e c y c l e . M g S 0 4 (mg/1 M g T + ) (mg/1 M g + + ) Lime as mg/1 C a ( 0 H ) 2 as • CaO A23 mg/1 pH ( C U . ) C o l o u r (Mass In (Mass Out ' .mg as P t ) :fig as P t ) % Remo' 07-22-1 100 300 0 2500 1875 2 10.83 10750 1420 1075 24.3 07-22-2 80* 240 60 2500 1875 2 11.40 7650 1420 765 46.1 07-22-3 80* 240 90 2500 1875 2 11.48 6550 1420 655 53.9 07-22-4 80* 240 120 2500 1875 2 11.50 4300 1420 430 69.7 07-22-5 80* 240 150 2500 1875 2 11.50 2600 1420 260 81.7 07-22-6 80* 240 300 2500 1875 2 10.90 1700 1420 170 88.0 R e c y c l e d S u p e r n a t a n t c o n t a i n e d 1250 mg/1 Mg io 25 mis ~ 30 mg M g + + 80% of the o r i g i n a l 300 mg/1 used i n t e s t i n g w i t h f r e s h c h e m i c a l s TABLE 21 JAR TESTS WITH RECYCLED M g + + - T-20 EFFLUENT I n i t i a l C o l o u r = 16,000 c u . I n i t i a l pH = 11.75 S u p e r n a t a n t pH = 7.85 S u p e r n a t a n t C o l o u r = 16,000 c u . J a r No. * R e c y c l e % Magnesium R e c y c l e MgSOa (mg/1 Mg) (mg/1 Mg) Lime mg/1 as C a ( 0 H ) 2 as CaO A23 mg/1 pH ( C U . ) C o l o u r (Mass In (Mass Out ,nig as Pt) -nig as Pt.) % Removal 07-22-7 100 300 ; 0 3750 2800 2 11. 83 6500 1420 650 54.2 07-22-8 100 300 0 5000 3750 2 12. 30 1900 1420 190 86.6 07-22-9 100 300 0 7500 5625 2 12. 50 620 1420 62 95.6 07-22-10 80 240 60 3750 2800 2 12. 23 1800 1420 180 87.3 07-22-11 80 240 60 5000 3750 2 12. 43 940 1420 94 93.4 07-22-12 80 240 60 7500 5625 2 12. 55 580 1420 58 95.9 * R e c y c l e d S u p e r n a t a n t c o n t a i n e d 1250 mg/1 Mg TABLE 22 JAR TESTS WITH RECYCLED M g + + - BKME I n i t i a l C o l o u r = 1500 c u . I n i t i a l pH = 7.5 S u p e r n a t a n t pH = 7.7 S u p e r n a t a n t c o l o u r = 9400 c u . J a r No. * R e c y c l e % Magnesium Recvcle+ - K M 9 S 0 4 ( f r e s h ) M 9 • % (mg/1 M q + + ) Lime mg/1 as C a ( 0 H ) 2 CaO F i n a l pH T o t a l C o l o u r + C o l o u r Out c u . % Remo 07-24-1 100 60 0 250 185 9.98 1850 1800 2.4 07-24-2 100 60 0 - 500 375 10.75 1850 354 80.8 07-24-3 100 60 0 - 750 560 11.49 1850 95 94.8 07-24-4 80 48 20 12 500 375 10.80 1740 296 82.9 07-24-5 80 48 20 12 625 475 11.32 1740 120 93.0 07-24-5 80 48 20 12 750 560 11.40 1740 109 93.7 07-24-7 Same as 07-24-3 e x c e p t 2 mg/1 o f A23 was added 11.49 1830 98 94.1 * The s u p e r n a t a n t r e c y c l e c o n t a i n e d 960 mg/1 Mg + C o l o u r was t h e c a l c u l a t e d combined c o l o u r o f t h e s u p e r n a t a n t and e f f l u e n t , ( c u . ) o -P=> 105 In e v a l u a t i n g t h e e f f i c i e n c y o f t h e r e c y c l e d M g + + , t e s t s ++ ++ were c o n d u c t e d a t the l e v e l o f 300 mg/1 Mg u s i n g 100% r e c y c l e d Mg , ++ and a t 240 mg/1 r e c y c l e d Mg w i t h v a r i o u s amounts o f makeup MgSOi,. The 80% r e c y c l e d M g + + seemed l i k e a r e a s o n a b l e l e v e l t o work w i t h i n t h e s e p r e l i m i n a r y t e s t s s i n c e i t was w e l l w i t h i n t h e range o f M g + + r e c o v e r i e s o b t a i n e d i n the p r e v i o u s b a t c h t e s t s . However, t he volumes o f s u p e r n a t a n t n e c e s s a r y t o p r o v i d e t h e 80% r e c y c l e d Mg dosage were s l i g h t l y l a r g e r than t h e y would be i n a f u l l s c a l e p r o c e s s where b e t t e r s l u d g e t h i c k e n i n g c o u l d be p r o v i d e d . T h i s meant t h a t o n l y about 65 t o 70% o f t h e t o t a l volume t r e a t e d i n t h e s e j a r t e s t s was raw waste, 20 t o 25% was r e c y c l e d s u p e r n a t a n t and 10% was l i m e s l u r r y . In o r d e r t o compare t he t e s t r e s u l t s , a l l c o l o u r removals were c a l c u l a t e d on t h e mass b a s i s , but no r e g a r d was g i v e n t o the volume o f s l u d g e g e n e r a t e d . ++ The f i r s t s e t o f r e s u l t s , u s i n g r e c y c l e d Mg a t v a r i o u s l e v e l s o f makeup MgSO^ p r e c i p i t a t e d by 2,500 mg/1 C a ( 0 H ) 2 , i s shown i n T a b l e 20. These r e s u l t s were f a i r l y d i s a p p o i n t i n g . P o o r e r c o l o u r removals r e s u l t e d i n a l l t e s t s . Even t h e use o f 240 mg/1 r e c y c l e d ++ ++ Mg w i t h 300 mg/1 f r e s h Mg gave o n l y 88% c o l o u r r e m o v a l . T h i s was e q u i v a l e n t t o t h e l e v e l a c h i e v e d by u s i n g o n l y 150 mg/1 o f f r e s h M g + + as shown by t h e r e s u l t s i n T a b l e 19. The o b v i o u s r e a s o n was the much lo w e r f i n a l pH r e a c h e d i n t h e s e t e s t s compared t o the t e s t s on the same e f f l u e n t w i t h f r e s h MgS0 4 ( T a b l e 19). Whereas a pH o f 12.15 was r e a c h e d w i t h 300 mg/1 f r e s h M g + + p l u s 2,500 mg/1 C a ( 0 H ) 2 , a pH o f o n l y 10.83 was r e a c h e d when 100% r e c y c l e d M g + + was used. From c a l c u l a t i o n s , i t was n o t e d t h a t t h e a l k a l i n i t y o f the combined raw waste p l u s c a r -b o n a t i o n s u p e r n a t a n t was about 2,900 mg/1, compared t o 1,800 i n the 106 e f f l u e n t a l o n e . T a b l e 21 shows the r e s u l t s o f j a r t e s t s aimed a t d e t e r m i n i n g whether i n c r e a s e d l i m e dosage would improve t h e c o l o u r r e d u c t i o n , o r i f p o s s i b l y t h e r e c y c l e d M g + + was n o t e f f e c t i v e a t any pH l e v e l . F o r t u n a t e l y , e x c e l l e n t r e s u l t s were o b t a i n e d f r o m t h e s e j a r t e s t s . ++ The r e l a t i v e e f f e c t o f the r e c y c l e d Mg can be seen by comparing t h e r e s u l t s i n T a b l e 21 w i t h t h o s e i n T a b l e 18; 300 mg/1 o f 100% r e c y c l e d M g + + w i t h 7,500 mg/1 C a ( 0 H ) 2 gave 95.6% c o l o u r removal compared t o o n l y about 80% removal w i t h t h e same amount o f l i m e a l o n e . In g e n e r a l , when the f i n a l c o a g u l a t i o n pH approached 12.3, c o l o u r removals o f 90 t o 95% were a c h i e v e d , both w i t h 100% r e c y c l e d M g + + (300 mg/1) and.with 80% r e c y c l e d M g + + , p l u s 20% makeup MgS0 4 (300 mg/1 as M g + + t o t a l ) . These r e s u l t s were s i m i l a r t o t h o s e o b t a i n e d i n t e s t i n g w i t h f r e s h c h e m i c a l s and i n d i c a t e t h a t t h e magnesium i n t h e s u p e r n a t a n t r e c y c l e i s i n d e e d a v a i l a b l e and i s an e f f e c t i v e s o u r c e o f M g + + f o r p r e c i p i t a t i o n w i t h l i m e . The r e l e a s e d c o l o u r i n t h e s u p e r n a t a n t was  not a major c o n c e r n . However, i n o r d e r t o r e a c h t h i s pH, a l i m e dosage o f 3,750 t o 5,000 mg/1 Ca(0H ) 2 was r e q u i r e d t o overcome t he a l k a l i n i t y i n t h e s e t e s t s compared t o 2,500 mg/1 C a ( 0 H ) 2 when the f r e s h MgSOit was used. I t s h o u l d be n o t e d t h a t t h e polymer dosage o f 2 mg/1 used i n t h e s e t e s t s was a g a i n c r i t i c a l i n a c h i e v i n g good s e t t l i n g . M o n i t o r i n g o f pH d u r i n g c o a g u l a t i o n showed t h a t i t s t a b i l i z e d w i t h i n f i v e m i n t u e s , i n most c a s e s , as i t d i d d u r i n g t e s t i n g w i t h f r e s h c h e m i c a l s . W h i l e f u r t h e r j a r t e s t i n g would be r e q u i r e d o v e r a w i d e r range o f o p e r a t i n g c o n d i t i o n s , t o a r r i v e a t optimum economic c o n d i t i o n s 107 f o r a l a r g e s c a l e a p p l i c a t i o n , t h e s e r e s u l t s were s u f f i c i e n t t o s e l e c t dosages f o r use i n subse q u e n t b a t c h t e s t s d u r i n g t h i s s t u d y . The ch e m i c a l dosages chosen f o r use i n Run #6 were 300 mg/1 M g + + (80% r e c y c l e and 20% MgSO^ makeup) a l o n g w i t h 5,000 mg/1 C a ( 0 H ) 2 (3,750 mg/1 CaO) and 2 mg/1 a n i o n i c polymer (Dow A-23). 6.2.1.3 BKME. In t h e s e t e s t s , s u p e r n a t a n t f r o m c a r b o n a t i o n Run #7 was used as the s o u r c e o f magnesium i o n s . The same i n i t i a l BKME sample was t r e a t e d i n Run #7, t h e s e j a r t e s t s , and Run #8. The i n i t i a l c h a r a c t e r i s t i c s o f t h i s e f f l u e n t and s u p e r n a t a n t a r e shown i n T a b l e s 12 and 15, r e s p e c t i v e l y . The M g + + c o n c e n t r a t i o n i n the r e c y c l e was 960 mg/1 and t h e c o l o u r was 9,400 C U . compared t o 1 ,500 C U . i n the u n t r e a t e d waste. T h e r e f o r e , a d d i n g 6.25 mis (100% r e c y c l e d M g + + ) o f s u p e r n a t a n t t o t h e 100 ml j a r s meant a c o l o u r i n c r e a s e f r o m 1 ,500 C U . t o 1 ,850 C U . i n t h e j a r t e s t s . When 80% r e c y c l e and 20% makeup was us e d , t h i s r e s u l t e d i n a combined i n i t i a l c o l o u r o f 1 ,740 C U . a t a r e c y c l e volume o f 5%. A g a i n i t h i s p e r c e n t a g e o f r e c y c l e d M g + + was w e l l w i t h i n t h e range o f ++ Mg r e c o v e r y o b t a i n e d i n the b a t c h t e s t s , and t h e 5% volume was c l o s e t o t h a t e x p e c t e d i n p r a c t i c e , as shown by the volume b a l a n c e s i n T a b l e 16. Based on the work w i t h T-20 e f f l u e n t , a j a r t e s t was s e t up to d e t e r m i n e t h e i n c r e a s e d l i m e dosage ( i f any) r e q u i r e d t o p r o v i d e t h e same 90 t o 95% c o l o u r removal u s i n g r e c y c l e d M g + + . R e s u l t s , shown i n T a b l e 22, were s i m i l a r t o t h o s e o b t a i n e d i n the T-20 e f f l u e n t t e s t s w i t h r e c y c l e ( T a b l e 2 1 ) . In t h e s e t e s t s t h e same 60 mg/1 and 500 mg/1 C a ( 0 H ) 2 , u s e d i n t e s t s w i t h f r e s h c h e m i c a l s , r e s u l t e d i n f i n a l pH v a l u e s o f o n l y 10.75 and 10.80, p r o v i d i n g c o l o u r r e d u c t i o n s o f 80.8 108 and 82.9% when 100% r e c y c l e and 80% r e c y c l e p l u s 20% makeup, r e s p e c t i v e l y , were used. However, when the pH was r a i s e d t o 11.3, by u s i n g 625 mg/1 C a ( 0 H ) 2 , 93.0% c o l o u r removal was a t t a i n e d . A t a h i g h e r l i m e l e v e l , even more c o l o u r was removed. These r e s u l t s can be compared t o t e s t s o f BKME t r e a t m e n t w i t h l i m e a l o n e and l i m e p l u s f r e s h magnesium i n T a b l e s 5 t o 8. A l t h o u g h s l i g h t l y d i f f e r e n t waste c h a r a c t e r i s t i c s were e v i d e n t i n t h e s e v a r i o u s t e s t s , i t can be c o n c l u d e d t h a t t h e r e c y c l e d s u p e r n a t a n t was an e f f e c t i v e s o u r c e o f Mg . The i n c r e a s e d l i m e r e q u i r e d t o r e a c h a pH n e a r 11.3 can be a t t r i b u t e d t o t h e i n c r e a s e i n i n i t i a l a l k a l i n i t y from 180 mg/1 t o a p p r o x i m a t e l y 360 mg/1 by the a d d i t i o n o f 5 mis o f s u p e r n a t a n t , h a v i n g an a l k a l i n i t y o f 3,650 mg/1. The s l u d g e i n t h e s e t e s t s s e t t l e d w e l l , and i n the one run where a polymer was us e d , no improvement i n s e t t l i n g o r c o l o u r removal r e s u l t e d . From t h e s e r e s u l t s , t h e dosage l e v e l s s e l e c t e d f o r use i n the n e x t b a t c h run were 60 mg/1 M g + + ( 8 0 % r e c y c l e and 20% makeup) a l o n g w i t h 625 mg/1 C a ( 0 H ) 2 (475 mg/1 CaO). 6.2.2 B a t c h D e c o l o u r i z a t i o n and Magnesium Re c o v e r y R e s u l t s 6.2.2.1 G e n e r a l B a t c h t e s t s w i t h BKME (Run #8) and T-20 e f f l u e n t (Run #6) were c a r r i e d o u t u s i n g c a r b o n a t i o n s u p e r n a t a n t f r o m Runs #7 and 5, r e s p e c t i v e l y . In Runs #7 and 8, BKME from t h e same c o n t a i n e r was t e s t e d , and i n Runs #5 and 6, T-20 e f f l u e n t from t h e same c o n t a i n e r was used. T h e r e f o r e , r e s u l t s o f Runs #8 and 7 can be compared d i r e c t l y , as can r e s u l t s o f Runs #6 and 5, t o e v a l u a t e t h e e f f e c t o f r e c y c l e d 109 s u p e r n a t a n t on the mass b a l a n c e s t h r o u g h o u t t he system. B a s i c a l l y , t h e o b j e c t i v e s were t h e same as f o r the b a t c h t e s t s u s i n g f r e s h c h e m i c a l s , o n l y more emphasis was p l a c e d on i d e n t i f y i n g any b e n e f i c i a l o r d e t r i m e n t a l e f f e c t s t he r e c y c l e d s u p e r n a t a n t had on t h e r e s u l t s . The same g e n e r a l s o u r c e s and s i n k s as i d e n t i f i e d i n F i g u r e 10 a p p l i e d t o t h e s e r u n s , remembering t h a t DOSE i s now 80% r e c y c l e d Mg . DOSE a l s o c o n t r i b u t e d t o the c o l o u r , Ca , a l k a l i n i t y and o t h e r components o f the raw e f f l u e n t . The r e s u l t s o f t h e two b a t c h t e s t s w i t h r e c y c l e d M g + + a r e p r e s e n t e d below and d i s c u s s e d m a i n l y i n the l i g h t o f any d i f f e r e n c e s between r e s u l t s o f Runs #7 and 8 (BKME) and Runs #5 and 6 (T-20 e f f l u e n t ) . 6.2.2.2 Batc h D e c o l o u r i z a t i o n A. Mass b a l a n c e s i ) BKME T a b l e 12 shows t h a t i n i t i a l e f f l u e n t c h a r a c t e r i s t i c s and c h e m i c a l dosages i n Runs #7 and 8 were e s s e n t i a l l y t h e same e x c e p t f o r the h i g h e r l i m e dosage used i n Run #8 (625 mg/1 i n s t e a d o f 500 mg/1 C a ( 0 H ) 2 ) . The r e s u l t i n g c h a r a c t e r i s t i c s o f t h e combined s u p e r n a t a n t and e f f l u e n t a r e n o t shown, but as d i s c u s s e d i n the j a r t e s t s e c t i o n , combined u n t r e a t e d c o l o u r was about 1 ,800 C U . and a l k a l i n i t y was a p p r o x i m a t e l y 360 mg/1 as C a C 0 3 . In Run #8, t h e c o a g u l a t i o n pH o f 11.4 was the same as i n Run #7, r e s u l t i n g i n 94.8% c o l o u r removal ( r e s i d u a l = 100 C U . ) c a l c u l a t e d f r o m an i n i t i a l c o l o u r o f 1 ,500 C U . T h i s was e q u i v a l e n t t o t h e 95.5% removal ( r e s i d u a l = 70 C U . ) a c h i e v e d w i t h f r e s h c h e m i c a l s , i f c a l -c u l a t e d f r o m an i n i t i a l c o l o u r o f 1 ,800 C U . Both e f f l u e n t s were s l i g h t l y o v e r t r e a t e d , a c c o r d i n g t o t h e c r i t e r i a e s t a b l i s h e d f o r t h i s 110 s t u d y , because o f t h e lower than normal i n i t i a l c o l o u r . I t i s i n t e r e s t ! * t o n o t e t h a t c a l c i u m l o s s e s i n t o t h e d e c o l o u r i z e d e f f l u e n t were s i g -n i f i c a n t l y lower i n Run #8, u s i n g r e c y c l e d s u p e r n a t a n t , than i n t h e p r e v i o u s run u s i n g f r e s h c h e m i c a l s . The 93 mg/1 C a + + measured i n Run #8 was a p p r o x i m a t e l y h a l f t h e 175 mg/1 i n Run #7 and as a p e r c e n t o f th e t o t a l mass l o s t , t h i s r e p r e s e n t e d a r e d u c t i o n f r o m 53.4 t o 23.2% i n t h e r e c y c l e r u n . T h i s can be a t t r i b u t e d t o the b e t t e r f l o e formed and t h e b e t t e r s e t t l i n g s l u d g e t h a t r e s u l t e d i n l e s s CaC0 3 f l o e c a r r y -o v e r when the r e c y c l e d m a t e r i a l was used. The f a c t t h a t a b e t t e r c o l o u r s l u d g e was formed i n Run #8 i s v e r i f i e d by the h i g h e r s o l i d s c o n c e n t r a t i o n (2.24% s o l i d s compared t o 1.25% i n Run #7), as shown i n T a b l e 14, and a l s o t h e lo w e r s l u d g e volume p r o d u c e d (5.2% o f t h e o r i g i n a l i n Run #8 compared t o 6.0% i n Run #7), shown i n T a b l e 16. The s m a l l d i f f e r e n c e i n M g + + l o s s e s between t he two runs (4.75 mg/1 i n Run #8 and 4.25 i n Run #7) i s w i t h i n t h e e x p e r i m e n t a l e r r o r o f t h e a n a l y t i c a l method used f o r M g + + d e t e r m i n a t i o n , so t h e r e was no s i g n i f i c a n t d i f f e r e n c e i n M g + + l o s s e s i n t o t h e t r e a t e d e f f l u e n t , i i ) T-20 E f f l u e n t A g a i n , T a b l e 12 shows t h a t i n i t i a l e f f l u e n t c h a r a c t e r i s t i c s and c h e m i c a l dosages i n Runs #5 and 6 were a p p r o x i m a t e l y t h e same e x c e p t f o r t h e 5,000 mg/1 Ca(0H ) 2 used i n Run #6, compared t o 2,500 mg/1 C a ( 0 H ) 2 used i n Run #5 w i t h f r e s h NlgSOi*. S i n c e t h e c o l o u r o f t h e s u p e r n a t a n t was 16,000 C.U., as was the u n t r e a t e d T-20 e f f l u e n t , t h e combined c o l o u r was unchanged. However, as mentioned i n the d i s -c u s s i o n o f the j a r t e s t s , t h e combined u n t r e a t e d a l k a l i n i t y o f Run #6 was about 2,900 mg/1, compared t o about 1,800 mg/1 i n t h e raw waste t r e a t e d i n Run #5. I l l The c o a g u l a t i o n pH o f 12.35 r e a c h e d i n Run #6 was s l i g h t l y h i g h e r than t h e pH o f 12.25 i n Run #5, r e s u l t i n g i n a r e s i d u a l c o l o u r o f 940 C U . and 900 C.U., r e s p e c t i v e l y . T h i s d i f f e r e n c e i s a g a i n w i t h i n t h e e x p e r i m e n t a l e r r o r o f t h e c o l o u r d e t e r m i n a t i o n , so ca n n o t be c o n s i d e r e d s i g n i f i c a n t . On a p e r c e n t mass b a s i s , 94.8% and 95.1% c o l o u r removal r e s u l t e d i n Runs #6 and 5, r e s p e c t i v e l y . In Run #6, t h e C a + + l o s s i n t o t h e d e c o l o u r i z e d e f f l u e n t was lower than i n any o f t h e T-20 e f f l u e n t runs w i t h f r e s h c h e m i c a l s . The C a + + c o n c e n t r a t i o n i n Run #6 was 313 mg/1 compared t o 390 mg/1 i n Run #5 and because t h e t o t a l mass o f C a + + i n Run #6 was h i g h e r , t h e p e r c e n t l o s t i n Run #6 (8.5%) was much lower t h a n i n Run #5 ( 2 1 . 1 % ) . ++ The r e d u c e d Ca l o s s e s i n t o t h e t r e a t e d e f f l u e n t can be a t t r i b u t e d t o t h e l a r g e r l i m e dosage u s e d , which r e s u l t e d i n b e t t e r s l u d g e c h a r a c t e r i s t i c s and l e s s CaC0 3 f l o e c a r r y o v e r . S l u d g e s e t t l i n g was e x c e l l e n t a t t h e 2 mg/1 polymer dosage used and the r e s u l t i n g 4% s o l i d s i n Run #6 c o l o u r s l u d g e was t w i c e t h a t a c h i e v e d i n Run #5 w i t h f r e s h c h e m i c a l s ( T a b l e 1 4 ) . R e s u l t a n t s l u d g e volume d e c r e a s e d from 22.8 t o 21.8% o f t h e o r i g i n a l when r e c y c l e d s u p e r n a t a n t was used as th e M g + + s o u r c e i n Run #6. The measured M g + + l o s s i n Run #6 was 3.6 mg/1 o r .9% o f the t o t a l . T h i s was w i t h i n t h e range f o u n d i n t h e t e s t s u s i n g f r e s h c h e m i c a l s , a l t h o u g h i t was s l i g h t l y h i g h e r than t h e 2.6 mg/1 (.6%) r e s u l t i n g i n Run #5. In a l l r u n s , t h e M g + + l o s s was s m a l l , and i t was d i f f i c u l t t o d e t e r m i n e from t h i s l i m i t e d d a t a whether t he use o f r e c y c l e d M g + + made a s i g n i f i c a n t d i f f e r e n c e t o M g + + l o s s e s i n t h e d e c o l o u r i z e d e f f l u e n t . 112 B. F a c t o r s a f f e c t i n g t h e mass b a l a n c e s i ) G e n e r a l The two r u n s , u s i n g r e c y c l e d s u p e r n a t a n t as a s o u r c e o f M g + + t o d e c o l o u r i z e BKME and T-20 e f f l u e n t , g e n e r a l l y c o n f i r m e d t h e f i n d i n g s o f t h e e x p e r i m e n t s u s i n g f r e s h c h e m i c a l s . U n t r e a t e d waste c h a r a c t e r i s t i c s , f i n a l c o a g u l a t i o n pH, m i x i n g speeds and ti m e s and s l u d g e c h a r a c t e r i s t i c s were i m p o r t a n t f a c t o r s . Most o f t h e s e have been m e n t i o n e d i n t h e p r e c e d i n g s e c t i o n and r e q u i r e T i t t l e f u r t h e r comment, i i ) C o a g u l a t i o n pH The e f f e c t o f the r e c y c l e d s u p e r n a t a n t c o l o u r and a l k a l i n i t y on e f f l u e n t t r e a t a b i l i t y was t o i n c r e a s e the l i m e dosage r e q u i r e m e n t s t o a c h i e v e a g i v e n c o a g u l a t i o n pH and c o l o u r r e m o v a l . I f t h e pH o f BKME was r a i s e d above 11.0 by an adequate l i m e dosage, 90 t o 95% c o l o u r removal r e s u l t e d when 60 mg/1 Mg was p r e s e n t , whether i n t h e form o f f r e s h MgS0 4 s o l u t i o n o r as Mg(HC0 3 ) 2 i n r e c y c l e d s u p e r n a t a n t . S i m i l a r l y , i f t h e pH o f T-20 e f f l u e n t was r a i s e d above 12.0 by l i m e , 90 t o 95% c o l o u r removal r e s u l t e d when 300 mg/1 M g + + was p r e s e n t whether i n the form o f f r e s h o r r e c y c l e d M g + + . i i i ) M i x i n g Speeds and Times In t h e t r e a t m e n t o f t h e s e e f f l u e n t s w i t h r e c y c l e d m a t e r i a l , s l o w e r m i x i n g speeds and s h o r t e r m i x i n g t i m e s p r o v e d t o be even more i m p o r t a n t than i n t r e a t m e n t w i t h f r e s h c h e m i c a l s . I f r a p i d m i x i n g ( a t 80 t o 100 rpm) was s t o p p e d one t o two m i n u t e s a f t e r t h e l i m e s l u r r y a d d i t i o n ( o r one minute a f t e r polymer a d d i t i o n , i f used) and f l o c -c u l a t i o n ( a t 30 t o 40 rpm) c o n t i n u e d f o r 5 t o 15 m i n u t e s , a good f l o e formed and s e t t l e d w e l l . In both r e c y c l e r u n s , t h e pH had s t a b i l i z e d 113 a f t e r a f i v e minute f l o c c u l a t i n g t ime and no improvement i n the o b s e r v e d f l o e c h a r a c t e r i s t i c s r e s u l t e d f r o m l o n g e r m i x i n g t i m e s , i v ) S l u d g e C h a r a c t e r i s t i c s The s l u d g e s f r o m runs u s i n g r e c y c l e d M g + + s e t t l e d f a s t e r t o a lower volume and h i g h e r p e r c e n t s o l i d s c o n c e n t r a t i o n , p r o b a b l y because o f t h e i r h i g h e r l i m e : magnesium r a t i o . The main e f f e c t o f t h i s was t o s i g n i f i c a n t l y r e d u c e the c a l c i u m l o s s e s i n t o t h e de-c o l o u r i z e d e f f l u e n t by p r e v e n t i n g CaC0 3 f l o e c a r r y o v e r . While a polymer was n o t r e q u i r e d i n t h e t r e a t m e n t o f BKME, i t was an i m p o r t a n t and n e c e s s a r y p a r t o f T-20 e f f l u e n t d e c o l o u r i z a t i o n . A dosage o f 2.0 mg/1 o f an a n i o n i c polymer (Dow A-23) was r e q u i r e d t o p r o v i d e adequate f l o e f o r m a t i o n and s e t t l i n g . 6.2.2.3 B a t c h S l u d g e C a r b o n a t i o n R e s u l t s A. Mass b a l a n c e s i ) G e n e r a l F o r c o n v e n i e n c e , t h e mass b a l a n c e r e s u l t s o f t h e runs w i t h r e c y c l e d s u p e r n a t a n t were i n c l u d e d i n the same t a b l e s as r e s u l t s o f the runs w i t h f r e s h c h e m i c a l s ( i . e . , i n Appendix D and T a b l e 1 4 ) . A l l th e mass b a l a n c e s were c a l c u l a t e d i n the same manner. The e f f l u e n t c h a r a c t e r i s t i c s and s u p e r n a t a n t q u a l i t y f o r each run can be seen i n T a b l e s 12 and 15, r e s p e c t i v e l y , i i ) C o l o u r 1. BKME. C a l c u l a t e d on t h e b a s i s o f 5% s o l i d s i n t h e f i n a l s l u d g e , t h e c a r b o n a t i o n s u p e r n a t a n t c o n t a i n e d 25.4% o f t h e t o t a l c o l o u r mass i n t h e system. T h i s was a c o l o u r r e l e a s e o f 26.8% from t h e c a r b o n a t e d c o l o u r s l u d g e . On t h e b a s i s o f a f i n a l s l u d g e s o l i d s c o n c e n t r a t i o n o f 60%, t h e s e v a l u e s were i n c r e a s e d t o 34.4% and 36.2%, 114 . r e s p e c t i v e l y . These r e s u l t s were i n the range f o u n d i n t e s t s u s i n g f r e s h c h e m i c a l s , but lower t h a n i n Run #7. T h i s was a t t r i b u t e d p a r t l y t o the h i g h e r c o l o u r s l u d g e c o n c e n t r a t i o n i n Run #8 w i t h r e c y c l e . S i n c e l e s s c o l o u r was r e c y c l e d , more was a c t u a l l y d i s p o s e d o f w i t h t h e f i n a l s l u d g e ( i . e . , 60.4 t o 69.4% o f the t o t a l , d e p e n d i n g on the f i n a l s l u d g e s o l i d s c o n c e n t r a t i o n ) . Even though the mass o f c o l o u r i n the c a r b o n a t i o n s u p e r -n a t a n t f r o m Run #8 was l e s s t h a n i n Run #7, t h e s u p e r n a t a n t c o l o u r c o n c e n t r a t i o n was h i g h e r . Due t o t h e h i g h e r f i n a l s l u d g e s o l i d s c o n c e n t r a t i o n a c t u a l l y a c h i e v e d i n Run #8 (6.33% s o l i d s ) , a lower r e c y c l e volume r e s u l t e d (3.8% o f t h e o r i g i n a l waste volume t r e a t e d ) . T h i s was lower than i n any o f the o t h e r runs and r e s u l t e d i n a s u p e r -n a t a n t c o l o u r o f 13,500 C U . compared t o 9,400 C U . i n 4.9% o f t h e volume i n Run #7. 2. T-20 E f f l u e n t . R e s u l t s o f Run #6 were j u s t as good as i n any o f t h e runs u s i n g f r e s h c h e m i c a l s and s l i g h t l y b e t t e r than Run #5 on t h e same e f f l u e n t . C o l o u r r e l e a s e was o n l y 16 t o 18%, d e p e n d i n g on t h e b a s i s o f t h e c a l c u l a t i o n . T h i s meant t h a t g r e a t e r u l t i m a t e d i s p o s a l o f c o l o u r r e s u l t e d i n t h i s r u n (77.6 t o 78.6% o f t h e t o t a l mass compared t o 70.3 t o 76.4% i n Run #5). In t h i s r u n , t h e r e c y c l e volume p e r c e n t a g e was l a r g e r than i n Run #5, due t o the e x t r e m e l y h i g h f i n a l s l u d g e c o n c e n t r a t i o n a c h i e v e d (31.1% s o l i d s ) . T h i s added d i l u t i o n , a l o n g w i t h the lower p e r c e n t r e l e a s e , r e s u l t e d i n a lower s u p e r n a t a n t c o l o u r c o n c e n t r a t i o n o f 12,550 C.U., compared t o 16,000 C U . i n Run #5. i i i ) Magnesium 1. BKME. In Run #8, the magnesium r e c o v e r y f r o m t h e 115 c a r b o n a t e d c o l o u r s l u d g e ( T a b l e 14, P a r t 2) was 62.6%, i f t h e f i n a l s l u d g e was 6.33% s o l i d s , o r 82.3% i f t he f i n a l s l u d g e was 60% s o l i d s . T h i s was a d e c r e a s e f r o m t h e 75.6 t o 91.1% r e c o v e r y a c h i e v e d i n the runs w i t h f r e s h c h e m i c a l s . I t i s t h o u g h t t h a t a c o m b i n a t i o n o f poor m i x i n g and h i g h e r s l u d g e s o l i d s c o n c e n t r a t i o n r e s u l t e d i n l o c a l i z e d a r e a s o f h i g h and low pH which was r e s p o n s i b l e f o r t h i s p o o r e r r e c o v e r y . However, o t h e r f a c t o r s , as d i s c u s s e d l a t e r , may a l s o a d v e r s e l y a f f e c t r e c o v e r y . The p o o r e r r e c o v e r y meant t h a t more M g + + was l o s t w i t h t h e f i n a l s l u d g e , a l t h o u g h t h e 15.2 t o 34.4% o f t o t a l mass was not too much g r e a t e r t h a n t h e 11.5 t o 26.2% l o s t i n Run #7 w i t h f r e s h c h e m i c a l s . I n t e r e s t i n g l y enough, when c a l c u l a t e d as the p e r c e n t o f the i n c i n e r a t e d f i n a l s l u d g e s o l i d s , t h e mass o f MgO i n t h e s l u d g e a c c o u n t e d f o r o n l y 3.7% o f t h e t o t a l ash w e i g h t , compared t o 5.2% i n Run #7. T h i s can be e x p l a i n e d by the b e t t e r CaO r e c o v e r y i n Run #6, i n c r e a s i n g t h e CaO.MgO r a t i o and d e c r e a s i n g t h e o v e r a l l MgO p e r c e n t o f the t o t a l ash w e i g h t . 2. T-20 E f f l u e n t . Magnesium r e c o v e r y f r o m t h e c a r b o n a t e d s l u d g e i n Run #6 was 56.7% o r 60.2%, d e p e n d i n g upon whether t he f i n a l s l u d g e was 31.1% o r 60% s o l i d s , r e s p e c t i v e l y . As i n t e s t s w i t h BKME, t h i s was l e s s t h a n t h e 68 t o 90% o b t a i n e d when f r e s h magnesium was used. A g a i n , t h i s seemed t o be r e l a t e d t o the p o o r e r m i x i n g combined w i t h t h e t h i c k e r c o l o u r s l u d g e ( 4 % s o l i d s ) and t h e e x t r e m e l y dense (31.1% s o l i d s ) f i n a l s l u d g e a c h i e v e d when r e c y c l e d s u p e r n a t a n t was used. In Run #6, t h e p e r c e n t o f t h e t o t a l mass o f M g + + i n t h e system l o s t i n the f i n a l s l u d g e was 36.4 t o 39.8, de p e n d i n g on the s l u d g e s o l i d s c o n t e n t . A l t h o u g h t h i s was h i g h e r than i n runs w i t h 116 f r e s h c h e m i c a l s , t h e MgO p e r c e n t o f the i n c i n e r a t e d f i n a l s l u d g e s o l i d s was o n l y 4.8%, compared t o 8.9% i n Run #5 because o f t h e b e t t e r c o n c u r r e n t CaO r e c o v e r y i n the r e c y c l e r u n . i v ) C a l c i u m 1. BKME. In o r d e r t o i n t e r p r e t t h e mass b a l a n c e p e r c e n t v a l u e s f o r C a + + , i t i s n e c e s s a r y t o take i n t o a c c o u n t t he i n c r e a s e d l i m e usage i n Run #8, a f a c t o r which r a i s e d t he t o t a l mass o f C a + + i n t h e system. However, from T a b l e 14, P a r t 2, i t can be seen t h a t o n l y 1.3% o f the Ca s e n t t o the c a r b o n a t o r was r e l e a s e d i n t o t he s u p e r -n a t a n t i n Run #8. T h i s was l e s s than h a l f t h a t r e l e a s e d i n any o f t h e o t h e r r u n s , r e p r e s e n t i n g a s i g n i f i c a n t d e c r e a s e i n C a + + l o s s . T h i s i n t u r n meant o v e r 90% C a + + r e c o v e r y i n t h e f i n a l s l u d g e o r 60.4 t o 69.4% ++ o f t h e t o t a l Ca i n the system. In Run #8, CaO a c c o u n t e d f o r 80.0% o f the mass o f t h e i n -c i n e r a t e d f i n a l s l u d g e s o l i d s , compared t o 73.9% i n Run #7. MgO and CaO i n Run #8 a c c o u n t e d f o r 83.7% o f t h e f i n a l s l u d g e s o l i d s ash w e i g h t , l e a v i n g 16.3% una c c o u n t e d f o r . T h i s was w i t h i n t h e range o f r e s u l t s f o u n d i n o t h e r r u n s . Runs #7 and 8 can be t h o u g h t o f as r e p r e s e n t i n g t h e f i r s t and second c y c l e o f a c o n t i n u o u s p r o c e s s . A comparison o f t h e f i n a l s u p e r n a t a n t q u a l i t y o f t h e s e runs ( T a b l e 1 5 ) , c o n s i d e r i n g t h e volume d i f f e r e n c e s ( T a b l e 16) and t h e i n c r e a s e d l i m e dosage used i n Run #8, i n d i c a t e s t h a t s u p e r n a t a n t q u a l i t y was s t a b i l i z i n g . The i n c r e a s e d a l k a l i n i t y and c o l o u r con-c e n t r a t i o n s i n Run #8 can be e x p l a i n e d by the d e c r e a s e i n c o l o u r s l u d g e p r o d u c t i o n and r e c y c l e volume. So, on a mass b a s i s , t h e y a r e about e q u i v a l e n t t o the r e s u l t s o f Run #7. 117 In a t h i r d b a t c h c y c l e , no f u r t h e r i n c r e a s e i n l i m e dosage would l i k e l y be r e q u i r e d , b u t more M g + + makeup would be needed because o f t h e p o o r e r r e c o v e r y i n Run #8. As d i s c u s s e d p r e v i o u s l y , p r o p e r d e s i g n and o p e r a t i o n o f a l a r g e s c a l e s l u d g e c a r b o n a t o r s h o u l d e n a b l e h i g h e r Mg r e c o v e r y , w h i l e m i n i m i z i n g Ca l o s s e s i n t o t h e s u p e r -n a t a n t . No s e r i o u s problems have been r e p o r t e d by r e s e a r c h e r s i n a c h i e v i n g g r e a t e r than 90% M g + + r e c o v e r y f r o m water t r e a t m e n t s l u d g e s . However, o n l y f u r t h e r t e s t i n g o f k r a f t e f f l u e n t s l u d g e s , on a l a r g e r s e m i - c o n t i n u o u s b a s i s , can s a t i s f a c t o r i l y c o n f i r m t h i s . 2. T-20 E f f l u e n t . Keeping i n mind t h a t t h e C a + + l e v e l i n Run #6 i s a l m o s t d o u b l e t h a t i n Run #5, T a b l e 14, P a r t 2 shows a s i g n i f i c a n t d e c r e a s e i n C a + + l o s s i n t o t h e s u p e r n a t a n t . O n l y 6.2 and 6.7% o f t h e C a + + i n the c a r b o n a t o r was r e l e a s e d w i t h f i n a l s l u d g e s o l i d s c o n t e n t s of.31.1 and 60%, r e s p e c t i v e l y . Compared t o the 15.8 and 21.5% r e l e a s e i n Run #5, t h i s was a s i g n i f i c a n t improvement t h a t can be a t t r i b u t e d t o t h e b e t t e r f i n a l s l u d g e s e t t l e a b i l i t y (due t o the h i g h e r l i m e l e v e l used i n Run #6). These l o w e r l o s s e s meant b e t t e r r e c o v e r y o f C a + + i n t h e f i n a l s l u d g e , (96.3 t o 96.7% compared t o 74.1 t o 79.8% i n Run #5). T h i s r e p r e s e n t e d an o v e r a l l r e c o v e r y o f C a + + o f 79.7 t o 80.0% o f t h e t o t a l mass i n the system (much more th a n any o t h e r r u n because o f t h e l a r g e r t o t a l mass). In Run #6, CaO a c c o u n t e d f o r 82.2% o f t h e mass o f the i n c i n e r a t e d f i n a l s l u d g e s o l i d s , compared t o 79.9% i n Run #5. T h i s meant t h a t t h e sum o f MgO and CaO i n Run #6 a c c o u n t e d f o r 87% o f t h e f i n a l s l u d g e s o l i d s a s h w e i g h t l e a v i n g 13% u n a c c o u n t e d f o r . A g a i n , comparing t h e s u p e r n a t a n t c h a r a c t e r i s t i c s r e s u l t i n g f r o m t h e two runs ( r e p r e s e n t i n g t h e f i r s t and second c y c l e o f a 118 c o n t i n u o u s p r o c e s s ) , t h e r e i s e v i d e n c e t h a t an e q u i l i b r i u m p o i n t was b e i n g r e a c h e d . A l k a l i n i t y and c o l o u r c o n c e n t r a t i o n d e c r e a s e d . No i n c r e a s e i n l i m e would l i k e l y be r e q u i r e d f o r the n e x t c y c l e , but a g a i n due t o poor M g + + r e c o v e r y a c h i e v e d i n Run #6, more magnesium makeup would be needed. However, t h e same f a c t o r s t h a t a f f e c t e d t e s t i n g w i t h f r e s h c h e m i c a l s a r e b e l i e v e d t o have c o n t r i b u t e d t o the l o w e r r e c o v e r y i n t h i s l a b o r a t o r y t e s t . Based on t h e good r e c o v e r i e s i n some o f t h e s e b a t c h t e s t s and t h e s u c c e s s o f r e s e a r c h e r s i n the U.S.A. t r e a t i n g s u r f a c e w a t e r s , t h e r e i s no r e a s o n t o e x p e c t t h a t magnesium and c a l c i u m r e c o v e r y s h o u l d be a s e r i o u s p r o b l e m i f p r o p e r d e s i g n and o p e r a t i o n o f t h e s l u d g e c a r b o n a t i o n s y s t e m i s p r o v i d e d . The e f f e c t s o f m i x i n g , s o l i d s c o n c e n t r a t i o n , and o t h e r i m p o r t a n t parameters a r e d i s c u s s e d n e x t . B. F a c t o r s a f f e c t i n g the s l u d g e c a r b o n a t i o n mass b a l a n c e  r e s u l t s u s i n g r e c y c l e d Mg i ) G e n e r a l Most o f t h e s e f a c t o r s have been d i s c u s s e d i n t h e p r e s e n t a t i o n o f t h e r e s u l t s i n t h e p r e v i o u s s e c t i o n . In g e n e r a l , t e s t s w i t h r e c y c l e v e r i f i e d t he i m p o r t a n c e o f t h e f a c t o r s i d e n t i f i e d from the r e s u l t s o f t h e runs u s i n g f r e s h c h e m i c a l s . Some f u r t h e r comments on t h e s e a r e made below under the a p p r o p r i a t e h e a d i n g s , i i ) F i n a l S l u d g e Volume The f i n a l s l u d g e volume a f f e c t e d t h e mass b a l a n c e c a l c u l a t i o n s . In both runs w i t h r e c y c l e d M g + + , t h e f i n a l s l u d g e s o l i d s c o n c e n t r a t i o n was o v e r 5%, so t h a t a l l mass b a l a n c e s were c a l c u l a t e d u s i n g two s i t u a t i o n s : the volume a t the a c t u a l s o l i d s c o n c e n t r a t i o n ; and t h e volume a t 60% s o l i d s . T h i s s h o u l d be k e p t i n mind when comparing the 119 r e s u l t s o f a l l t h e r u n s . In any c a s e , T a b l e 14 shows t h e e f f e c t o f d i f f e r e n t f i n a l s o l i d s c o n c e n t r a t i o n on the v a r i o u s mass b a l a n c e s , i i i ) C a r b o n a t i o n pH F u r t h e r i n f o r m a t i o n on the p r e f e r r e d c a r b o n a t i o n pH can be g a i n e d by e x a m i n i n g t h e r e s u l t s o f c a r b o n a t i o n m o n i t o r i n g i n Run #8 on BKME and Run #6 on T-20 e f f l u e n t , shown g r a p h i c a l l y i n F i g u r e s 13 and 14, r e s p e c t i v e l y . The same g e n e r a l c o l o u r , M g + + , and C a + + p r o f i l e s r e s u l t e d as i n t h e runs w i t h f r e s h c h e m i c a l s . However, t h e magnesium c u r v e s had not l e v e l l e d o f f as c o m p l e t e l y i n Runs #8 and 6 as t h e y had i n Runs #7 and 5 ( F i g u r e s 11 and 1 2 ) . T h i s p o i n t s t o the p o s s i b l e b e n e f i t o f c a r b o n a t i n g t o a pH s l i g h t l y below 7.5 and p a r t i a l l y e x p l a i n s t h e lower M g + + r e c o v e r i e s i n t h e s e t e s t s , i v ) Time The c o n c e n t r a t i o n s o f c o l o u r , M g + + and C a + + a g a i n u s u a l l y changed d u r i n g t h e s l u d g e s e t t l i n g p e r i o d , but l i t t l e f u r t h e r i n f o r m a t i o n on t h e a f f e c t s o f time c o u l d be g a i n e d f r o m t h e s e two r u n s . v) C o l o u r S l u d g e and F i n a l S l u d g e S o l i d s C o n c e n t r a t i o n The i m p o r t a n c e o f t h e s e i n t e r r e l a t e d f a c t o r s became more a p p a r e n t d u r i n g t h e runs w i t h r e c y c l e d m a t e r i a l . The c o l o u r s l u d g e s o l i d s c o n c e n t r a t i o n was a p p r o x i m a t e l y t w i c e t h a t o b t a i n e d i n the c o r r e s p o n d i n g runs w i t h f r e s h c h e m i c a l s ( i . e . , 2.24 f o r BKME and 4.03 f o r T-20 e f f l u e n t ) . The f i n a l s l u d g e c o n c e n t r a t i o n s were more than t w i c e as h i g h (6.33% i n Run #8 w i t h BKME and an a s t o u n d i n g 31.1% i n Run #6 w i t h T-20 e f f l u e n t ) . The e f f e c t o f t h e more c o n c e n t r a t e d c o l o u r s l u d g e f e e d t o th e c a r b o n a t o r on c o l o u r r e l e a s e was not as s i g n i f i c a n t as i n d i c a t e d 120 o f o -0 -1000-1 800 H Q 600-^ E + + j? 400 200 H •o C O L O U R -o C a + + -,+ + 120 h-100 D) J , + + CO o r 14000 h12000 10000 8000 h6000 -4000" •2000 L 0 73 6 CL ZD O _J o u FIGURE 13. p H vs. C O L O U R , M g + + A N D C a + + IN T H E S U P E R N A T A N T F R O M A B K M E S L U D G E C A R B O N A T I O N U S I N G R E C Y C L E D M A G N E S I U M * C h a n g e In Concen t r a t i on A n d pH Dur ing S l udge S e t t i n g 121 r 13000 - , r 1- i 11 10 9 8 P H F IGURE 14 p H V s C O L O U R , M g + + A N D C a + + I N T H E S U P E R N A T A N T F R O M A T - 2 0 S L U D G E C A R B O N A T I O N U S I N G R E C Y C L E D M A G N E S I U M . * C H A N G E IN C O N C E N T R A T I O N A N D p H DURING S L U D G E S E T T L I N G 122 by r e s u l t s o f e a r l i e r r u ns o f more d i l u t e s l u d g e . The c o l o u r r e l e a s e s i n Runs #8 and 6 were 2 t o 5% l e s s than i n Runs #7 and 5, i n d i c a t i n g t h a t some b e n e f i t w i t h r e s p e c t t o c o l o u r i s g a i n e d f r o m a c o n c e n t r a t e d s l u d g e f e e d t o the c a r b o n a t o r ; however, t h e r e a p p e a r s t o be an upper l i m i t t o t h i s ( l i k e l y about 5% s o l i d s ) . I t was d i f f i c u l t t o s e p a r a t e the e f f e c t s o f c o l o u r s l u d g e c o n c e n t r a t i o n and f i n a l s l u d g e c o n c e n -t r a t i o n , s i n c e a p o r t i o n o f t h e c o l o u r s l u d g e u l t i m a t e l y becomes f i n a l s l u d g e a f t e r c a r b o n a t i o n . However, i n the two runs w i t h r e c y c l e , t h e f o r m a t i o n o f a v i s i b l e CaCG"3 s l u d g e b l a n k e t , t h a t t e n d e d t o s e t t l e , even d u r i n g c a r b o n a t i o n , i s b e l i e v e d r e s p o n s i b l e f o r t h e r e d u c e d Mg r e c o v e r i e s i n Runs #8 and 6. The r e d u c e d C a + + l o s s e s i n t o t h e s u p e r -n a t a n t a r e a l s o a t t r i b u t e d t o t h i s d e n s e r s l u d g e t h a t h i n d e r e d m i x i n g , c a u s i n g a n o n - u n i f o r m s l u d g e pH i n t h e c a r b o n a t i o n column, v i ) M i x i n g The problems e n c o u n t e r e d w i t h m i x i n g were d i s c u s s e d i n t h e s e c t i o n on f r e s h c h e m i c a l s . As i n d i c a t e d above, t h e s e problems were m a g n i f i e d i n t h e t e s t s where v e r y c o n c e n t r a t e d s l u d g e was c a r b o n a t e d . I t i s l i k e l y t h a t u n i f o r m pH was not r e a c h e d i n the c a r b o n a t o r , and hence some o f the Mg(0H) 2 d i d not have a chance t o s o l u b i l i z e i n the a r e a s o f h i g h e r pH. L i k e w i s e , p o c k e t s o f low pH c a u s e d u n e x p e c t e d l y h i g h CaC0 3 d i s s o l u t i o n i n some t e s t s . C a r b o n a t o r d e s i g n and o p e r a t i o n a p p e a r s t o be one o f t h e most i m p o r t a n t c o n s i d e r a t i o n s i n a c h i e v i n g good magnesium and c a l c i u m r e c o v e r y . Complete m i x i n g must be p r o v i d e d whether i n a b a t c h o r c o n t i n u o u s s l u d g e c a r b o n a t i o n system. C o u p l e d w i t h a good C 0 2 d i f f u s e r d e s i g n , complete m i x i n g s h o u l d a c h i e v e a more u n i f o r m s l u d g e pH, r e s u l t i n g i n more e f f i c i e n t M g + + r e c o v e r i e s and lower C a + + l o s s e s . F u r t h e r t e s t s a r e needed t o v e r i f y t h i s . 123 CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS 7.1 C o n c l u s i o n s 1. A c o m b i n a t i o n o f low magnesium and low li m e dosages can a c h i e v e b e t t e r d e c o l o u r i z a t i o n o f k r a f t m i l l e f l l u e n t s than much g r e a t e r amounts o f l i m e a l o n e . 2. Reasonable dosages o f magnesium and l i m e can p r o v i d e 90-95% d e c o l o u r i z a t i o n o f both c a u s t i c e x t r a c t i o n e f f l u e n t and b i o l o g i c a l l y t r e a t e d combined e f f l u e n t from a b l e a c h e d k r a f t mi 11. 3. R e c y c l e d s u p e r n a t a n t f r o m c o l o u r s l u d g e c a r b o n a t i o n p r o v i d e s an e f f e c t i v e s o u r c e o f magnesium i o n s , d e s p i t e i t s r e l a -t i v e l y h i g h c o l o u r and a l k a l i n i t y . 4. The magnesium r e c o v e r i e s and r e c a l c i n e d l i m e q u a l i t i e s , a c h i e v e d i n t h i s s t u d y by c a r b o n a t i n g t h e p r i m a r y c o l o u r s l u d g e , show t h a t t h i s p r o c e s s w a r r a n t s f u r t h e r i n v e s t i g a -t i o n on a l a r g e r , c o n t i n u o u s b a s i s . 7.2 Recommendations F o r F u t u r e R e s e a r c h 7.2.1 Bench S c a l e a. F u r t h e r j a r t e s t i n g i s r e q u i r e d o v e r a wide range o f i n f l u e n t c o n d i t i o n s , to i d e n t i f y a r e l a t i o n s h i p between waste c h a r a c -t e r i s t i c s , c o a g u l a t i o n pH, l i m e and magnesium dosage r e q u i r e -ments. T h i s i n f o r m a t i o n i s e s s e n t i a l f o r o p t i m i z a t i o n o f any c o n t i n u o u s , magnesium c o a g u l a t i o n system. b. I t i s a n t i c i p a t e d t h a t dosage r e q u i r e m e n t s cfor good s e t t l i n g w i l l be d e c r e a s e d i f t h e u n t r e a t e d waste s o l i d s c o n t e n t i s h i g h e r . T h e r e f o r e , a ' j a r t e s t i n g program s h o u l d be c a r r i e d o u t t o e v a l u a t e t h e t r e a t a b i l i t y o f t o t a l BKME b e f o r e and 124 a f t e r p r i m a r y s e t t l i n g , as w e l l as p r i o r t o b i o l o g i c a l t r e a t m e n t and s h o u l d i n c l u d e m o n i t o r i n g o f BOD and SS, i n a d d i t i o n t o c o l o u r . 7.2.2 P i l o t S c a l e C o n c u r r e n t w i t h the l a b o r a t o r y s t u d i e s c o n d u c t e d t h i s y e a r , a 20 Igpm p i l o t , p l a n t has been assembled on p o r t a b l e s k i d s and i s now b e i n g o p e r a t e d a t the Canada C e n t r e f o r I n l a n d Waters (CCIW). A s c h e m a t i c d i agram o f the p l a n t i s shown i n F i g u r e 15. a. I n i t i a l work on t h e p i l o t p l a n t s h o u l d be c o n d u c t e d a t CCIW, u s i n g a p r e p a r e d d y e - l i g n o s o l m i x t u r e as a c o l o u r s o u r c e , t o t e s t the o p e r a t i o n and c o n t r o l o f the v a r i o u s u n i t p r o c e s s e s ; I i n a d d i t o n , i t c o u l d be used t o p o i n t o u t r e q u i r e d e q u i p -ment m o d i f i c a t i o n s p r i o r t o f i e l d i n s t a l l a t i o n . b. The emphasis o f t h e p i l o t p l a n t program s h o u l d be on gener-a t i n g economic and d e s i g n d a t a f o r a f u t u r e f u l l - s c a l e system. The key t o economic v i a b i l i t y l i e s i n c h e m i c a l r e c o v e r y and r e -use. T h e r e f o r e , the main q u e s t i o n s t h a t remain to be answered by p i l o t s c a l e t e s t i n g a r e : i ) I s s l u d g e c a r b o n a t i o n a t e c h n i c a l l y and e c o n o m i c a l l y f e a s i b l e method o f magnesium r e c o v e r y f o r r e u s e ? . i i ) Is s l u d g e c a r b o n a t i o n , f o l l o w e d by d e w a t e r i n g and i n c i n e r a -t i o n , a t e c h n i c a l l y and e c o n o m i c a l l y f e a s i b l e method o f l i m e  r e c o v e r y f o r r e u s e ? In o r d e r to p r o v i d e answers to t h e s e q u e s t i o n s , i t w i l l be n e c e s s a r y t o r e l o c a t e the p i l o t p l a n t a t a k r a f t m i l l f o r a p e r i o d o f s e v e r a l months. PRIMARY "COLOUR" CLARIFIER #2 #1 t FLOCCULATORS COLOUR SLUDGE [Mg (OH)2+ C 3 C O 3 + COLOUR] DUAL MEDIA FILTERS RECARBONATOR RECARBONATOR *1 -H C 0 2 SLURRY RECYCLE RECARB-ONATION CLARIFIER -5> #2 C02 RECARB-ONATION SURGELJ TANK SLUDGE ( CaC0 3) | PROPOSED SLUDGE *»1 DEWATERING (VACUUM FILTER) and INCINERATION (ROTARY KILN) LIME • #2 RAW WASTEWATER H« # 1 I Mg + + MAKE-UP RAPID MIXERS SUPERNATANT [Mg(HC03)2] RECYCLE CO; FINAL EFFLUENT BATCH SLUDGE CARBON-ATOR/ THICKENER FINAL | SLUDGE I (CaC03 + I COLOUR) SLUDGE RECYCLE L_ RECOVERED CaO RECYCLED F I G U R E 15. S C H E M A T I C O F T H E E P S M A G N E S I U M C O A G U L A T I O N P ILOT P L A N T 126 A n s w e r i n g t h e f i r s t q u e s t i o n w i l l i n v o l v e " }") T e s t i n g s e v e r a l s o u r c e s o f f r e s h M g + + t o f i n d the l e a s t ex-p e n s i v e make-up c h e m i c a l . These s h o u l d i n c l u d e i ) MgO o r M g ( 0 H ) 2 i i ) MgC0 3.3H 20 i i i ) D o l o m i t i c l i m e 2) M o n i t o r i n g e f f l u e n t q u a l i t y and M g + + makeup r e q u i r e m e n t s through a number o f magnesium r e c o v e r y and r e u s e c y c l e s . The q u e s t i o n o f l i m e r e c o v e r y and r e u s e i s one t h a t has not been f o r m a l l y a d d r e s s e d t o t h i s p o i n t . T h i s w i l l r e q u i r e the a d d i t i o n o f a s l u d g e h a n d l i n g s y s t e m t o t h e e x i s t i n g p i l o t p l a n t . S u i t a b l e equipment i s a v a i l a b l e a t CCIW f o r f i e l d use. The c a r b o n a t e d f i n a l s l u d g e w i l l p r o b a b l y have a s o l i d s c o n t e n t o f 10-25% on l e a v i n g the b a t c h c a r b o n a t o r - t h i c k e n e r , so f u r t h e r d e w a t e r i n g to 50-60% s o l i d s , on a vacuum f i l t e r p r i o r to i n c i n e r a t i o n , w i l l be needed. R e c a l c i n e d l i m e q u a l i t y s h o u l d be m o n i t o r e d t h r o u g h s e v e r a l c y c l e s o f r e c o v e r y and r e u s e t o check f o r b u i l d u p o f magnesium, c h l o r i d e and o t h e r i m p u r i t i e s . F l u e gas c o n t a i n i n g C 0 2 s h o u l d be e v a l u a t e d f o r e f f l u e n t and s l u d g e c a r b o n a t i o n , f o r comparison w i t h use o f pure C 0 2 . The o n - s i t e p i l o t p l a n t s t u d y s h o u l d i n c l u d e i n f l u e n t and e f f l u e n t m o n i t o r i n g f o r such parameters as BOD, SS, and r e s i n and f a t t y a c i d s , and t o x i c i t y , i n a d d i t i o n t o c o l o u r , s i n c e a s i g n i f i c a n t r e d u c t i o n o f t h e s e p o l l u t a n t s can be a n t i c i p a t e d . 7.2.3 Concept f o r F u l l S c a l e I m p l e m e n t a t i o n I f t he p r o p o s e d p i l o t p l a n t t e s t i n g program i s c e n t e r e d around ++ ++ t i g h t mass b a l a n c e s f o r Mg , Ca , C o l o u r , SS, e t c . , s u f f i c i e n t d a t a s h o u l d be a c q u i r e d f o r making more d e t a i l e d economic e s t i m a t e s f o r f u l l -127 s c a l e use and comparison t o o t h e r p r o c e s s e s . O p e r a t i o n o f a p i l o t p l a n t o f t h i s s i z e s h o u l d g e n e r a t e s u f f i c i e n t d e s i g n d a t a t o a l l o w s c a l e - u p to a 10 m i l l i o n g a l l o n / d a y t r e a t m e n t f a c i l i t y , a c c o r d i n g t o Lund (1971). I f t e c h n i c a l l y and e c o n o m i c a l l y a c c e p t a b l e , t he p r o c e s s c o u l d be implemented i m m e d i a t e l y a t the Skookumchuck M i l l o f C r e s t b r o o k F o r e s t P r o -d u c t s , s i n c e t h e y a re c u r r e n t l y under B r i t i s h Columbia P o l l u t i o n C o n t r o l Board o r d e r s t o reduce e f f l u e n t c o l o u r . A l s o , a c c o r d i n g t o G r i f f i n ( 1 9 7 5 ) , t h e r e a r e a t l e a s t two o t h e r m i l l s i n B r i t i s h Columbia t h a t r e q u i r e e f f l u e n t d e c o l o u r i z a t i o n . In a d d i t i o n , i t s h o u l d be kept i n mind t h a t t h i s magnesium c o a g u l a t i o n p r o c e s s has a b r o a d e r a p p l i c a t i o n than s i m p l y c o l o u r removal. The i n f o r m a t i o n on magnesium and l i m e r e c o v e r y and r e u s e o b t a i n e d from t h i s s t u d y o f h i g h l y - c o l o u r e d , p u l p m i l l e f f l u e n t w i l l p o i n t t o the f e a s i b i l i t y o f p r o c e s s use f o r removal o f o t h e r , l e s s moxious t y p e s o f i m p u r i t i e s , such as f l u o r i d e , s i l i c a ? t r a c e m e t a l s , p h o s p h o r u s , dye o r suspended s o l i d s , to name a few. I f removal e f f i c i e n c i e s o f BOD, SS, and t o x i c i t y p r o ve t o be as h i g h as e x p e c t e d , i t i s q u i t e p o s s i b l e t h a t magnesium c o a g u l a t i o n c o u l d be c a p a b l e o f r e p l a c i n g b i o l o g i c a l t r e a t m e n t i n some s i t u a t i o n s . In any c a s e , t he proven p o t e n t i a l o f magnesium c o a g u l a t i o n t o c l a r i f y s u r f a c e water o r m u n i c i p a l and i n d u s t r i a l wastewaters s h o u l d w a r r a n t f u r t h e r i n v e s t i g a t i o n o f t h i s r e l a t i v e l y new t e c h n o l o g y . 128 BIBLIOGRAPHY 1. APHA, AWWA, WPCF, " S t a n d a r d Methods f o r the E x a m i n a t i o n o f Water  and Wastewater", 13th E d i t i o n , (1971). 2. AWWAiInc., "Water Q u a l i t y and Treatment. A Handbook o f P u b l i c  Water Supplies": McGraw-Hill Book Co., NY 3 r d Ed., p. 559 (1971 ). 3. B.C. R e s e a r c h , "Survey o f C o l o u r D i s c h a r g e s i n V a r i o u s P u l p M i l l s i n B r i t i s h C o l umbia", P r e p a r e d f o r Water Resources Branch o f Department o f Lands, F o r e s t s and Water R e s o u r c e s , V i c t o r i a , B.C., (Feb. 1973). 4. 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S p r u i l l , E.L. - "Long Term E x p e r i e n c e w i t h C o n t i n e n t a l Can's C o l o u r Removal System", T a p p i E n v i r o n m e n t a l C o n f e r e n c e (1974). S t o c k n e r , J.G., C l i f f , D.D., Munro, K., "The E f f e c t s c o f P u l p M i l l E f f l u e n t on P h y t o p l a n k t o n P r o d u c t i o n i n B.C. C o a s t a l Waters" F i s h e r i e s and M a r i n e S e r v i c e T e c h n i c a l R e p o r t No. 578, (1975). Stumm, W., and Morgan J . J . , A q u a t i c C h e m i s t r y , Woley - I n t e r s c i e n c e N.Y., N.Y. (1970). Swanson, J.W., D u g a l , H.S., Buchanan, M.A., D i c k e y , E..E. - " K r a f t E f f l u e n t C o l o r C h a r a c t e r i z a t i o n B e f o r e and A f t e r S t o i c h i o m e t r i c Lime T r e a t m e n t s " Env. P r o t . Tech. S e r . , EPA-R2-73-141 (Feb. 1973). T c h o b a n o g l o u s , G., " P h y s i c a l and Chemical P r o c e s s e s f o r N i t r o g e n Removal: T h e o r y and A p p l i c a t i o n s , " p r e s e n t e d a t 12th S a n i t a r y Eng.,.Conf., U n i v e r s i t y o f I l l i n o i s (Feb. 1970). Thompson, C.G., S i n g l e y , J . E . , and B l a c k , A.P., "Magnesium Car-bonate A R e c y c l e d C o a g u l a n t " , J o u r n . AWWA P a r t I , p. 11 - 19 ( J a n . 1972). P a r t I I , p. 93 - 99 (Feb. 1972). Thompson, C.G. w i t h B l a c k , Crow and E i d s n e s s I n c . , C o n s u l t i n g E n g i n e e r s , Montgomery Alabama, a P e r s o n a l Communication (Dec. 1975). T r a v e r s A., and N o u v e l . "On t h e S o l u b i l i t y o f Magnesium H y d r o x i d e a t High T e m p e r a t u r e s " Compt. Rend ( F r . ) . 188: 499 (1929). T y l e r , M.A., and F i t z g e r a l d , A.D. - "A Review o f C o l o u r R e d u c t i o n T e c h n o l o g y i n P u l p and Paper M i l l E f f l u e n t s " , P r e s e n t e d a t 58th Annual M e e t i n g , T e c h n i c a l S e c t i o n CPPA (1972). 134 V i n c e n t , D.L. " C o l o u r Removal from B i o l o g i c a l l y T r e a t e d P u l p and Paper M i l l E f f l u e n t s " CPAR P r o j e c t No., 210, F i n a l R e p o r t ( A p r i l 1974), Walden, C C , D i r e c t o r o f B r i t i s h Columbia R e s e a r c h , Vancouver, B.C., P e r s o n a l Communication (Dec. 1975). APPENDIX A DEVELOPMENT. OF TEST PROCEDURES 136 1. General Because c o a g u l a t i o n w i t h magnesium-plus-lime i s a r e l a t i v e l y new p r o c e s s , many o f t h e l a b o r a t o r y t e s t p r o c e d u r e s had t o be worked o u t as the s t u d y p r o g r e s s e d . C o n s i d e r a b l e time was a l s o s p e n t i n v e r i f y i n g the a n a l y t i c a l methods used f o r t h e s e s p e c i f i c k r a f t m i l l e f f l u e n t s . I t was thou g h t t o be w o r t h w h i l e , t h e r e f o r e , t o o u t l i n e i n some d e t a i l t h e t e s t p r o c e d u r e s d e v e l o p e d d u r i n g t h i s s t u d y , i n the hope o f p r o v i d i n g a b a s i s f o r f u t u r e r e s e a r c h i n t o t h i s p r o c e s s . 2. Development o f J a r T e s t P r o c e d u r e s The f a c t o r s e x p e c t e d to a f f e c t c o l o u r removal e f f i c i e n c y i n -c l u d e d : ( i ) time ( o f r a p i d m i x i n g , f l o c c u l a t i o n and s e t t l i n g ) ; ( i i ) speed ( o f r a p i d m i x i n g and f l o c c u l a t i o n ) ; ( i i i ) pH s t a b i l i z a t i o n d u r i n g coagu-l a t i o n ; ( i v ) dosage and method o f magnesium i o n a d d i t i o n , and (v) dosage and method o f l i m e a d d i t i o n . I t has been f a i r l y w e l l e s t a b l i s h e d by V i n c e n t (1974) t h a t f o r e f f e c t i v e magnesium c o a g u l a t i o n , M g + + i o n s must be added i n a s o l u b l e form, p r i o r t o l i m e a d d i t i o n . D u r i n g p r e l i m i n a r y j a r t e s t i n g , b o t h mag-nesium and l i m e were added d u r i n g r a p i d m i x i n g f o r f a s t d i s p e r s i o n t h r o u g h o u t the wastes. Lime was added as d r y r e a g e n t grade C a ( 0 H ) 2 « a n d M g + + was added as a s o l u t i o n o f Mg C l 2 - 6 H 2 0 o r MgS0 4.7H 20 i n the e a r l i e r t e s t s and dosage l e v e l s used were s i m i l a r t o t h o s e e s t a b l i s h e d by V i n c e n t (1974). The f i r s t j a r t e s t s t u d i e s were c a r r i e d o u t u s i n g 1;00 ml samples i n 150 ml beak e r s s t i r r e d on l a b o r a t o r y m a g n e t i c s t i r r e r s . * These s m a l l t e s t s were s e l e c t e d i n o r d e r t o c o n s e r v e e f f l u e n t s a m p l e s , s i n c e o n l y 50 -mf was needed f o r c o l o u r a n a l y s i s and because o f t h e i r ease o f pe r f o r m a n c e . * F i s h e r F l e x a - M i x Model 16 137 M i x i n g speed o f t h e s e m i x e r s was v e r y f a s t ( ^ 1 0 0 rpm on r a p i d m i x i n g ) and f l o c c u l a t i o n was f a s t e r than d e s i r e d f o r good f l o e f o r m a t i o n . To g a i n more c o n t r o l o f m i x i n g speeds a s t a n d a r d Phipps and B i r d L a b o r a t o r y s t i r r e r * was m o d i f i e d s l i g h t l y by r e p l a c i n g the custom p a d d l e s w i t h s m a l l e r p r o p e l l e r s . T h i s p r o v e d t o be an e x c e l l e n t a p p a r a t u s f o r the pur-poses o f t h i s s t u d y and gave r e s u l t s comparable t o 1 l i t r e j a r t e s t s and b a t c h d e c o l o u r i z a t i o n t e s t s . I t was e x p e c t e d t h a t r a p i d m i x i n g and f l o c c u l a t i n g r e q u i r e m e n t s would be m a i n l y dependent on pH s t a b i l i z a t i o n a f t e r the l i m e a d d i t i o n . pH m o n i t o r i n g o f s e v e r a l e f f l u e n t s showed t h a t the pH u s u a l l y s t a b i l i z e d by the end o f one minute r a p i d m i x i n g . F i g u r e s 1 and 2 a r e t y p i c a l pH s t a b i l i z a t i o n c u r v e s f o r T-20 and BKME. r e s p e c t i v e l y . F u r t h e r t e s t i n g r e v e a l e d t h a t c o l o u r removal e f f i c i e n c y was not improved by l o n g e r r a p i d m i x i n g o r f l o c c u l a t i o n t i m e s . In f a c t , the s m a l l amount o f d a t a , p r e s e n t e d here i n T a b l e s 1 and 2, i n d i c a t e d t h a t p r o l o n g e d r a p i d mix {>} min a f t e r a d d i t i o n o f each c h e m i c a l ) r e d u c e d d e c o l o u r i z a -t i o n e f f i c i e n c y as d i d e x c e s s i v e f l o c c u l a t i n g t i m e s ( ^ 1 5 min). I t appears t h a t the magnesium h y d r o x i d e f l o e f o r m a t i o n i s e s s e n -t i a l l y i n s t a n t a n e o u s , as the pH re a c h e s about 11, and t h e r e i s no advan-t a g e t o l o n g e r m i x i n g times as f a r as c o l o u r removal e f f i c i e n c y i s con-c e r n e d . On the c o n t r a r y , e x c e s s i v e m i x i n g d e s t r o y e d t he f l o e p r o d u c e d and r e s u l t e d i n d e c r e a s e d c o l o u r r e d u c t i o n and a p o o r e r s e t t l i n g s l u d g e . L a t e r t e s t s on both T-20 and BKME v e r i f i e d t h e need f o r s h o r t e r m i x i n g times and l e s s v i g o r o u s a g i t a t i o n . Based on t h e s e f i n d i n g s , t h e f o l l o w i n g p r o c e d u r e was a d o p t e d f o r magnesium-plus-lime c o a g u l a t i o n t e s t i n g . 1. F o r a l l p r e l i m i n a r y c o a g u l a t i o n t e s t s , 100 ml samples o f e f f l u e n t i n 150 ml b e a k e r s were mixed e i t h e r on l a b o r a t o r y * Phipps and B i r d I n c . , Richmond V i r g i n i a 138 13 12 rap id mix F L O C C U L A T I O N m g + + D O S E = 3 0 0 m g / / Mg++ ( a s M g C l 2 - 6 H 2 0 ) L i m e D O S E = 2 5 0 0 mg/ i C a ( O H ) 2 C a ( O H ) 2 a d d e d 6 8 10 12 14 16 T I M E (min ) Fig.1 pH STABILIZATION OF T-20 EFFLUENT AFTER LIME ADDITION. 12 r 11 10 P H 8 r ap i d m i x - F L O C C U L A T I O N ® • m g + + D O S E = 6 0 m g / i M g + + (as M g C I 2 - 6 H 2 0 ) L i m e D O S E = 5 0 0 m g / i C a ( O H ) 2 C a ( O H ) 2 a d d e d 0 6 8 10 T I M E (min) 12 14 16 FIGURE 2. pH STABILIZATION OF BKME AFTER LIME ADDITION. 139 TABLE 1 EFFECT OF RAPID MIX .TIME ON COLOUR.REMOVAL E f f l u e n t Type - BKME - I n i t i a l C o l o u r = 1900 Magnesium Dose - 60 mg/1 Mg (MgCT ?.6H 90) I n i t i a l pH = 8.2 Lime Dose - 500 mg/1 C a ( O H ) 9 - c No. Rapid Mix A f t e r Mg A d d i t i o n (Min.) A f t e r C a ( 0 H ) ? A d d i t i o n c F l o c c u l a t i o n (min) S e t t l i n g (min) pH C o l o u r C U . % Removal 24-1 ! 1/6 15 60 11.1 190 90.0 24-2 1 1/6 15 60 11.1 170 91.1 24-3 1 1 15 60 11.1 160 91.6 24-4 1 1 15 60 11.1 180 90.5 24-5 1 3 15 60 11.1 200 89.5 24-6 1 3 15 60 11.1 220 88.4 TABLE 2 EFFECT OF FLOCCULATION TIME ON COLOUR REMOVAL E f f l u e n t Type - BKME I n i t i a l C o l o u r = 1900 Magnesium Dose - 60 mg/1 Mg (MgClp.6rL0) I n i t i a l pH = 8.2 Lime Dose - 500 mg/1 C a ( 0 H ) 9 c L No. 25-1 25-2 25-3 25-4 25-5 25-6 R a p i d Mix (Min.) A f t e r Mg A f t e r C a ( 0 H ) 2 A d d i t i o n A d d i t i o n F l o c c u l a t i o n (min) 15 15 30 30 60 60 S e t t l i n g (min) pH C o l o u r C U . % Removal 60 60 60 60 60 60 210 200 230 240 240 240 88.9 89.5 87.9 87.4 87.4 87.4 140 magnetic s t i r r e r s o r on t h e m o d i f i e d P h i p p s and B i r d L a b o r a -t o r y d e s c r i b e d p r e v i o u s l y . 2. In some l a t e r j a r t e s t s , aimed a t more a c c u r a t e e v a l u a t i o n o f p r o c e s s v a r i a b l e s , 1 l i t r e samples o f e f f l u e n t were t e s t e d on an u n m o d i f i e d P h i p p s and B i r d L a b o r a t o r y S t i r r e r . 3. When magnesium and l i m e were used, they were both added d u r i n g r a p i d m i x i n g (80-100 rpm) 1 minute a p a r t , w i t h mix-i n g c o n t i n u i n g f o r 1 minute a f t e r l i m e a d d i t i o n . When a s e t t l i n g a i d was a l s o u sed, i t was added q u i c k l y ( ^ 3 0 secon d s ) a f t e r l i m e a d d i t i o n and r a p i d m i x i n g was c o n t i n u e d f o r 1 minute. 4. F l o c c u l a t i o n was c a r r i e d o u t a t a speed o f 30-40 rpm f o r 5 minutes i n most t e s t s a l t h o u g h even s l o w e r speeds a re recommended f o r f u t u r e work. The time was e x t e n d e d t o 15 minutes i n some e x p e r i m e n t s , but t h i s p r o v e d t o be o f l i t t l e v a l u e , so i t was d i s c o n t i n u e d . 5. S e t t l i n g o f BKME was e s s e n t i a l l y complete a f t e r 30 m i n u t e s , but i n most c a s e s , 1 hour s e t t l i n g was a l l o w e d b e f o r e decan-t i n g 50 mis f o r pH and c o l o u r measurement. S l u d g e from T-20 e f f l u e n t s e t t l e d s l o w l y , b ut 50 mis c o u l d be o b t a i n e d a f t e r 1 hour i n most c a s e s . The optimum m i x i n g c o n d i t i o n s d e r i v e d f o r magnesium-plus-lime c o a g u l a t i o n a r e summarized below. Time Speed R a p i d M i x i n g 1 min + 1 min 80-100 rpm F l o c c u l a t i o n 5-15 min 30-40 rpm S e t t l i n g 1 hour 0 rpm 141 T h i s p r o c e d u r e was e s t a b l i s h e d t o d e v e l o p t e n t a t i v e c h e m i c a l dosage r e q u i r e m e n t s and o p e r a t i n g c o n d i t i o n s f o r use d u r i n g d e t a i l e d b a t c h d e c o l o u r i z a t i o n and magnesium r e c o v e r y s t u d i e s . Only one s i g n i f i -c a n t change was made d u r i n g t h e b a t c h s t u d i e s . S ludge s e t t l i n g time i n th e s e s t u d i e s v a r i e d f r o m 1% hours t o o v e r n i g h t i n one c a s e , t o f a c i l i t a t e c o n c u r r e n t t e s t i n g which had t o be c a r r i e d o u t d u r i n g t h e s e l o n g runs. In a l l but t h e f i r s t T-20 e x p e r i m e n t s , an a n i o n i c polymer was added a f t e r the l i m e to a i d i n s e t t l i n g . 3. Method o f Magnesium A d d i t i o n I t was supposed t h a t , as l o n g as s o l u b l e magnesium i o n s a r e a v a i l a b l e i n the e f f l u e n t f o r p r e c i p i t a t i o n as Mg(OH),, on l i m e a d d i t i o n , the s o u r c e o f t h e s e i o n s i s i m p o r t a n t o n l y from an economic v i e w p o i n t . In o r d e r t o v e r i f y t h i s s u p p o s i t i o n and s e l e c t a s u i t a b l e method o f M g + + a d d i t i o n f o r e x p e r i m e n t a t i o n , a b r i e f 1 l i t r e j a r t e s t was c a r r i e d o u t . The r e s u l t s a r e summarized i n T a b l e 3. There was e s s e n t i a l l y no d i f f e r e n c e i n c o l o u r removal e f f i c i e n c y between t he t h r e e cases where t h e M g + + was s o l u b l e i n t h e e f f l u e n t . How-e v e r , when a s l u r r y o f p r e f o r m e d Mg(0H)2 was u s e d , t he e f f i c i e n c y was s i g n i f i c a n t l y r e duced. T h i s c o n f i r m s t h a t MgCOH^ p r e c i p i t a t i o n " i n s i t u " i s n e c e s s a r y f o r optimum d e c o l o u r i z a t i o n . MgS0 4.7H 20 s t o c k s o l u t i o n was chosen f o r s ubsequent t e s t s . 4. Method o f Lime A d d i t i o n In a m i l l s c a l e a p p l i c a t i o n o f t h i s t e c h n o l o g y , i t i s a l m o s t c e r -t a i n t h a t CaO would be s l a k e d and added as a s l u r r y ; however, s e v e r a l o p t i o n s a r e open f o r s l u r r y i n g w ater. In o r d e r t o s e l e c t a l i m e a d d i t i o n t e c h n i q u e f o r the c u r r e n t s t u d y , s e v e r a l methods were compared f o r c o l o u r removal e f f i c i e n c y . T h e . r e s u l t s o f t h i s b r i e f e v a l u a t i o n a r e shown i n T a b l e 4. TABLE 3 COLOUR. REMOVAL USING.DIFFERENT. SOURCES OF MAGNESIUM E f f l u e n t Type: B i o l o g i c a l l y T r e a t e d BKME I n i t i a l C o l o u r : 2254 C U . I n i t i a l pH : 7.30 Magnesium S o u r c e * Lime S o u r c e ^ PH C o l o u r C U . - % Removal M g C l 2 . 6 H 2 0 S t o c k S o l ' n d r y C a ( 0 H ) 2 11.05 169 92.5 11.09 169 92.5 MgS0 4.7H 20 S t o c k Sol'n 10.97 167 92.6 H II II 11.06 161 92.9 MgO + H 2 S 0 4 " / 10.98 167 92.6 II II II 11.02 181 92.0 MgO powder (10% s l u r r y A " 11.58 440 80.1 i n d i s t i l l e d H ?0) " 11.56 523 76.8 * 60 mg/1 M g T T added i n a l l c a s e s + 500 mg/1 r e a g e n t C a ( 0 H ) 2 added d r y i n a l l c a s e s / MgO was s o l u b l i z e d w i t h H 2 S 0 4 (1 mg Mg0:2.5 ml H 2 S 0 4 ) A S l u r r y mixed 15 minutes p r i o r t o a d d i t i o n - 1000 mis e f f l u e n t + 100 mis e f f l u e n t + 100 mis s l u r r y , so c o l o u r c o r r e c t e d f o r d i l u t i o n ( x ^ - ) 143 TABLE 4 COLOUR REMOVAL USING DIFFERENT.SOURCES OF LIME E f f l u e n t Type: B i o l o g i c a l l y T r e a t e d BKME I n i t i a l C o l o u r : 2175 C U . I n i t i a l pH: 7.8 Magnesium S o u r c e * Lime S o u r c e + pH- " r , n . % Removal MgS0 4.7H 20 S t o c k S o l ' n . Reagent C a ( O H ) 2 - d r y 11.30 157 92.8 Reagent C a ( 0 H ) 2 - 11.28 164 92.5 (10% s l u r r y i n d i s t i l l e d H 20) Reagent C a ( 0 H ) 2 / 11.15 168 92.3 (10% s l u r r y i n h o t raw e f f l u e n t ) T e c h n i c a l grade CaO ( d r y ) 11.04 383 82.4 T e c h n i c a l grade CaO - i n 11.80 163 92.5 a 10% s l u r r y o f c o l d d i s t i l -l e d H 20 T e c h n i c a l grade CaO i n a 11.28 173 92.1 10% s l u r r y o f h o t d i s t i l l e d H 20 Crushed c a l c i n e r p r o d u c t A 10.75 347 84.1 ( d r y ) C r u s hed c a l c i n e r p r o d u c t i n 10.92 263 87.9 10% s l u r r y o f c o l d d i s t i l -l e d H 20 Crushed c a l c i n e r p r o d u c t i n 10.30 1088 50.0 10% s l u r r y o f hot d i s t i l -l e d H 20 * 60 mg/1 added i n a l l c a s e s + 500 mg/1 as C a ( O H ) 2 added i n a l l c a s e s A' C a l c i n e r p r o d u c t f r o m t h e C r e s t b r o o k F l u o s o l i d s C a l c i n e r were c r u s h e d i n the l a b . / A l l s l u r r i e s were s t i r r e d 15 minutes p r i o r t o a d d i t i o n and hot s l u r r i e s were = 200°F o r 93°C C o l o u r . 144 I t can be seen t h a t , when a h i g h p u r i t y r e a g e n t grade Ca(0H)2 was used, i t made l i t t l e d i f f e r e n c e whether i t was added d r y , i n a d i s -t i l l e d w a t er s l u r r y o r i n a s l u r r y w i t h t h e raw e f f l u e n t . A s l u r r y o f t e c h n i c a l grade CaO gave equal p e r f o r m a n c e . In t h i s e x p e r i m e n t , c a l c i n e r p r o d u c t c r u s h e d i n the l a b gave v a r i a b l e r e s u l t s , d epending on method o f a d d i t i o n . When added dry o r i n a c o l d w a t e r s l u r r y , c o l o u r removal was s l i g h t l y l e s s than C a ( 0 H ) 2 o r t e c h . CaO. T h i s i s b e l i e v e d t o be t h e case because o f d i f f i c u l t y i n o b t a i n i n g a f i n e powder from t h e c a l c i n e r p r o d u c t i n the l a b . I t i s a l s o worth n o t i n g t h a t use o f hot s l u r r i e s r e s u l t e d i n re d u c e d c o l o u r r e m o v a l . F o r p u rposes o f t h i s r e s e a r c h , d ry r e a g e n t Ca(0H)2 was chosen f o r use i n e a r l y work, f o r c o n v e n i e n c e and r e p r o d u c i b i l i t y , b u t l a t e r , r e a g e n t Ca(0H)2 i n a s l u r r y , w i t h a volume o f d i s t i l l e d w a t e r equal; t o 10%.of the t o t a l volume t r e a t e d , was used; i t was f e l t t h a t l i m e d i s p e r -s i o n c o u l d be a c c o m p l i s h e d w i t h l e s s v i o l e n t m i x i n g and thus improve s l u d g e s e t t l i n g . 5. Development o f Batc h D e c o l o u r i z a t i o n and Magnesium Recovery T e s t P r o c e d u r e A l l the p r e l i m i n a r y i n v e s t i g a t i o n s d e s c r i b e d so iar were aimed a t e s t a b l i s h i n g a p p r o p r i a t e c o a g u l a t i o n p r o c e d u r e s and ch e m i c a l dosage l e v e l s , f o r use i n a c h i e v i n g the main o b j e c t i v e o f t h i s r e s e a r c h ( t o determine t h e p e r c e n t magnesium r e c o v e r y and c o l o u r r e l e a s e d u r i n g c a r -b o n a t i o n o f t h e c o l o u r s l u d g e ) . The b a t c h s t u d i e s were composed o f two c o n s e c u t i v e , y e t s e p a r a t e p a r t s : 1. D e c o l o u r i z a t i o n ( a volume o f e f f l u e n t l a r g e enough to g e n e r a t e a p p r o x i m a t e l y 2 l i t r e s o f p r i m a r y " c o l o u r " s l u d g e was t r e a t e d i n a l a r g e c o n t a i n e r ) and 2. Sl u d g e c a r b o n a t i o n ( C 0 2 gas was b u b b l e d t h r o u g h t h e c o l l e c t e d c o l o u r s l u d g e i n a 2 - l i t r e , g r a d u a t e d C y l i n d e r ) . The main o b j e c -145 t i v e o f t h e s e b a t c h s t u d i e s was t o o b t a i n mass b a l a n c e s o f c o l o u r , c a l c i u m magnesium around the e n t i r e system. The p r o c e d u r e s used i n t h i s phase o f the s t u d y a r e based on th o s e used by Thompson e t a l ( 1 9 7 2 ) , d u r i n g t h e i r t r e a t m e n t o f w a t e r s u p p l i e s . Many m o d i f i c a t i o n s i n a p p a r a t u s and p r o c e d u r e s were made d u r i n g the c o u r s e o f the s t u d y . A l t h o u g h t h e e a r l y t e s t s on both BKME and T-20 e f f l u e n t p r o v i d e d l i t t l e u s e f u l d a t a , each run exposed t e c h n i q u e s n e e d i n g improvement and g r a d u a l l y a wo r k a b l e p r o c e d u r e e v o l v e d . The d e t a i l e d p r o c e d u r e d e v e l o p e d f o r magnesium r e c o v e r y t e s t runs c o n s i s t e d o f the f o l l o w i n g sequence: 1. P a r t o f a day, p r i o r to a r u n , was used t o p r e p a r e f o r the run. T h i s i n c l u d e d : (a) E s t i m a t i n g t h e amount o f sample t o be t r e a t e d so t h a t the c o l o u r s l u d g e volume a f t e r 2 hours s e t t l i n g , would be a p p r o x i m a t e l y 2 l i t r e s . (b) D e t e r m i n i n g t h e amount o f M g + + s t o c k s o l u t i o n to be added. ( c ) D e t e r m i n i n g the amount o f Ca(0H)2 t o be added. (d) P l a c i n g the r e q u i r e d e v a p o r a t i n g d i s h e s and gooch c r u c i b l e s i n t o t h e 103°C d r y i n g oven. (e) S e t t i n g t h e e f f l u e n t sample o u t o f t h e 4°C c o o l e r , t o a l l o w a slow r e t u r n to room te m p e r a t u r e by the time o f the "main run" t h e n e x t morning. ( f ) P r e p a r i n g a l l the equipment f o r t h e "main r u n " . 2. In o r d e r t o c l o s e l y s i m u l a t e s e t t l i n g times t h a t would e x i s t i n a l a r g e s c a l e s y s t e m , i t was d e c i d e d t o a l l o w 2 hours s e t t l i n g f o r c o l o u r and f i n a l s l u d g e . T h i s would 146 a l l o w a b a t c h d e c o l o u r i z a t i o n , f o l l o w e d (2 hours l a t e r ) by a b a t c h c a r b o n a t i o n o f t h e c o l o u r s l u d g e . Then a f t e r 2 more hours o f s e t t l i n g , a n a l y s e s o f t h e f i n a l s l u d g e and s u p e r n a t a n t c o u l d be done. D u r i n g s e t t l i n g p e r i o d s , a n a l y s e s o f raw d e c o l o u r i z e d e f f l u e n t were c a r r i e d o u t . , F i n a l l y , a l l samples f o r a s h i n g were p l a c e d i n a m u f f l e f u r n a c e f o r o v e r n i g h t i n c i n e r a t i o n a t 700°C. The main run u s u a l l y took a t l e a s t 10 hours o f c o n t i n u o u s work.. The d e t a i l s o f t h e main run a r e as f o l l o w s : (a) Pour i n t o the t r e a t m e n t v e s s e l ( b u c k e t ) an amount o f c o l o u r e d wastewater to g i v e the d e s i r e d f i n a l volume a f t e r a l l c h e m i c a l a d d i t i o n s , p l u s 500 mis t o be removed f o r a n a l y s i s . (b) A f t e r the raw e f f l u e n t had r e a c h e d room, t e m p e r a t u r e and had been mixed to i n s u r e u n i f o r m i t y , a 500 ml r e p r e s e n -t a t i v e sample was e x t r a c t e d f o r a n a l y s i s as u n t r e a t e d e f f l u -e n t ( I N ) . ( c ) pH and a l k a l i n i t y (IN) were d e t e r m i n e d i m m e d i a t e l y . (d) The c o a g u l a t i o n was then c a r r i e d o u t a c c o r d i n g t o the p r o c e d u r e s a l r e a d y o u t l i n e s and, i n most c a s e s , m o n i t o r e d f o r pH and c o n d u c t i v i t y changes d u r i n g c o a g u l a t i o n . (e) Immediately a f t e r f l o c c u l a t i o n , a 25 ml sample was t a k e n f o r a l k a l i n i t y , and f i l t e r e d t h r o u g h 40 Whatman p a p e r , which was r e t a i n e d f o r a s h i n g . ( f ) D u r i n g c o l o u r s l u d g e s e t t l i n g , a n a l y s e s o f (IN) were comp l e t e d f o r t e m p e r a t u r e and c o n d u c t i v i t y . Samples were p r e p a r e d f o r c o l o u r and f o r M g + + and C a + + d e t e r m i n a t i o n , 147 a f t e r dry a s h i n g as d e s c r i b e d p r e v i o u s l y . (g) A f t e r s e t t l i n g f o r 2 hours i n most c a s e s , the d e c o l o u r -i z e d e f f l u e n t was c a r e f u l l y syphoned i n t o a l a r g e c o n t a i n e r and a sample o f t h e composite was taken f o r a n a l y s i s as t r e a t e d e f f l u e n t (OUT). (h) The pH, a l k a l i n i t y , c o n d u c t i v i t y and t e m p e r a t u r e o f (OUT) were d e t e r m i n e d i m m e d i a t e l y , w h i l e sample p r e p a r a t i o n f o r c o l o u r , M g + + and C a + + was done l a t e r . ( i ) The c o l o u r s l u d g e was then mixed w e l l and a sample taken f o r % s o l i d s d e t e r m i n a t i o n and d r y a s h i n g . The rem a i n d e r was t r a n s f e r r e d t o a 2 " l i t r e g r a d u a t e d c y l i n d e r f o r c a r b o n a t i o n . C o l o u r , s l u d g e volume, pH and c o n d u c t i v i t y i n t h e c y l i n d e r were r e c o r d e d . ( j ) In a l l runs e x c e p t #4, the s l u d g e was sampled a t i n t e r -v a l s d u r i n g c a r b o n a t i o n t o m o n i t o r a l k a l i n i t y , c o l o u r , M g + + and C a + + changes as t h e pH d e c r e a s e d . S l u d g e i n the c y l i n d e r was c o n t i n u o u s l y m o n i t o r e d f o r pH and c o n d u c t i v i t y . T h i s p r o c e d u r e was aimed a t i d e n t i f y i n g some p r a c t i c a l c o n t r o l mechanisms f o r use i n a l a r g e s c a l e i n s t a l l a t i o n , ( i . e . d e t e r m i n e when t o s t o p c a r b o n a t i n g ) . In Run #4, the s l u d g e was c a r b o n a t e d t o a p r e d e t e r m i n e d t a r g e t pH (7.5) and i n -c l u d e d no s a m p l i n g d u r i n g c a r b o n a t i o n . T h i s was t o see how f a s t c a r b o n a t i o n c o u l d be c a r r i e d o u t . COg f e e d r a t e was n o t s t r i c t l y c o n t r o l l e d and appeared t o f l u c t u a t e w i t h t i m e . T h e r e f o r e , pH, r a t h e r than t i m e , was a b e t t e r c o n t r o l v a r i a b l e f o r t h e p r o c e s s . D u r i n g runs where s a m p l i n g was c o n d u c t e d , some samples were syphoned out a t p r e d e t e r m i n e d s l u d g e pH's. Time, s l u d g e c o n d u c t i v i t y 148 and pH were r e c o r d e d . The sample was i m m e d i a t e l y f i l t e r e d t h r o u g h 40 Whatman p a p e r , pH and a l k a l i n i t y were then d e t e r -mined on the f i l t r a t e . Samples o f f i l t r a t e were then s e t a s i d e f o r p r e p a r a t i o n f o r c o l o u r , M g + + , and C a + + a n a l y s i s . A l l sample f i l t e r p a p e r s were saved and dry ashed f o r M g + + and C a + + d e t e r m i n a t i o n . In t h e s e r u n s , c a r b o n a t i o n was c o n t i n u e d t o a s l u d g e pH o f 6.5-7.5, i n most cases a t a slow r a t e (1-2 h r s ) t o a l l o w time f o r sample a l k a l i n i t y measurements. In Run #5 where no samples were taken,, c a r b o n a t i o n t o a f i n a l s l u d g e pH o f 7.5 took about h hour. (k) In most c a s e s , F i n a l S l u d g e s e t t l i n g was a l l o w e d 2 hours (sometimes more) b e f o r e c a r e f u l l y s y p h o n i n g o f f the s u p e r n a t a n t . S u p e r n a t a n t and F i n a l S l u d g e volume were r e -c o r d e d . (1) A composite sample o f the s u p e r n a t a n t was t a k e n . A p o r t i o n was f i l t e r e d f o r pH, a l k a l i n i t y , and c o l o u r , and an u n f i l t e r e d p o r t i o n f o r c o n d u c t i v i t y , t e m p e r a t u r e and dry a s h i n g , f o r M g + + and C a + + . (m) F i n a l s l u d g e was sampled f o r % s o l i d s and 10 mis dry ashed f o r M g + + and C a + + . (n) C o l o u r d e t e r m i n a t i o n on a l l samples was u s u a l l y s t a r t e d d u r i n g f i n a l s l u d g e s e t t l i n g and f i n i s h e d s h o r t l y a f t e r , (o) F i n a l l y , a f t e r o b t a i n i n g d ry w e i g h t o f both s l u d g e s , a l l samples f o r d r y a s h i n g were p l a c e d i n t h e f u r n a c e and the t i m e r s e t to a l l o w c o o l i n g by morning. APPENDIX B SETTLEABILITY OF T-20 EFFLUENT AND POLYMER SELECTION 150 Because o f the p o o r s e t t l e a b i l i t y o f T-20 e f f l u e n t , a few s e t t l i n g a i d s were t e s t e d on t h i s e f f l u e n t . The r e s u l t s o f s e t t l i n g , t e s t s a r e shown i n T a b l e 1. O n l y Dow's a n i o n i c polymer P u r i f l o c A-23 gave a s i g n i f i c a n t improvement, so f u r t h e r t e s t i n g t o m i n i m i z e t h e dosage r e q u i r e m e n t s were c o n d u c t e d a c c o r d i n g t o t h e m a n u f a c t u r e r ' s d i r e c t i o n s (Dow, 1967). The r e s u l t s o f 100 ml s e t t l i n g t e s t s w i t h P u r i f l o c A-23 are shown i n T a b l e 2. These t e s t . r e s u l t s were somewhat e r r a t i c , due t o the f a c t t h a t l i m e was added d r y and r a p i d m i x i n g speeds v a r i e d s l i g h t l y between the j a r s on the m a g n e t i c s t i r r e r s . A f t e r t h i s , l i m e was added as a s l u r r y and 100 ml j>ar t e s t s were c o n d u c t e d on the m o d i f i e d s i x - p l a c e l a b o r a t o r y s t i r r e r . A polymer a d d i t i o n o f 1-2 mg/1 appeared t o be adequate f o r good s e t t l i n g o f the T-20 s l u d g e . F u t u r e j a r t e s t s o f T-20 e f f l u e n t used 2 mg/1 o f A-23 as a s e t t l i n g a i d . TABLE 1 SETTLEABILITY OF T-20 EFFLUENT WITH VARIOUS SETTLING AIDS A l l j a r s were made up t o 1.0 l i t r e w i t h M g + + = 300 mg/1 and C a ( 0 H ) o = 2500 mg/1* O b s e r v a t i o n s J a r No. S e t t l i n g A i d Dosage Sludge Volume (mis) A f t e r Time (min) 0 10 20 30 40 50 60 90 14 hr. 1 Be t z 835 A 10 mg/1 1000 820 660 590 500 460 430 400 280 2 Dow P u r i f l o c A-23 10 mg/1 S e t t l e d o u t i m m e d i a t e l y a f t e r r a p i d mix t o 200 mis. 3 Ca(.0H) 2 2500 mg/1 1000 930 880 800 620 540 510 460 300 4 N a 2 S i 0 3 . 9 H 2 0 36.4 mg/1 1000 910 780 680 580 500 490 420 290 5 None 1000 960 900 820 630 580 510 460 300 R a p i d Mix ( 2 min a f t e r Mg a d d i t i o n ) (80 rpm) ( 2 min a f t e r C a ( 0 H ) 2 a d d i t i o n ) ( 2 m i n . a f t e r s e t t l i n g a i d a d d i t i o n ) F l o c c u l a t e ( 5 min - 10 rpm) (30 rpm) * e x c e p t i n J a r No. 3 where an e x t r a 2500 mg/1 was added as a s e t t l i n g a i d 152 TABLE 2 MINI SETTLING.TESTS USING PURIFLOC A-23 AS A SETTLING AID * ir No. Mg ** C a ( 0 H ) 2 A-23 O b s e r v a t i o n s S l u d g e Volume (mis) a f t e r t i m e (min (mg/1) (mg/1) (mg/1). 0 15 30 45 60 120 1 300 2500 0.1 100 90 80 75 62 43 2 300 2500 0.5 100 79 59 45 40 30 3 300 2500 1.0 100 4 2 + 38 35 30 28 4 150 2500 0.5 100 60 45 40 36 30 5 150 2500 1.0 100 82 70 59 48 33 6 150 2500 2.0 100 80 62 50 42 30 100 ml beakers added as the d r y powder Ra p i d mix was n o t as v i g o r o u s and good f l o e formed d u r i n g t h i s t i m e , r e s u l t i n g i n b e t t e r s e t t l i n g . APPENDIX C EXAMPLE CALCULATION TO DERIVE THE SLUDGE AND SUPER-NATANT VOLUME RESULTING FROM SLUDGE DEWATERING TO 5% and 60% SOLIDS 154 Run #7 a) T e s t r e s u l t s : C S . volume = 1.80 1 C S . sample = .05 1 C S . t o Carb. = 1.75 1 Samples (4 x .050) = .20 1 F i n a l S u p e r n a t a n t (Sp) r + F i n a l S l u d g e (F.S.) = 1.55 1 F i n a l s l u d g e volume a f t e r s e t t l i n g = .49 1 F i n a l S u p e r n a t a n t Volume = 1.06 1 A c t u a l % S o l i d s o f F.S. o b t a i n e d = 3.19 1 A c t u a l F.S. Volume o b t a i n e d = .49 1 b) C a l c u l a t i o n a t any F.S. P e r c e n t S o l i d s F.S. volume a t x% s o l i d s = a c t u a l F.S. v o l . x a c t u a l F.S. % s o l i d s R e c y c l e volume x% s o l i d s F i n a l s u p e r n a t a n t volume + Volume s u p e r n a t a n t samples taken d u r i n g c a r b o n a t i o n c) C a l c u l a t i o n a t F.S. s o l i d s = 5% F.S. volume a t 5% s o l i d s S u p e r n a t a n t volume R e c y c l e volume d) C a l c u l a t i o n s a t F.S. s o l i d s = 60% F.S. volume a t 60% s o l i d s S u p e r n a t a n t volume R e c y c l e volume .49 1 x 3.19% 5% ' 1.237 1 ,313 1 1.237 1 + .200 1 = 1.437 1 .49 1 x 3.19% = .026 1 1.524 1 1.524 1 + .200 1 = 1.724 1 APPENDIX D SLUDGE CARBONATION MONITORING DATA 156 Run #1 Volume (1) % S o l i d s C o l o u r S l u d g e 1.0 .99 F i n a l S l u d g e .2 3.65 Sample Time pH o f S l u d g e pH o f T o t a l ++ Conduct- C o l o u r mg Ca Min. i n C y l i n d e r F i l t r a t e A l k a l i n i t y ( C a C 0 o ) i v i t y JA S c u . mg/1 mg/1 C.S. 0 11.20 11.20 155 17500 220 10.5 78 s l 60 7.1 7.8 N.A. N.A. 6260 525 41 75 6.7 7.4 2400 27000 7175 525 88 S F 195 N.A. 7.3 2250 27000 5575 525 93 Run #2 C o l o u r S l u d g e F i n a l SIudge Volume (1) 2. 0 .225 % S o l i d s • 46 2.1 C.S. 0 11.00 11.1 135 18500 240 5.6 156 s i 32 8.7 8.9 950 N.A. 4700 248 71 S 2 44 8.0 8.2 750 N.A. 4620 250 68 S3 65 7.5 7.9 1050 N.A. 4545 250 68 S 4 85 6.5 7.5 1300 N.A. 3750 263 158 S F 205 N.A. 7.0 1400 28000 3790 263 192 * e x c e p t SF which i s pH o f u n f i l t e r e d f i n a l s u p e r n a t a n t a f t e r s l u d g e s e t t l i n g 157 .SLUDGE CARBONATION MONITORING DATA Run #3 C o l o u r S l u d g e F i n a l S l u d g e Volume (1) 2,0 ,5 % S o l i d s 1.7 3,83 * Sample Time pH o f S l u d g e pH o f T o t a l Conduct- + + + + min. i n C y l i n d e r F i l t r a t e A l k a l i n i t y i v i t y C o l o u r Mg Ca (CaCOo) yS c u . mg/1 mg/1 C S . 0 12.1 12.3 s l 16 10.0 10.4 S 2 36 9.0 9.2 S 3 64 8.0 8.2 S 4 90 7.2 7,7 S F 210 7.2 7.55 1775 70400 1450 3.64 344 1566 50100 11755 182 410 2318 51000 15270 298 250 3602 64200 13440 412 610 6077 98000 9470 470 1400 6578 90000 12825 552 1750 Run #4 "..: -•• C o l o u r S l u d g e F i n a l S l u d g e Volume, ,(1) 2.0 .4 % S o l i d s 1.2 3.45 C S . 0 11.9 12.1 1250 74000 810 4.7 510 s l 6 10.5 N.A. N.A. 42000 N.A. N.A. N.A. S 2 13 9.5 N.A. N.A. 47800 N.A. N.A. N.A. S 3 16 8,5 N.A. N.A. 51600 N.A. N.A. N.A. S 4 25 7.5 N.A. N.A. 61000 N.A. N.A. N.A. S F 205 7.3 7.3 N.A. 6100 11820 734 1240 * e x c e p t SF which i s pH o f u n f i l t e r e d f i n a l s u p e r n a t a n t a f t e r s l u d g e s e t t l i n g 158 SLUDGE CARBONATION MONITORING DATA Run #5 C o l o u r S l u d g e F i n a l S l u d g e Volume (1) 2.0 1.0 % S o l i d s 2.04 2.6 Sample * Time pH o f S l u d g e min. i n C y l i n d e r pH o f F i l t r a t e T o t a l A l k a l i n i t y ( C a C 0 3 ) Conduct-i v i t y liS C o l o u r c u . Mg mg/1 ea + " mg/' C.S. 0 12.1 12.1 1600 79500 900 2.6 390 S l 16 10.5 11.5 815 56000 1250 12.5 66 S 2 27 9.3 9.65 2445 40000 14500 700 200 S 3 47 8.5 8.60 4510 N.A.+ 17700 1000 340 S 4 60 8.0 820 5510 N.A.+ 17700 1150 540 S 5 80 7.5 7.95 6700 95000 16000 1200 980 S F 350 7.7 7.9 6830 91000 16000 1250 1050 Run #6 C o l o u r S l u d g e F i n a l S l u d g e Volume (1) 1.380 .160 % S o l i d s 4.03 31.1 C.S. 0 12.32 12.38 2050 10100 940 3.6 313 S l 16 10.5 10.9 1130 59000 4000 42 107 S 2 29 9.5 9.75 2130 60000 10150 320 135 S 3 38 8.2 8.45 4135 75000 12100 750 280 S 4 48 7.5 7.88 6580 80000 10150 900 775 S F 168 N.A. 7.7 5765 80000 12550 900 865 + C o n d u c t i v i t y probe p i u g g e d * e x c e p t SF which i s pH o f u n f i l t e r e d f i n a l s u p e r n a t a n t a f t e r s l u d g e s e t t l i n g 159 SLUDGE CARBONATION..MONITORING DATA Run #7 C o l o u r S l u d g e F i n a l S l u d g e Volume (1) 1,75 .490 % S o l i d s 1,26 3.19 Sample Time min. * pH o f S l u d g e i n C y l i n d e r pH o f F i l t r a t e T o t a l A l k a l i n i t y ( C a C 0 3 ) ' Conduct-^ i v i t y ' C o l o u r ' Mg yS c u . mg/1 ^ C a+ + mg/1 C.S. 0 11.4 11.4 180 18000 70 4.25 175 S l 17 10.5 10.75 125 15500 160 5.6 70 S 2 50 9.5 9.7 1150 21000 2800 312.5 70 S 3 69 8.4 8.45 3500 40000 7700 850 90 S 4 73 7.5 8.0 3500 43500 9000 950 100 S F 223 7.7 7.8 3650 43500 9400 960 135 Run #8 Volume (1) C o l o u r S l u d g e F i n a l S l u d g e 1.30 .350 % S o l i d s 2.24 6.33 C.S. 0 11.4 11.4 155 16000 100 4.75 93 S l 10 10.4 10.4 130 13000 460 37.5 75 S 2 21 9.5 9.65 1475 23000 3900 400 60 S 3 44 8.4 8.5 3525 40000 10600 900 90 S 4 55 7.5 7.8 3825 43000 13600 970 106 S F 175 7.6 7.8 4375 43000 13500 980 no A, * e x c e p t SF which i s pH o f u n f i l t e r e d f i n a l s u p e r n a t a n t a f t e r s l u d g e s e t t l i n g . 

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