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Two-stage anaerobic digestion of hog wastes 1975

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TWO-STAGE ANAEROBIC DIGESTION OF HOG WASTES BY ADRIAN C. DUNCAN B.A.Sc, U n i v e r s i t y of B r i t i s h Columbia, 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in the Department of C i v i l Engineering We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1975 In presenting th i s thesis in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i lab le for reference and study. I further agree that permission for extensive copying of th i s thes is for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of th is thes i s fo r f i nanc ia l gain shal l not be allowed without my writ ten permission. Department of C-l\/lL t ^ 6 / Af ^ ff, /A;' Q The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 ABSTRACT The present trend towards concentrated land-use animal farming has given rise to a number of new animal waste disposal problems due to the very high strength of liquid wastes from such f a c i l i t i e s . One treatment alternative applicable to these wastes is anaerobic digestion. A study was undertaken to determine the anaerobic digestion characteristics of waste from a high-density hog-raising f a c i l i t y . Earlier work had provided treatment efficiency data for a single-stage, laboratory- scale anaerobic reactor, as well as giving certain design c r i t e r i a for anaerobic lagoons. The present study was intended to provide a measure of the increase in treatment efficiency obtained through use of a two-stage anaerobic reactor, again on a laboratory scale, and to give information regarding biodegrada- b i l i t y of the settled sludge. The effect of variations in temperature and detention time was included in the study, as was an investigation of vola- t i l e acids, total organic carbon, and copper toxicity due to the use of brass fittings in test apparatus. Conclusions reached on the basis of this study were that the two- stage system gives a slightly higher loading capacity, due to improved settling capability, but the effluent from the second c e l l is s t i l l of higher strength than is often desirable for discharge to receiving waters. The settled solids were found to be degradable to a limited extent only, and thus most of them w i l l require physical removal from a lagoon. No significant correlation was found between BOD, COD, and TOC and copper levels were found to reach significant levels in the reactors. i i This report was based on laboratory findings only. No correlation between laboratory-scale and f i e l d results was attempted in the study. i i i TABLE OF CONTENTS ABSTRACT i i LIST OF TABLES v i LIST OF FIGURES v i i . ACKNOWLEDGEMENT v i i i • CHAPTER 1. INTRODUCTION 1 1.1 General Discussion 1 1.2 Fundamentals of Anaerobic Digestion 3 1.3 Need for Further Research 4 1.4 Separation of Settled Solids and Supernatant 6 1.5 Sludge Build-up and Gas Production 7 CHAPTER 2. EXPERIMENTAL PROCEDURE 9 2.1 General Discussion 9 2.2 Establishment and Operation of the Batch Systems ... 10 2.3 Establishment and Operation of the Two-Stage Systems 13 2.4 Testing Procedure for the Influents and Effluents .. 15 2.5 Testing Procedure for the Evolved Gases 18 2.6 Summary 18 CHAPTER 3. RESULTS OF BATCH TESTS 20 3.1 Introduction 20 3.2 General Discussion of Procedure 20 3.3 Gas Production and Analysis 21 3.4 Relationship of Methane Production to BOD and COD Removal 24 3.5 Relationship of Methane Production to Volatile Solids Removal 29 iv CHAPTER 4. TWO-STAGE DIGESTER RESULTS 35 4.1 Introduction 35 4.2 General Discussion 35 4.3 Effectiveness of Modifications to Cell 36 4.4 Overall Treatment Efficiency 42 4.5 Settling vs. Biological Degradation 43 4.6 Relative Importance of 1st and 2nd Cell 48 4.7 Single-Stage vs. Two-Stage System 49 4.8 Copper Concentrations 55 CHAPTER 5. VOLATILE ACIDS AND TOTAL ORGANIC CARBON RESULTS 59 5.1 Introduction 59 5.2 Volatile Acids and pH Levels in Anaerobic Systems ... 59 5.3 Volatile Acids and pH in the Two-Stage Digesters .... 61 5.4 Total Organic Carbon Measurements in the Two-Stage Digesters 69 CHAPTER 6. CONCLUSIONS AND RECOMMENDATIONS 74 6.1 Introduction 74 6.2 Conclusions from Batch Test Results 74 6.3 Conclusions from Two-Stage Continuous-Feed Digester Results 75 6.4 Conclusions Regarding Two-Cell Vs. One-Cell Systems 75 6.5 Conclusions from Copper, Volatile Acids, pH and Total Organic Carbon Tests 76 6.6 Recommendations 77 BIBLIOGRAPHY 79 APPENDIX A. Sample Calculations 81; v LIST OF TABLES TABLE I. Average Raw Waste C h a r a c t e r i s t i c s 36 TABLE I I . Average E f f l u e n t C h a r a c t e r i s t i c s 37 TABLE I I I . Comparison of Percent Removals i n O r i g i n a l 25 I C e l l and Modified 12.5 £ C e l l 38 TABLE IV. Comparison of Mass Removed Per Unit C e l l Volume for 25 I Single-Stage C e l l and 12.5 I F i r s t C e l l of Double-Cell Digester 39 TABLE V. Percentage BOD Removals 44 TABLE VI. Percentage COD Removals 44 TABLE VII. Percentage V o l a t i l e Solids Removals 45 TABLE VIII. Percentage To t a l Solids Removals 45 TABLE IX. Percentage BOD Removals Due to S e t t l i n g and B a c t e r i o l o g i c a l Action 46 TABLE X. Percentage COD Removals Due to S e t t l i n g and B a c t e r i o l o g i c a l Action 46 TABLE XI. Percentage V o l a t i l e Solids Removal Due to S e t t l i n g and B a c t e r i o l o g i c a l Action 47 TABLE XII. Percentage To t a l Solids Removal Due to S e t t l i n g and B a c t e r i o l o g i c a l Action 47 TABLE XIII. Comparison of Single- and Two-Stage Systems (LDT=50 days). 50 TABLE XIV. Comparison of Single- and Two-Stage Systems (LDT=25 days). 50 TABLE XV.. Comparison of 1st C e l l Only Vs. 1st and 2nd C e l l s at 25-Day LDT 52 TABLE XVI. Copper Concentrations i n Two-Stage Digester C e l l s 56 v i LIST OF FIGURES FIGURE 1.1 Mechanism of Anaerobic Sludge Digestion 5 FIGURE 2.1 Single-Stage Digester, Showing P o s s i b i l i t y of Short- C i r c u i t i n g During Feeding 11 FIGURE 2.2 Two-Stage Digester, Showing Flow Pattern During Feeding 12 FIGURE 3.1 Methane Production Rate Vs. Time 22 FIGURE 3.2 Cumulative Methane Production Vs. Time 23 Figure 3.3 COD Values Vs. Time 25 FIGURE 3.4 BOD Values Vs. Time 26 FIGURE 3.5 COD Removal Vs. Methane Production 28 FIGURE 3.6 BOD Removal Vs. Methane Production 30 FIGURE 3.7 V o l a t i l e Solids Vs. Time (Batch Tests) 31 FIGURE 3.8 V o l a t i l e Solids Reduction Vs. Gas Production 33 FIGURE 5.1 V a r i a t i o n of V o l a t i l e Acid Concentration 60 FIGURE 5.2 V o l a t i l e Acids Vs. Time f o r Two-Stage Digester Number 1 (30°C) 62 FIGURE 5.3 V o l a t i l e Acids Vs. Time f o r Two-Stage Digester Number 2 (23°C) 63 FIGURE 5.4 V o l a t i l e Acids Vs. Time for Two-Stage Digester Number 3 (10°C) 64 FIGURE 5.5 pH Vs. Time f or Two-Stage Digester Number 1 (30°C) 65 FIGURE 5.6 pH Vs. Time f or Two-Stage Digester Number 2 (23°C) 66 FIGURE 5.7 pH Vs. Time f or Two-Stage Digester Number 3 (10°C) 67 FIGURE 5.8 BOD Vs. TOC f o r Raw Feed and Digester E f f l u e n t s 71 FIGURE 5.9 COD Vs. TOC for Digester E f f l u e n t 72 v i i ACKNOWLEDGEMENT The author i s deeply g r a t e f u l to his supervisor, Dr. W.K. Oldham, for h i s assistance, patience and encouragement during the course of the study. The author i s also g r a t e f u l f o r the help received from L i s a McDonald, Gary B i r t w h i s t l e , David Bond, and Richard Brun. Special thanks go to Linda Blaine for her excellent typing of the f i n a l report. This thesis i s dedicated to the author's father, Dr. J.P. Duncan, without whose encouragement i t might never have been written. Vancouver, B.C. August, 1975 v i i i 1 CHAPTER 1. INTRODUCTION 1.1 General Discussion During the last fifteen years or so there has been a marked trend among farmers involved in the breeding and raising of animals towards con- centration of their stock on ever-decreasing areas of land. This process, while bringing considerable economic benefits to the farmer, has led to cer- tain new agricultural problems, not the least of which i s that of animal waste disposal. Previously, wastes could be spread on arable land as f e r t i - l i s e r , but clearly this concept i s not applicable, except in special instances, in the case of concentrated land-use animal farms. Today i t is generally accepted that some form of biological waste treatment system i s a prerequi- site in the design of such farms''"'"''. Biological treatment takes many forms, but basically there are two major classifications: a) Aerobic_treatment — employs micro-organisms which require dissolved oxygen to obtain their energy. In this instance some indirect means must clearly be found of supplying and distributing sufficient oxygen throughout the system to keep the bacteria f u l l y active. b) Anaerobic_treatment — employs micro-organisms which do not require dissolved oxygen, but employ fermentation, or anaerobic . respiration, to obtain their energy. In this case, there exists no oxygen supply problem, which greatly simplifies matters from a mechanical standpoint. 2 From these brief observations i t becomes clear that anaerobic treatment does offer several advantages over aerobic treatment when con- sidered for use in farming. For a typical treatment f a c i l i t y , such as a lagoon, the anaerobic system w i l l give a lower i n i t i a l cost, and usually a lower maintenance cost, as i t does not require the aeration and mixing equip- ment essential to the aerobic process, and also requires no power to function. Maintenance time, also, w i l l be minimal with the anaerobic system, as mechani- cal parts are reduced to the bare minimum, a few valves and pipes being a l l that are required. However, the anaerobic system, for a l l i t s simplicity, does have i t s drawbacks, among which may be counted the odor problem often associated with anaerobic treatment. This is due to the production of hydro- gen sulphide gas which accompanies the digestion process. The anaerobic sys- tem also tends to give a somewhat inferior effluent quality compared to an [2] aerobic system . Thus i t is not uncommon to find an aerobic pond following .an anaerobic pond in situations where a very high effluent quality is a requirement. The effluent quality problem is offset, however, by the fact that anaerobic systems operate under a very high loading rate, which is important when one considers the high strength of the animal wastes with which farmers are concerned. Thus i t may be seen that both methods have their drawbacks, and careful thought must be given in any design project to the choice of system, having due regard to the type and strength of waste, the location and size of the f a c i l i t y , and the quality of effluent required. This study concerns i t s e l f with investigations into the anaerobic system only. Anaerobic lagooning is at f i r s t sight a very attractive proposi- tion. A l l that is required is an area near the farm buildings sufficient to 3 accommodate a lagoon or group of lagoons large enough to handle the waste output of the farm and produce an effluent of quality acceptable to the appropriate regulatory agencies. No mixing of air-supply equipment i s required. If the lagoon is i n i t i a l l y well constructed, maintenance costs w i l l be largely determined by the frequency at which accumulated sludge has to be removed from the bottom of the lagoons. The rate of sludge build-up is thus of great importance, and a primary objective of the research pro- ject under discussion was to learn something of the degree to which sludge is biologically degraded in such a lagoon. A knowledge of this would enable an estimate to be made regarding maintenance costs for many years ahead, as sludge which is not biologically degraded w i l l eventually have to be physi- cally removed. Clearly the anaerobic lagoon is the simplest possible treatment f a c i l i t y for a high-concentration agricultural operation, and such lagoons [3] have been widely used with success . A good example of an operation of this kind,' using the anaerobic lagoon system, is to be found at Abbotsford, B.C., in the Fraser Valley. Here the National Hog Center has a large indoor pig-raising f a c i l i t y . The treatment system on that farm provided the wastewater for this present study, as well as giving a check on performance under actual, as opposed to laboratory, operating conditions. 1.2 Fundamentals of Anaerobic Digestion The mechanism of anaerobic sludge digestion is shown in Figure 1.1. There are two different groups of bacteria involved in the anaerobic chain. The f i r s t group are known as the "acid-forming bacteria". These take the organic materials in the waste, which are f i r s t l i q u i f i e d by extra-cellular 4 enzymes, and convert them to v o l a t i l e acids, such as a c e t i c , propionic and but y r i c . The second group of micro-organisms are known as the "methane- forming b a c t e r i a " . They take the v o l a t i l e acids already produced by the acid-forming b a c t e r i a and ferment them further to form gaseous products, the main constituents of which are methane and carbon dioxide. Some nitrogen and hydrogen sulphide are also produced at t h i s stage, due to reduction of n i t r a t e s and sulphates, etc. However, these are present only as trace gases i n a well-operating system that does not have to treat wastes high i n ni t r a t e s or sulphates. Obviously both l i n k s . i n the chain are equally important, as the acids formed i n the f i r s t stage s t i l l exert BOD and COD on the receiving waters. The true reduction of the waste occurs only i n the second stage. Thus i t becomes important to understand the response of both acid-formers and gas-formers to changing conditions. I t i s generally assumed that the rate of reaction i s co n t r o l l e d by the rate at which v o l a t i l e acids are con- [2] verted to methane and carbon dioxide . Thus, system f a i l u r e , which occurs when there i s an imbalance i n the process, r e s u l t s i n a build-up of i n t e r - mediate v o l a t i l e acids. To check on t h i s , the v o l a t i l e acids and pH were c a r e f u l l y monitored i n t h i s study, as they are important i n d i c a t o r s of how w e l l the second phase i s proceeding. 1.3 Need.for Further Research During the summer and f a l l of 1970, the C i v i l Engineering Department of The University of B r i t i s h Columbia undertook research intended to provide more de t a i l e d information regarding the design and operation of anaerobic [4] lagoons than had previously been a v a i l a b l e . The program employed a num- ber of single-stage anaerobic digesters, each of twenty-five l i t r e s capacity, 5 SLUDGE INSOLUBLE ORGANIC MATERIAL EXTRACELLULAR ENZYMES SOLUBLE ORGANIC MATERIAL ACID-FORMING BACTERIA VOLATILE ACIDS + CO. H„ OTHER PRODUCTS BACTERIAL CELLS INTERMEDIATE PRODUCTS CH. 4 + co 2 GAS-FORMING BACTERIA ENDOGENOUS >RESPIRATION TO OTHER PRODUCTS + BACTERIA CELLS FIGURE 1.1 Mechanism of Anaerobic Sludge Digestion. 6 which were fed with samples of raw waste obtained from the previously- mentioned National Hog Center at Abbotsford, B.C. A l l the important opera- t i o n a l and treatment-efficiericy'indices were monitored, and the e f f e c t on the system of changes i n operating conditions was investigated. As a r e s u l t of t h i s work, c e r t a i n recommendations for design of anaerobic lagoons were outlined, and a number of recommendations for future studies were made. The present study i s based on three of those recommendations. 1.4 Separation of Settled Solids and Supernatant The topic was recommended for further study i n the previous [4] report . It was f e l t that much of the e f f i c i e n c y of treatment was due to s e t t l i n g out of the s o l i d s i n the waste, and that b i o l o g i c a l degradation was of secondary importance i n the production of a high-quality e f f l u e n t . Thus any increase i n s e t t l i n g e f f i c i e n c y should prove worthwhile. The problem [4] encountered with the single-stage digesters used i n the previous study was that gas lenses would form i n the sludge at the bottom, and would eventually u p l i f t the s o l i d s , mixing them with the supernatant. These r e - suspended s o l i d s would be flushed out with the e f f l u e n t , contributing to lower o v e r a l l e f f i c i e n c y . I t was therefore decided to b u i l d , f o r the present work, a series of two-stage digesters, approximating on a laboratory scale the two-stage lagoon arrangement used i n the Abbotsford operation. In the l a t t e r case, the main lagoon was followed by a smaller lagoon serving as a f i n a l s e t t l i n g and p o l i s h i n g chamber. The pairs of c e l l s used i n the laboratory were both of the same capacity, twelve and one-half l i t r e s each, giving an i d e n t i c a l t o t a l volume to the single-stage units [4] used i n the previous research . The f i r s t c e l l would trap the bulk of the s o l i d s by s e t t l i n g , and much of the b i o l o g i c a l a c t i v i t y would occur 7 here. The supernatant from t h i s c e l l would be run into a second c e l l which would not exhibit as much b i o l o g i c a l a c t i v i t y , but which would serve as a s e t t l i n g chamber, as i t would be le s s subjected to the self-mixing process already described. Thus it;;was expected that some idea of the e f f e c t of increased s e t t l i n g e f f i c i e n c y would be obtainable from the study. 1.5 Sludge Build-up and Gas Production These topics were recommended for further i n v e s t i g a t i o n s i n recommen- [4] dations B and D of the previous study . There were a number of reasons for undertaking such an i n v e s t i g a t i o n . The f i r s t of these was to determine what percentage of the s e t t l e d s o l i d s were " f i x e d " , that i s , would not respond to b i o l o g i c a l treatment, but would accumulate on the bottom of the lagoon, eventually having to be removed mechanically. The second, as outlined above, was that a major objective of the present work was to gain some idea of the r e l a t i v e importance of s e t t l i n g as opposed to b i o l o g i c a l a c t i v i t y i n the degradation process. The only measure of b i o l o g i c a l a c t i v i t y r e a d i l y a v a i l a b l e was gas production and an a l y s i s ; thus gas production had to be linked to COD, BOD and v o l a t i l e s o l i d s removal. Figures existed f or these values for domestic wastes, but they would not necessarily apply to concen- trated animal wastes. [4] To obtain t h i s information, i t was recommended that a ser i e s of batch tests be undertaken, with the various parameters such as BOD, COD, etc., measured i n a f u l l y mixed condition. The temporal reduction i n s o l i d s and oxygen demand could then be g r a p h i c a l l y linked to the amount of gas formed, and unit production figures obtained from these. Also, the propor- t i o n of s o l i d s remaining i n the c e l l s a f t e r b i o l o g i c a l degradation was e s s e n t i a l l y complete would give the desired information regarding the proportion of " f i x e d " s o l i d s . This recommendation formed the basis f o r remainder of the present study. 9 CHAPTER 2. EXPERIMENTAL PROCEDURE 2.1 General Discussion I n i t i a l l y , much time was saved by the use of material and equip- [4] ment from the former experiments . The single-stage digesters previously used were s t i l l i n operation, and hence s t i l l contained v i a b l e organisms. These organisms were used as seed f o r the ongoing research program. Two of the single-stage a c r y l i c digesters were used as the containers f o r the batch experiments. The new two-stage units had to be constructed. Three of these two-stage units were b u i l t . For the batch t e s t s , i t was decided for several reasons to run two concurrent experiments. F i r s t l y , a comparison could be made on the r e s u l t s of one test against those of the other, giving a greater degree of ce r t a i n t y about the r e s u l t s . Secondly, the National Hog Center had for some months been using Acti-Zyme , an enzyme addit i v e intended to stimulate b i o l o g i c a l a c t i v i t y and prevent sludge build-ups at the i n l e t s to the lagoons. This [4] material had not been present i n the samples taken f o r the previous tests , but i t would be present i n the samples used f o r the new serie s of t e s t s . Thus, some idea of i t s e f f e c t , i f any, on the anaerobic a c t i v i t y would be useful. The batch tests were accordingly f i l l e d with waste obtained, by s p e c i a l arrangement, free from Actizyme. One of the batch tests was treated with Acti-Zyme according to the manufacturers i n s t r u c t i o n s , and the other was l e f t untreated. Each batch test unit was f i l l e d to the twenty-four and one- ha l f l i t r e l e v e l i n i t i a l l y . As samples were taken weekly, the volume decreased. Allowance was made for t h i s i n a l l c a l c u l a t i o n s . The batch tests were both Manufactured by Actizyme Co., Box 188, Three Rivers, C a l i f o r n i a , U.S.A. 10 run at room temperature, which held f a i r l y constant at around 22° Ce l s i u s . The two-stage systems were s p e c i a l l y designed to prevent carry- over of suspended s o l i d s from the f i r s t c e l l to the second c e l l , or from the second c e l l to the e f f l u e n t . The old c e l l s had i n l e t and outlet valves at the same l e v e l . Thus short c i r c u i t i n g of some of the i n f l u e n t waste to the e f f l u e n t valve was very possible (Figure 2.1). The new digesters had b a f f l e s i n front of the transfer pipe i n both f i r s t and second-stage c e l l s . This e f f e c t i v e l y prevented any s h o r t - c i r c u i t i n g of t h i s kind (Figures 2.2 and 2.3). The three two-stage units were run at 30° Celsius, room tempera- ture, and 10° Celsius r e s p e c t i v e l y . Thermostatically c o n t r o l l e d heating tapes were used to heat the high-temperature system, while the low-temperature system was equipped with a set of cooling c o i l s i n each c e l l . Thermostats s i m i l a r to those used i n the high-temperature unit c o n t r o l l e d the pumping of cold water through these c o i l s . An immersion r e f r i g e r a t i o n unit kept the cooling water r e s e r v o i r at around 4° C e l s i u s , and submersible e l e c t r i c pumps were used f o r water c i r c u l a t i o n . Thermometers i n the digester l i d s enabled a check on temperature to be kept at a l l times. The objective of the study was to use t h i s and other laboratory equipment to obtain the necessary data f o r a l l objectives of the i n v e s t i g a - t i o n . The experimental procedure may be broken down conveniently as follows. 2.2 Establishment and Operation of the Batch Systems As previously mentioned, the c e l l s from the previous single-stage 11 FIG. 2 - I S INGLE-STAGE DIGESTE R, SHOWING POSSIBILITY OF SHORT-CIRCUITING DURING FEEDING. o Outlet Inlet 12 FIG. 2 - 2 TWO-STAGE DIGESTER,SHOWING FLOW PATTERN DURING FEEDING. FIG. 2 - 3 PLAN OF FLOW DURING FEEDING 13 tests were s t i l l set up i n the laboratory, and s t i l l contained sludge. Although gas production had f a l l e n o f f to p r a c t i c a l l y n i l , i t was f a i r l y safe to assume that a v i a b l e culture of b a c t e r i a s t i l l existed i n t h i s sludge. The two digesters to be used for the batch tests were therefore drained u n t i l only an inch or so of sludge remained. Fresh raw waste from the Abbotsford farm was then used to r e - f i l l the digesters up to the twenty- four and one-half l i t r e l e v e l . Digester number 1 was f i l l e d with wastewater which was s p e c i a l l y c o l l e c t e d to be free of Acti-Zyme, while digester number 2 was treated with an Acti-Zyme concentration of 0.00625% as recommended by the manufacturer. It may be stated that the use of Acti-Zyme on the farm had proved e f f e c t i v e i n keeping pipe blockages and sludge build-ups on the lagoons to a minimum. It was i n t e r e s t i n g to note i n view of t h i s , that, while unit number 1 took twenty-five days to become b i o l o g i c a l l y a c t i v e , number 2 started immediately. Also, as may be seen from the gas production rate curve (Figure 3.1), number 2 had, throughout the t e s t , a higher gas produc- t i o n rate. This w i l l be further discussed i n the next chapter, but c e r t a i n l y on the basis of these i n d i c a t i o n s , the use of Acti-Zyme i n systems of t h i s type would seem to be b e n e f i c i a l . 2.3 Establishment and Operation of the Two-Stage Systems In the case of the two-stage u n i t s , l i t t l e or no a c t i v i t y could be obtained at f i r s t , despite the addition of seed material from the s t i l l a c t ive s i n g l e - c e l l u n i t s . The problem was that the acid-forming b a c t e r i a began to work at once, and produced enough a c i d to send the pH of the sys- tem down to the region of 6.6 to 6.8 i n the primary c e l l s . The pH of the 14 secondary c e l l s held i n the 7.1 - 7.2 range. Previous studies have found that, f o r a balanced system to be achieved, the optimum pH i s i n the order of 6.8 to 7.2, depending on system operating conditions. In the case under discussion, the acid-formers were f a r outperforming the gas-formers i n the primary c e l l s and i n doing so were giving r i s e to a pH range which made i t extremely u n l i k e l y that a balanced state could be achieved. Thus, a f t e r four weeks, i t was decided to r a i s e the pH of the digester contents with lime. Just a f t e r t h i s decision was taken, however, the 30°C digester suddenly exhibited a r i s e i n pH on i t s own, and began to function w e l l . The other two, both of which were at room temperature for s t a r t i n g purposes, remained at t h e i r former unfavourable pH l e v e l s , so a f t e r another two weeks they were treated with lime (CaCOH)^) to a r t i f i c i a l l y r a i s e the pH. Lime was added 0.1 gm. at a time and thoroughly mixed: t h i s process was continued u n t i l the pH was r a i s e d to about' 7.2. This process was spread over several days to avoid shock to the system. At the new pH, both digesters began to function w e l l . The 10°C digester was given a further month to become established, during which time i t was cooled down to 10°C i n a seri e s of 1°C increments of temperature, i n order to give the organisms ample time to acclimatise. Feeding was done once a day and was accomplished by means of a funnel and spigot mounted i n the l i d . No a i r could pass into the digester gas space through t h i s , and the outlet into the digester was a two-opening one which gave h o r i z o n t a l flow i n opposite d i r e c t i o n s , thus minimising s w i r l i n the chamber and disturbance of the bottom deposits (Figure 2.3). It was anticipated that t h i s would tend to reduce s o l i d s transfer from the primary to the secondary c e l l . E f f l u e n t was drawn o f f from the secondary c e l l at the time of feeding.the primary c e l l to maintain a constant l e v e l i n the digesters and e f f e c t a uniform transfer of supernatant from the '. ' 15 primary to the secondary c e l l s . To ensure complete independence of operation of the two c e l l s , the transfer valve between the two was kept closed except during feed addition. I n i t i a l l y , i t had been intended to use a rotary wire screen i n the primary c e l l s to disturb the sludge j u s t s u f f i c i e n t l y to prevent lensing and self-mixing as encountered i n the previous t e s t s . However, i t was found impossible with the equipment a v a i l a b l e to run t h i s screen slowly enough to prevent lensing without i t s e l f causing re-suspension of s o l i d s by mixing. This idea was therefore dropped, and the systems were operated e n t i r e l y undisturbed except by s e l f - a c t i o n i n the sludge. 2.4 Testing Procedure f o r the Influents and E f f l u e n t s The raw waste used i n the t e s t s was obtained i n exactly the same [4] manner as i n the previous ser i e s of tests , with eight-hour composite samples being taken from the manhole clos e s t to the lagoons i n the main o u t f a l l sewer. Polyethylene carboys were used as sample containers, and these were stored i n a r e f r i g e r a t i o n unit u n t i l needed f o r feeding the experimental u n i t s . Unfortunately, during the t e s t s , the farm at Abbotsford was closed down and evacuated due to an outbreak of v i r u s disease among the hog population, and so enough waste to supply the second h a l f of the t e s t s had to be gathered and stored at once, while there was s t i l l a waste- producing population i n the farm. The raw waste samples varied i n strength throughout.the t e s t s , but there were no marked abnormalities among the c h a r a c t e r i s t i c s of the long-term storage samples, as opposed to the previous . short-term storage ones. The only problem was a shortage of feed during the f i n a l stages of the experiment, which l i m i t e d the scope of the work 16 somewhat. 1 Grab samples were used to test the digester e f f l u e n t . E f f l u e n t from both primary and secondary c e l l s of the two-stage systems was tested. Tests were performed weekly for most parameters, and under equilibrium con- d i t i o n s only for the remainder. The object of the experiment was to achieve an equilibrium state at each of several feeding rates, and to run a l l r e l e - vant tests to measure digester performance at these feeding rates. Feeding rates used were 0.5 A/day, 1.0 it/day, and 1.5 H/day, giving a s i m i l a r range [4] of detention times to that used i n the previous research Both mixed raw waste and digester supernatant were tested. In the case of the batch t e s t s , the contents of the digester were f u l l y mixed before t e s t i n g , as actual q u a n t i t i e s , not concentrations, were required i n t h i s case. I n i t i a l l y , some i r r e g u l a r i t y was found i n the batch test r e s u l t s , but exten- sion and standardisation of the pre-sampling mixing time solved t h i s pro- blem. A l l tests were c a r r i e d out according to the procedures given i n Standard M e t h o d s ^ , and further explained i n Chemistry f o r Sanitary Engineers^"'. In the case of both the batch tests and the two-stage digesters, tests run ro u t i n e l y on the ef f l u e n t s were: 1) PH 2) Biochemical Oxygen Demand (BOD) 3) Chemical Oxygen Demand (COD) 4) To t a l and V o l a t i l e Solids (TS and VS) 5) Kjeldahl Nitrogen, both T o t a l and Organic. D i l u t i o n of the samples was necessary, due to the high strength of 17 the waste. These were ar r i v e d at using the previous work as a basis and modifying the figures through repeated t r i a l s . In the case of the two-stage digesters, tests run r o u t i n e l y on the i n f l u e n t and e f f l u e n t i n addition to the above were: 6) V o l a t i l e Acids 7) T o t a l Organic Carbon. The v o l a t i l e acids data were expected to be of great i n t e r e s t / as i n d i c a t o r s of m i c r o - b i o l o g i c a l conditions i n the test u n i t s . V o l a t i l e Acids analysis was accomplished using a Hewlett-Packard Model 5752 B gas chromatograph with a s i x - f o o t by 1/8 inch diameter s t a i n l e s s s t e e l column packed with Porapack Q 50 - 80 Mesh packing, containing 2 percent phosphoric acid. Total Organic Carbon was determined using a Beckman Model 915 T o t a l Organic Carbon Analyser. The data generated was intended to provide a c o r r e l a t i o n between BOD, COD and TOC. I t i s f e l t by some that TOC may, at some time, become a standard t e s t , perhaps even supplanting the comparatively lengthy and involved BOD and COD tests Hence t h i s test was also included to a s s i s t a link-up to possible future r e s u l t s . In addition to the tests previously mentioned, tests run occasionally or at equilibrium only were: 8) T o t a l Phosphate 9) Copper Concentration 10) N i t r a t e 11) A l k a l i n i t y . 18 For the purposes of t h i s study, nutrient values were of l i t t l e importance, as they were studied i n some d e t a i l i n the previous experiments. A check was kept on them i n the present case, but no more. The copper con- centration was of i n t e r e s t , because i t was f e l t that dissolved copper from the brass f i t t i n g s i n the digesters, and also from the copper c o i l s i n the cold digester, might reach concentrations s u f f i c i e n t to have a harmful e f f e c t on the operation of the digesters. 2.5 Testing Procedure for the Evolved Gases The evolved gases were analyzed both q u a l i t a t i v e l y and q u a n t i t a t i v e l y . The Hewlett-Packard gas chromatograph was used to obtain gas composition f i g u r e s . These r e s u l t s would enable methane production to be calculated and t h i s was to be related to s o l i d s and BOD removal. Also they gave some i n d i c a t i o n as to the s t a b i l i t y of the system. To obtain volumes of evolved gas, the same water-displacement [4] tubes used i n the previous work were employed. Later, a p a i r of Alexander Wright and Co., Model M 809 LT Hyde Pattern wet-meters were obtained and used for t h i s purpose. Gas production for the two-stage units was measured only a f t e r equilibrium had been achieved. Continuous use of the wet-meters was not considered advisable from a corrosion standpoint. Samples for gas composition analysis were taken by means of syringes from sampling ports i n s t a l l e d i n the gas outlet l i n e which connected the treatment unit to the gas meter. 2.6 Summary No great d i f f i c u l t i e s , other than the shortage of feed near the 19 end of the t e s t , were encountered during the course of the experiment. A l l equipment worked w e l l , and s u f f i c i e n t data was obtained. The analysis of this, data and discussion of r e s u l t s i s presented i n the following chapters. 20 CHAPTER 3. RESULTS OF BATCH TESTS 3.1 Introduction The f i r s t objective i n the case of the batch tests was to quantita- t i v e l y r e l a t e the rate of methane production to the rates of reduction of s o l i d s , BOD and COD. The second objective was to study the rate and degree of b i o l o g i c a l degradation of the accumulated s o l i d s . Such understanding would enable a good estimate to the made both of the r e l a t i v e importance of bio-degradation as opposed to s e t t l i n g , and of the proportion of s o l i d s which would eventually have to be removed from the lagoon by p h y s i c a l means. [21 Eckenfelder , among others, gives figures f or the r e l a t i o n s h i p between gas production and reduction of s o l i d s , BOD and COD, but a check on these for the p a r t i c u l a r waste was f e l t to be necessary. 3.2 General Discussion of Procedure Gas production was measured on a cumulative basis by taking the twenty-four hour production, converting to STP (having regard to laboratory temperature and pressure) and adding to the previous day's cumulative t o t a l . Hence the t o t a l amount of gas produced was known at any time. Composition of the gas was checked also at weekly i n t e r v a l s . Thus methane production was at a l l times monitored. To check the reduction of s o l i d s , BOD, COD, etc., the f u l l y mixed contents were sampled and analysed weekly. Knowing the concentrations, and the volume remaining i n the digester,, the actual quantity of s o l i d s , BOD, and COD remaining could e a s i l y be cal c u l a t e d . From the data thus obtained, i t was easy to check the published figures for methane production, 21 versus BOD, COD and s o l i d s removal. 3.3 Gas Production and Analysis Some d i f f i c u l t y was experienced i n obtaining consistent readings on the gas chromatograph at f i r s t , but p r a c t i s e i n technique improved t h i s s i t u a t i o n a f t e r a time. The r e s u l t s showed considerable v a r i a t i o n , so an average value was used i n c a l c u l a t i o n s . Values obtained were as follows: Methane: 56 - 65% — avg. 60% Carbon Dioxide: 33 - 42% — avg. 38% Nitrogen: 0.5 - 1.5% — avg. 1% Water: 0.5 - 1.5% — avg. 1% Hydrogen Sulphide: trace Due to the d i f f i c u l t i e s mentioned previously, r e s u l t s were not obtained with any c e r t a i n t y near the beginning of the t e s t s , but the above figures are good f or the time at which production was at a peak and was consistent. Hence the use of the 60% f i g u r e f o r methane content seems T 81 j u s t i f i e d , and accords with accepted figures With regard to volumetric production, Figure 3.1 shows the rate of methane production against time, and Figure 3.2 shows the cumulative methane production. As previously mentioned, the enzyme-treated digester number 2 produced more gas at a l l times than number 1, which was not so treated. However, i t may be mentioned that the s o l i d s content i n i t i a l l y present i n number 2 was appreciably higher than that i n number 1 due to differences i n the s o l i d s content of the raw waste used and t h i s may have some bearing on the higher gas production. T l M E - D A Y S  24 As mentioned e a r l i e r , some d i f f i c u l t y was i n i t i a l l y encountered i n obtaining uniform r e s u l t s from the chemical t e s t s . The mixing of the digester contents was found to be at f a u l t and a standard procedure was adopted of giving the digester 20 minutes of rapid mixing p r i o r to sampling. A f t e r t h i s , l e s s trouble was experienced and so the cumulative gas production p l o t was started from the f i r s t day of t h i s standard procedure, and a l l figures and conclusions drawn from the tests taken a f t e r t h i s time. The gas rate curve exhibits a f a i r l y t y p i c a l form. If i t i s assumed that the gas production rate i s an i n d i c a t o r of b a c t e r i a l population, then one would expect the curve seen on Figure 3.1. The resurgence of pro- duction near the end of the t e s t , at around the 50-day mark, may be due to a change-over of the system to endogenous r e s p i r a t i o n as the substrate becomes depleted. In any case, endogenous r e s p i r a t i o n i s c e r t a i n l y a f a c t o r i n the c l o s i n g phases of a batch test such as t h i s one, and hence r e s u l t s from t h i s portion of the curve may be suspect as f a r as t h e i r a p p l i c a t i o n to a continuous system i s concerned. This w i l l be further referred to i n subsequent sections of the present chapter. 3.4 Relationship of Methane Production to BOD and COD Removal The decreases i n e f f l u e n t BOD and COD with time are shown i n Figures 3.3 and 3.4 r e s p e c t i v e l y . Even a f t e r standardised s t i r r i n g was introduced, the COD test r e s u l t s showed considerable f l u c t u a t i o n . This was due l a r g e l y to the nature of the waste. Wood chips, small pieces of c l o t h and s t r i n g , sawdust, and large lumps of s o l i d surface crust were a l l present i n the digester contents. The presence of a small piece of wood, for example, can s i g n i f i c a n t l y a f f e c t the COD t e s t , causing a higher value to be read than would be the case i f i t were not present. FIG.3-3 COD VALUES VS. TIME 25 10,000 T I M E - D A Y S 27 To allow somewhat for these f l u c t u a t i o n s , a f i t t e d curve was drawn through the points obtained f o r both digesters, p l a c i n g more emphasis on the points obtained from digester number 1, which exhibited f ar l e s s f l u c t u a t i o n than number 2. However, the points from number 2 lay f a i r l y close to t h i s l i n e also, with the exception of some towards the end of the te s t . Using the gas production data and c a l c u l a t i n g the amount of COD removed by b a c t e r i a l a ction as i n Appendix A, Figure 3.5 was developed to show COD removal versus methane production. The f i t t e d COD curve of Figure 3.3 was used i n conjunction with the mean methane production from the two t e s t s , representing an average of both COD removal and gas production [9] from the two un i t s . A l i n e having the slope given by Lawrence and McCarty 3 of 5.62 f t . of methane per l b . of COD destroyed (35.15 I of methane per 100 gm. of COD destroyed) was drawn for comparison. The Lawrence and McCarty fig u r e i s based on t h e o r e t i c a l as well as experimental considera- tions. The l i n e obtained from the average of the two tests correlated quite well with t h i s t h e o r e t i c a l slope, with a tendency toward a s l i g h t l y lower production of methane per unit of COD destroyed. The l i n e s obtained from the i n d i v i d u a l gas production figures from the two digesters are shown also on Figure 3.5, showing that unit number 2 apparently reduced much less COD per unit of methane produced than number 1. However, due to the e r r a t i c nature of the COD r e s u l t s , the approach of taking the average value f o r gas production and applying i t to the f i t t e d COD curve seems to be j u s t i f i e d . On the basis of these f i n d i n g s , i t can be stated that the use of the f i g u r e of 0.35 mi methane per mg of COD destroyed i s p e r f e c t l y j u s t i f i e d i o o 0 100 200 300 C O D R e d u c e d B i o l o g i c a l l y - g rams i n c a l c u l a t i o n s involving the p a r t i c u l a r waste under t e s t . This f i g u r e was accordingly employed i n subsequent c a l c u l a t i o n s . An i d e n t i c a l procedure was followed i n c a l c u l a t i n g BOD removal as r e l a t e d to gas production. The two tests were again averaged out with regard to BOD removal and gas production. The curve obtained i s shown i n Figure 3.6, along with the l i n e obtained from the Lawrence and McCarty fig u r e given above L J. This should hold good for BOD^ ( B o r > u i t ) a s w e l l a s COD, since the b a c t e r i a remove BOD only, and COD i s the sum of the BOD and those c o n s t i - i-l Li tuents which can be chemically oxidised only. In the case of the BOD r e s u l t s , much more consistency was obtained than with the COD r e s u l t s , as the small pieces of wood, hog h a i r s , etc., are not r e a d i l y bio-degradable and thus do not a f f e c t the BOD test as they would the COD t e s t . As may be seen from Figure 3.6, the r e s u l t s agreed quite well [91 with the f i g u r e given by Lawrence and McCarty ., e s p e c i a l l y when averaged out. Thus, i t may again be stated that, on the basis of these r e s u l t s , the use of the figure 0.35 ml methane per mg of BOD destroyed i s f u l l y j u s t i f i e d i n c a l c u l a t i o n s concerning the hog waste i n question. 3.5 Relationship of Methane Production to V o l a t i l e Solids Removal V o l a t i l e s o l i d s removal was calculated as i n Appendix A. Since a l l samples were taken f u l l y mixed, the concentration of the samples was the same as the o v e r a l l concentration i n the digester, enabling the weight of v o l a t i l e s o l i d s i n the digester to be calculated for the date of sampling. Figure 3.7 shows the v o l a t i l e s o l i d s concentration i n the two digesters p l o t t e d against time, s t a r t i n g from the beginning of the improved mixing i n the digesters for sampling purposes. To allow for the f l u c t u a t i o n s i n the F I G . 3 - 6 BOD REMOVAL VS. METHANE PRODUCTION. FIG. 3 - 7 VOLATILE SOLIDS VS. TIME (BATCH TESTS). Vo - sanos 3 i I I V I O A 32 r e s u l t s , f i t t e d curves were drawn through the points obtained, and these were used to obtain the f i g u r e s f o r v o l a t i l e s o l i d s reduction as r e l a t e d to methane production. Figure 3.8 shows the gas production r e l a t e d to the removal of v o l a t i l e s o l i d s , with the published range of values p l o t t e d f o r comparison. [2] 3 Eckenfelder gives a range of 17 to 20 f t . of gas produced per l b . of VS destroyed with a methane content of around 65%. This i s equivalent to 69 to 81 £ of methane produced per 100 gm. of VS destroyed. In the same work, Eckenfelder notes that these values are a maximum, assuming complete conver- sion of v o l a t i l e s o l i d s to methane. This may explain why the slope obtained [21 i n i t i a l l y f o r digester number 2 i s lower than the Eckenfelder figures , although the same digester gave a higher gas production per unit of BOD [91 destroyed than the Lawrence and McCarty f i g u r e , as seen from Figure 3.6. V o l a t i l e s o l i d s may be destroyed by conversion to v o l a t i l e acids, with no [21 production of methane and no reduction i n BOD . The low i n i t i a l slope i s probably therefore due to a lag i n e f f i c i e n c y of the methane-forming bac- t e r i a as opposed to the acid-forming b a c t e r i a . A f t e r the f i r s t two points [2] on the curve, the slope steepens to match that given by Eckenfelder . The r e s u l t s from digester number 1 agreed quite well with the Eckenfelder value. Eckenfelder's f i g u r e was" accordingly used i n further c a l c u l a t i o n involving the hog waste under t e s t . With regard to s o l i d s degradation i n the batch units used i n the t e s t s , o v e r a l l v o l a t i l e s o l i d s reduction from s t a r t to f i n i s h was 19% f o r digester number 1 and 22% for digester number 2. Since gas produc- t i o n had f a l l e n o f f to nearly zero jLn both cases at the conclusion of te s t i n g , any further v o l a t i l e . s o l i d s reduction would be of an extremely F I G . 3 - 8 33 34 long-term nature. The v o l a t i l e / t o t a l s o l i d s r a t i o i n both cases averaged out at approximately 69% at the end of the t e s t , i n d i c a t i n g that at l e a s t 30% of the remaining s o l i d s were d e f i n i t e l y non-degradable i n nature. Thus, allowing a digester or lagoon to stand unfed w i l l not prove of much value i n reducing sludge accumulations. 35 CHAPTER 4. TWO-STAGE DIGESTER RESULTS 4.1 Introduction The primary objective of the flow-through experiment was to acquire knowledge concerning the r e l a t i v e performance of a two-cell anaerobic sys- tem as compared to a single-stage system of the type employed i n previous [4] research . Results obtained from the two systems at corresponding L i q u i d Detention Times (LDT) were compared. A further area of study was the deter- mination of the manner and degree of the contribution of each of the two c e l l s i n a two-stage system to the o v e r a l l treatment e f f i c i e n c y . 4.2 General Discussion As previously mentioned, the one problem encountered during the test program was a shortage of feed during the l a t t e r stages of the experi- ment. A f t e r completion of the tests for the 50-day and 25-day LDT stages, i t was r e a l i s e d that i n s u f f i c i e n t feed remained to complete the whole schedule of t e s t i n g on a l l u n i t s . A dec i s i o n was therefore made to run the f i n a l 17-day LDT test on the 30°C and 10°C digesters only. Even with t h i s l i m i t a t i o n , only 13 days of feeding could be completed with the remaining feed. Thus there i s considerable doubt that equilibrium was achieved or even approached on the 17-day LDT. In f a c t , the e f f l u e n t BOD, COD and s o l i d s values were s t i l l r i s i n g , although not r a p i d l y , at the conclusion of the t e s t . Hence the r e s u l t s f or percentage removals on the 17-day LDT are probably a l i t t l e high. The amounts of methane produced by the immediate (24-hour) degradation of the feed material, and by the continuing degradation of the 36 accumulated sludge were .determined by multiplying the t o t a l methane produc- t s t i o n by percentages determined i n previous research with s i m i l a r sub- st r a t e . The c h a r a c t e r i s t i c s of the raw feed and e f f l u e n t s r e s p e c t i v e l y are shown i n Tables I and I I . These data form the basis f o r r e s u l t s discussed herein. TABLE I. Average Raw Waste C h a r a c t e r i s t i c s . THEORETICAL LDT DAYS BOD,, ppm COD ppm SOLIDS % 50 4410 24,100 T 2.964 V 2.146 25 6185 25,232 T 2.696 V 1.508 17 4113 20,500 T 2.049 V 1.459 4.3 Effectiveness of Modifications to C e l l The two-stage flow-through digesters had two c e l l s each of 12.5 Z capacity, giving the same 25 Z capacity of the previously used single-stage [4] units . The f i r s t c e l l of the two-stage system could be considered as a h a l f - s i z e single-stage unit f o r comparison purposes. Comparative perfor- mance should thus be achieved i n a given s i n g l e - c e l l reactor used i n pre- [4] vious research and the f i r s t c e l l of a two-stage reactor being fed at ha l f the d a i l y rate of the s i n g l e - c e l l reactor. Percentage removal figures f o r the two-cell types are shown i n Table I II and mass removal figures i n Table IV. 37 TABLE I I . Average E f f l u e n t C h a r a c t e r i s t i c s . BOD ppm TEMP LDT DAYS^\^^ 30°C 23°C 10°C 1st CELL 2nd CELL 1st CELL 2nd CELL 1st CELL 2nd CELL 50 565 470 777 578 3583 2266 25 1035 850 1735 1352 5966 5680 17 1450 850 4100 4660 COD ppm TEMP LDT^ S\. DAYS^v. 30°C 23°C 10°C 1st CELL 2nd CELL 1st CELL 2nd CELL 1st CELL 2nd CELL 50 6780 5203 7630 6120 10,626 7,983 25 6995 5750 7680 6190 12,710 11,363 17 7690 6330 11,020 10,200 TOTAL AND V( )LATILE SOL IDS % TEMP LDT^v. DAYS^V. 30°C 23°C 10°C 1st CELL 2nd CELL 1st CELL 2nd CELL 1st CELL 2nd CELL T 5 0 V 0.657 0.400 0.545 0.320 0.661 0.402 0.651 0.362 0.717 0.467 0.631 0.408 T 25 ° V 0.676 0.411 0.569 0.329 0.728 0.450 0.617 0.366 0.769 0.492 0.705 0.438 T 1 7 V 6.730 0.455 0.629 0.379 0.909 0.629 0.780 0.525 TABLE I I I . Comparison of Percent Removals i n O r i g i n a l 25 I C e l l and Modified 12.5 £ C e l l . 12.5 DAY LDT UNIT 25 I CELL 12.5 I CELL (MODIFIED) Temp °C 30 23 10 30 23 10 % BOD Rem. 84.0 80.5 27.0 83.2 72.0 3.6 % COD Rem. 77.0 78.0 52.0 72.2 69.6 49.6 % VS Rem. : 79.0 80.5 74.5 72.7 73.0 67.4 % TS Rem. 74.0 74.0 69.5 74.9 73.0 71.5 25 DAY LDT UNIT 2 5 I CELL 12.5 CELL (MODIFIED) Temp 0 C 30 23 10 30 23 10 % BOD Rem. 88.0 85.0 43.5 " 87.2 82.4 18.7 % COD Rem. 82.5 81.5 64.5 71.9 68.3 55.7 % VS Rem. 79.5 81.0 79.0 81.4 81.2 78.2 % IS Rem. 73.5 74.0 73.0 77.8 77.6 75.8 TABLE IV. Comparison of Mass Removed Per Unit C e l l Volume f o r 25 I Single-stage C e l l and 12.5 I F i r s t C e l l of Double-Cell Digester. 25 DAY LDT UNIT 25 I CELL 12.5 I MODIFIED CELL Temp °C 30 23 10 30 23 10 * BGT>5 Removed 344 332 169 153 145 33 COD Removed 985 976 769 692 658 538 VS Removed 661 674 657 698 697 671 TS Removed 829 834 823 922 921 898 12.5 DAY LDT UNIT 25 I CELL 12.5 £ MODIFIED CELL Temp °C 30 23 10 30 23 10 * B0D5 Removed 334 320 107 206 178 8 COD Removed 1009 1022 681 729 702 500 VS Removed 601 612 566 438 423 406 TS Removed 803 803 754 808 787 770 A l l removals i n mg/Jt of cell/day. 40 Points to note were: a) Percentage BOD removals were almost i d e n t i c a l at the 30°C temperature, but dropped o f f much more sharply with decreasing temperature In the case of the f i r s t 12.5 £ c e l l of the two- stage system. On a mass basis, the removal of the 12.5 £ c e l l never approached that of the 25 £ c e l l . This may be due to the fa c t that the BOD of the material used as feed f o r the 12.5 £ c e l l was very much lower than that used for the 25 £ c e l l . The r e s u l t i n g decrease i n the amount of a v a i l a b l e substrate per unit of c e l l volume could give r i s e to the lower mass removal figures obtained. Also, any s h o r t - c i r c u i t i n g during feeding of the c e l l would lower the actual mass feeding rate f o r the c e l l i n terms of raw waste feed, as the amount of raw waste remaining i n the c e l l a f t e r feeding would be lower. This would favour the single-stage digesters, as the two-stage c e l l s were designed to avoid s h o r t - c i r c u i t i n g . However, t h i s e f f e c t would be o f f s e t to an indeterminate degree by the increase i n e f f l u e n t strength due to the s h o r t - c i r c u i t e d raw feed. b) COD removal, both on a percentage and mass ba s i s , was greatly superior f o r the 25 £ c e l l as compared to the 12.5 £ two-stage c e l l . Again the two-stage c e l l type f e l l o f f i n removal e f f i c i e n c y much more with decreasing temperature than did the 25 £ c e l l . This decrease i n effectiveness of treatment with decreasing temperature noted also f o r BOD, can only be due to decrease of b i o l o g i c a l a c t i v i t y with decreasing temperature. Since both two-stage and single-stage systems were subjected to s i m i l a r temperature v a r i a t i o n s , the fact that the two-stage system responded more noticeably can only be due to a differ e n c e 41 In the nature of the b a c t e r i a l population i n the c e l l . The ba c t e r i a i n the two-cell system were less tolerant of tempera- ture changes. Since no b a c t e r i o l o g i c a l studies were undertaken i n the course of t h i s research, the actual nature of the b a c t e r i a present i n both cases i s unknown. c) T o t a l s o l i d s removal was, on a percentage basis, s l i g h t l y better f o r the 12.5 £ two-stage type of c e l l , and was s i g n i f i - cantly better on a mass basis f o r t h i s c e l l . V a r i a t i o n with temperature was only s l i g h t i n the case of t o t a l s o l i d s removal. V o l a t i l e s o l i d s removal was s l i g h t l y better f o r the 12.5 £ c e l l at the 25-day LDT, but was not as good at the 12.5-day LDT as that obtained with the 25 £ c e l l . From the t o t a l s o l i d s removal f i g u r e s , i t can be stated that the modified c e l l does give somewhat improved s e t t l i n g e f f i c i e n c y as predicted. The v a r i a t i o n i n removal e f f i c i e n c i e s of the other parameters can be a t t r i - buted to differences i n the composition of the feed material, which was con- siderable i n the case of BOD content, and to s h o r t - c i r c u i t i n g i n the 25 £ c e l l s . The BOD concentration of the feed used.in the two-stage c e l l s was less than [4] h a l f that used i n the single-stage experiments and thus there was much l e s s r e a d i l y degradable material present. This would not a f f e c t parameters such as t o t a l s o l i d s removal, which are dependent l a r g e l y on s e t t l i n g , but could appreciably a f f e c t parameters such as BOD which can be expected to be more dependent on b a c t e r i o l o g i c a l action. S h o r t - c i r c u i t i n g , which was probably present during feeding of the 25 £ c e l l s would give better apparent r e s u l t s for these c e l l s due to the actual feeding rate being lower than the t h e o r e t i c a l l e v e l . These points w i l l be discussed further i n the subsequent sections of th i s t h e s i s . 42 4.4 Overall Treatment E f f i c i e n c y The r e s u l t s obtained from percentage removal i n each c e l l of the two-cell u n i t s , and for the o v e r a l l system, are shown i n Tables V - VIII. Results noted were: a) Removal e f f i c i e n c y decreased with decreasing LDT: t h i s e f f e c t was more marked from 25 days to 17 days than from 50 days to 25 days. The decrease was not nearly so pronounced i n the case of s o l i d s removal as i n the cases of COD and, p a r t i c u l a r l y , BOD. This seems to i n d i c a t e that much of the BOD and a c e r t a i n portion of the COD i s of a non-settleable nature, depending on b a c t e r i o l o g i c a l action f o r removal. The figures f o r the 10°C digester, which showed minimal b a c t e r i o l o g i c a l a c t i o n , support t h i s view, as a v o l a t i l e s o l i d s removal of 64% was obtained with e s s e n t i a l l y no BOD removal at the 17—day LDT. The conclu- sion here i s that most of the s e t t l e a b l e v o l a t i l e s o l i d s are [4] e s s e n t i a l l y non-degradable, as found i n the e a r l i e r research and i n the batch tests discussed i n Chapter 3. b) A s i m i l a r e f f e c t was noticed f or the decrease i n e f f i c i e n c y with decreased temperature. Again the decrease i n s o l i d s removal was somewhat l e s s pronounced than the decrease i n COD removal, and very much less pronounced than the decrease i n BOD removal. Since the only f a c t o r i n the treatment process s i g n i f i c a n t l y affected by temperature i s b i o l o g i c a l a c t i o n , i t may be concluded that most of the s o l i d s and much of the COD are removed by s e t t l i n g , but that supernatant 43 biological action removes most of the BOD. This conclusion i s further examined in the following section. 4.5 Settling vs. Biological Degradation Tables IX - XII show the percentage BOD removal figures broken down into removals due to immediate bacteriological action and removals due to settling. These figures were arrived at by taking the 24-hour methane produc- tion due to the raw feed addition and multiplying this by the appropriate coefficient as discussed in Chapter 3 to obtain the amount of each parameter biologically degraded each day. Knowing the feed and effluent characteristics, the total removal per day could be simply calculated, and from this the removal due to settling was obtained. A l l figures were then converted to percentages. Points to note were: a) Immediate bacteriological degradation of vo l a t i l e solids was of very small importance compared to the settling effect. Removal due to bacteriological degradation never exceeded 13% of the total removal, and this was only achieved at the 30°C tempera- ture. b) The proportion of COD removal due to bacteriological action was somewhat higher than that found for solids, reaching a maximum of 19.3% of the total removal, also at the 30°C temperature, but again the settlement factor predominated. c) BOD removal was far more dependent on immediate bacteriological degradation than either solids or COD. At the 30°C temperature, TABLE V. Percentage BOD Removals. \ . TEMP LDT \ . DAYS^X. 1st CELL 2nd CELL OVERALL 30 23 10 30 23 10 30 23 10 50 87.2 82.4 18.7 2.1 4.5 29.8 89.3 86.9 48.5 25 83.2 72.0 3.6 \ 4.1 6.2 4.6 87.3 78.2 8.2 17 64.7 0 14.7 0 79.4 0 TABLE VI. Percentage COD Removals. TEMP L D T ^ \ DAYS^\. 1st CELL 2nd CELL OVERALL 30 23 10 30 23 10 30 23 10 50 71.9 68.3 55.9 6.5 6.3 11.0 78.4 74.6 66.9 25 72.2 69.6 49.6 4.9 5.6 5.3 77.1 75.2 54.9 17 62.5 46.3 6.6 3.9 69.1 50.2 45 TABLE VII. Percentage V o l a t i l e Solids Removals. TEMP°C LDT DAYS 1st CELL 2nd CELL OVERALL 30 23 10 30 23 10 30 23 10 50 81.4 77.6 78.2 3.7 0.4 2.7 85.1 78.0 80.9 25 72.7 73.0 67.4. 5.5 4.1 3.6 78.2 77.1 71.0 17 68.9 56.9 5.2 7.1 74.1 64.0 TABLE VIII. Percentage Total Solids Removals. TEMP°C LDT ̂ " v . DAYS 1st CELL 2nd CELL OVERALL 30 23 10 30 23 10 30 23 10 50 r 77.8 77.6 75.8 5.8 0.4 2.9 83.6 78.0 78.7 25 •74.9' 73.0 71.5 7.4 4.1 2.4 82.3 77.1 73.9 17 64.4 55.7 13.6 6.3 78.0 62.0 TABLE IX. Percentage BOD Removals Due to S e t t l i n g and B a c t e r i o l o g i c a l Action. N. TEMP°C LDT DAYS 1st CELL 2nd CELL OVERALL 30 23 10 30 23 10 30 23 10 >°1 75.3 11.9 61.2 21.2 12.0 6.7 0 2.1 3.1 1.4 0 29.8 75.3 14.0 64.3 22.6 12.0 36.5 » s 44.6 38.6 34.1 37.9 3.3 0.3 4.1 0 2.4 3.8 0 4.6 47.9 38.9 36.5 41.7 3.3 4.9 1 7 s 57.6 7.1 0 0 8.6 6.1 0 0 66.2 13.2 0 0 B = B a c t e r i o l o g i c a l S = S e t t l i n g TABLE X. Percentage COD Removals Due to S e t t l i n g and B a c t e r i o l o g i c a l Action. TEMP°C LDT DAYS 1st CELL 2nd CELL OVERALL 30 , 23 10 30 23 10 30 23 10 » s 13.8 58.1 11.2 57.1 2.2 53.7 0 6.5 0.6 5.7 0 11.0 13.8 64.6 11.8 62.8 2.2 64.7 «I 10.9 61.3 8.4 61.2 0.8 48.8 1.0 3.9 0.6 5.0 0 5.3 11.9 65.2 9.0 66.2 0.8 54.1 » s 11.6 50.9 0.9 45.4 1.7 4.9 0 3.9 13.3 55.8 0.9 49.3 47 TABLE XI. Percentage V o l a t i l e Solids Removal Due to S e t t l i n g and B a c t e r i o l o g i c a l Action. TEMP°C LDT DAYS 1st CELL 2nd CELL OVERALL 30 23 10 30 23 10 30 23 10 7.9 73.5 6.4 74.8 1.2 77.0 0.0 3.7 0.3 1.5 0.0 2.7 7.9 77.2 6.7 76.3 1.2 79.7 9.3 63.4 7.1 63.1 0.7 66.7 0.9 4.6 0.5 0.0 3.6 10.2 68.0 7.6 68.2 0.7 70.3 » I ' • 8.3 60.6 0.6 56.3 • 1.2 4.0 0.0 7.1 9.5 ' 64.6 0.6 63.4 B = B a c t e r i o l o g i c a l S = S e t t l i n g TABLE XII. Percentage Total Solids Removal Due to S e t t l i n g and B a c t e r i o l o g i c a l Action. ^ ^ ^ ^ TEMP°C LDT DAYS ' 1st CELL 2nd CELL OVERALL 30 23 10 30 23 10 30 23 10 *> s 5.7 72.1 4.6 73.0 0.9 74.9 0.0 5.8 0.2 0.2 0.0 2.9 5.7 77.9 4.8 73.2 0.9 77.8 » s 5.2 69.7 4.0 69.0 0.4 71.1 0.5 6.9 0.3 3.8 0.0 2.4 5.7 76.6 4.3 72.8 0.4 73.5 » s . 5.9 58.5 0.5 55.2 0.9 12.7 0.0 6.4 6.8 71.2 0.5 61.5 48 a maximum of 84% of the t o t a l removal was achieved by b a c t e r i a i n the supernatant. A f a i r portion of the BOD did appear to be s e t t l e a b l e , however, with a maximum of 41.7% removal due to s e t t l i n g being obtained i n the 10°C reactor. The n e g l i g i b l e removals for the 10°C digester at the 25- and 17-day LDT values are probably due to the fact that the BOD of the raw feed was decreasing with time at the end of the 25-day LDT and throughout the 17-day LDT. This would c l e a r l y give r i s e to decreased apparent removal figures f o r t h i s digester, e s p e c i a l l y since the 10°C digester would be the slowest by f a r to react to changes i n feed strength. The decrease i n feed strength would not show as soon i n the e f f l u e n t of the 10°C digester as i n that of the others. Thus the r e s u l t s from the 10°C digester are probably suspect, and more r e l i a n c e can be placed on the r e s u l t s from the other two un i t s . d) At a given LDT, t o t a l s o l i d s removal by s e t t l i n g was e s s e n t i a l l y independent of temperature. Differences i n t o t a l s o l i d s removal were mainly due to b a c t e r i o l o g i c a l reduction of non-settleable v o l a t i l e s o l i d s . Much of the gas produced was due to the degradation of s e t t l e d v o l a t i l e s o l i d s , but t h i s was allowed for as previously mentioned. 4.6 Relative Importance of 1st and 2nd C e l l Tables V - XII show removal figures f o r each c e l l of the system as well as the o v e r a l l removals. From these tables the following points are noted: 49 a) The f i r s t of the two c e l l s was responsible for the major portion of the removal of a l l parameters. The general trend was to a greater second-cell contribution at a lower LDT, which would be l o g i c a l l y expected due to the r i s i n g strength of the 1st c e l l e f f l u e n t at lower LDT supplying more feed material to the second c e l l . b) B a c t e r i o l o g i c a l a c t i v i t y i n the second c e l l was minimal at any temperature, and non-existent at 10°C. The contribution of the second c e l l was thus almost e n t i r e l y due to s e t t l i n g . c) The contribution of the second c e l l to treatment e f f i c i e n c y , although small, was nonetheless s i g n i f i c a n t i n view of the high raw feed strength. For a waste of the strength used i n t h i s experiment, a 1% reduction i n any of the important parameters represents a worthwhile gain i n e f f l u e n t q u a l i t y . 4.7 S i n g l e - s t a g e vs. Two-Stage System Two methods of comparison were employed. The f i r s t method was [4 to compare r e l a t i v e performance figures obtained from the e a r l i e r research and from the present study at corresponding LDT values. These figures are presented i n Tables XIII and XIV. The d i f f i c u l t y here was that only two corresponding LDT's, 50 days and 25 days, were a v a i l a b l e , and the data from the 50-day LDT was rather incomplete for the single-stage t e s t . A further point was the p r o b a b i l i t y , discussed i n Section 4.3, of varying r e s u l t s due to feed c h a r a c t e r i s t i c s and short c i r c u i t i n g . A second method of comparison was therefore used also. This e n t a i l e d the consideration of the r e s u l t s from the f i r s t c e l l of the flow-through systems as being-from a single-stage TABLE XIII. Comparison of Single- and Two-Stage Systems. (LDT =50 days) UNIT SINGLE-STAGE (25 H) TWO-STAGE (25 I) Temp°C 30 23 10 30 23 10 % BOD Removed 89.0 87.0 — 89.3 86.9 48.5 % COD Removed 83.0 80.5 — 78.4 74.6 66.9 % VS Removed — 85.1 78.0 80.9 % TS Removed — 83.6 78.0 78.7 TABLE XIV. Comparison of Single- and Two-Stage Systems. (LDT = 25 days) UNIT SINGLE-STAGE (25 I) TWO-STAGE (25 £) Temp°C 30 23 10 30 23 10 % BOD Removed 88.0 85.0 43.5 87.3 78.2 8.2 % COD Removed 82.5 81.5 64.5 77.1 75.2 54.9 % VS Removed 79.5 81.0 79.0 78.2 77.1 71.0 % TS Removed 73.5 74.0 73.0 82.3 77.1 73.9 51 unit of 12.5 I capacity. At a feeding rate of 0.5 H/day, t h i s gave an LDT of 25 days, which could be compared with the r e s u l t s from the t o t a l system of two c e l l s at a feeding rate of 1.0 &/day, giving a corresponding 25-day LDT. The only problem here would be the p o s s i b i l i t y of the volume differ e n c e of the two systems giving r i s e to some scale e f f e c t . Also only one LDT, that of 25 days, could be compared. The r e s u l t s f o r t h i s comparison are shown i n Table XV. [4] Regarding the comparison of the o r i g i n a l single-stage t e s t s and the two-stage t e s t s , points to note are: a) BOD removal was almost i d e n t i c a l at the 30°C temperature f o r both systems, but decreased much more r a p i d l y with decreasing temperature at the same LDT i n the case of the two-stage t e s t . b) The v a r i a t i o n i n COD removal figures was somewhat wider, the single-stage unit giving considerably better removal at both LDT's. c) V o l a t i l e s o l i d s removal at the 25-day LDT, the only one f o r which comparison was po s s i b l e , was of a very s i m i l a r l e v e l at 30°C, but again the two-stage unit f e l l o f f i n performance more r a p i d l y with decreasing temperature. d) To t a l s o l i d s removal was considerably better f o r the two-stage system at the two higher temperatures of 30°C and 23°C, but was almost i d e n t i c a l at 10°C. Again the two-stage unit f e l l o f f i n performance with decreasing temperature, while the single-stage unit remained e s s e n t i a l l y constant regardless of temperature. TABLE XV. Comparison of 1st Cell,Only Vs. 1st and 2nd C e l l s at 25-Day LDT. Percentage Basis UNIT 1st CELL (12.5 I) • TWO-STAGE (25 I) Temp°C 30 23 10 30 23 10 % BOD Removed 87.2 82.4 18.7 87.3 78.2 8.2 % COD Removed 71.9 68.3 55.9 77.1 75.2 54.9 % VS Removed 81.4 81.2 7.8.2 78.2 75.8 71.0 % TS Removed 77.8 77.6 75.8 82.3 77.1 73.9 Mass Removed Basis UNIT 1st CELL (12.5 I) TWO-STAGE (25 I) kTemp°C 30 23 10 30 23 10 * BOD Removed 153 145 33 213 193 20 COD Removed 692 658 538 779 761 554 VS Removed 698 697 671 471 456 428 TS Removed 922 921 898 850 831 796 A l l removals i n mg/1 of cel l / d a y . 53 From these observations, i t may be stated that the two-stage units did give better s e t t l i n g e f f i c i e n c y , but v a r i a t i o n s i n the composi- t i o n of the feed used i n the two tests caused lower e f f i c i e n c i e s to be recorded f or removal of parameters other than t o t a l s o l i d s . The r e s u l t s could well be explained i f i t were the case, for example, that there was less s e t t l e a b l e BOD and COD i n the waste used for the two-stage t e s t s , thus placing more emphasis on b i o l o g i c a l action as the chief removal f a c - tor . The fa c t that removal f e l l o f f with temperature f o r the two-stage system, but did no do so to any great extent i n the single-stage system indicates that there was more dependence on b a c t e r i o l o g i c a l action for the two-stage removals than for the single-stage. This would c l e a r l y be t i e d up with feed c h a r a c t e r i s t i c s such as s e t t l e a b l e v o l a t i l e s o l i d s , dissolved BOD or COD as opposed to s e t t l e a b l e BOD or COD. C l e a r l y non- s e t t l e a b l e , BOD, COD or v o l a t i l e s o l i d s can only be removed by b a c t e r i a l action, so a higher l e v e l of these parameters i n s o l u t i o n or suspension would c e r t a i n l y give r i s e to a more marked temperature e f f e c t . Thus the i n d i c a t i o n i s that, although the two-stage system does give improved s e t t l i n g , as indicated by the t o t a l s o l i d s removal, the percentage removals of BOD, COD and v o l a t i l e s o l i d s as a r e s u l t of t h i s w i l l be considerably influenced by r e l a t i v e l y minor changes i n raw waste c h a r a c t e r i s t i c s . With regard to the comparison of the f i r s t c e l l only against the t o t a l two-cell system, both at 25-day LDT, points to note are: a) On a percentage basis, BOD removal was again, the same at the 30°C temperature, but f e l l o f f more r a p i d l y f o r the two-stage 54 system with decreasing temperature. However, the BOD of the feed used at 25-day LDT for the two-cell system was considerably higher than that used for the run from which the 25-day LDT figures were obtained for the f i r s t c e l l only. As a r e s u l t of t h i s , on a mass basis the two-cell system gave considerably better removal figures except at the 10°C temperature, at which temperature the BOD removal was very small i n both cases. b) On both a percentage and a mass basis, the COD removal f or the two-cell system were considerably better again with the excep- t i o n of the 10°C u n i t s , which were almost i d e n t i c a l i n removal e f f i c i e n c y . c) On a percentage basis, t o t a l s o l i d s removal was very s i m i l a r f o r both systems, although on a mass basis the f i r s t c e l l alone gave better r e s u l t s . This can be explained by the fac t that the feed t o t a l s o l i d s concentration was higher f or the f i r s t c e l l only, than f or the two-cell system. I f the percentage of s e t t l e a b l e s o l i d s were the same i n both cases, as seems not unreasonable, then c l e a r l y the lower t o t a l incoming s o l i d s l e v e l f o r the two- stage unit would r e s u l t i n a lower t o t a l s o l i d s mass removal, as found i n t h i s t e s t . The same observation holds true f o r v o l a t i l e s o l i d s removals, which were f a r lower i n the case of the two-cell system than f o r the f i r s t - c e l l only. In t h i s instance, lower removals were obtained both on a percentage and a mass basis for the two-cell system. On the basis of these r e s u l t s , i t would appear that there i s l i t t l e d i f f e r e n c e i n s e t t l i n g capacity of the two-cell system and the f i r s t c e l l of 55 that system working alone. The two-cell system gave considerably better per- formance on a BOD - COD removal b a s i s , despite the absense of appreciably improved s e t t l i n g , i n d i c a t i n g that, compared to the f i r s t c e l l alone, the two-stage system i s somewhat more e f f e c t i v e b i o l o g i c a l l y . Again, feed compo- s i t i o n differences appear to have a marked e f f e c t on the r e s u l t s obtained. A true evaluation of the r e l a t i v e merits of the two systems would require a series of concurrent tests on both s i n g l e - and two-stage systems of the same type using i d e n t i c a l feed material. These, and other conclusions, w i l l be discussed i n Chapter 6. 4.8 Copper Concentrations Towards the end of the s e r i e s of tests made on the two-stage digesters, i t was decided to sample the c e l l contents and determine the l e v e l s of copper present due to corrosion of the brass digester f i t t i n g s and copper cooling c o i l s . This was not done i n the case of the e a r l i e r T41 tests , but i t would be of i n t e r e s t since copper has a s i g n i f i c a n t e f f e c t on the BOD t e s t , causing reduction i n apparent BOD values, and w i l l also i n h i b i t b a c t e r i o l o g i c a l action. Copper concentrations were determined by the atomic absorption technique using samples of the f u l l y mixed digester contents. Two separate sets of analyses were made, the f i r s t at three months from the conclusion of the experiment and the second at the conclusion of t e s t i n g . Results are shown i n Table XVI. [2] Eckenfelder gives data i n d i c a t i n g that the r e s u l t s given by the BOD,, test w i l l be reduced up to 50% by the presence of 4.0 mg/£ copper. Levels below 1.0 mg/£ copper have a r e l a t i v e l y i n s i g n i f i c a n t e f f e c t on t h i s t e s t . However, McKee and Wolfe state that the concentration necessary 56 to reduce BOD,, by 50% has been v a r i o u s l y determined at between 8.4 mg/£ and 35 mg/£. Thus there i s a considerable d i v e r s i t y of information regarding t h i s topic. TABLE XVI. Copper Concentrations i n Two-Stage Digester C e l l s CELL NUMBER COPPER CONCENTRATION (mg/£ Cu)* Test Number 1 Test Number 2 } 30°C 1.67 4.30 0.67 1.66 2^2 } 2 3 ° C 2.80 6.30 4.38 1.53 } 10°C 11.60 3.20 16.12 1.90 Test Number 1 — 3 months p r i o r to end of experiment. Test Number 2 — at end of experiment. *A11 samples f u l l y mixed and acid-digested to ensure a l l copper i n s o l u t i o n . With regard to t o x i c i t y of copper to micro-organisms, McKee and Wolfe'"'^ report that copper concentrations as low as 0.1 to 0.5 mg/ft are toxic to c e r t a i n micro-organisms. The Committee on Water Quality C r i t e r i a recorded''"'""'""' that sewage organisms i n p a r t i c u l a r are i n h i b i t e d to 50% of oxygen u t i l i s a t i o n by 21 ppm copper. The copper i n h i b i t s a c t i v i t y by tying up the proteins i n the key enzyme systems, preventing these from reacting ,, [12]. normally With regard to the r e s u l t s of the tests reported herein, the copper concentrations are c e r t a i n l y i n the range reportedly required to cause 57 s i g n i f i c a n t decreases i n the r e s u l t s given by the BOD test and to cause some i n h i b i t i o n of m i c r o b i o l o g i c a l a c t i v i t y . In p a r t i c u l a r , the l e v e l s encountered i n digester number 3 (!0°C) which had cooling c o i l s made of pure copper, were high, enough not only to a f f e c t the BOD t e s t , but to s i g n i f i c a n t l y a f f e c t u t i l i s a t i o n of oxygen by sewage organisms. Since no m i c r o b i o l o g i c a l studies were undertaken to c l a s s i f y species of b a c t e r i a present, no estimate can be made as to the actual e f f e c t of the copper upon the r e s u l t s of these t e s t s , but i t can be said that the r e s u l t s were almost c e r t a i n l y affected to some degree. It i s s i g n i f i c a n t that the 10° digester showed p r a c t i c a l l y no gas production throughout the test s . However, the measured BOD of the e f f l u e n t from t h i s unit was close to that of the raw waste, as reported e a r l i e r i n t h i s chapter, so in d i c a t i o n s are that the BOD test was not affected to a serious degree. [4] The single-stage digesters used i n the e a r l i e r work were con- structed of i d e n t i c a l materials. Hence, the comparative r e s u l t s between the two,forming the main basis of t h i s study,should s t i l l be valid,although absolute values may be suspect. It should also be noted that with the exception of c e l l s 2-1 and 3-1, the copper concentrations i n the other c e l l s . f e l l s i g n i f i c a n t l y over the three months between the f i r s t and second t e s t s . This indicates that copper was not going into s o l u t i o n as f a s t as i t had been e a r l i e r . This was undoubtedly due to the formation of a layer of copper compounds on the metal surfaces which formed a b a r r i e r between the copper and the digester contents. When the c e l l s were emptied at the conclusion of t e s t i n g , a crust of corrosion products was i n fa c t found on a l l m e t a l l i c f i t t i n g s . I t i s reasonable to conclude that operation over a longer period 58 of time would probably bring copper concentrations down to acceptable l e v e l s . However, the use of brass or copper f i t t i n g s i n experiments of t h i s nature i s c l e a r l y shown by these r e s u l t s to be undesirable and p o t e n t i a l l y ruinous to the obtaining of accurate absolute values for experimental data. Stain- less s t e e l should be u t i l i s e d whenever possible. 59 CHAPTER 5. VOLATILE ACIDS AND TOTAL ORGANIC CARBON RESULTS 5.1 Introduction Neither V o l a t i l e Acids nor To t a l Organic Carbon (TOC) were [4] studied i n depth i n the previous work , due to lack of s u i t a b l e equip- ment. However, the necessary instruments were a v a i l a b l e f o r the present work, and accordingly i t was decided to study these parameters i n some d e t a i l . [131 I t has been reported by McGhee that v o l a t i l e acids l e v e l s i n batchrfed systems vary considerably, reaching a peak approximately 4 hours a f t e r feeding and f a l l i n g back to a base l e v e l at about 16 hours a f t e r feeding. The peak i s t y p i c a l l y greater than the base l e v e l by a factor of 5 or 6. Figure 5.1 shows a t y p i c a l curve of v o l a t i l e a c i d concentration versus the number of hours a f t e r feeding; the data are taken from the work [131 of McGhee . The v o l a t i l e a c i d data presented herein are base-level f i g u r e s , since they were determined immediately p r i o r to feeding. 5.2 V o l a t i l e Acids and pH Levels i n Anaerobic Systems [12] It i s considered that a v o l a t i l e acids l e v e l above 2,000 mg/5, i n anaerobic systems i s an i n d i c a t i o n that trouble i s imminent. This r i s e i n v o l a t i l e acids can depress pH to the point where the methane b a c t e r i a are severely i n h i b i t e d , and thus cannot keep pace with the acid-formers. In t h i s s i t u a t i o n , the v o l a t i l e acids w i l l continue to r i s e , and the pH w i l l f a l l further, r e s u l t i n g i n a t o t a l cessation of methane production and an upset i n the system. 60 FIG. 5-1 VARIATION OF VOLATILE ACID CONCENTRATION. (AFTER McG.HEE [ l* 3) 61 V o l a t i l e acids are not i n themselves toxic to methane-forming ba c t e r i a , as laboratory studies have shown that i t i s possible to operate a digester at l e v e l s of up to 20,000 mg/£ v o l a t i l e acids as long as the pH [12] i s maintained above 6.5 . I n such cases, the rate of matabolism of the methane-formers i s l i m i t e d by the concentration of soluble cations added [12] while n e u t r a l i s i n g the v o l a t i l e acids to the desired pH . Lime i s commonly used for t h i s purpose, and i t was employed during t h i s experiment as reported e a r l i e r . Lime i s most su i t a b l e for t h i s purpose, as calcium i s the least soluble cation usable i n a n e u t r a l i s i n g s i t u a t i o n , and thus causes the l e a s t possible upset to the methane formers; i t i s also very cheap and r e a d i l y a v a i l a b l e . 5.3 V o l a t i l e Acids and pH i n the Two-stage Digester The measured v o l a t i l e acids l e v e l s i n the two-stage digesters are presented i n Figures 5.2 to 5.4, with pH data being presented i n Figures 5.5 to 5.7. Points to note are: a) The v o l a t i l e acids concentrations i n the 30°C and 23°C digesters were generally r e l a t i v e l y low, i n the 100-600 mg/£ range, i n d i c a t i n g that both b i o l o g i c a l systems were functioning i n a stable manner. The pH.of these two digesters was also i n a stable range, being of the order of 7.2 - 7.6. At no time did the v o l a t i l e acids l e v e l s of either system exceed 1,200 mg/£, except f o r number 1-1, which went to 3,500 when the feed rate was changed from 1.0 to 1.5 £/day. Recovery was quick from t h i s load, however, as the pH at t h i s time remained at around 7.3, i n d i c a t i n g a very well-buffered system.  Q I O V O I 1 3 0 V S V 9 J i ! l s a i o v 3 1 I 1 V 1 0 A Q l O v * 0 I 1 3 D V S\/ 9 J i ! l / 6 " J S Q I D V 3 1 I 1 V 1 0 A 65 FIG. 5 - 5 pH VS. TIME FOR 2- STAGE DIGESTER # I ( 3 0 ° C ) . CM I O X3 m a T3 c a> O CM O O O CO o O T J CO • >- < O CO 2 o o CO CD 'st CM Hd O CO CO CO CD   68 b) The second c e l l s of the 30°C and 23°C digesters exhibited l e s s f l u c t u a t i o n i n v o l a t i l e acids than did the f i r s t c e l l s . This would be expected, as they were not subject to the d i r e c t shock loading of raw waste. At no time did these second c e l l s i n d i - cate any tendency to upset, although i t must be borne i n mind that b a c t e r i o l o g i c a l a c t i v i t y i n the second c e l l s was very l i m i t e d , as reported e a r l i e r . This l i m i t e d a c t i v i t y cannot be a t t r i b u t e d to upset conditions on the basis of these r e s u l t s , but must have been due to l i m i t e d a v a i l a b i l i t y of substrate to further b i o l o g i c a l degradation. c) The 10°C digester gave r e s u l t s for both v o l a t i l e acids and pH which indicated upset conditions. V o l a t i l e acids l e v e l s of 2,000 to 3,000 mg/ft were encountered, together with pH values down i n the 6.8 - 6.9 range. I t can be stated that conditions i n t h i s unit were not conducive to good metabolism of the methane - formers, and i n f a c t there was very l i t t l e methane produced from these c e l l s . The v o l a t i l e acids did r i s e with increased feed rate, but remained i n the same general range, i n d i c a t i n g that the acid-formers themselves were i n a state of i n h i b i t i o n . This may have been due to.the copper l e v e l s reported e a r l i e r , but the temperature was doubtless a factor here. d) The two digesters (30°C - 23°C) which operated i n the stable range of pH and v o l a t i l e acids values had no trouble adjusting to the 1.0 H/day feed rate and the 30°C digester adapted well to the 1.5 Z/day rate. I t was unfortunate that lack of feed material, prevented more extensive t e s t i n g of the 1.5 Z/day 69 rate. In t h i s regard, i t would c e r t a i n l y have been most i n s t r u c t i v e to observe the response of the 23°C digester to the 1.5 Z/day feeding rate, but the shortage of feed material at the close of the test forced the abandonment of feeding of one digester and the 23°C unit was chosen. e) The 30°C digester showed an abrupt jump i n v o l a t i l e acids concentration at the change of feed rate to 1.5 H/day, but recovered quickly, i n d i c a t i n g excellent system s t a b i l i t y . 5.4 Total Organic Carbon Measurements i n the Two-Stage Digesters The T o t a l Organic Carbon analyser has been developed over the past decade or so into a very precise instrument capable of giving excellent and reproducible r e s u l t s with a minimum expenditure of time and e f f o r t . A sample can be tested i n two minutes quite e a s i l y . Thus, i t i s f e l t by many to be a f a r better test than the currently-used BOD and COD analyses. Robbins, Howells, and K r i z have conducted an extensive program of research into the use of TOC i n c h a r a c t e r i s a t i o n of swine wastes. Their published conclusions''"'"^'"'"'''' may be stated as follows: a) TOC i s a more reproducible and convenient test for swine wastes than either BOD or COD. b) The BOD test i n p a r t i c u l a r i s not applicable to characterisa- t i o n of concentrated swine wastes and lagoon e f f l u e n t s , due to the presence of toxic substances, high s o l i d s contents and to errors associated with the high d i l u t i o n requirements for 70 t e s t i n g . c) No general B0D/T0C c o r r e l a t i o n was found for concentrated swine wastes and e f f l u e n t s , although once the e f f l u e n t s were d i l u t e d by runoff, TOC r e s u l t s could be used to y i e l d a fig u r e f o r BOD for estimating purposes. In addition, the TOC test i s f a r more convenient for t h i s than the BOD t e s t . d) The v a r i a t i o n of the BOD/TOC r a t i o gives an i n d i c a t i o n of the presence of toxic materials, as the BOD test responds most markedly to these, whereas TOC does not. e) The actual value of the BOD/TOC r a t i o gives an i n d i c a t i o n of the ease of biodegradation and degree of s t a b i l i s a t i o n of a swine wastewater. In general, t h i s r a t i o i s le s s than one, except f o r raw wastes or those which are not s t a b i l i s e d to a great degree. For the present work, TOC tests were performed r o u t i n e l y on both feed material and e f f l u e n t from a l l three digesters. Hence, a wide range of TOC, BOD- and COD values was generated for comparison purposes. These data were plotted as shown i n Figures 5.8 and 5.9 for BOD and COD respec- t i v e l y . The r e s u l t s f or BOD agreed w e l l with the observations recorded r 151 by Robbins et a l L J. The BOD/TOC r a t i o varied from 1.35 down to 0.22 with a " b e s t - f i t " mean of 0.57. The f l u c t u a t i o n could be caused by the presence of the copper a f f e c t i n g the BOD t e s t , based on the t o x i c i t y i n d i c a t i o n theory. Robbins et a l ^ " ^ reported r a t i o s ranging from 0.41 to 1.25 for concentrated e f f l u e n t s .  72 FIG. 5 - 9 COD VS.TOC FOR DIGESTER EFFLUENTS . 73 A s i m i l a r v a r i a t i o n was found f o r COD. The COD/TOC r a t i o varied from 1.8 to 3.35 with a " b e s t - f i t " mean of 2.71 Since no other work has apparently been done on COD and TOC c o r r e l a t i o n s , no comparison can be made here. It should be noted that a regression equation was not obtained for these curves, as the wide spread of r e s u l t s would make t h i s a meaning- less exercise. The chief conclusion i n fac t i s that due to a number of factors not i d e n t i f i e d , c o r r e l a t i o n between BOD and COD with TOC i s very poor f o r concentrated swine wastes. The e f f l u e n t s had BOD/TOC r a t i o s of well under one i n most cases, i n d i c a t i n g that they were well s t a b i l i s e d i n general. There was no marked difference i n d i s t r i b u t i o n between f i r s t and second c e l l e f f l u e n t s , and thus these could not meaningfully be plo t t e d as separate graphs. 74 CHAPTER 6. .CONCLUSIONS AND RECOMMENDATION 6.1 Introduction In t h i s chapter, the basic findings of the study are summarised. The conclusions reached as a r e s u l t of t h i s study may conveniently be d i s - t r i b u t e d under several d i s t i n c t subheadings. These are presented i n the following sections. 6.2 Conclusions from Batch Test Results a) On the basis of the batch test r e s u l t s , the use of the figu r e [91 given by Lawrence and McCarty , 0.35 ml methane produced per mg of COD or BOD^ destroyed, i s f u l l y j u s t i f i e d i n c a l c u l a - tions involving anaerobic digestions of hog wastes. b) The same tests indicated that the use of the figures published [2] by Eckenfelder , 69 to 81 £ of methane produced per 100 gm of V o l a t i l e Solids destroyed, i s f u l l y j u s t i f i e d i n c a l c u l a t i o n s involving anaerobic digestion of hog wastes. c) The p r a c t i s e of allowing a lagoon to stand unfed f o r a period of time i n order to reduce s o l i d s build-up i s of l i m i t e d value, as the majority of s o l i d s i n hog waste of the type employed i n these tests are not r e a d i l y biodegradable. The main benefit r e s u l t i n g from allowing a lagoon to stand would be reduction of sludge volume i n the lagoon r e s u l t i n g from gravity c o n s o l i - dation of the s o l i d s layer i n the lagoon, which i s occurring anyway whether the lagoon i s fed or not. B i o l o g i c a l degradation 75 would be of secondary Importance. 6.3 Conclusions from Two-Stage Continuous-Feed Digester Results a) S e t t l i n g i s the major removal mechanism i n the case of COD and s o l i d s removal. However, BOD removals are dependent on b i o l o g i c a l action to a considerable extent. As a r e s u l t of these observations, i t i s c l e a r that waste c h a r a c t e r i s t i c s play a major r o l e i n governing treatment e f f i c i e n c y of a system. Based on r e s u l t s presented i n t h i s report, i t may be stated that r e l a t i v e l y minor changes ,in waste c h a r a c t e r i s t i c s can have an appreciable e f f e c t on system e f f i c i e n c y . BOD removal i n p a r t i c u l a r i s very s e n s i t i v e to changing conditions i n both the treatment process and the waste c h a r a c t e r i s t i c s . b) The f i r s t c e l l i n a two-cell system achieves most of the removal obtained i n the system. The contribution of the second c e l l i s small by comparison, but with a high-strength waste such as hog waste, even small percentage removals are s i g n i f i c a n t on a mass basis . The second c e l l i n these tests exhibited very l i t t l e b i o l o g i c a l a c t i v i t y , i n d i c a t i n g that the supernatant from the f i r s t c e l l was not r e a d i l y bio-degradable by the anaerobic process despite i t s high r e s i d u a l BOD. Second-cell removals were almost e n t i r e l y due to s e t t l i n g . 6.4 Conclus ions Regarding Two—Cell Vs. One—Cell Systems a) The two-cell system does give improved s e t t l i n g e f f i c i e n c y and s o l i d s removal capacity. Whether or not t h i s improves BOD or 76 COD removal s i g n i f i c a n t l y w i l l depend to a great extent on waste c h a r a c t e r i s t i c s such as s e t t l e a b l e BOD and COD, dissolved BOD and COD, r e l a t i v e percentage of v o l a t i l e and t o t a l sus- pended s o l i d s , etc. b) The two-cell system does have a higher capacity f or removal on a mass basis, of BOD and COD, based on the comparison of removal e f f i c i e n c y of the f i r s t c e l l only against that of both c e l l s (same t o t a l detention time i n each system). Thus, the unit loading capacity of the two-cell system i s somewhat higher. 6.5 Conclusions from Copper, V o l a t i l e Acids, pH and T o t a l Organic Carbon Tests a) The anaerobic system can accept changes i n loading rate without upset conditions developing, provided temperatures are-kept up i n a range favourable to b i o l o g i c a l a c t i v i t y . At the 10°C temperature, the methane-forming b a c t e r i a are adversely affected, and are unable to cope with sudden upswings i n v o l a t i l e acids concen- t r a c t i o n s , or i n fa c t to keep up at a l l with the acid formers, even at steady feed rates. b) Lack of b i o l o g i c a l a c t i v i t y observed i n the second c e l l s was not due to upset conditions, and must therefore have been due to l i m i t e d a v a i l a b i l i t y of substrate,, or some other f a c t o r not determined. c) The use of brass or copper f i t t i n g s i n equipment used for anaerobic digestion gives r i s e to s e r i o u s l y high l e v e l s of copper i n the substrate, and i s p o t e n t i a l l y disastrous from 77 the point of view of obtaining meaningful r e s u l t s . d) There i s no r e a d i l y i d e n t i f i a b l e c o r r e l a t i o n between BOD, COD, and TOC. 6.6 Recommendat ions a) In the design of an anaerobic lagoon system for hog wastes, use of two lagoons of a given t o t a l volume, rather than a sin g l e lagoon of the same volume, should be considered where space permits, as loading capacity and removal e f f i c i e n c y w i l l be increased for a very small increase i n the area of the lagoon system. b) In design of such a system, waste c h a r a c t e r i s t i c s should be thoroughly investigated, as they w i l l play a major r o l e i n determining the e f f i c i e n c y of treatment which can be obtained from a given lagoon system. The waste should be tested for such c h a r a c t e r i s t i c s as s e t t l e a b l e BOD and COD, s e t t l e a b l e s o l i d s , and t r e a t a b i l i t y of the raw supernatant r e s u l t i n g from s e t t l i n g out of the s o l i d s . This should form an impor- tant part of any future laboratory research a l s o . c) Anaerobic lagoons w i l l only exhibit worthwhile b i o l o g i c a l a c t i v i t y at temperatures above approximately 20°C. At 10°C, there i s p r a c t i c a l l y no a c t i v i t y . Hence, for year-round operation, such lagoons should be considered as s e t t l i n g ponds only, and the two-stage system i s better suited to t h i s than the single-stage system. 78 d) A l l laboratory equipment used i n anaerobic research should have s t a i n l e s s s t e e l f i t t i n g s , as copper and brass both cause unfavourably high l e v e l s of copper to dissolve i n the substrate. e) Consideration should be given to the possible improvements i n treatment e f f i c i e n c y r e s u l t i n g from aeration of the second lagoon. The supernatant from the f i r s t c e l l of a two-cell system i s not, on the basis of t h i s study, r e a d i l y degradable by anaerobic b a c t e r i a , but may well respond to aerobic t r e a t - ment. 79 BIBLIOGRAPHY [1] Loehr, R.C.; "The Challenge of Animal Waste Management", Animal Waste Management (1969), C o r n e l l U n i v e r s i t y , pgs. 17-22. [2] Eckenfelder, W.W.; Water Quality Engineering for P r a c t i s i n g Engineers (1970), Barnes and Noble, Inc., New York. [3] Loehr, R.C; "Effl u e n t Quality From Anaerobic Lagoons Treating Feedlot Wastes", Journal WPCF, Vol. 39, No. 3 (March 1967), pgs. 384-391. [4] Nemeth, L.; "Anaerobic Treatment Analysis of Concentrated Hog Wastes", M.A.Sc. Thesis, University of B r i t i s h Columbia ( A p r i l 1972). [5] APHA, AWWA, WPCF; "Standard Methods for the Examination of Water and Wastewater" (1965), American Public Health Association, Inc., 12th E d i t i o n . [6] Sawyer, C.N. and McCarty, P.L.; "Chemistry for Sanitary Engineers" (1967), McGraw-Hill, 2nd E d i t i o n , New York. [7] Emery, R.M., Welch, E.B., and Christman, R.F.; "The Total Organic Carbon Analyzer and i t s A p p l i c a t i o n to Water Research", Journal WPCF, Vol. 43, No. 9 (1971), pgs. 1834-1844. 18] Taiginides, E.P., Baumann, E.R., Johnson, H.P., and Hazen, T.E.; "Anaerobic Digestion of Hog Wastes", Journal of A g r i c u l t u r a l Engineering Research ( B r i t i s h ) , V o l. 8, No. 4 (1963), pgs. 327-333. [9] Lawrence, A. and McCarty, P.L.; "Kinetics of Methane Fermentation i n Anaerobic Waste Treatment", Technical Report No. 75, Department of C i v i l Engineering, Stanford U n i v e r s i t y , Stanford, C a l i f o r n i a (1967). [10] McKee and Wolfe; "Water Quality C r i t e r i a " , C a l i f o r n i a State Water Resources Control Board, P u b l i c a t i o n 3-A (1963). [11] "Water Quality C r i t e r i a 1972 - A Report of the Committee on Water Quality C r i t e r i a " , EPA P u b l i c a t i o n R3-73-033 (March 1973). [12] McKinney, R.E.; "Microbiology for Sanitary Engineers", McGraw-Hill (1962). [13] McGhee, T.J.; " V o l a t i l e Acid Concentration i n Batch Fed Anaerobic Digesters", Water and Sewage Works, Vol. 118, No. 5 (May 1971), pgs. 130-132. [14] Robbins, J.W.D., Howells, D.H., and K r i z , G.J.; "Stream P o l l u t i o n From Animal Production Units", Journal WPCF, Vol. 44, No. 8 (August 1972), pgs. 1536-1544. 80 [15] Robbins, J.W.D., K r i z , G.J., and Howells, D.H.; "Total Organic Carbon Determination on Swine Waste E f f l u e n t s " , Transactions of the ASAE, Vol. 15 (1972), pgs. 105-109. APPENDIX A. Sample Calculations a) Amount of COD removed by b a c t e r i o l o g i c a l action i n batch tests Volume of l i q u i d i n digester at s t a r t of experiment = 23.7 Z (after removal of test sample) COD at s t a r t = 30,600 ppm. •: T o t a l COD i n digester = 23.7 x 30,600 = 726,000 mg. At time of next sampling: 0.2 Z sample taken, leaving volume of 23.5 Z i n digester. COD of sample = average COD i n digester since sample taken f u l l y mixed. COD = 27,200 ppm. COD remaining i n digester = 27,200 x 23.5 = 640,000 mg. COD removed i n sample = 0.2 x 27,200 = 5,440 mg. •: COD removed by b a c t e r i o l o g i c a l action = Drop i n COD i n digester - COD removed i n sample. COD removed b a c t e r i o l o g i c a l l y = (726,000 - 640,000 ) - 5,440 mg = 80,560 mg. b) I d e n t i c a l procedure followed to ca l c u l a t e BOD and VS removed by b a c t e r i o l o g i c a l a c t i o n .

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