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Improvements in the aerobic digestion of waste activated sludge through chemical control of mixed liquor… Anderson, Bruce Campbell 1989

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IMPROVEMENTS IN THE AEROBIC DIGESTION OF WASTE ACTIVATED SLUDGE THROUGH CHEMICAL CONTROL OF MIXED LIQUOR pH: PILOT-SCALE INVESTIGATIONS by BRUCE CAMPBELL ANDERSON B.Sc.(Biological Sciences), The University of Toronto, 1977 M.ASc.(Civi l Engineering), The University of British Columbia, 1982 A THESIS SUBM ITTED IN P A R T I A L F U L F I L 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 T H E D E G R E E O F D O C T O R O F P H I L O S O P H Y in T H E F A C U L T Y O F G R A D U A T E STUDIES (Department of Civi l Engineering) We accept this thesis as conforming to the required standard T H E U N I V E R S I T Y O F BRIT ISH C O L U M B I A December 1989 © Bruce Campbell Anderson, 1989 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of C i v i l E n g i n e e r i n g The University of British Columbia Vancouver, Canada Date December 5, 1989  DE-6 (2/88) ABSTRACT P i l o t - s c a l e a m b i e n t and l ow t e m p e r a t u r e r e s e a r c h i n t o t h e enhancement o f a e r o b i c d i g e s t i o n o f w a s t e a c t i v a t e d s l u d g e , t h r o u g h c o n t r o l o f m i x e d l i q u o r pH (MLpH), was p e r f o r m e d u s i n g an e x t e n d e d a e r a t i o n and 3 a h i g h r a t e w a s t e s l u d g e . To o f f s e t MLpH d e c r e a s e s e n c o u n t e r e d d u r i n g n i t r i f i c a t i o n , Ca (OH) 2 and NaHC0 3 were u s e d t o c o n t r o l MLpH i n t h e s e r i e s pH 6, 7 and 8. The p e r f o r m a n c e and b e h a v i o u r o f t h e d i g e s t e r s , u n d e r b o t h c o n t r o l l e d and u n c o n t r o l l e d MLpH c o n d i t i o n s , were m o n i t o r e d t h r o u g h p a r a m e t e r s r e l a t e d t o v o l a t i l e mass r e d u c t i o n , s l u d g e mass m e t a b o l i s m , q u a l i t y o f d i g e s t e d e n d -p r o d u c t and s o l u b l e c h a r a c t e r i s t i c s o f t h e d i g e s t e r e f f l u e n t . V o l a t i l e mass r e d u c t i o n was s i g n i f i c a n t l y a f f e c t e d by MLpH c o n t r o l , u n d e r c e r t a i n c o n d i t i o n s . Improvements i n r e d u c t i o n p e r f o r m a n c e o f >100% o v e r t h e u n c o n t r o l l e d c o n d i t i o n were n o t e d , d e p e n d i n g on s l u d g e o r i g i n ; howeve r , i t was c o n c l u d e d t h a t o n l y c e r t a i n t e m p e r a t u r e r ange s s h o u l d be t a r g e t e d f o r t h e most e f f e c t i v e u se o f MLpH c o n t r o l , s i n c e use o f MLpH c o n t r o l i n r a nge s w h e r e i n l i t t l e improvement w o u l d be r e a l i z e d was f e l t t o be u n e c o n o m i c a l . A n a l y s i s o f v o l a t i l e mass r e d u c t i o n r a t e s , b a s e d on a s e r i e s o f 1 day b a t c h c o n d i t i o n s ( n e c e s s i t a t e d by t he r e a c t o r f l o w scheme and t h e v a r i a b i l i t y o f t h e d i g e s t i o n p r o c e s s ) , d e m o n s t r a t e d t h a t r e a c t o r p e r f o r m a n c e o s c i l l a t e d a r o u n d a mean p e r f o r m a n c e v a l u e ; MLpH c o n t r o l a c t e d t o r e d u c e t h e s e o s c i l l a t i o n s , s u c h t h a t t h e d i g e s t e r s p e r f o r m e d c l o s e r t o t h e mean v a l u e more o f t h e t i m e . T e m p e r a t u r e s e n s i t i v i t y c o e f f i c i e n t s were q u i t e v a r i a b l e , and a s i n g l e v a l u e d i d n o t d e s c r i b e a l l s i t u a t i o n s . I t was p r o p o s e d t h a t 9 was i n f l u e n c e d by d i g e s t i o n s y s t e m , o p e r a t i n g t e m p e r a t u r e , s l u d g e t y p e and MLpH l e v e l . The u se o f t h i s c o e f f i c i e n t f o r d e t e r m i n i n g t h e o p e r a t i n g r ange s most s u i t a b l e f o r MLpH c o n t r o l was a d v a n c e d . i i The f a t e o f t h e n u t r i e n t s n i t r o g e n and pho spho ru s were g r e a t l y i n f l u e n c e d b y MLpH c o n t r o l . The u se o f C a ( 0 H ) 2 r e s u l t e d i n l e s s r e l e a s e o f p h o s p h o r u s f r o m t h e s o l i d p h a s e , w i t h s u b s e q u e n t l y l o w e f f l u e n t P 0 4 - P c o n c e n t r a t i o n s i n t h e n e u t r a l MLpH r a n g e . The d rawback o f t h i s r e a c t i o n was f o u n d t o be t h e p r o d u c t i o n o f i n e r t i n o r g a n i c s l u d g e s o l i d s , t h e r e b y i l l u s t r a t i n g t h e n e e d f o r a t r a d e - o f f be tween t h e v a r i o u s b e n e f i t s and d rawback s o f t h e enhanced d i g e s t i o n p r o c e s s . N i t r i f i c a t i o n p r o c e e d e d a t a l l t e m p e r a t u r e s , and i n c o n d i t i o n s p r e v i o u s l y t h o u g h t t o be i n h i b i t o r y t o t h e c h e m o l i t h o t r o p h i c o r g a n i s m s . D i g e s t e r e f f l u e n t q u a l i t y was i m p r o v e d t h r o u g h MLpH c o n t r o l , b u t s u b s t a n t i a l c o n c e n t r a t i o n s o f NO x-N were o b s e r v e d unde r some c o n d i t i o n s . B a s ed on d i r e c t c o m p a r i s o n s w i t h p r e v i o u s l a b - s c a l e r e s e a r c h , i t was c o n c l u d e d t h a t t he enhancement p r o c e s s h a d v e r y good p o t e n t i a l f o r i m p l e m e n t a t i o n a t t h e f u l l - s c a l e l e v e l , e i t h e r f o r t h e improvement o f e x i s t i n g u n d e r d e s i g n e d p r o c e s s e s , o r f o r t h e i n i t i a l d e s i g n o f more e f f i c i e n t a e r o b i c d i g e s t i o n f a c i l i t i e s . i i i TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i v L I ST OF TABLES . v i i L I ST OF FIGURES x i ACKNOWLEDGEMENTS x v GLOSSARY OF TERMS x v i 1. INTRODUCTION 1 1.1 P r o j e c t Need 1 1.2 R e s e a r c h A p p r o a c h and O b j e c t i v e s 4 2. OPERATING THEORY AND LITERATURE REVIEW 7 2.1 A e r o b i c D i g e s t i o n P r o c e s s D e s c r i p t i o n 7 2.2 B e n e f i t s / D e t r i m e n t s and P r o c e s s K i n e t i c s o f A e r o b i c D i g e s t i o n . .• , 13 2.3 R e c e n t R e s e a r c h I n A e r o b i c S l udge D i g e s t i o n 19 2.4 E f f e c t s o f E n v i r o n m e n t on A e r o b i c S l udge D i g e s t i o n . . . 25 2 .4 .1 pH L e v e l 26 2 . 4 . 2 T e m p e r a t u r e 28 2.5 P r e v i o u s R e s e a r c h i n t o T e m p e r a t u r e and pH E f f e c t s on A e r o b i c D i g e s t i o n 31 2.6 J u s t i f i c a t i o n o f P r e s e n t R e s e a r c h 43 3. EXPERIMENTAL SET-UP AND ANALYTICAL TECHNIQUES 45 3.1 Raw S l u d g e S o u r c e s 45 3.2 Raw S l u d g e C o l l e c t i o n P r o c e d u r e s and P r e p a r a t i o n . . . . 50 3 .2.1 E x t e n d e d A e r a t i o n S l udge 50 3 .2 .2 H i g h Ra te S l udge 51 3.3 L a b o r a t o r y Se tup 52 3.4 D i g e s t e r O p e r a t i o n 54 3.5 D i g e s t e r pH C o n t r o l 57 3.6 E x p e r i m e n t a l Sequence 59 3.7 A n a l y t i c a l and S a m p l i n g T e c h n i q u e s 61 3.8 Sample P r e s e r v a t i o n . . 73 3.9 S t a t i s t i c a l Methods 73 i v TABLE OF CONTENTS ( c o n t . ) Page 4 . RESULTS AND DISCUSSION 75 4.1 I n t r o d u c t i o n 75 4 .2 V a l i d i t y o f Compar i s on s be tween T e m p o r a l l y D i s c r e t e E x p e r i m e n t s 76 4 . 2 . 1 Raw S l udge Q u a l i t y - E x t e n d e d A e r a t i o n S l u d g e . . 77 4 . 2 . 2 Raw S l udge Q u a l i t y - H i g h R a t e S l udge 85 4 . 2 . 3 A e r o b i c D i g e s t e r M i x e d L i q u o r pH V a r i a b i l i t y . . 93 4 . 2 . 3 . 1 E x t e n d e d A e r a t i o n S l udge E x p e r i m e n t s . . 95 4 . 2 . 3 . 2 H i g h R a t e S l udge E x p e r i m e n t s 101 4 . 2 . 4 C o n c l u s i o n s 106 4 . 3 A e r o b i c D i g e s t i o n P e r f o r m a n c e : S o l i d s R e d u c t i o n and D i g e s t i o n K i n e t i c s . 107 4 . 3 . 1 V a r i a b l e s U sed t o D e s c r i b e P r o c e s s E f f i c i e n c y 107 4 . 3 . 2 V o l a t i l e Mass R e d u c t i o n 116 4 . 3 . 3 A n a l y s i s o f V a r i a n c e 143 4 . 3 . 4 Endogenous Decay C o e f f i c i e n t s 148 4 . 4 A e r o b i c D i g e s t i o n P e r f o r m a n c e : C h a r a c t e r i s t i c s o f D i g e s t e d S l udge 170 4 . 4 . 1 S o l i d s 170 4 . 4 . 2 C a r b o n , N i t r o g e n and Pho spho ru s 190 4 . 4 . 2 . 1 B iomass Ca rbon C o n t e n t and Remova l R a t e s 190 4 . 4 . 2 . 2 N i t r o g e n Forms and N i t r i f i c a t i o n . . . . 198 4 . 4 . 2 . 3 Pho spho ru s 218 4 . 4 . 3 S l udge Mass M e t a b o l i s m 230 4 . 4 . 3 . 1 D e f i n i t i o n s and A s s u m p t i o n s 230 4 . 4 . 3 . 2 A s s e s smen t o f S l udge Mass M e t a b o l i s m . . 234 4 .5 A e r o b i c D i g e s t i o n P e r f o r m a n c e : S o l u b l e C h a r a c t e r i s t i c s . 252 4 . 5 . 1 N i t r o g e n and Pho spho ru s 252 4 . 5 . 2 Ca rbon and D i s s o l v e d S o l i d s 258 4 .6 A e r o b i c D i g e s t i o n P e r f o r m a n c e : Summary 261 5. CONCLUSIONS AND RECOMMENDATIONS 271 5.1 C o n c l u s i o n s 271 5.2 Recommendat ions f o r F u r t h e r R e s e a r c h 282 REFERENCES 285 APPENDICES 296 A 297 - R e a c t o r and Raw S l udge TSS and TVSS D a t a 298 - D i s s o l v e d S o l i d s D a t a Summary 314 - M o v i n g A v e r a g e Mass B a l a n c e R e d u c t i o n D a t a 330 - Summary o f TVSS Mass R e d u c t i o n u s e d f o r ANOVA 339 v TABLE OF CONTENTS ( c o n t . ) Page APPENDICES ( c o n t . ) B 341 - Suspended S o l i d s D e t e r m i n a t i o n : Method C o m p a r i s o n . . . . 342 - Example o f V a r i a b i l i t y w i t h i n S o l i d s Measurements . . . . 345 C 350 - Summary o f O p e r a t i n g D a t a 351 C a r b o n 351 N i t r o g e n and Pho spho ru s 361 S p e c i f i c N i t r i f i c a t i o n R a t e s 386 I n d i c e s o f S l udge Mass M e t a b o l i s m 388 D 414 - SpOUR V a r i a b i l i t y o v e r a 24 h o u r p e r i o d 415 - SpOUR and B0D 5 as I n d i c a t o r s o f S l udge Mass A c t i v i t y . . 418 v i L IST OF TABLES T a b l e Page 3.1 Raw Waste S l udge C o m p a r i s o n 49 3.2 D i g e s t e r E x p e r i m e n t a l C o n d i t i o n s , E x t e n d e d A e r a t i o n S l udge 60 3.3 D i g e s t e r E x p e r i m e n t a l C o n d i t i o n s , H i g h R a t e S l u d g e 61 3.4 Sample P r e s e r v a t i o n and S t o r a g e T e c h n i q u e s 74 4.1 Raw S l u d g e C h a r a c t e r i s t i c s : E x t e n d e d A e r a t i o n S l u d g e , S o l u b l e P a r a m e t e r s 78 4 .2 Raw S l udge C h a r a c t e r i s t i c s : E x t e n d e d A e r a t i o n S l u d g e , S p e c i f i c B iomass P a r a m e t e r s 79 4 . 3 Raw S l udge C h a r a c t e r i s t i c s : E x t e n d e d A e r a t i o n S l u d g e , P h y s i c a l P a r a m e t e r s 80 4 . 4 Raw S l u d g e S o l i d s C h a r a c t e r i s t i c s : E x t e n d e d A e r a t i o n S l u d g e 80 4 .5 Raw S l udge C h a r a c t e r i s t i c s : H i g h R a t e S l u d g e , T e m p o r a l I n f l u e n c e s . . . . 86 4 .6 Raw S l udge C h a r a c t e r i s t i c s : H i g h R a t e S l u d g e , S o l u b l e P a r a m e t e r s 87 4 .7 Raw S l udge C h a r a c t e r i s t i c s : H i g h Ra te S l u d g e , S p e c i f i c B iomass P a r a m e t e r s 88 4 .8 Raw S l udge C h a r a c t e r i s t i c s : H i g h Ra te S l u d g e , P h y s i c a l P a r a m e t e r s 89 4 .9 Raw S l udge S o l i d s C h a r a c t e r i s t i c s : H i g h R a t e S l u d g e 89 4 .10 P e r c e n t TVSS R e d u c t i o n : E x t e n d e d A e r a t i o n S l u d g e , O v e r a l l Mass B a l a n c e B a s i s 117 4 .11 P e r c e n t TVSS R e d u c t i o n : E x t e n d e d A e r a t i o n S l u d g e , A v e r a g e M o v i n g Mass B a l a n c e B a s i s 118 4 .12 P e r c e n t TVSS R e d u c t i o n : H i g h R a t e S l u d g e , O v e r a l l Mass B a l a n c e B a s i s 119 4 . 13 P e r c e n t TVSS R e d u c t i o n : H i g h Ra te S l u d g e , A v e r a g e M o v i n g Mass B a l a n c e B a s i s 120 v i i L IST OF TABLES ( c o n t . ) T a b l e Page 4 . 1 4 A e r o b i c D i g e s t i o n S o l i d s R e d u c t i o n P e r f o r m a n c e C o m p a r i s o n : S e l e c t e d R e f e r e n c e s 130 4 .15 Raw S l udge L o a d i n g R a t e V a r i a b i l i t y 136 4 .16 A e r o b i c D i g e s t i o n P e r f o r m a n c e C o m p a r i s o n : E x t e n d e d A e r a t i o n S l u d g e , E f f e c t o f pH C o n t r o l on Mass R e d u c t i o n 139 4 .17 A e r o b i c D i g e s t i o n P e r f o r m a n c e C o m p a r i s o n : H i g h R a t e S l u d g e , E f f e c t o f pH C o n t r o l on Mass R e d u c t i o n 140 4 .18 A n a l y s i s o f V a r i a n c e - Example C a l c u l a t i o n 145 4 .19 A n a l y s i s o f V a r i a n c e - Summary 146 4 . 20 Summary o f Mean Endogenous Decay C o e f f i c i e n t s , E x t e n d e d A e r a t i o n S l udge 159 4 .21 Summary o f Mean Endogenous Decay C o e f f i c i e n t s , H i g h R a t e S l u d g e 160 4 .22 A e r o b i c S l u d g e D i g e s t i o n K i n e t i c Decay C o e f f i c i e n t s From P r e v i o u s R e s e a r c h . 163 4 .23 Summary o f Tempe ra tu re S e n s i t i v i t y C o e f f i c i e n t s (8) . . . 168 4 . 24 Changes i n N o n v o l a t i l e Suspended S o l i d s and R e q u i r e d C h e m i c a l Doses t o M a i n t a i n D i g e s t e r T a r g e t pH, E x t e n d e d A e r a t i o n S l udge . . 181 4 .25 Changes i n N o n v o l a t i l e Suspended S o l i d s and R e q u i r e d C h e m i c a l Doses t o M a i n t a i n D i g e s t e r T a r g e t pH, H i g h R a t e S l udge . 182 4 .26 C h a r a c t e r i s t i c s o f D i g e s t e d S l u d g e : C a r b o n , E x t e n d e d A e r a t i o n S l udge 191 4 .27 C h a r a c t e r i s t i c s o f D i g e s t e d S l u d g e : C a r b o n , H i g h R a t e S l udge 192 4 .28 C a r b o n Remova l Summary: COD B a s i s . . . 196 4 .29 N i t r o g e n D a t a Summary: E x t e n d e d A e r a t i o n S l u d g e 199 4 . 30 N i t r o g e n D a t a Summary: H i g h R a t e S l udge 200 4 .31 A l k a l i n i t y Con sumpt i on d u r i n g N i t r i f i c a t i o n 218 v i i i L I ST OF TABLES ( c o n t . ) T a b l e Page 4 .32 Pho spho ru s D a t a Summary: E x t e n d e d A e r a t i o n S l u d g e . . . . 219 4 . 33 Pho spho ru s D a t a Summary: H i g h R a t e S l udge 220 4 . 3 4 I n d i c e s o f S l udge Mass M e t a b o l i s m , E x t e n d e d A e r a t i o n S l u d g e 239 4 .35 I n d i c e s o f S l udge Mass M e t a b o l i s m , H i g h R a t e S l u d g e . . . 240 4 .36 S o l u b l e C h a r a c t e r i s t i c s , E x t e n d e d A e r a t i o n S l u d g e . . . . 254 4 .37 S o l u b l e C h a r a c t e r i s t i c s , H i g h R a t e S l udge 255 4 .38 Maximum C o n c e n t r a t i o n s o f S o l u b l e P a r a m e t e r s 256 4 .39 A e r o b i c D i g e s t i o n P e r f o r m a n c e Summary 263 A l S o l i d s D a t a , B I /20 -1 E x p e r i m e n t 298 A2 S o l i d s D a t a , B I /20 -2 E x p e r i m e n t 299 A3 S o l i d s D a t a , B I /20 -3 E x p e r i m e n t . . 300 A4 S o l i d s D a t a , B I/12 E x p e r i m e n t 301 A5 S o l i d s D a t a , B I/5 E x p e r i m e n t 303 A6 S o l i d s D a t a , SQ/20-1 E x p e r i m e n t 305 A7 S o l i d s D a t a , SQ/20-2 E x p e r i m e n t 306 A8 S o l i d s D a t a , SQ/12 E x p e r i m e n t 307 A9 S o l i d s D a t a , SQ/5 E x p e r i m e n t 309 A10 Raw S l u d g e S o l i d s D a t a , A l l E x p e r i m e n t s 311 A l l D i s s o l v e d S o l i d s D a t a Summary, A l l E x p e r i m e n t s 314 A12 P e r c e n t TVSS Mass R e d u c t i o n , M o v i n g A v e r a g e Mass B a l a n c e B a s i s , B I/20 -1 E x p e r i m e n t 330 A13 P e r c e n t TVSS Mass R e d u c t i o n , Mov ing. A ve r a ge Mass B a l a n c e B a s i s , B I /20 -2 E x p e r i m e n t . 331 A14 P e r c e n t TVSS Mass R e d u c t i o n , M o v i n g A v e r a g e Mass B a l a n c e B a s i s , B I /20 -3 E x p e r i m e n t . . . . . 332 i x L I ST OF TABLES ( c o n t . ) T a b l e Page A15 P e r c e n t TVSS Mass R e d u c t i o n , Mov i n g A v e r a g e Mass B a l a n c e B a s i s , B I/12 E x p e r i m e n t 333 A16 P e r c e n t TVSS Mass R e d u c t i o n , M o v i n g A v e r a g e Mass B a l a n c e B a s i s , B I/5 E x p e r i m e n t 334 A17 P e r c e n t TVSS Mass R e d u c t i o n , M o v i n g A v e r a g e Mass B a l a n c e B a s i s , SQ/20-1 E x p e r i m e n t 335 A18 P e r c e n t TVSS Mass R e d u c t i o n , M o v i n g A v e r a g e Mass B a l a n c e B a s i s , SQ/20-2 E x p e r i m e n t 336 A19 P e r c e n t TVSS Mass R e d u c t i o n , M o v i n g A ve r a ge Mass B a l a n c e B a s i s , SQ/12 E x p e r i m e n t 337 A20 P e r c e n t TVSS Mass R e d u c t i o n , M o v i n g A v e r a g e Mass B a l a n c e B a s i s , SQ/5 E x p e r i m e n t . . . 338 A21 Summary o f Mass R e d u c t i o n u s e d f o r A n a l y s i s o f V a r i a n c e . 339 B l S o l i d s V a l u e s f o r D e t e r m i n a t i o n o f A n a l y t i c a l P r e c i s i o n 345 CI Summary o f O p e r a t i n g D a t a : Ca rbon 351 C2 Summary o f O p e r a t i n g D a t a : N i t r o g e n and Pho spho ru s . . . . 361 C3 S p e c i f i c N i t r i f i c a t i o n R a t e s 386 C4 Summary o f O p e r a t i n g D a t a : I n d i c e s o f S l udge Mass M e t a b o l i s m 388 x L IST OF FIGURES F i g u r e Page 3.1 UBC P i l o t P l a n t C o n f i g u r a t i o n 47 3.2 P i l o t S c a l e D i g e s t e r S c h e m a t i c 53 4 .1 D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : ( B I /20 -1 E x p e r i m e n t ) 96 4 .2 D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : ( B I /20 -2 E x p e r i m e n t ) 96 4 .3 D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : ( B I /20 -3 E x p e r i m e n t ) 97 4 . 4 a D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (B I/12 E x p e r i m e n t ) 97 4 .4b D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (B I/12 E x p e r i m e n t ) 98 4 . 5 a D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (B I/5 E x p e r i m e n t ) 98 4 .5b D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (B I/5 E x p e r i m e n t ) 99 4 .6 D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (SQ/20-1 E x p e r i m e n t ) 103 4 . 7 D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (SQ/20-2 E x p e r i m e n t ) 103 4 . 8 a D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (SQ/12 E x p e r i m e n t ) 104 4 . 8b D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (SQ/12 E x p e r i m e n t ) 104 4 . 9 a D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (SQ/5 E x p e r i m e n t ) 105 4 .9b D i g e s t e r M i x e d L i q u o r pH V a r i a t i o n s : (SQ/5 E x p e r i m e n t ) 105 4 .10 S o l i d s Mass B a l a n c e i n A e r o b i c D i g e s t e r s 108 4 .11 D e t e r m i n a t i o n o f N o n d e g r a d a b l e VSS 112 4 .12 A b s o l u t e D i g e s t e r P e r f o r m a n c e C o m p a r i s o n , E x t e n d e d A e r a t i o n S l udge 123 x i L I ST OF FIGURES ( c o n t . ) F i g u r e Rage 4 . 13 A b s o l u t e D i g e s t e r P e r f o r m a n c e C o m p a r i s o n , H i g h R a t e S l u d g e 123 4 . 14 SRT and T e m p e r a t u r e E f f e c t s on D i g e s t e r P e r f o r m a n c e , E x t e n d e d A e r a t i o n S l udge 126 4 .15 SRT and T e m p e r a t u r e E f f e c t s on D i g e s t e r P e r f o r m a n c e , H i g h R a t e S l udge 126 4 .16 O v e r a l l Deg ree -Day s C o m p a r i s o n 127 4 .17 R e l a t i v e P e r f o r m a n c e C o m p a r i s o n , E x t e n d e d A e r a t i o n S l udge . . . . 141 4 .18 R e l a t i v e P e r f o r m a n c e C o m p a r i s o n , H i g h R a t e S l u d g e . . . . 141 4 .19 D a i l y K i n e t i c Decay C o e f f i c i e n t s , B I /20 -1 E x p e r i m e n t 151 4 .20 O r d e r e d K i n e t i c Decay C o e f f i c i e n t s , B I /20 -1 E x p e r i m e n t 151 4 .21 O r d e r e d K i n e t i c Decay C o e f f i c i e n t s , B I /20 -2 E x p e r i m e n t 154 4 .22 O r d e r e d K i n e t i c Decay C o e f f i c i e n t s , B I /20 -3 E x p e r i m e n t 154 4 .23 O r d e r e d K i n e t i c Decay C o e f f i c i e n t s , B I/12 E x p e r i m e n t 155 4 . 24 O r d e r e d K i n e t i c Decay C o e f f i c i e n t s , B I/5 E x p e r i m e n t 155 4 .25 O r d e r e d K i n e t i c Decay C o e f f i c i e n t s , SQ/20-1 E x p e r i m e n t 156 4 .26 O r d e r e d K i n e t i c Decay C o e f f i c i e n t s , SQ/20-2 E x p e r i m e n t 156 4 .27 O r d e r e d K i n e t i c Decay C o e f f i c i e n t s , SQ/12 E x p e r i m e n t 157 4 .28 O r d e r e d K i n e t i c Decay C o e f f i c i e n t s , SQ/5 E x p e r i m e n t 157 4 .29 E f f e c t o f T e m p e r a t u r e and pH on Decay C o e f f i c i e n t s , E x t e n d e d A e r a t i o n S l udge . 161 x i i L I ST OF FIGURES ( c o n t . ) F i g u r e Page 4 . 30 E f f e c t o f T e m p e r a t u r e and pH on Decay C o e f f i c i e n t s , H i g h R a t e S l udge 161 4 .31 T e m p e r a t u r e S e n s i t i v i t y C o e f f i c i e n t s , E x t e n d e d A e r a t i o n S l u d g e , C a ( 0 H ) 2 C o n t r o l 166 4 .32 T e m p e r a t u r e S e n s i t i v i t y C o e f f i c i e n t s , E x t e n d e d A e r a t i o n S l u d g e , NaHC0 3 C o n t r o l 166 4 .33 T e m p e r a t u r e S e n s i t i v i t y C o e f f i c i e n t s , H i g h R a t e S l u d g e , C a ( 0 H ) 2 C o n t r o l 167 4 . 34 T e m p e r a t u r e S e n s i t i v i t y C o e f f i c i e n t s , H i g h R a t e S l u d g e , NaHC0 3 C o n t r o l 167 4 .35 D i g e s t e r and Raw S l udge TVSS, B I /20 -1 E x p e r i m e n t 171 4 .36 D i g e s t e r and Raw S l udge TVSS, B I /20 -2 E x p e r i m e n t 171 4 .37 D i g e s t e r and Raw S l udge TVSS, B I /20 -3 E x p e r i m e n t 172 4 .38 D i g e s t e r and Raw S l udge TVSS, B I/12 E x p e r i m e n t 172 4 .39 D i g e s t e r and Raw S l udge TVSS, BI/5 E x p e r i m e n t 173 4 . 40 D i g e s t e r and Raw S l udge TVSS, SQ/20-1 E x p e r i m e n t 173 4 .41 D i g e s t e r and Raw S l udge TVSS, SQ/20-2 E x p e r i m e n t 174 4 .42 D i g e s t e r and Raw S l udge TVSS, SQ/12 E x p e r i m e n t 174 4 .43 D i g e s t e r and Raw S l udge TVSS, SQ/5 E x p e r i m e n t 175 4 . 44 M i x e d L i q u o r V o l a t i l e F r a c t i o n , E x t e n d e d A e r a t i o n S l u d g e 177 4 .45 M i x e d L i q u o r V o l a t i l e F r a c t i o n , H i g h R a t e S l udge 177 4 .46 Changes i n NVSS R e l a t e d t o C h e m i c a l Dose, E x t e n d e d A e r a t i o n S l udge 184 4 .47 Changes i n NVSS R e l a t e d t o C h e m i c a l Dose, H i g h R a t e S l u d g e 184 4 .48 C h e m i c a l Dose t o M a i n t a i n T a r g e t pH, E x t e n d e d A e r a t i o n S l u d g e 187 4 .49 C h e m i c a l Dose t o M a i n t a i n T a r g e t pH, H i g h R a t e S l u d g e 187 x i i i L I ST OF FIGURES ( c o n t . ) F i g u r e Page 4 . 50 E f f e c t s o f pH and T e m p e r a t u r e on N i t r i f i c a t i o n R a t e , E x t e n d e d A e r a t i o n S l udge 210 4 .51 E f f e c t s o f pH and Tempe ra tu re on N i t r i f i c a t i o n R a t e , H i g h R a t e S l udge 210 4 .52 E f f e c t s o f pH and T e m p e r a t u r e on N i t r i f i c a t i o n Amount, E x t e n d e d A e r a t i o n S l udge 215 4 .53 E f f e c t s o f pH and Tempe ra tu re on N i t r i f i c a t i o n Amount, H i g h R a t e S l udge 215 4 . 54 E f f e c t s o f pH and T e m p e r a t u r e on Pho spho ru s R e l e a s e , E x t e n d e d A e r a t i o n S l udge 226 4 .55 E f f e c t s o f pH and T e m p e r a t u r e on Pho spho ru s R e l e a s e , H i g h R a t e S l udge 226 4 .56 E f f e c t s o f pH and T e m p e r a t u r e on A c t i v i t y (SpOUR), E x t e n d e d A e r a t i o n S l udge 244 4 .57 E f f e c t s o f pH and T e m p e r a t u r e on A c t i v i t y (SpOUR), H i g h R a t e S l udge 244 4 .58 E f f e c t s o f pH and T e m p e r a t u r e on A c t i v i t y ( ATP ) , E x t e n d e d A e r a t i o n S l udge 247 4 .59 E f f e c t s o f pH and Tempe ra tu re on A c t i v i t y ( ATP ) , H i g h R a t e S l u d g e 247 4 . 60 G e n e r a l T r end s i n S o l u b l e Components , N i t r o g e n and P h o s p h o r u s , B o t h S l u d g e s 256 4 .61 P e r f o r m a n c e C o m p a r i s o n R e l a t i v e t o C h e m i c a l Dose, E x t e n d e d A e r a t i o n S l udge 268 4 .62 P e r f o r m a n c e C o m p a r i s o n R e l a t i v e t o C h e m i c a l Dose, H i g h R a t e S l udge 268 D l ( a - c ) SpOUR V a r i a b i l i t y o v e r 24 H o u r s , E x t e n d e d A e r a t i o n S l u d g e 415 t o 417 D2 SpOUR and B0D 5 as I n d i c a t o r s o f S l udge Mass A c t i v i t y , E x t e n d e d A e r a t i o n S l udge 418 D3 SpOUR and B0D 5 as I n d i c a t o r s o f S l udge Mass A c t i v i t y , H i g h R a t e S l udge 418 x i v ACKNOWLEDGEMENTS The a u t h o r w o u l d l i k e t o e x p r e s s h i s g r a t i t u d e t o t h e many i n d i v i d u a l s who have a s s i s t e d h i m i n t h e e x e c u t i o n and c o m p l e t i o n o f t h i s r e s e a r c h p r o g r am. F i r s t , I t h a n k my w i f e P a t r i c i a , whose u n d e r s t a n d i n g and encou ragement p r o v i d e d t h e i d e a l e n v i r o n m e n t i n w h i c h t o c o m p l e t e t h i s d i s s e r t a t i o n . I am v e r y g r a t e f u l t o my r e s e a r c h s u p e r v i s o r , D r . D.S. M a v i n i c , f o r h i s encou ragement o f my e n r o l m e n t i n t h e Ph.D. p r og r am i n C i v i l E n g i n e e r i n g , and f o r h i s c o n t i n u i n g h e l p , a d v i s e and g e n e r a l u n d e r s t a n d i n g d u r i n g t h e e x t e n d e d p e r i o d o f t h i s r e s e a r c h . Thanks a l s o go t o t h e members o f my r e s e a r c h c o m m i t t e e , D r . W.K. Oldham, D r . K . J . H a l l and D r . D.G. K i l b u r n , f o r t h e i r d i r e c t i o n and c o n s i d e r a b l e t e c h n i c a l a d v i s e i n t h e f o r m u l a t i o n and e x e c u t i o n o f t h e r e s e a r c h p r o g r a m . I w o u l d l i k e t o t h a n k t h e many g r a d u a t e s t u d e n t s who have h e l p e d , i n some c a p a c i t y , d u r i n g t h i s r e s e a r c h . S p e c i a l g r a t i t u d e must be e x t e n d e d t o my " s l u d g e b u d d i e s " , C h r i s J e n k i n s and C r a i g P e d d i e , who know e x a c t l y what r e s e a r c h d e a l i n g w i t h s l u d g e i s a l l a b o u t , and who were i n v a l u a b l e i n d i s c u s s i o n s i n v o l v i n g t h e v a r i o u s p r o j e c t s o n g o i n g i n t h e S l u d g e Management G roup . As w e l l , I w o u l d l i k e t o e x t e n d my s i n c e r e g r a t i t u d e t o P a t Co leman, f o r h i s h e l p i n t h e s t a t i s t i c a l a n a l y s i s o f t h e d a t a . Susan L i p t a k , P a u l a P a r k i n s o n , Sue J a s p e r and t h e s t a f f o f t h e E n v i r o n m e n t a l E n g i n e e r i n g L a b o r a t o r y were a l w a y s a v a i l a b l e f o r d i s c u s s i o n s o f a t e c h n i c a l o r p e r s o n a l n a t u r e , and I t h a n k them f o r t h a t . I w o u l d a l s o l i k e t o t h a n k t h e o p e r a t i n g p e r s o n n e l a t t he t r e a t m e n t p l a n t s u s e d i n t h i s r e s e a r c h ( t h e U .B .C . p i l o t - p l a n t , t h e C e n t r a l S . T . P . i n Squami sh , and t h e A g a s s i z S . T . P . ) . As w e l l , g r a t i t u t e i s e x t e n d e d t o A l e x B i r k b e c k and t h e t e c h n i c a l s t a f f a t B.C. R e s e a r c h , f o r t h e i r h e l p i n sample a n a l y s i s . F i n a l l y , t h e a u t h o r w o u l d l i k e t o e x p r e s s h i s g r a t i t u d e t o t h e N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l o f Canada, t h e B r i t i s h C o l u m b i a S c i e n c e C o u n c i l and t h e H.R. M a c M i l l a n f a m i l y , f o r t h e i r f i n a n c i a l a s s i s t a n c e i n s u p p o r t o f t h i s r e s e a r c h . x v GLOSSARY OF TERMS ATP a d e n o s i n e - 5 ' - t r i p h o s p h a t e ANOVA a n a l y s i s o f v a r i a n c e B0D 5 b i o c h e m i c a l oxygen demand ( 5 - d a y ) BVSS b i o d e g r a d a b l e v o l a t i l e s u spended s o l i d s B i o - P b i o l o g i c a l n u t r i e n t r e m o v a l p r o c e s s °C d e g r e e s C e l s i u s COD c h e m i c a l oxygen demand d day h h o u r HRT h y d r a u l i c r e t e n t i o n t i m e L l i t r e x mean v a l u e fig m i c r o g r a m mg m i l l i g r a m NVSS . n o n v o l a t i l e s u spended s o l i d s ( a l s o c a l l e d FSS, f i x e d s u spended s o l i d s ) n number o f measurements OUR, DOUR d i s s o l v e d oxygen u p t a k e r a t e raw s l u d g e , w a s t e s l u d g e . . . u n s t a b i l i z e d , u n d i g e s t e d s l u d g e t a k e n d i r e c t l y f r o m t h e t r e a t m e n t p r o c e s s DO . . r e s i d u a l d i s s o l v e d oxygen SRT . . . . . s o l i d s r e t e n t i o n t i m e ( s l u d g e age) SpOUR s p e c i f i c oxygen u p t a k e r a t e s s t a n d a r d d e v i a t i o n T IC t o t a l i n o r g a n i c c a r b o n x v i GLOSSARY OF TERMS ( c o n t . ) TKN t o t a l K j e l d a h l n i t r o g e n TOC t o t a l o r g a n i c c a r b o n TP t o t a l pho spho ru s TSS t o t a l s u spended s o l i d s TVSS t o t a l v o l a t i l e s u spended s o l i d s LR TVSS l o a d i n g r a t e k d TVSS k i n e t i c r e d u c t i o n c o e f f i c i e n t (base e) UBC 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 x v i i CHAPTER 1 INTRODUCTION 1.1 P r o j e c t Need Today , t h e r e m o v a l o f o r g a n i c and/o r i n o r g a n i c c o n s t i t u e n t s f r o m d o m e s t i c and i n d u s t r i a l w a s t e w a t e r s c a n be a c c o m p l i s h e d i n a v a r i e t y o f ways , i . e . p h y s i o - c h e m i c a l l y ; b i o l o g i c a l l y ; a c o m b i n a t i o n o f schemes, o r s p e c i a l i z e d i n n o v a t i v e t e c h n o l o g y . A l l o f t h e s e c a n be e n g i n e e r e d t o p e r f o r m w i t h a h i g h d e g r e e o f e f f i c i e n c y , and r e s u l t i n r e l a t i v e l y c l e a n e f f l u e n t s t h a t a r e s u i t a b l e f o r d i s c h a r g e t o t h e e n v i r o n m e n t . However, t h e m a t e r i a l s removed d u r i n g t h e t r e a t m e n t p r o c e s s a r e u s u a l l y c o n t a i n e d w i t h i n a s l u d g e w h i c h must be f u r t h e r t r e a t e d b e f o r e i t t o o c a n be d i s c h a r g e d . A c c o r d i n g t o a r e c e n t r e p o r t by t h e Wate r P o l l u t i o n C o n t r o l F e d e r a t i o n t o a U n i t e d S t a t e s c o n g r e s s i o n a l s ubcommi t t ee i n v e s t i g a t i n g t h e r e s e a r c h p rog rams o f t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency , one o f t h e most c r i t i c a l a r e a s o f w a t e r p o l l u t i o n c o n t r o l , i n w h i c h t h e r e i s a c o n t i n u e d need f o r r e s e a r c h , i s t he t r e a t m e n t and d i s p o s a l o f m u n i c i p a l s l u d g e s ( G l o y n a , 1 982 ) . The a u t h o r e m p h a s i z e d t h i s p o i n t by s u g g e s t i n g t h a t t h e p r o b l e m o f s l u d g e management i s l a r g e l y u n r e s o l v e d , and f u r t h e r s t a t e s t h a t t h e r e i s a l a c k o f i n f o r m a t i o n a t e ven t h e most b a s i c l e v e l s o f s l u d g e c h a r a c t e r i z a t i o n , i . e . t h e f u n d a m e n t a l p r o p e r t i e s o f s l u d g e s and t he f a c t o r s t h a t i n f l u e n c e them. From t h e r e p o r t , i t i s s t a t e d t h a t s l u d g e quantities have i n c r e a s e d i n t h e r e c e n t p a s t , b u t s l u d g e quality has d e c r e a s e d as a d i r e c t r e s u l t o f t he e x i s t i n g and i m p r o v i n g l e v e l s o f w a s t e w a t e r t r e a t m e n t ; as w e l l , t h e o p t i o n s f o r s l u d g e d i s p o s a l have been s e v e r e l y r e s t r i c t e d due t o r e g u l a t i o n s e n a c t e d t o p r o t e c t t h e e n v i r o n m e n t f r o m t h e s e k i n d s o f u n d e s i r a b l e r e s i d u a l m a t e r i a l s . As an example o f t h e need f o r f u r t h e r r e s e a r c h i n t o t h e p r o p e r t i e s o f t h e s e w a s t e w a t e r t r e a t m e n t r e s i d u a l s , and f u r t h e r t o t h e above r e p o r t , D i c k 2 (1982) p r o p o s e d an e x t e n s i v e l i s t o f t h e r e s e a r c h a r e a s he f e l t r e q u i r e d t he most immed i a t e a t t e n t i o n , i n o r d e r t o b o t h b e t t e r u n d e r s t a n d and b e t t e r d e a l w i t h t h e s e s l u d g e s . Among t h e p r o p o s e d a r e a s o f r e s e a r c h w e r e : 1. F u n d a m e n t a l s t u d i e s o f t h e s u r f a c e p r o p e r t i e s o f s l u d g e p a r t i c l e s , 2. A s u r v e y o f t h e s o u r c e s o f h e a v y m e t a l s and t o x i c o r g a n i c s i n t h e s l u d g e , 3. The i n f l u e n c e o f r e t u r n s t r e ams f r o m s l u d g e t r e a t m e n t on w a s t e w a t e r t r e a t m e n t p l a n t o p e r a t i o n s , 4 . P r e d i c t i o n o f t h i c k e n i n g and d e w a t e r i n g p e r f o r m a n c e t h r o u g h f u n d a m e n t a l and r e a d i l y m e a s u r a b l e p r o p e r t i e s , 5. R e l a t i n g m o l e c u l a r s t r u c t u r e and e n v i r o n m e n t a l c o n d i t i o n s t o t h e p e r f o r m a n c e o f b i o l o g i c a l s t a b i l i z a t i o n , 6. E p i d e m i o l o g i c a l e v i d e n c e t o a s s e s s t h e a d v i s a b i l i t y o f u s i n g s p e c i f i c p r o c e s s e s f o r i n a c t i v a t i o n o f v i r u s e s and p a t h o g e n s i n s l u d g e s , 7. Methods f o r d e t o x i f y i n g s l u d g e s , 8. The h a r v e s t i n g o f u s e f u l c o n s t i t u e n t s f r o m t h e t r e a t m e n t r e s i d u a l s , 9. Improvement i n p r o c e s s p e r f o r m a n c e mode l s and c o s t d a t a m o d e l s , and 10. Deve lopment o f r e l a t i v e r i s k a s s e s smen t m e t h o d o l o g i e s f o r t h e t r e a t m e n t o f t h e s e r e s i d u a l m a t e r i a l s . The b a s i c i m p o r t a n c e o f t he i n c o r p o r a t i o n o f s l u d g e t r e a t m e n t and d i s p o s a l i n t o t h e o v e r a l l w a s t e w a t e r t r e a t m e n t scheme c a n n o t be o v e r e m p h a s i z e d . U n f o r t u n a t e l y , u n t i l r e c e n t l y most o f t h e a t t e n t i o n o f r e s e a r c h e r s has been d i r e c t e d t o w a r d t h e t r e a t m e n t o f t h e l i q u i d i n f l u e n t s t r e a m , and as i n d i c a t e d b y t h e above l i s t o f r e s e a r c h n e e d s , n o t enough has b e e n done as f a r as s t a b i l i z a t i o n and u l t i m a t e d i s p o s a l o f t h e t r e a t m e n t r e s i d u a l s a r e c o n c e r n e d . D i c k (1974) s u g g e s t e d t h a t t h e p r o b l e m s o f s l u d g e 3 t r e a t m e n t and d i s p o s a l f a i l e d t o t i t i l l a t e t h e i m a g i n a t i o n o f r e s e a r c h e r s o r t o a t t r a c t t h e a t t e n t i o n o f d e s i g n e n g i n e e r s , a p r o b l e m t h a t he saw w o r s e n i n g i n t h e f u t u r e as a r e s u l t o f more s t r i n g e n t c o n t r o l s on w a s t e w a t e r t r e a t m e n t p l a n t d i s c h a r g e s . O t h e r s s h a r e t h e o p i n i o n o f a d e t e r i o r a t i n g s i t u a t i o n , w h i l e a t t h e same t i m e c a l l i n g f o r i n c r e a s e d r e s e a r c h and g o v e r n m e n t a l i n f l u e n c e t o a l l e v i a t e t h e p r o b l e m ( J o h n s o n , 1979; K a l i n s k e , 1978; C h r i s t e n s e n , 1 9 8 2 ) . The i m p o r t a n c e o f t h e n e e d f o r i m p r o v e d r e s i d u a l s management and d i s p o s a l i s f u r t h e r e m p h a s i z e d when t h e economic c o n s i d e r a t i o n s o f s l u d g e a r e t a k e n i n t o a c c o u n t . R a n d a l l et al. (1971) s u g g e s t e d t h a t t h e d a i l y c o s t s o f s l u d g e d i s p o s a l may e x c e e d t h e c o s t o f any o t h e r s i n g l e p r o c e s s i n t he t r e a t m e n t p l a n t . More s p e c i f i c a l l y , H a s i t and V e s i l i n d (1981) and H a s i t et al. ( 1 9 8 1 ) , q u o t i n g a r e p o r t p u b l i s h e d by t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency , s t a t e t h a t t h e c o s t s o f s l u d g e management ( i n c l u d i n g s l u d g e h a n d l i n g w i t h i n a p l a n t , t r a n s p o r t o f s l u d g e s t o a d i s p o s a l s i t e and u l t i m a t e d i s p o s a l o f t h e r e s i d u e ) c o m p r i s e 35% o f t h e c a p i t a l c o s t s and 55% o f t h e a n n u a l o p e r a t i o n and m a i n t e n a n c e c o s t s o f a w a s t e w a t e r t r e a t m e n t p l a n t . R e c e n t l y , Evans and F i l m a n (1988) c a l c u l a t e d t h a t t h e s l u d g e h a n d l i n g c o s t s i n a number o f O n t a r i o w a s t e w a t e r t r e a t m e n t p l a n t s a c c o u n t e d f o r 50-60% o f o v e r a l l o p e r a t i o n and m a i n t e n a n c e c o s t s , e x c l u s i v e o f t h e f i n a l c o s t s o f d i s p o s a l . U s i n g t h e s e e s t i m a t e s , i t w o u l d seem l o g i c a l t h a t t h e a v e r a g e r e s i d u a l s management s y s t e m s h o u l d be as w e l l d e s i g n e d as p o s s i b l e , i n o r d e r t o m i n i m i z e c o s t s ; s u c h i s n o t g e n e r a l l y t h e c a s e , howeve r . I t i s an economic r e a l i t y i n t he s m a l l e r c o m m u n i t i e s t h a t b u d g e t a r y c o n s t r a i n t s w i l l d i c t a t e t he deg ree o f r e s i d u a l s management, and a l l t o o o f t e n , t h e s e r e s i d u a l s a r e s i m p l y l e f t t o d e a l w i t h t h e m s e l v e s . L i m i t a t i o n s i n o p e r a t i n g f und s u s u a l l y mean t h a t t h e l i q u i d e f f l u e n t w i l l r e c e i v e t he most 4 a t t e n t i o n , and t h e r e s u l t a n t s l u d g e w i l l be d e a l t w i t h when t i m e and money a l l o w . T h e r e f o r e , a l t h o u g h s l u d g e management r e s e a r c h does have need o f t h e o r e t i c a l r a t i o n a l d e s i g n and m o d e l l i n g s y s t e m s , s u c h as t h a t p r e s e n t e d by R i c h ( 1 9 8 7 ) , i t i s i m p o r t a n t t h a t t h i s t y p e o f r e s e a r c h a l s o t a k e i n t o a c c o u n t t h e p r a c t i c a l and economic r e a l i t i e s o f t h e s m a l l e r c o m m u n i t i e s i n any a s s e s s m e n t o f s l u d g e d i s p o s a l p r a c t i c e s . 1.2 R e s e a r c h A p p r o a c h and O b j e c t i v e s W i t h t h e above d i s c u s s i o n i n m i n d , t h e p r e s e n t p r o j e c t was u n d e r t a k e n t o examine methods o f i m p r o v i n g t he o p e r a t i o n o f one a s p e c t o f r e s i d u a l s management, t h a t b e i n g a e r o b i c d i g e s t i o n o f wa s t e a c t i v a t e d s l u d g e . An o n -g o i n g r e s e a r c h p rog r am a t t he 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 s c o n c e r n e d w i t h t h e improvement o f v a r i o u s k i n d s o f wa s t e s l u d g e s t a b i l i z a t i o n , s u c h t h a t t h e y c a n be r e a d i l y and c o s t e f f e c t i v e l y i n t e g r a t e d i n t o t he o v e r a l l t r e a t m e n t scheme, r a t h e r t h a n b e i n g , as d e s c r i b e d p r e v i o u s l y , an unwelcome b u r d e n t o be d e a l t w i t h a t t h e end o f t h e p r o c e s s t r a i n . S p e c i f i c a l l y , t h e p r e s e n t r e s e a r c h examined t h e b e h a v i o u r o f a number o f wa s t e b i o l o g i c a l s l u d g e s ( f r o m t h e a c t i v a t e d s l u d g e p r o c e s s ) , when t h e m i x e d l i q u o r pH i n t he d i g e s t e r was a r t i f i c i a l l y r a i s e d t o l e v e l s above t h o s e u s u a l l y f o u n d i n t h e f i e l d s i t u a t i o n . The e f f i c i e n c y o f t h e d i g e s t i o n p r o c e s s i s u s u a l l y e v a l u a t e d by t he mass o f v o l a t i l e b i o l o g i c a l m a t e r i a l removed f r o m t h e s l u d g e upon d i g e s t i o n , t h r o u g h t h e a u t o o x i d a t i o n o f t h i s mass d u r i n g endogenous r e s p i r a t i o n . As t h i s p r o c e e d s , t h e pH o f t he d i g e s t i n g mass i s r e d u c e d t h r o u g h b o t h t h e c o n s u m p t i o n o f a l k a l i n i t y and t h e p r o d u c t i o n o f weak a c i d s (as a d i r e c t r e s u l t o f n i t r i f i c a t i o n o f t h e ammonia l i b e r a t e d d u r i n g endogenous r e s p i r a t i o n ) . I n a w e a k l y b u f f e r e d s y s t e m , t h e s e o n g o i n g r e a c t i o n s c a n r e s u l t i n a n o r m a l 5 d i g e s t i o n m i x e d l i q u o r pH as l ow as pH 3. However, s i n c e t h e d i g e s t i n g mass i s b i o l o g i c a l i n n a t u r e , i t was r e a s o n e d t h a t t h i s l o w e n v i r o n m e n t a l pH c o u l d p r o v e d e t r i m e n t a l t o t h e o r g a n i s m s i n q u e s t i o n , and a more n e u t r a l pH l e v e l w o u l d p r o v i d e a b e t t e r e n v i r o n m e n t f o r t h e o x i d a t i v e m i c r o o r g a n i s m s ; t h i s i n t u r n c o u l d l e a d t o enhanced p r o c e s s e f f i c i e n c y . T h i s h y p o t h e s i s i s c o n t r a r y t o t h e f i n d i n g s o f some r e s e a r c h e r s , who s u g g e s t t h a t t h e d i g e s t i o n p r o c e s s i s u n h i n d e r e d a t l ow e n v i r o n m e n t a l pH l e v e l s ( Lawton and Norman, 1964; K o e r s , 1 9 7 9 ) . A t p r e s e n t , t h e r e a r e a t l e a s t 20 a e r o b i c d i g e s t e r s ( t r e a t i n g wa s t e a c t i v a t e d s l u d g e ) i n u se i n t he p r o v i n c e o f B r i t i s h C o l u m b i a f o r t he t r e a t m e n t and s t a b i l i z a t i o n o f wa s t e s l u d g e , s e r v i n g an e s t i m a t e d p o p u l a t i o n o f 150,000 ( M i n i s t e r o f S u p p l y and S e r v i c e s Canada, 1981 ) . P r o b l e m s i n t h e o p e r a t i o n o f t h e s e u n i t s a r e common, due e i t h e r t o i n i t i a l u n d e r s i z i n g o f t h e a e r a t i o n b a s i n i t s e l f , o r i n e f f i c i e n c y i n t h e d i g e s t i o n p r o c e s s ( D a l e W e t t e r , Waste Management B r a n c h , B.C. M i n i s t r y o f t h e E n v i r o n m e n t ; p e r s o n a l c o m m u n i c a t i o n ) . W i t h t h e p r e s e n t r e s e a r c h , t h e a u t h o r hoped t o a d d r e s s and c o r r e c t t h e s e p r o b l e m s i n a c o s t e f f e c t i v e manner. I t i s assumed t h a t t h e number o f a e r o b i c d i g e s t e r s w i l l i n c r e a s e i n t he p r o v i n c e (and c o u n t r y as a w h o l e ) i n t he n e a r f u t u r e , as t he need f o r e f f i c i e n t s l u d g e t r e a t m e n t becomes more p r e s s i n g . The b e n e f i t s f r o m t h i s r e s e a r c h may be e x p e c t e d t o a c c r u e i n a v a r i e t y o f a r e a s , e . g . a more f u l l y o x i d i z e d , s t a b l e end p r o d u c t may be p r o d u c e d ; a l o w e r s l u d g e mass may r e s u l t i n c o s t s a v i n g s b o t h i n i n i t i a l c o n s t r u c t i o n and f i n a l d i s p o s a l ; and an improvement i n d i g e s t e r s u p e r n a t a n t q u a l i t y may o c c u r . Th rough t h i s r e s e a r c h , t h e a u t h o r hoped t o d e m o n s t r a t e t h e a p p l i c a b i l i t y and u s e f u l n e s s o f t h e enhancement o f t h e a e r o b i c d i g e s t i o n o f w a s t e a c t i v a t e d s l u d g e t h r o u g h c o n t r o l o f d i g e s t e r m i x e d l i q u o r pH. 6 The b a s 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 p rog r am may t h e r e f o r e be s ummar i zed as f o l l o w s : 1. To i n v e s t i g a t e t h e f e a s i b i l i t y o f i m p r o v i n g t h e a e r o b i c d i g e s t i o n o f w a s t e a c t i v a t e d s l u d g e , as a s s e s s e d by a number o f p h y s i c a l and b i o l o g i c a l p a r a m e t e r s , t h r o u g h t h e c o n t r o l o f m i x e d l i q u o r pH l e v e l s i n t h e d i g e s t e r a t t h e p i l o t - s c a l e l e v e l ; 2. To compare t h e r e l a t i v e b e n e f i t s and d e t r i m e n t s o f t h e u se o f two s p e c i f i c pH c o n t r o l l i n g c h e m i c a l s i n t h e enhancement p r o c e s s ; 3. To s t u d y t h e b e h a v i o u r o f wa s t e s l u d g e f r o m a b i o l o g i c a l n u t r i e n t r e m o v a l a c t i v a t e d s l u d g e p r o c e s s u n d e r t h e c o n d i t i o n s o f t h i s r e s e a r c h , and t o compare t h i s p e r f o r m a n c e w i t h a more common s h o r t r e t e n t i o n t i m e wa s t e s l u d g e ( w i t h p a r t i c u l a r emphas i s on t h e changes i n s l u d g e s o l i d s f r a c t i o n s and t h e f a t e o f t h e n u t r i e n t s n i t r o g e n and p h o s p h o r u s ) ; 4 . To s t u d y t he e f f e c t s o f l ow o p e r a t i n g t e m p e r a t u r e s on t h e p r o c e s s o f a e r o b i c s l u d g e d i g e s t i o n , and t h e i n f l u e n c e o f m i x e d l i q u o r pH c o n t r o l i n t h e s e s u b - o p t i m a l c o n d i t i o n s ; and 5. To compare t h e r e s u l t s o f t h i s r e s e a r c h w i t h t h o s e f r om a s m a l l e r l a b o r a t o r y - s c a l e e x p e r i m e n t , i n an a t t e m p t t o a s s e s s t h e s c a l e - u p p o t e n t i a l o f t h e enhanced d i g e s t i o n p r o c e s s . 7 CHAPTER 2 OPERATING THEORY AND LITERATURE REVIEW T h e r e has b e e n e x t e n s i v e work done i n t h e f i e l d o f a e r o b i c d i g e s t i o n o f w a s t e w a t e r s l u d g e s , and d e t a i l e d r e v i e w s o f t h e h i s t o r y and deve l opmen t o f t h e p r o c e s s , a l o n g w i t h t h e i n f l u e n c e o f s e v e r a l p r o c e s s v a r i a b l e s , c a n be f o u n d e l s e w h e r e ( P a g o r i a , 1977; K o e r s , 1979; Sanchez B a i n , 1982; J e n k i n s , 1 988 ) . No s u c h r e v i e w i s a t t e m p t e d h e r e i n , g i v e n t h e numerous and d i v e r s e t y p e s o f d i g e s t e r o p e r a t i o n s and raw s l u d g e s o u r c e s one e n c o u n t e r s i n t h e f i e l d ; r a t h e r , t h i s c h a p t e r w i l l p r o v i d e an o v e r v i e w o f t h e d i g e s t i o n p r o c e s s , as i t p e r t a i n s t o t h e o b j e c t i v e s o f t h e p r e s e n t r e s e a r c h . A t t h e same t i m e , a b r i e f u p d a t e on t h e more r e c e n t r e s e a r c h i s p r o v i d e d . I n t h e c a s e where a r e f e r e n c e has p a r t i c u l a r r e l e v a n c e t o t h e r e s u l t s o f t h i s r e s e a r c h , i t w i l l be d i s c u s s e d where t h e s e r e s u l t s a r e p r e s e n t e d i n t h e m a i n t e x t . 2.1 A e r o b i c D i g e s t i o n P r o c e s s D e s c r i p t i o n The p r o c e s s o f a e r o b i c d i g e s t i o n o f wa s t e a c t i v a t e d s l u d g e i s e s s e n t i a l l y a c o n t i n u a t i o n o f t h e a c t i v a t e d s l u d g e p r o c e s s , w h e r e i n t he wa s t e s l u d g e mass ( e x c e s s b i o l o g i c a l s o l i d s ) grown i n t h e w a s t e w a t e r t r e a t m e n t p o r t i o n o f t h e p r o c e s s i s p l a c e d i n an a d d i t i o n a l a e r a t i o n v e s s e l . The re i s , howeve r , a m a j o r d i f f e r e n c e be tween t h e two p r o c e s s e s , as f a r as t h e o n g o i n g b i o c h e m i c a l r e a c t i o n s a r e c o n c e r n e d . I n t h e w a s t e w a t e r t r e a t m e n t p o r t i o n , s o l u b l e and s o l u b i l i z a b l e o r g a n i c m a t e r i a l s c o n t a i n e d i n t h e i n f l u e n t w a s t e w a t e r a r e u t i l i z e d by t h e c h e m o o r g a n o t r o p h i c and/o r c h e m o l i t h o t r o p h i c m i c r o o r g a n i s m s ( t h e a c t i v a t e d s l u d g e ) as e x t e r n a l s u b s t r a t e , f o r m a i n t e n a n c e and g r o w t h f u n c t i o n s ; p a r t o f t h i s s u b s t r a t e i s o x i d i z e d t o , among o t h e r p r o d u c t s , C0 2 and e n e r g y . T h i s e n e r g y , s t o r e d i n t h e f o r m o f ATP, i s i n t u r n 8 u s e d i n c o n j u n c t i o n w i t h t h e o x i d a t i o n o f t h e r e m a i n i n g s u b s t r a t e f o r t h e s y n t h e s i s o f new c e l l b i o m a s s . These r e a c t i o n s c a n be d e s c r i b e d by e q u a t i o n s 2.1 and ( s t o i c h i o m e t r i c a l l y ) 2.2 , w h e r e i n t h e new s l u d g e c e l l mass i s r e p r e s e n t e d by t h e e m p i r i c a l f o r m u l a C 5 H 7 N0 2 (Hoover e t al., 1 9 5 2 ) : S i m p l e R e s p i r a t i o n O r g a n i c s u b s t r a t e + 0 2 => C0 2 + H 20 + e n e r g y + end p r o d u c t s . . . ( 2 . 1 ) S y n t h e s i s ( A s s i m i l a t i o n ) n (C x H y 0 z ) + n ( x + y / 4 - z / 2 - 5 ) 0 2 + nNH 3 => (C 5 H 7 N0 2 ) n + n ( x - 5 ) C 0 2 + n / 2 ( y - 4 ) H 2 0 . . . ( 2 . 2 ) When t h e e x t e r n a l o r g a n i c s u b s t r a t e i s i n e x c e s s , t h e s e two r e a c t i o n s may o c c u r w i t h l i t t l e r e s t r i c t i o n . , a s s um ing t h e r e a r e no o t h e r g r o w t h - l i m i t i n g f a c t o r s p r e s e n t ; t h i s i s t h e p r i n c i p a l o f l i n e a r b a c t e r i a l g r o w t h . T h i s t y p e o f g r o w t h i s , howeve r , u n d e s i r a b l e i n w a s t e w a t e r t r e a t m e n t , s i n c e t he e x c e s s b i omas s p r o d u c e d w o u l d p r e s e n t a d i s p o s a l p r o b l e m . I n t h e n a t u r a l wa s t e t r e a t m e n t s i t u a t i o n ( d e p e n d i n g on t he h y d r a u l i c c o n d i t i o n s ) , t h e e x t e r n a l s u b s t r a t e i s n o t i n e x c e s s , and t h i s t e n d s t o l i m i t t h e b a c t e r i a l g r o w t h . I n t h i s c a s e , t h e b a c t e r i a may be f o r c e d t o u t i l i z e o t h e r a v a i l a b l e s u b s t r a t e s i n o r d e r t o o b t a i n t h e r e q u i r e d ene r g y t o m a i n t a i n g r owth f u n c t i o n s . A common s o u r c e o f t h i s a d d i t i o n a l s u b s t r a t e i s t he i n t e r n a l c e l l u l a r m a t e r i a l s o f t h e b a c t e r i u m i t s e l f , and t h e p r o c e s s o f o x i d a t i o n o f t h e s e s u b s t r a t e s has been t e r m e d autooxidation o r endogenous respiration ( B e n e f i e l d and R a n d a l l , 1 980 ) . I f t h i s i s c a r r i e d t o i t s u l t i m a t e c o n c l u s i o n , i n t h e s i t u a t i o n where t h e r e i s l i t t l e o r no e x t e r n a l s u b s t r a t e a v a i l a b l e , t h e s l u d g e m i c r o o r g a n i s m s w i l l be f o r c e d t o r e l y a l m o s t e x c l u s i v e l y on endogenous r e s p i r a t i o n t o o b t a i n 9 t h e r e q u i r e d e n e r g y f o r t h e i r l i f e p r o c e s s e s . I n t h i s p h a s e , t h e o r g a n i s m s d e p l e t e any r e a d i l y d e g r a d a b l e i n t e r n a l m a t e r i a l s t o m a i n t a i n c e l l i n t e g r i t y . I n t h e e x t r eme c a s e o f e x t e n d e d s t a r v a t i o n , some c e l l s may n o t be a b l e t o m a i n t a i n t h i s i n t e g r i t y , and w i l l undergo l y s i s , w h e r e i n i n t e r n a l c e l l u l a r m a t e r i a l s w i l l be r e l e a s e d b a c k t o t h e b u l k l i q u i d , t o be u t i l i z e d by o t h e r o r g a n i s m s . The n e t r e s u l t i s an o v e r a l l d e c r e a s e i n t h e u n o x i d i z e d p o r t i o n o f t h e s l u d g e mass. These endogenous r e a c t i o n s c o m p r i s e t h e p r i n c i p a l o f a e r o b i c d i g e s t i o n , w h i c h may be d e f i n e d as t h e c o n v e r s i o n o f d e g r a d a b l e o r g a n i c w a s t e w a t e r s l u d g e s , b y a e r o b i c b i o l o g i c a l mechan i sms , t o o x i d i z e d e n d - p r o d u c t s (Adams and E c k e n f e l d e r , 1 981 ) . E q u a t i o n s 2.3 ( g e n e r a l i z e d ) and 2.4 ( s t o i c h i o m e t r i c ) d e s c r i b e t h e s e r e a c t i o n s w i t h i n t h e c h e m o o r g a n o t r o p h i c m i c r o o r g a n i s m s ( M e t c a l f and Eddy, 1 979 ) : S l udge c e l l s + 0 2 ^ N o n d e g r a d a b l e c e l l m a t e r i a l s + C0 2 + H 20 + NH 3 . . . ( 2 . 3 ) C 5 H 7 N0 2 + 50 2 => 5C0 2 + 2H 20 + NH 3 . . . ( 2 . 4 ) I t i s n o t e d t h a t e n e r g y i s an i m p o r t a n t p r o d u c t o f t h i s r e a c t i o n . I t ha s been s u g g e s t e d t h a t o r g a n i s m s o f h i g h e r t r o p h i c l e v e l s c a n p l a y an i m p o r t a n t r o l e i n t h e d i g e s t i o n p r o c e s s , t h r o u g h s u c h a c t i o n s as g r a z i n g ( P a g o r i a , 1 9 7 7 ) . However, g i v e n t h e comp lex n a t u r e o f t h e h e t e r o g e n e o u s s l u d g e mass, and t h e many c o n c u r r e n t r e a c t i o n s o n g o i n g w i t h i n t h i s mass, no a t t e m p t ha s b e e n made t o q u a n t i f y t h e s e o r g a n i s m s ' c o n t r i b u t i o n s t o t he mass r e d u c t i o n a s p e c t o f t h e d i g e s t i o n p r o c e s s ; as s u c h , any d e s c r i p t i o n o f t he d i g e s t i o n p r o c e s s i s g e n e r a l l y l i m i t e d t o t h e b a c t e r i a w i t h i n t h e s l u d g e mass. Unde r t h e p r o p e r c o n d i t i o n s w i t h i n t h e a e r o b i c d i g e s t e r ( i n t e rms o f r e t e n t i o n t i m e , oxygen s u p p l y , l i q u i d t e m p e r a t u r e and b u l k l i q u i d pH l e v e l ) , 10 c h e m o l i t h o t r o p h i c b a c t e r i a ( n i t r i f y i n g m i c r o o r g a n i s m s ) a r e a b l e t o u t i l i z e t he i n o r g a n i c n i t r o g e n l i b e r a t e d t h r o u g h t h e endogenous r e s p i r a t i o n o f c e l l u l a r m a t e r i a l s b y t h e c h e m o o r g a n o t r o p h s , f o r t h e s y n t h e s i s o f new c e l l mass. T h i s two s t a g e p r o c e s s c a n be d e s c r i b e d by t he f o l l o w i n g e q u a t i o n s , w i t h c e l l mass r e p r e s e n t e d b y C 5 H 7 N0 2 (U .S . EPA, 1 975 ) : 55NH 4 + + 760 2 + 109HCO 3 => C 5 H 7 N0 2 + 54N0 2 + 57H 20 + 104H 2 C0 3 . . . ( 2 . 5 ) 400N0 2 + N H 4 + + 4H 2 C0 3 + HC0 3 + 1950 2 =» C 5 H 7 N0 2 + 3H 20 + 400N(V . . . ( 2 . 6 ) W h i l e t h e c h e m o l i t h o t r o p h i c p o p u l a t i o n may o n l y be a s m a l l f r a c t i o n o f t h e t o t a l b a c t e r i a l b i omas s i n a g r o w i n g c u l t u r e w i t h e x t e r n a l s u b s t r a t e , i t may become much more s i g n i f i c a n t i n t h e a e r o b i c d i g e s t i o n p r o c e s s , g i v e n t he l o n g r e t e n t i o n t i m e s common i n t h e s e s y s tems ( w h i c h i n t u r n a r e enough t o s a t i s f y t h e g r o w t h r a t e s o f t he c h e m o l i t h o t r o p h s ) . The s i g n i f i c a n c e o f t h i s w i l l be d i s c u s s e d l a t e r i n t h i s C h a p t e r . E q u a t i o n 2.7 r e p r e s e n t s t h e a e r o b i c d i g e s t i o n p r o c e s s when a v i a b l e p o p u l a t i o n o f n i t r i f y i n g c h e m o l i t h o t r o p h i c o r g a n i s m s i s p r e s e n t : C 5 H 7 N0 2 + 70 2 => 5C0 2 + 3H 20 + N0 3 ' + H + + e n e r g y . . . ( 2 .7 ) The p r o c e s s and r e a c t i o n s t h a t c o m p r i s e endogenous m e t a b o l i s m i n b a c t e r i a ha s b e e n w e l l r e s e a r c h e d i n t h e f i e l d o f m i c r o b i o l o g y , u s i n g pu r e c u l t u r e t e c h n i q u e s . Lamanna (1963) s u g g e s t e d t h a t endogenous m e t a b o l i s m c o u l d be d e f i n e d as " t h e sum o f a l l t h e c h e m i c a l a c t i v i t i e s p e r f o r m e d by o r gan i sms i n t h e ab sence o f u t i l i z a b l e e x t r a c e l l u l a r m a t e r i a l s s e r v i n g as s o u r c e s o f e n e r g y and b u i l d i n g s t o n e s f o r a s s i m i l a t i o n and g r o w t h . W a t e r , m o l e c u l a r o x y g e n , and n o n c a r b o n a c e o u s and n o n - n i t r o g e n o u s m i n e r a l s a l t s may be p r e s e n t i n t h e e x t e r n a l e n v i r o n m e n t and p a r t i c i p a t e i n endogenous m e t a b o l i s m . I n n u t r i t i o n a l t e r m s , endogenous m e t a b o l i s m c a n be v i e w e d as encompa s s i n g a l l t h o s e c h e m i c a l a c t i v i t i e s engaged i n b y t h e s t a r v i n g c e l l " . I t ha s b e e n s u g g e s t e d t h a t t h e s e endogenous r e a c t i o n s may o c c u r s i m u l t a n e o u s l y w i t h r e s p i r a t i o n , i n t h e p r e s e n c e o f an e x t e r n a l s u b s t r a t e ( i n some s p e c i e s i t has been f o u n d t h a t t h e endogenous r e s p i r a t o r y a c t i v i t y i s n o t d e c r e a s e d i n t h e p r e s e n c e o f exogenous s u b s t r a t e s ; C l i f t o n , 1 9 6 3 ) ; however , i t i s g e n e r a l l y a g r e e d t h a t , when no exogenous s u b s t r a t e s a r e p r e s e n t , endogenous r e s p i r a t o r y a c t i v i t y w i l l p r o v i d e t he m a j o r i t y o f e n e r g y f o r b a c t e r i a l f u n c t i o n s . These s t u d i e s have l e a d t o t h e t h e o r y o f an e n e r g y o f m a i n t e n a n c e d u r i n g n u t r i e n t l i m i t a t i o n , w h e r e i n i t i s c o n c e i v e d t h a t a d e f i n i t e amount o f e n e r g y must be expended b y t h e c e l l , f o r t h e m a i n t e n a n c e o f m i c r o b i a l i n t e g r i t y , w i t h o u t g r owth o r d e a t h o c c u r r i n g . F o r t h i s , t h e m o b i l i z a t i o n o f endogenous s u b s t r a t e s must f u r n i s h t h e r e q u i r e d e n e r g y t o a l l o w v a r i o u s c e l l u l a r a c t i v i t i e s t o c o n t i n u e ( i e . r e s y n t h e s i s o f m a c r o m o l e c u l e s , o s m o t i c r e g u l a t i o n o f s o l u b l e c o n s t i t u e n t s , h e a t l o s s t o t h e e x t e r n a l e n v i r o n m e n t and t h e m a i n t e n a n c e o f c e l l w a l l i n t e g r i t y ) . I n t h i s way, t h e v i a b i l i t y o f t he c e l l i s m a i n t a i n e d , e ven unde r s t a r v a t i o n c o n d i t i o n s (Dawes and R i b b o n s , 1964; M a r r e t al., 1963) . W h i l e t h e e n e r g y o f m a i n t e n a n c e may be s u f f i c i e n t t o a l l o w t h e c e l l t o r e m a i n v i a b l e d u r i n g s h o r t p e r i o d s o f s t a r v a t i o n , i t has b e e n o b s e r v e d t h a t t h e s e c e l l s w i l l e v e n t u a l l y e x p e r i e n c e l o s s o f v i a b i l i t y a f t e r p r o l o n g e d e x p o s u r e t o s t a r v a t i o n c o n d i t i o n s . I t has been s u g g e s t e d t h a t t h i s l o s s o f v i a b i l i t y c o u l d r e s u l t f r o m a p r o g r e s s i v e d e s t r u c t i o n o f t h e m a c r o m o l e c u l e s u s e d as endogenous s u b s t r a t e s ( i e . p r o t e i n s ) . I n t h e p r e s e n c e o f exogenous s u b s t r a t e s , w h i c h s e r v e as e n e r g y s o u r c e s f o r t h e c e l l , t h e amino a c i d s r e l e a s e d d u r i n g p r o t e i n t u r n o v e r ( i n t h e d e g r a d a t i o n o f s u r p l u s p r o t e i n ) c a n be r e s y n t h e s i z e d b a c k i n t o p r o t e i n when r e q u i r e d . T h i s ha s b e e n r e f e r r e d t o as a r e d i s t r i b u t i o n p r o c e s s . However, w i t h o u t an exogenous s u b s t r a t e s o u r c e , some o f t h e r e l e a s e d amino a c i d s may be r e s p i r e d , t o p r o v i d e t h e e n e r g y (as ATP) n e c e s s a r y f o r p r o t e i n r e s y n t h e s i s . T h i s w i l l e v e n t u a l l y l e a d t o a d e p l e t i o n o f t h e t u r n e d - o v e r p r o t e i n s , w h i c h i n t u r n has b e e n i m p l i c a t e d i n t h e l o s s o f g r o w t h a b i l i t y by t h e o r g a n i s m , o r a l o s s o f v i a b i l i t y (Ma r r e t al., 1963; C l i f t o n , 1 963 ) . I t ha s a l s o been o b s e r v e d , howeve r , t h a t s p e c i e s w h i c h have been s t a r v e d f o r p r o l o n g e d p e r i o d s , and a r e p r e s u m a b l y e x i s t i n g i n v a r i o u s s t a t e s o f c e l l u l a r d i s o r g a n i z a t i o n as a r e s u l t o f m a c r o m o l e c u l a r t u r n o v e r , may s t i l l be c a p a b l e o f r e c o v e r y unde r a p p r o p r i a t e c o n d i t i o n s ( P o t t s , 1 9 6 9 ) . T h i s a b i l i t y t o r e c o v e r w i l l have o b v i o u s i m p l i c a t i o n s f o r the s u r v i v a l o f t h e s p e c i e s i n q u e s t i o n . L o s s o f v i a b i l i t y i s a l s o t h o u g h t t o l e a d t o t h e p r o c e s s o f c r y p t i c g r o w t h w i t h i n t h e b a c t e r i a l c u l t u r e . Once a c e l l " d i e s " , t h e i n t e g r i t y o f t he c e l l w a l l c a n no l o n g e r be m a i n t a i n e d , and t h e i n t e r n a l c e l l u l a r components w i l l be r e l e a s e d t o t h e b u l k l i q u i d ; t h e s e m a t e r i a l s a r e t h o u g h t t o be a b l e t o s e r v e as e x t e r n a l s u b s t r a t e s f o r t h e r e m a i n i n g c e l l s . A g a i n , t h i s p r o c e s s i s t h o u g h t t o have an i n f l u e n c e on t h e o v e r a l l s u r v i v a l o f a s t a r v i n g p o p u l a t i o n ( P o s t g a t e and H u n t e r , 1962; P o s t g a t e and C a l c o t t , 1 985 ) . W h i l e i t i s d i f f i c u l t t o g e n e r a l i z e abou t s u r v i v a l mechanisms be tween t h e c o u n t l e s s s p e c i e s o f o r g a n i s m s s t u d i e d i n t h i s f i e l d , i t i s g e n e r a l l y a g r e e d t h a t endogenous m e t a b o l i c a c t i v i t y may be v i e w e d as a p r o c e s s w h e r e i n v a r i o u s c e l l u l a r c o n s t i t u e n t s ( i e . p r o t e i n s , RNA, g l y c o g e n and po l y -/3 -h y d r o x y b u t y r a t e , as s u g g e s t e d by C a m p b e l l e t al., 1963) a r e d e p l e t e d d u r i n g s t a r v a t i o n c o n d i t i o n s . The e x t e n t o f t h i s d e g r a d a t i o n w i l l depend on t h e 13 i n i t i a l c o n c e n t r a t i o n o f t h e i n d i v i d u a l s u b s t r a t e s , w h i c h w i l l i n t u r n be d e t e r m i n e d by t h e n u t r i t i o n a l s t a t u s o f t h e o r g a n i s m . A t t h e same t i m e , t h e s e c o n s t i t u e n t s a r e t h o u g h t t o be d i v i d e d i n t o c l a s s e s o f e s s e n t i a l and e x p e n d a b l e , as f a r as t h e i r a v a i l a b i l i t y as endogenous s u b s t r a t e s i s c o n c e r n e d . The a b i l i t y o f an o r g a n i s m t o u se t h e s e c o n s t i t u e n t s t o m a i n t a i n c e l l m e t a b o l i c f u n c t i o n s w i l l d e t e r m i n e t h e s u r v i v a l o f t h a t o r g a n i s m . I n t h e a e r o b i c d i g e s t i o n o f wa s t e a c t i v a t e d s l u d g e , t h e h e t e r o g e n e o u s m i c r o b i a l makeup o f t h e s l u d g e mass makes c o m p a r i s o n s w i t h t h e o r i e s d e r i v e d f r o m p u r e c u l t u r e e x p e r i m e n t s d i f f i c u l t . I t c a n be p r o p o s e d , howeve r , t h a t t h e b a s i c i d e a o f s l u d g e mass r e d u c t i o n i n t he d i g e s t e r must b e , i n who l e o r i n p a r t , a r e s u l t o f t h e endogenous m e t a b o l i s m o f t he s l u d g e m i c r o o r g a n i s m s , g i v e n t h e l i m i t e d amounts o f exogenous s u b s t r a t e s n o r m a l l y p r e s e n t i n t h e d i g e s t e r . From t h i s , i t c a n a l s o be p r o p o s e d t h a t t h e same k i n d s o f b i o c h e m i c a l r e a c t i o n s , d i s c u s s e d above , a r e r e s p o n s i b l e f o r t h i s mass r e d u c t i o n . A e r o b i c d i g e s t i o n c a n g e n e r a l l y be d e s c r i b e d as b e i n g a p r o c e s s whereby t he b i o m a s s i s " g r o w i n g f r o m w i t h i n " , i e . g r owth e n e r g y i s o b t a i n e d f r o m t h e endogenous r e s p i r a t i o n o f c e l l u l a r m a t e r i a l s . I f d i g e s t i o n i s a l l o w e d t o p r o c e e d f o r a s u f f i c i e n t l y l o n g p e r i o d o f t i m e , most o f t h e e a s i l y o x i d i z a b l e m a t e r i a l w i l l be u t i l i z e d f o r t h e m a i n t e n a n c e o f m i c r o b i a l m e t a b o l i s m , t he o v e r a l l c e l l mass w i l l d e c r e a s e , and t he r e m a i n i n g c e l l mass w i l l c o n s i s t , t o a c o n s i d e r a b l e d e g r e e , o f i n e r t ( n o n - o x i d i z a b l e ) m a t e r i a l o f s u c h a l ow ene r g y s t a t e as t o be c o n s i d e r e d b i o l o g i c a l l y s t a b l e and s u i t a b l e f o r d i s p o s a l . 2.2 B e n e f i t s / D e t r i m e n t s and P r o c e s s K i n e t i c s o f A e r o b i c D i g e s t i o n Adams and E c k e n f e l d e r (1981) compared t h e a e r o b i c d i g e s t i o n p r o c e s s w i t h t h a t o f t h e more c o n v e n t i o n a l a n a e r o b i c p r o c e s s , i n an a t t e m p t t o d e l i n e a t e 14 t h e a d v a n t a g e s and d i s a d v a n t a g e s o f t h e a e r o b i c s y s t e m . T h i s c o m p a r i s o n was o f a g e n e r a l n a t u r e , and d i d n o t t a k e i n t o a c c o u n t t h e p r o c e s s v a r i a b l e s a f f e c t i n g t h e a e r o b i c p r o c e s s ( i e . p r o c e s s c o n f i g u r a t i o n , t y p e o f raw s l u d g e , p r e v a i l i n g e n v i r o n m e n t a l c o n d i t i o n s , e t c . ) ; n o n e t h e l e s s , t h e y s u g g e s t e d t h e f o l l o w i n g a d v a n t a g e s f o r t h e a e r o b i c d i g e s t i o n s y s t e m : 1. A b i o l o g i c a l l y s t a b l e e n d - p r o d u c t i s p r o d u c e d . 2. T h i s s t a b l e e n d - p r o d u c t i s r e l a t i v e l y i n n o c u o u s , t h e r e b y e n c o u r a g i n g l a n d d i s p o s a l i n h o l d i n g l a g o o n s o r by s p r a y i r r i g a t i o n . 3. Due t o s i m p l i c i t y o f c o n s t r u c t i o n , c a p i t a l c o s t s f o r t h e a e r o b i c s y s t e m a r e r e l a t i v e l y l ow compared w i t h a n a e r o b i c d i g e s t i o n and o t h e r s o l i d s - h a n d l i n g schemes. 4 . A e r o b i c a l l y d i g e s t e d s l u d g e g e n e r a l l y p o s s e s s e s good d e w a t e r i n g c h a r a c t e r i s t i c s , i t d r a i n s w e l l when p l a c e d on s and beds and i s r e s i s t a n t t o r e w e t t i n g d u r i n g r a i n f a l l . 5. V o l a t i l e s o l i d s r e d u c t i o n e q u i v a l e n t t o t h a t o b t a i n e d by a n a e r o b i c d i g e s t i o n i s p o s s i b l e w i t h a e r o b i c s y s tems t r e a t i n g s e c o n d a r y s l u d g e s . 6. S u p e r n a t a n t l i q u o r s f r o m a e r o b i c d i g e s t i o n p o s s e s s a l o w e r BOD c o n t e n t t h a n t h o s e f r o m a n a e r o b i c d i g e s t i o n . The a e r o b i c s u p e r n a t a n t i n most c a s e s has been f o u n d t o p o s s e s s a BOD l e s s t h a n 100 mg/L. T h i s advan tage i s s i g n i f i c a n t b e c a u s e many 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 p l a n t s have been o v e r l o a d e d due t o r e c y c l i n g o f t h e h i gh -BOD s u p e r n a t a n t l i q u o r s f r o m a n a e r o b i c d i g e s t e r s . 7. Fewer o p e r a t i o n a l p r o b l e m s e x i s t f r o m a e r o b i c d i g e s t i o n when compared w i t h t he more comp lex a n a e r o b i c s y s t e m . T h e r e f o r e , l o w e r m a i n t e n a n c e c o s t s c a n be e x p e c t e d and l e s s s k i l l e d l a b o u r r e q u i r e d w i t h t he a e r o b i c f a c i l i t y . 15 8. I t ha s b e e n r e p o r t e d t h a t t h e a e r o b i c a l l y d i g e s t e d s l u d g e has a h i g h e r f e r t i l i z e r c o n t e n t t h a n t h a t r e s u l t i n g f r o m a n a e r o b i c d i g e s t i o n . Among t h e d i s a d v a n t a g e s o f t h e a e r o b i c d i g e s t i o n s y s t e m w e r e : 1. The re a r e h i g h e r power c o s t s , l e a d i n g t o h i g h e r o p e r a t i n g c o s t s i n c o m p a r i s o n w i t h a n a e r o b i c d i g e s t i o n . The d i f f e r e n c e i n o p e r a t i n g c o s t i s n o t s i g n i f i c a n t w i t h s m a l l e r t r e a t m e n t p l a n t s , b u t i s i m p o r t a n t w i t h t h e l a r g e r f a c i l i t i e s . 2. G r a v i t y t h i c k e n i n g p r o c e s s e s f o l l o w i n g a e r o b i c d i g e s t i o n t e n d t o g e n e r a t e h i g h s o l i d s 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 . 3. Some s l u d g e s w h i c h have been a e r o b i c a l l y d i g e s t e d have n o t been o b s e r v e d t o d e w a t e r e a s i l y by t h e vacuum f i l t r a t i o n p r o c e s s . 4 . S i n c e t h e p r o c e s s i s a e r o b i c , no methane gas i s p r o d u c e d f o r r e c o v e r y as a b y - p r o d u c t . 5. The s o l i d s r e d u c t i o n e f f i c i e n c y o f t h e a e r o b i c d i g e s t e r may v a r y w i t h e x t r eme changes i n amb i en t t e m p e r a t u r e , w h i c h s u b s e q u e n t l y a f f e c t s t h e a e r a t i o n b a s i n t e m p e r a t u r e ( i n a u t o t h e r m a l p r o c e s s e s ) . B e n e f i e l d and R a n d a l l ( 1 9 8 0 ) , i n a s i m i l a r c o m p a r i s o n , s u g g e s t e d t h a t t h e a e r o b i c p r o c e s s may g e n e r a l l y be c o m p e t i t i v e w i t h t h e a n a e r o b i c p r o c e s s , g i v e n t h a t t h e c o n d i t i o n s f o r t he r e q u i r e d m i c r o b i a l a c t i v i t y a r e a d e q u a t e . V e s i l i n d (1980) s u g g e s t e d t h a t , s i n c e t h e a e r o b i c s t a b i l i z a t i o n p r o c e s s has t h e ad van t a ge o f a c o m p l e t e f o o d c h a i n and a m i x e d and v a r i e d e c o l o g y ( i n c l u d i n g some a n a e r o b i c and f a c u l t a t i v e o r g a n i s m s ) , i t i s a more r e s i l i e n t s y s t e m t h a n t h e a n a e r o b i c p r o c e s s , and l e s s s u s c e p t i b l e t o p r o c e s s u p s e t . S i m i l a r l y , Benedek e t al. (1972) s u g g e s t e d t h a t t h e m a i n r e a s o n f o r t he w i d e s p r e a d u se o f t h e a e r o b i c s y s t e m was t h a t i n d u s t r i a l w a s t e s o f t e n c o n t a i n 16 h a r m f u l c o n c e n t r a t i o n s o f t o x i c m a t e r i a l s o r pH l e v e l s , w h i c h w o u l d be l e s s u p s e t t i n g t o t h e a e r o b i c s y s t e m . I n t h e w a s t e w a t e r t r e a t m e n t f i e l d , a g r e a t d e a l o f r e s e a r c h e f f o r t has b e e n made i n t h e m o d e l l i n g o f a e r o b i c m i c r o b i a l a c t i v i t y , as r e l a t e d t o s l u d g e mass a c c u m u l a t i o n i n t h e a c t i v a t e d s l u d g e p r o c e s s , and t h e l o s s o f s l u d g e mass i n t h e a e r o b i c d i g e s t e r . No r e a l c on sen su s e x i s t s p e r t a i n i n g t o t h e " c o r r e c t " mode l o f t h e s e r e a c t i o n s , and t h i s i s n o t s u r p r i s i n g g i v e n t h e f a c t t h a t t h e r e d u c t i o n o f b i o l o g i c a l m a t e r i a l i n t he a e r o b i c d i g e s t e r i s a f u n c t i o n o f s e v e r a l s e p a r a t e and c o n c u r r e n t p h y s i c a l and b i o c h e m i c a l r e a c t i o n s , i e . endogenous m e t a b o l i s m , p r e d a t i o n / g r a z i n g , and c r y p t i c g r o w t h . The c o m p l e x i t y o f t h e i n t e r r e l a t i o n s h i p s w i t h i n t h e a e r o b i c d i g e s t e r f o r c e s t h e m o d e l l e r t o s i m p l i f y h i s a p p r o a c h , and i t i s g e n e r a l l y assumed t h a t endogenous m e t a b o l i s m i s r e s p o n s i b l e f o r t h e m a j o r i t y o f t h e mass r e d u c t i o n t a k i n g p l a c e . I t ha s commonly b e e n s u g g e s t e d t h a t , i n a b i o l o g i c a l s y s t e m , n e t c e l l mass a c c u m u l a t i o n c a n be e x p r e s s e d as a f u n c t i o n o f b o t h i n c r e a s e t h r o u g h s y n t h e s i s and d e c r e a s e t h r o u g h endogenous m e t a b o l i s m , as e x p r e s s e d i n e q u a t i o n M = q u a n t i t y o f a c t i v e mass i n t h e s y s t e m (VSS ) , S — q u a n t i t y o f s u b s t r a t e removed f r o m s y s t e m (BOD), Ka = f r a c t i o n o f removed s u b s t r a t e w h i c h i s s y n t h e s i z e d i n t o new a c t i v e mass, and ko = f r a c t i o n o f a c t i v e mass i n t h e s y s t e m w h i c h i s consumed, p e r day , by endogenous m e t a b o l i s m . 2 . 8 : S l u d g e a c c u m u l a t i o n dM d t . . . ( 2 . 8 ) whe re : I n t h e a e r o b i c d i g e s t e r , i t i s f u r t h e r assumed t h a t t h e s o l u b l e o r g a n i c s u b s t r a t e i n t he l i q u i d medium has been d e p l e t e d ( i e . no exogenous s u b s t r a t e s 17 a r e p r e s e n t ) , and a l l s t o r e d o r a d s o r b e d m a t e r i a l s have b e e n m e t a b o l i z e d , so t h a t t h e s l u d g e mass o r g a n i s m s a r e unde r s t a r v a t i o n c o n d i t i o n s . From t h i s , e q u a t i o n 2.8 becomes : dM d t -k„M o r , upon i n t e g r a t i o n : £ - e o r I n £ = - k d t . . . ( 2 . 9 ) " o Mo E q u a t i o n 2.9 r e p r e s e n t s a f i r s t o r d e r r e a c t i o n , w h e r e i n k d i s t h e r a t e c o n s t a n t f o r a u t o - o x i d a t i o n ( K o e r s , 1 979 ) , a l s o r e f e r r e d t o as t h e endogenous d e c a y c o e f f i c i e n t . Benedek e t al. (1972) s u g g e s t e d t h a t t h i s r e a c t i o n o n l y h o l d s f o r t h e b a t c h o r p l u g - f l o w r e a c t o r c o n f i g u r a t i o n , and i f a c o n t i n u o u s -f e e d c o m p l e t e l y m i x e d r e a c t o r i s u s e d , e q u a t i o n 2.9 must be m o d i f i e d t h r o u g h a mass b a l a n c e o f s o l i d s a r o u n d t he s y s t e m . The e x p r e s s i o n o f mass r e d u c t i o n w o u l d t h e n become: I - rhs- • •< 2- 1°> W i t h o u t s o l i d s r e c y c l i n g , t h e t e r m t ' i s e q u a l t o t he h y d r a u l i c r e t e n t i o n t i m e ; i f , h oweve r , t h e s y s t e m u se s t he p o s t - t h i c k e n i n g and r e c y c l i n g o f s l u d g e mass s o l i d s , K o e r s (1979) s u g g e s t e d t h a t t ' s h o u l d be t a k e n as t h e a ve r a ge r e s i d e n c e t i m e o f t h e s l u d g e s o l i d s i n t h e s y s t em ( i e . t h e " s l u d g e a ge " w i t h i n t h e s y s t e m , c a l c u l a t e d as t h e mass o f s o l i d s u n d e r a e r a t i o n d i v i d e d by t he mass o f s o l i d s w a s t e d f r om the s y s t em on a d a i l y b a s i s ) . Many m o d i f i c a t i o n s t o t h e s e e q u a t i o n s have been made, s i n c e no one e q u a t i o n has b e e n d e r i v e d t h a t i s u n i v e r s a l l y a p p l i c a b l e i n t h e a e r o b i c d i g e s t i o n f i e l d , g i v e n t h e numerous d e s i g n v a r i a b l e s i n v o l v e d . Benedek e t al. (1972) and Adams e t al. (1974) s u g g e s t e d t h a t b i o d e g r a d a b l e s u spended s o l i d s (BVSS, b e i n g t h e d i f f e r e n c e be tween t o t a l v o l a t i l e s u spended s o l i d s and t h e i r n o n - d e g r a d a b l e p o r t i o n , w h i c h i s assumed t o be c o n s t a n t t h r o u g h o u t t h e a e r a t i o n p e r i o d ) be u s e d when q u a n t i f y i n g s l u d g e mass r e d u c t i o n r a t e s , s i n c e c o m p l e t e o x i d a t i o n o f t h e s l u d g e mass s o l i d s was n o t p o s s i b l e w i t h i n a r e a s o n a b l e l e n g t h o f t i m e . They p r o p o s e d t h a t t h e u se o f t h i s v a r i a b l e w o u l d b e t t e r mode l t h e r e d u c t i o n i n b i o l o g i c a l s l u d g e mass. B e n e f i e l d and R a n d a l l (1978, 1980) d i s a g r e e d w i t h t h i s a p p r o a c h , howeve r , on a number o f c o u n t s . They a r g u e d t h a t t he r e a c t i o n r a t e c o e f f i c i e n t , as d e t e r m i n e d b y t h e above method , was b a s e d on t h e r a t e o f b i o d e g r a d a b l e s o l i d s r e d u c t i o n , r a t h e r t h a n on t h e magn i t ude o f t h e s e r e d u c t i o n s . A t t h e same t i m e , t h e y s u g g e s t e d t h a t t h e amount o f b i o d e g r a d a b l e s o l i d s v a r i e d d e p e n d i n g on d i g e s t e r c o n d i t i o n s , and t h e r e f o r e a c o e f f i c i e n t d e r i v e d i n t h i s manner w o u l d have l i m i t e d use i n t h e c o m p a r i s o n o f r e a c t i o n r a t e s b e t w e e n d i f f e r e n t d i g e s t i o n s y s t e m s . The a u t h o r s f u r t h e r s u g g e s t e d t h a t t he c o n c e p t o f a c o n s t a n t amount o f n o n d e g r a d a b l e m a t e r i a l s i n t h e r e a c t o r was i n c o r r e c t , h a v i n g o b s e r v e d c o n s i d e r a b l e l o s s e s o f f i x e d s u spended s o l i d s i n t h e i r r e s e a r c h . These r e d u c t i o n s were h y p o t h e s i z e d t o be a r e s u l t o f t he s o l u b i l i z a t i o n o f m i c r o b i a l m a t e r i a l s r e l e a s e d f r o m t h e c e l l s upon l y s i s . These d i s a g r e e m e n t s l e d B e n e f i e l d and R a n d a l l t o p r o p o s e a new mode l o f a e r o b i c d i g e s t i o n a c t i v i t y , b a s e d on t o t a l s u spended s o l i d s ( T S S ) . They assumed t h a t o n l y t h e d e g r a d a b l e a c t i v e f r a c t i o n o f t h e TSS d e c r e a s e d d u r i n g a e r o b i c d i g e s t i o n , and t h r o u g h t h e u se o f t h e i r m o d e l , t h e i m p o r t a n c e o f t he p h y s i o l o g i c a l s t a t e o f t h e b i omas s was a c c o u n t e d f o r i n t h e a n a l y s i s . They d i d , howeve r , have d i f f i c u l t y i n p r o v i d i n g r e l i a b l e d e s i g n p r o c e d u r e s b a s e d on t h i s m o d e l , and t he e s t i m a t i o n o f s l u d g e mass a c t i v e f r a c t i o n was s u s p e c t . 19 G rady and L i m (1980) s u g g e s t e d t h a t t h e r e were two d i f f e r e n t fo rms o f decay t a k i n g p l a c e d u r i n g a e r o b i c d i g e s t i o n , i e . decay o f l i v i n g c e l l s a t a s p e c i f i c r a t e , and decay o f dead c e l l s a t a s e p a r a t e and s p e c i f i c r a t e . A l t h o u g h t h e a u t h o r s r e c o g n i z e d t h e i n h e r e n t v a r i a b i l i t y i n s o l i d s r e d u c t i o n s as a r e s u l t o f s l u d g e t y p e and o p e r a t i n g c o n d i t i o n s , t h i s a p p r o a c h was l i m i t e d b y t h e same p r o b l e m s d i s c u s s e d f o r t h e mode l o f Adams et al. ( 1 9 7 4 ) . From t h e above b r i e f d i s c u s s i o n , i t c a n be s e e n t h a t t h e r e i s a g r e a t d e a l o f d i s a g r e e m e n t p e r t a i n i n g t o t h e e s t i m a t i o n o f r e a c t i o n r a t e s w i t h i n t he a e r o b i c d i g e s t e r . A t t he same t i m e , t h e r e i s l i t t l e c o n s e n s u s on t he e x a c t p a r a m e t e r most u s e f u l i n t h i s a s s e s s m e n t , g i v e n t h e l a r g e number o f v a r i a b l e s i n d i g e s t e r o p e r a t i o n on