The thermophilic anaerobic digestion process

I¢%ter Research rot. 1I. pp. 129 to 1-t3. Pergamon Press t977. Printed in Great B~tain.

REVIEW PAPER THE THERMOPHILIC ANAEROBIC DIGESTION PROCESS H. O. BUHR Department of Chemical Engineering. University of Cape Tov,n. Rondebosch. CP 7700, South Africa and J. F. ANDREWS Department of Civil Engineering, University of Houston, Houston, TX 77004. U.S.A.

(Received 30 March 1976) Abstract---The paper presents a review of the thermophilic anaerobic digestion process, which has been successfully employed in Los Angeles, U.S.A., and Moscow, U.S.S.R. The chief advantages of operating at higher, thermophilic temperatures are lower detention times, improved dewaterability of the sludge, and increased destruction of pathogenic organisms. It is anticipated that operation of the thermophilic process wilt require closer control than is conventionally practised. A dynamic process model is proposed, and the expected response of a thermophilic digester to changes in feed rate, concentration and temperature is illustrated.

NOMENCLATURE

a,b,c,d A,B, AI, A,_ t"i

(COz)o (CO,)T, Cr D E~. E2 Fo Ft G H÷

HCO~ HS ! kd kr K.

stoichiometric coefficients constants molality of ith species, mol kg- t concentration of dissolved CO_,, mol 1-t concentration of dissolved CO2 at equilibrium, tool I-l concentration of total carbonic species, (HCO~ + CO2o), mol 1-* gas volume conversion factor, 1 m o l - t activation energies for synthesis and degradation, respectively, J m o l - t liquid feed rate to digester, 1 d - t effluent withdrawal rate, 1 d - t net growth rate per unit mass of organisms, d - t hydrogen ion concentration, tool 1-t concentration of bicarbonate ion, mol 1- t un-ionized substrate (volatile acids) concentration, mol 1-t ionic strength, ½X; ci ~ , tool kg-x biological decay coefficient, d - t rate constant for death due to toxicity, d-t dissociation constant for acetic acid, tool

PH:o

Pr Q Qcs.,, Qco2 R Rs R~ SSr t ' T

Tx V V6

X Ycm:x

Yco,.:x

I- ~

KH Kt Kt.a K, Kt NHI Nr p (prefix) Pco; w.R. 11/2--~,

dissociation constant for ammonia, mol I- l Henry's Law constant for CO2/water, mol 1- t atm- 1 inhibition coefficient, mol Ioverall mass transfer coefficient for COz, d -x saturation coefficient, mol 1-t first dissociation coefficient for carbonic acid system, mol 1-t concentration of ammonium ion, mol 1- t concentration of total nitrogen species, mol 1- t negative log partial pressure of CO, in gas phase, atm

Y,~:x

Y,v:s Z

7

7H" t.t t~ 0 ~Subscript 0 129

vapor pressure of water at operating temperature, atm total pressure in gas phase, atm rate of dry gas flow from digester, 1 d - t production rate of methane and carbon dioxide, respectively, in digester off-gas, ld-t gas constant, J tool-t K - t rate of biolo~cal COs production per unit digester volume, mol 1- * d - t rate of CO, transfer to gas phase per unit digester volume, mol 1- t d - t ionized substrate (volatile acids) concentration, tool 1-x total substrate (volatile acids) concentration, tool 1-' time, d absolute temperature, °K concentration of conservative toxic material, mol 1-1 digester liquid volume, I volume of gas space in digester, 1 organism concentration, mol 1-* yield of methane per mole of organisms formed carbon dioxide yield rate of nitrogen consumption per mole of organisms formed yield of organisms per mole of substrate consumed net concentration of (cations-anions) other than C O l , H ÷, HCO~, N H I , O H - and S-, tool l - t dielectric constant activity coefficient activity coefficient for hydrogen ion specific growth rate, d maximum specific growth rate, d-1 temperature, :C ionic valence input conditions

130

H.O. Burro and J. F. ANDREWS INTRODUCTION

There has been an interest in thermophilic anaerobic digestion since at least i930 when Rudotfs & Heukelekian (1930) conducted bench scale tests on the batch process. Since that time. several other investigators have studied the process in the laboratory and there have been at least four plant scale investigations. Operation of anaerobic digesters in the thermophilic range of temperatures (50-60-C) offers several potential advantages over conventional mesophilic {30--38"C) operation. Included among these are: Ca) increased reaction rates with respect to the destruction of organic solids. (b) increased efficiency with respect to the fraction of organic solids destroyed. (c) improved solids-liquid separation. (d) increased destruction of pathogenic organisms. Increased reaction rates would permit the use of lower detention times, thereby decreasing capital costs, and increased organic solids destruction would decrease the mass of solids for ultimate disposal while simultaneously yielding larger quantities of methane gas for in-plant energy requirements. There are, of course, interactions between these two items, in that the fraction of organic solids destroyed is a function of residence time for both thermophilic and mesophilic digestion. As an example of increased reaction rates-, conversion from mesophilic to thermophilic digestion in Moscow, U.S.S.R. (Popova & Bolotina, 1964) permitted the detention time to be decreased from 18 to 9 days with a reduction in total gas output of only 3 to 4 ° . Improved solids-liquid separation is of importance if digested sludge is to be dewatered prior to further processing or ultimate disposal. In 1953 Garber (1954) studied the vacuum filtration of thermophilic (50~C) anaerobic digested sludge and reported greatly improved vacuum filter yields for thermophilic as compared to mesophilic (29 °) sludge, together with a lower coagulant demand. Improved solids-liquid separation would also be of value in land application of digested sludge, by decreasing the quantity of liquid sludge for disposal and thereby lowering the cost of transport to the disposal site. The increased destruction of pathogenic organisms at thermophilic temperatures is of special significance in light of the current trend toward land disposal of digested sewage sludge. In 1962, Popova & Bolotina (1964) discussed the use of thermophilic (51°C) anaerobic digestion in Moscow, U.S.S.R., and stated, "The most essential advantage of this process is the sanitary quality of the thermophilic sludge. According to the sanitary officials of the health department, viable eggs of helminths are absent from such a sludge." The public health aspects of the disposal of digested sludge on land are of considerable concern throughout the world and in this connection it should be noted that pasteurization of digested sludge prior to land disposal is being used at the Niersverband in West Germany (Kugel, 1972a, b) and was being con-

sidered for a large treatment plant in Vienna in 1971 Won der Emde & Muller. 1972). However. according to the U.S.S.R. experience, pasteurization would appear to be unnecessaD ' when thermophilic anaerobic digestion is practised. Possible disadvantages of the thermophilic process, on the other hand. may include the following: (a) High energy requirement for heating. (b) Poor supernatant quality. (c) Poor process stability. The use of the process in Moscow, U.S.S.R., as well as simple heat balances indicate that the energy requirements lbr heating are not excessive and can easily be supplied by the produced gas. An exception would be the digestion of sludge with a low solids concentration. There are mixed reports in the literature concerning the quality of the supernatant produced in thermophilic anaerobic digestion (Fischer & Greene, 1945: Garber, 1954). However, it appears likely that the thermophilic supernatant will contain larger quantities of dissolved materials. There are also mixed reports (Fischer & Greene, 1945; Heukelekian & Kaplovsky, 1948; Garber 1954) concerning process stability, especially with respect to perturbations in temperature. However, Garber (1954) experienced no difficulty with process stability thus indicating, at least for large installations, that the process can be operated in a stable manner. Nevertheless, it is expected that poor process stability could be the most significant disadvantage of the process since this is also one of the major problems encountered with the mesophilic process. It is well known that as biological processes approach environmental extremes (pH. salinity, temperature, etc.), fewer species are able to survive and the process becomes less resistant, or more unstable, with respect to change, it would therefore be expected that the thermophilic anaerobic digestion process might not be as stable as mesophilic digestion and. accordingly, adequate process control would be an important requirement for the successful application of thermophilic digestion as a treatment operation. The purpose of this paper is twofold. First, to review the results presented by a number of investigators, in order to identify the operating characteristics and the advantages that may accrue from the application of the thermophilic anaerobic digestion process and, second, to present a dynamic model for the process as a firs't step toward a study of the operational stability and controllability of the process. PREVIOUS W O R K

Bench scale studies

In 1930, Rudolfs & Heukelekian (1930) studied the batch digestion of primary municipal sludge at thermophilic temperatures. Using thermophilic digestion, the yield of gas per gram of volatile matter added was higher and a greater percentage of the volatile matter was destroyed. The composition of the gas was

The thermophilic anaerobic digestion process not materially affected by digestion at the higher temperatures. They concluded that the digestion of primaD" municipal sludge at temperatures of 4%55-C was feasible and that the time required for digestion of seeded solids in this temperature range was materially shorter than for the mesophitic temperature range, provided that the seed sludge had been produced under thermophilic conditions. In further studies, Heukelekian 1t930~ found that gasification was essentially complete in 14 days at a temperature of 50:C. Eleven and twelve days were required, respectively, for temperatures of 55 and 60:C. Temperatures above 60:C resulted in a retardation of gasification. The gas yield/g of volatile matter added, volatile matter reduction and decomposition of nitrogenous substances was greater and the decomposition of fats slightly less for 14 days digestion at 50-C than for 35 days digestion at 22:C. Fair & Moore 11932, 1937) studied the batch digestion of both primary and waste activated sludge, and concluded that the digestion time could be substantially shortened by thermophilic digestion. They reported an optimum temmperature of 50~C for both primary and waste activated sludge. In 1948, Heukelekian & Kaplovsky used batch digesters to study the effect of changes of temperature on thermophilic digestion. Their experimental procedure was to make pulse changes (durations of 2-5 days) from the normal operating temperature of 50~C down to either 40 or 20'C. Similar experiments were also performed on seed sludge developed at 40"C: however, in addition, upward pulse changes from 40 to 5 0 C were made on these digesters. These workers observed that rates of gasification were greatly affected by the drop in temperature for both the 40 and 50~C digesters, with gasification completely stopping at 20:C. However, no lasting effect on the subsequent digestion was noted when the digesters were returned to their original operating temperatures. The results obtained from the experiments in which the 40:(2 digesters were raised to temperatures of 50°C were somewhat inconclusive. The digestion continued at this higher temperature; in most cases, however, the time required for digestion was somewhat longer than if no change had been made to the higher temperature. Golueke (1958) studied the effects of temperature on the digestion of primary sludge using bench scale apparatus but with daily feeding and mixing. All of his studies were conducted at a detention time of 30 days and an organic loading of 1,4 kg volatile matter m-3d -t (0.091bft-3d-~). As would be expected at this long detention time and low solids loading, there was no appreciable difference in solids destruction for temperatures ranging from 35 to 5YC. However, very little solids destruction was observed at 65~C. Gas production rates, gas composition, and general sludge appearance were also similar at temperatures ranging from 35 to 60-C. Two significant differences were that the sludge produced at 50 and 60°C had substantially

131

better dewatering characteristics, as measured by the amount of coagulant required, and sludges produced at the higher temperatures had higher volatile acid concentrations. There was an especially sharp increase from approximately 500 mgl-~ of volatile acids at 50:C to 21./00mg 1-~ at 55~C. Golueke also observed that when a population was not well established in a particular digester, it became very sensitive to any abrupt drop in temperature, which occasionally happened because of a temporary failure of equipment. Thus, a drop of 5~C lasting 16--18 h in the 55:C digester resulted in a decline in destruction of volatile matter from 49.9 to 38";, over a 4-day material balance period: similar reductions were experienced at 60 and 55~C. In 1961, Malina (1961) studied the effects of temperature on the digestion of waste activated sludge in bench scale apparatus using daily feeding and continuous gas recirculation for mixing. At a temperature of 52.5-C he was able to obtain 42°0 volatile matter destruction at a detention time of 6 days and an organic loading of 4.8kg volatile matter m-3d -* (0.3 lb ft- 3d- 1). Corresponding reductions in volatile matter under the same conditions but at temperatures of 42.5 and 32.5:C were 41 and 39~-o, respectively. He also observed higher volatile acids concentrations at the higher temperatures and somewhat less gas production at 52.5C than at 32.5~C, with gas production at 42.5-C being lower than at either of the other two temperatures. Mixed results have been reported for the thermophitic anaerobic digestion of industrial wastes. In tests on a desulphated waste from the production of alcohol from sugar cane molasses, Stander & Elsworth (1950} reported appreciably higher percentages of organic carbon converted to CH~ and CO2 for thermophilic (55~C) than for mesophilic (33°C) digestion (57.8 vs 48.3°0). At high sulphate concentrations (5800 mg 1- t SO.d, however, thermophilic digestion was inhibited to a marked extent, giving an average conversion of 37.4°0 as against 46.8°.:0 for mesophilic digestion. Anaerobic digestion of a yeast waste containing 1800 mg 1- t SO.~ {Stander, 1950) also showed a lower percentage conversion for thermophilic than for mesophilic digestion (138.8 vs 53.7"Q. Basu & Leclerc 11975) studied the anaerobic digestion of a beet sugar molasses distillery waste containing 200 mg 1- ~ of zinc and 29 mg 1- t of copper and found only a very slight improvement for a digestion temperature of 55~C as compared to 35°C. Other bench scale studies on thermophilic anaerobic digestion include those of Pohland & Bloodgood (1963), Buraczewski (1964), Iwai, et al. (1964), Matsumoto & Endo (1965) and Maly & Fadrus (1971). Plant scale sttdies

Fischer & Greene (1945) have reported on plant scale studies of thermophilic (54=C) anaerobic digestion of primary sludge at Aurora, Illinois in 1931. Apparently there was no artificial mixing and the

132

H.O. BUHR and J. F. A:,;DREws

digesters ~ere stratified. The detention period. measured on the ba.,fis of solids detention, was 12.9 days and the organic loading was 0.45 kg volatile matter m --~d - ~ !0.028 lb ft- 3d- i ). They observed higher solids destruction in the thermophilic digester (56.4",,t than in a mesophilic digester 150.5°o). the latter being operated at a detention time of 12.5 days and a temperature of 32-C. The composition of gas from both digesters was approximately the same: however, the thermophilic digester produced somewhat less gas ( 1.07 vs 1.26 m3j per kg of volatile matter destroyed (17.2 vs 20.2 ft 3 lb-l). The thermophilic sludge had a significantly hieher solids concentration 18.65 vs _.9 ~) although it dewatered less rapidly in Buchner funnel tests using FeCI 3 as a coagulant. The supernatant from the thermophilic digester was not of as high a quality as the supernatant from the mesophilic digester. From 1942 to 1944, Fischer & Greene (1945) also studied thermophilic anaerobic digestion in Jackson, Michigan. They used three-stage digestion with a feed sludge consisting of one part of primary sludge to three parts of activated sludge. The mixture contained 5.2°0 total solids of which 64?0 was volatile. The digesters were not artificially mixed and supernatant was withdrawn only from the tertiary tank. Operating temperatures of the two parallel units were 29 and 52~C, respectively, with only the primary tanks being heated. The digesters were fed every other day. The detention period in the primary tanks was 27 days and the organic loadings were 0.53 kg volatile matter m - 3 d - t (0.033 lb ft-3d - t). Higher volatile solids destruction (44.2 vs 38.0',~'o) was obtained in the primary stage of the thermophilic digester and gas production/kg of volatile matter destroyed was also somewhat higher (1.08 vs 1.0 m3). The thermophitic sludge from the primary tank had a o.. higher solids concentration (3.8 vs 3-.3/0) and the supernatant from the third stage of the thermophilic unit was of higher quality than that from the third stage of the mesophilie unit. Fischer & Greene noted that a rise or drop of 3~C or more during a relatively short period greatly affected the digestion process in the temperature range of 46--52~C. This variation was far more serious in the 46--49~C range and could cause a temporary complete cessation in digester activity. The most extensive plant-scale test of thermophilic anaerobic digestion in the U.S. was conducted by Garber (1954, 1973! from 1953 to 1957 at the Los Angeles Hyperion plant. The feed sludge to the digesters consisted of a mixture of approximately 70°,o primary and 30°-o waste activated sludge with the mixture having a solids concentration of 6.4°;; of which 75'~i; was volatile. Sludge heating was by direct steam injection and mixing was by draft tube mixers and/or recirculation pumps. Temperatures of 29, 38 and 49~C were studied at detention times of 12 and 24 days and organic loadings of 2.1 and 3.8 kg volatile solids

m - 3d - I iO.13 and 0.24 lb R - 3d - t l. The digesters were fed once or twice each day. Approximately 5~":, of the volatile solids were destroved for both loadings and detention times at 49~C. This was equal to or better than that obtained at other temperatures. The reduction in ether solubles and gas production kg of volatile solids destroyed was approximately the same at all temperatures. Volatile acids concentrations were higher in the thermophitic digesters, ranging from 600 to 800 mg1-1 whereas concentrations at other temperatures ranged from 100 to 200 mg I- 1. The thermophilic sludge had a higher solids concentration (4.6-5.1%) when compared with digested sludge at the other temperatures (3.1-3.9°,,). In Garber's opinion, the major advantage of the thermophilic process was the production of a sludge with improved dewatering characteristics and lower coagulant demand. He obtained vacuum filter yields for the thermophilic sludge of 270gm-2h-X (6.31bft-2h -*) with a coagulant dosage of 3.4~'~ FeCI3 as compared to yields for the mesophilic sludge of 7 0 g m - Z h -~ (l.71bft-2h -~) with a higher coagulant dosage of 6.5 percent FeCI3. It was considered that these higher filter yields and lower coagulant demands were due to a smaller percentage of fine materials {65 vs 807;, passing 200 mesh) and a different morphology and structure of the organisms at thermophilic temperatures. In renewed tests since 1972, Garber et al. (1975) have again operated a thermophilic digester at 46-51~C. They report that the operational difficulty level of the thermophilic process is essentially similar to that under mesophilic conditions, although a greater sensitivity to temperature change is apparent. Sharp increases in volatile acids concentration were experienced whenever the digester temperature approached 52:C. Improved filtration yields, similar to those obtained earlier, are reported. The thermophilic filtrate, however, was of poorer quality than that of the mesophilic sludge, showing increases of 50-100,'..,, in concentrations of ether solubles, COD, nitrogen, phosphorus, and heavy metals. Garber (1976) further indicates that examination of sludge samples for bacterial content has shown that thermophilic operation results in sharply lower numbers of potentially pathogenic bacteria in the digested sludge. At the first conference of the International Association on Water Pollution Research in 1962, Popova & Bolotina (1964) reported on the use of thermophilic anaerobic digestion in a i million m3d- 1 (260 MGD) treatment plant in Moscow, U.S.S.R. Plant scale tests on thermophilic anaerobic digestion were first made in 1944, and in 1958 all of the digesters were converted to thermophilic operation at a temperature of 51~C. The feed sludge was a mixture of primary and waste activated sludge with a total solids concentration ranging from 3 to 7";:, of which 70°~ was volatile. Heating and mixing were by steam injection with recirculation of the steam-sludge mixture.

The thermophilic anaerobic digestion proces~ Conversion from mesophilic to thermophilic operation permitted a decrease in detention time from 18 to 9 days and an increase in organic loading from 1.65 up to 3.5 kg volatile matter m-3d -t, with an organic solids destruction of up to 50",~. However. Popova & Boloti'na considered the primary advantage of the thermophilic process to be the sanitary quality of the thermophilic sludge produ~d. Research at the plant had shown that mesophilic digested sludge retained up to 20% viable helminth eggs: after thermophilic digestion no viable eggs could be found. The sanitary quality of the sludge was of particular concern since it had become the practice to apply the liquid sludge directly to agricultural land. Thermophilic anaerobic digestion is still used in Moscow as well as in other areas of the U.S.S.R. !Rosenkranz, 1974). An example of the utilization of thermophilic anaerobic digestion for industrial wastes is that reported by Ono 11965) for the treatment of wastes from alcohol distillation as practised in Japan (Fermentation Research Institute, undated). This is a hot, strong waste with 3-6'~0 volatile matter, a BOD of 20,000-50,000 mg 1- ~ and an ionic strength approximately that of sea water. Full-scale thermophilic digestion was practised at 10 plants. Temperatures used were 53-54:C with detention times of 4-7 days and maximum organic toadings up to 16 kg volatile matter m - Sd- t (1.0 Ib ft- 3 d - 1). Eighty-ninety ')o of the influent BOD is removed in the process with the production of gas having a methane content of 50-60'~;;.

Summary of previous work Reaction rates and organic solids destruction. The preponderance of the results presented for both bench and plant scale studied show that the rate of destruction of organic solids is higher at thermophilic temperatures than at mesophilic temperatures. Consequently, at equal residence times a larger fraction of the organic solids is destroyed although the difference may not be noticeable at longer residence times. However, the maximum hydraulic and organic Ioadings for thermophilic digestion have not been established as Torpey 11955} has done for the mesophilic process. Torpey, working at pilot scale, demonstrated that stable operation could be maintained at 36:C with detention times as low as 3.2 days and organic loadings as high as 14kg volatile matter m-Sd -~ (0.87 Ib ft-3d - t). Solids-liquid separation. The literature indicates that in all cases the thermophilic digested sludge had a higher solids concentration than mesophilic digested sludge. The work of Garber et aL (1954, 1975) and Golueke 119581 also indicate better filtration rates with a lower coagulant demand. In Garber's opinion, this was the major advantage of the process. However, the work of Fischer and Greene (1945) at Aurora, Illinois indicates somewhat lower filtration rates.

133

Garber indicated that he obtained a poorer quality supernatant for the thermophilic process. Fischer and Greene obtained a poorer quality supernatant in their Aurora studies: ho~vever, in their work at Jackson, they report one instance in which the supernatant was of higher quality and another in which the supernarant was of poorer quality than that obtained in the mesophilic process. In all cases, volatile acids concentrations were higher in thermophilic digesters than in mesophilic digesters. On balance it thus appears that a poorer quality supernatant may be expected for thermophilic digestion. Unfortunately, no comparisons of the sludge dewatering characteristics or supematant quality of thermophilic and mcsophilic digested sludges are presented in the report on the Moscow thermophilic digesters. Destruction oJpathoyenic oryani.sms. In the disposal of digested sludge upon land. considerable attention must be devoted to the possibility of creating public health hazards. In 1971. the Water Pollution Control Federation (1971) issued a new edition of its manual of practice on utilization of municipal wastewater sludge, in which art entire chapter was devoted to the hygienic aspects of sludge utilization. Mesophilic anaerobic digestion greatly reduces the number of pathogens in sludge: however, some do survive and digested sludge is frequently stored for several months to provide additional reduction of pathogens as well as further dewatering. The WPCF manual recommends that digested sludge should be heated to 5TC for 1 h or dried for 12-15 months before use as a general fertilizer. It would appear, in light of the reports by Popova & Bolotina (1964) and Garber (1976), that sludge digested at 50'C for a period of 10 or more days would be at least as safe since organism destruction is a time-temperature phenomenon. In this respect, thermophilic anaerobic digestion may be advantageously compared with composting in which pathogens are destroyed, at least in part, by the high temperatures generated in the composting operation. The WPCF manual should be consulted for a more detailed discussion of previous work on the hygienic aspects of digested sludge and compost. Additional references on this topic which do not appear in the WPCF manual include papers by Kelley & Sanderson (1959), Liebmann (1965) and Palif (1973). Energy for heatin 9. Objections have been raised to therm6philic anaerobic digestion, based on the belief that the cost of heating the reactor would be excessive. These objections can be refuted on a qualitative basis by the use of the process in Moscow, U.S.S.R., which is obviously in a cold climate. On a quantitative basis, the objections can be refuted by a typical heat balance which will show that the heat energy produced in the gas is normally at least 100°,;> in excess of the major digester heat requirements. The largest component, by far, is the heat required to bring the sludge to its operating temperature.

13-t

H.O. Bt.:HRand J. F. ANDREWS

E n e r ~ requirements could be reduced by increasing the solids concentration in the feed sludge, thus reducing the amount of sludge to be heated, and by better digester insulation. A portion of the heat in the digested sludge might also be recovered by using the digester effluent to preheat the incoming feed sludge. In general, surplus energy would be available for other uses in the treatment plant. However, the amount of surplus energy available would be greatly dependent upon the type of system used for heating the sludge and in this connection it should be noted that all of the plant scale thermophilic digestion studies reported have used direct steam injection which is a highly efficient system. It is expected that many existing plants would have insufficient heating capacity to maintain thermophilic temperatures since they have been designed for mesophilic operation. Auxiliary heaters would therefore be required. Fischer & Greene (1945) found this to be the case in their studies at Jackson, Michigan, in which they supplemented their existing conventional hot water heating system with an auxiliary direct steam injection system. Garber (1973) was unable to meet his heating requirements during the winter months when blower engines were down for routine maintenance and steam production was therefore at a minimum. Effects of fluctuations in temperature. There is a widespread feeling that the mesophilic digestion process is very sensitive to sudden changes in temperature and that the thermophilic process may be even more so. However, the results reported in the literature are contradictory upon this point. Fischer & Greene (1945) noted that a rise or drop of 3-C or more during a relatively short period greatly affected the thermophilic process and could cause a temporary complete cessation in digestion activity, Golueke (1958) also noted that when a population in a digester was not well established, it became very sensitive to any abrupt change in temperature. Heukelekian & Kaplovsky (1948) imposed pulse changes in temperature on batch thermophilic digesters and noted that although a drop in temperature resulted in a greatly decreased rate of gasification, there was no lasting effect on the subsequent digestion when the digesters were returned to their original operating temperature. Garber (1954) reported that the thermophilic process was quite stable and resistant to upset, with temperature drops of 5~C in 48 h producing no unusual changes. Operation at the higher temperatures did, however, appear to be more sensitive to temperature change, with the sensitivity increasing with temperature (Garber, 1976). Speece & Kem (1970) studied the effects of short term (h) temperature variations on methane production using bench scale mesophilic (35:C) digesters. Their detention time was 20 days and the digesters were fed raw sludge once/day. They noted that the methane production rate was particularly sensitive to decreases in temperature and practically ceased when

the temperature was dropped to 20:C. However, gasification rapidly resumed when the temperature was returned to its original level of 35~C. They also conducted an experiment in which the volatile acids were artificially increased at the same time as the temperature was increased from 35 to 45-C. The increase in temperature resulted in a rapid reduction in the volatile acids concentration and they therefore suggested that a temporary increase in digester temperature could serve to restore balanced digestion. Although the literature is contradictory, it does indicate that fluctuations in temperature can cause problems for thermophilic digesters and requires further study. However, there is e~idence that the problems are not insurmountable. Temperature stability may be enhanced by the provision of automatic controls. Good process control and an adequate understanding of the influence of operating variables, particularly temperature, on the biological process, will be important factors in the successful utilization of the thermophilic anaerobic digestion process. PROCESS MODEL Digital computer simulation provides a convenient means of investigating t',e performance of a process under various operating conditions, in a relatively short time. Depending on the inherent accuracy of the process model and process parameters employed, the simulation results may or may not be quantitatively correct, but they nevertheless provide an indication of process trends and permit the identification of likely areas of stability or instability. A reliable dynamic process model may further be used to evaluate the potential effectiveness of alternative control stategies for meeting a variety of process upset conditions, before these strategies are applied in practice. A dynamic model to describe the characteristics of the anaerobic digestion process has been evolved over the past 10 yr and is discussed in more detail in papers by Andrews (1969), Andrews & Graef (1971) and Graef & Andrews (1974a, b). The model was developed from material balances on the biological, liquid and gas phases of a completely-mixed anaerobic digester, and the latest version is summarized in Fig. 1. The components on which material balances were made are given below: (a) Biological Phase: Organisms. (b) Liquid Phase: Volatile acids, bicarbonate and dissolved carbon dioxide, ammonia, other cations and anions, conservative toxic material. Ic) Gas Phase: Carbon dioxide, methane. The model has been kept as simple as possible by considering the conversion of volatile acids to methane and carbon dioxide as the rate limiting step. It was also assumed that there was no lag phase or inhibition by products. The model is restricted to a pH below 8 and does not consider the precipitation or dissolution of solid chemical phases such as calcium carbonate, or inhibition by un-ionized ammonia.

The thermophilic anaerobic digestion process Two key features of the model are the use of an inhibition function in lieu of the Monod function to relate volatile acids concentration and specific growth rate for the methane bacteria (equation 1), and consideration of the un-ionized fraction of the volatile acids as both the growth limiting substrate and inhibiting agent. # =

K~

HS

(i)

K__ where ,u = specific growth rate S] = maximum specific growth rate K~ and Kr = saturation and inhibition coefficients HS = concentration of un-ionized substrate. The use of an inhibition function is an important modification since it enables the model to predict process failure by high concentrations of volatile acids at residence times exceeding the washout residence time. Consideration of the un-ionized fraction of the volatile acids as the inhibiting agent resolves the conflict which has existed in the literature as to whether inhibition is caused by high volatile acids concentration or low pH. Since the concentration of unionized acids is a function of both total volatile acids concentration and pH, both are therefore of importance. In order to assign numerical values to the physical constants employed in the model, it was assumed that volatile acids may be approximated as acetic acid. The relationship for substrate conversion to methane and carbon dioxide may therefore be based on the stoichiometry for acetic acid, as given in equation 2: CH.~COOH + a NH3 ~ a CsHvNO2 +bCO2+cCH,+dH20

(2)

where a = 0.0227, from organism yield reported by Lawrence & McCarty (1965), h = c = 0.943, and d = 0.069. This equation shows a 1:1 relationship between the methane and carbon dioxide produced. In practice this relationship will be influenced by the additional methane generated by volatile acids of a higher order than acetic acid, as well as by the carbon dioxide synthesized during the acid formation step. Thus, in order to approximate more closely the true distribution between methane and carbon dioxide obtained in domestic sewage sludge digestion, a 10°-o reduction in CO., and a 10°-~ increase in CH, was arbitrarily assumed. These stoichiometric coefficients, expressed in appropriate units, are shown in Table 1, together with the other parameters used in the model. Nitrogen was assumed to enter the digester in the form of ammonia, and the conversion of organic to inorganic nitrogen was thus not included in the model. Some ammonia is consumed according to

135

reaction (21 above, while the remainder occurs as NH3 or NH~ ions in the digester liquor. The concentration of un-ionized NH3 is negligible in the pH range considered here. but the con~ntration of NH.,7 plays an important part in determining the bicarbonate alkalinity, as indicated in Fig. 1. Effect of temperature on model parameters It is well established (lngraham. 19621 that the net rate of bacterial growth increases with temperature. According to available experimental data iMonod, 1942; Brock, 19671 there is a range of temperatures over which the growth rate is approximately linear with the reciprocal of the absolute temperature, and may be described by an Arrhenius relationship. Above a certain maximum temperature, growth rate falls off rapidly. This maximum may be in the region of 2OC for the so-called psychrophilic bacteria, 40~C for mesophiles, and 60:C for thermophiles. Allen (1950) and Ingraham (1962j point out, however, that these divisions are no more than convenient means of reference and that, in practice, there is no natural distinction between these groups. The occurrence of the temperature maximum was explained by Hinshelwood (1946) as being the result of two competitive processes, namely, synthesis and degradation. He suggested that each process may be taken to conform to the Arrhenius law so that net or efl~ective growth rate is of the form: At exp(-Ev/RT) - A, exp(-E2/RT).

(3)

The value of the activation energy for degradation, E2, is appreciably greater than El; thus, at the lower temperatures of the growth range, the rate of degradation is small, and the effect of temperature is mainly observed on synthetic processes. As the temperature increases, inactivation of enzymes and denaturation of proteins assume greater importance, and a maximum in the net growth rate is reached. Further increases in temperature cause a sharp drop in growth rate, to zero. A similar conclusion was reached by Allen (1950) who showed that lysis of cells, when substrate is exhausted, is sharply temperature dependent, and postulated that the maximum temperature is reached when thermal enzyme destruction overtakes enzyme synthesis. The net growth rate per unit mass of methanogenic organisms may be expressed as: G = U - kd

(4)

where F~ is the ~owth rate of organisms due to substrate utilization and ka is a decay coefficient, representing a decrease in organism mass from all causes. In accordance with the suggestion of Hinshelwood (1946) the temperature variation of both ~ and kd will be expressed in an Arrhenius form, with the constants chosen such that the rate of degradation is much lower than the rate of synthesis at the lower temperatures, but overtakes synthesis in the vicinity of 60°C.

H. O. Bt:rm and J. F. ANDREWS

136

Table 1. Biological and physico-chemical parameters employed in the mathematical model Reference Yr s = I(~.x = !q:o: x = l,c, ~ ~ = kr = K~= Ki = fi = ka = D = , PH.O

Lawrence & McCarty, 1965 Equation 2 See text See text

0.0227 mole X:mole substrate converted t.0 mole NH~ consumed mole X produced 37.4 mole CO2 mole X produc~xt 45.7 mole CH.~ mole X produced not used I x 10-*molcl -t 6.67 x 10-*molel - t 0.324 cxp',0.06 (0-35)i days- t 0.02 exp ~0.14 (0-35P, days- 1 241 mole- t at 20:C and I a t m

= exp 12.0

Simulations Simulations See text See text Ideal gas law

4014 ) (0 +- ~,34.6)J+atm

Perry. 1950"

842.9 ~ m o l e l - t a t m - t K n = exp --8.1403 + (0 + 151.5).1 = 100days - t

Kca

Lange, 1967* Stephen&Stephen. 1963* Estimated

Dissociation constants : Ka = 10 ~-pK,, + Py + P:'"'~ KA = I ( Y - p K , - ~:, + p:',.I

K I = 10~-t'Kt + p7 + pTu-) where pK,, pK a pKt

(--

p;,,' = ,4~2

\Q t+v't

= 4~2(1 It"'"

A= B= ¢= = =

Robinson & Stokes, 1959 Robinson & Stokes, 1959 Robinson & Stokes, 1959

= 1170.48/T- 3.1649 + 0.013399 T = 2835.76/T - 0.6322 + 0.001225 T = 3404.71/T - 14.8435 + 0.032786 T 0.2 I

)

Butler, 1964

x/l + 9B~ ' l )

Butler, 1964

1.825 x 10~ ( ¢ T ) - t 5 50.3 (¢T) -°': Dielectric constant 87.74 - 0.400080 + 9.398 x 1 0 - 4 0 2 - 1.410 x 1 0 - 6 0 3 Ionic valence = 1 (for ions considered here)

Butler, 1964 Butler, 1964 Harned & Owen, 1958

* Least-squares fit of available data to Antoine equation.

Adoption of this type of temperature relationship infers that the difference between the m a x i m u m temperatures for thermophilic and mesophilic cultures may be explained either by differences in the rate of synthesis, as suggested by Allen (1950), or by the greater heat stability and resulting lower rate of degradation of thermophiles, or both. A n o t h e r approach would be to introduce a temperature dependent maintenance energy coefficient in the substrate balance equation (Matsch6 & Andrews, 1973). The rate of synthesis, /~, is expressed in e q u a t i o n 1, and involves the three parameters, /~, K, and Kt. It would be unrealistic, given our present limited data, to attempt to define the variation of each of these parameters with temperature, a n d the same c o m m e n t applies to the other biological parameters such as the yield coefficients, etc. Accordingly it is proposed, for the purposes of the present study, to incorporate a temperature variation into the parameters /~ a n d ka only, and to choose constant values for all the others. The temperature relationships proposed are s h o w n in Table 1. Base conditions were chosen to correspond

with values reported by Lawrence & McCarty (1965) for mesophiles, at 35°C. The temperature coefficient for growth was estimated from available data (Lawrence & McCarty, 1965; Maly & Fadrus, 1971; Coultate & Sundaram, 1974), while that for k., was chosen by simulation studies to agree with the data of Golueke (1958), as discussed later. The resulting curve of net growth rate versus temperature is s h o w n in Fig. 2, and exhibits a m a x i m u m at a b o u t 60°C. T e m p e r a t u r e relationships for the physico-chemical parameters involved are well d o c u m e n t e d a n d are also given in Table 1. These equations were obtained directly from the literature, or, in the two cases indicated, by fitting available data to the Antoine equation. SIMULATIONS W i t h i n the limits of accuracy of the biological parameters chosen, the model may now be used for predicting dynamic as well as steady-state behaviour of a thermophilic anaerobic digestion system. This is

The thermophilic anaerobic digestion process

t37

GAS PHASE

PT" D

_I

dPco{ dt

V PT P ~G RG

"

0

"

OCH ~

+

o

I PCOz

HS - (H+|(S-) Ka H+

K H,

=

Fo, Fz,

;

NH~

(HCO 1- }

dC T d-~ RG

= .

(COz) O

~

KL a {(COz) D CT -

HS,

RG, QCH.

K a + {H+)

ST

(H+) K A + {H+I

NT

H÷, z, HCO~-,

F~ 9-- {Tx~ - Tx)

RB

CO~ D, NH~ +

RG

(COz)o }

(HCOI-)

:

~

~'dV .

Fo - F,

(CO2) D =

KH 9CO 2

IST, NT, RB, QCH~

BIOLOGICAL PHASE

VF_.£ (X~ " X)

Ks ,

+

TX, V

IdX ~,

dTx d-i- "

;

(CTo - C T )

-

I

Rs

(Z} + (NH~ +) - (S-}

F~ V (z~ - z)

=

CTo. TXo, Zl

D V

Ka

S-

(HCO1 ) dZ

=

PHASE

K~ (COz) D

KA, KI, RLa,

I

iI v, LIQUID

K a,

I

QCO:

:

OCOz

~T

~PT- PH:o)

PCO,

"

VG' PHzO

'~

u X

-

kd X

- k T TX

KI ,

kd, k T, Y, o, Fa, Xo, q STo' NTo

u

z-

=

-+

~~S

-+-

H_.%s

x,s T

KI

RB

:

YCO2/X u X

;

QCH~

-

D V YCHw/X u X

dST

u

d--~ "

FV-£ (STo - ST)

~

dNT

Fo ~- (NT~ - NT)

Y~/X u X

"

X

Fig. 1. Summary of mathematical model.

done by specifying digester temperature, feed and withdrawal flow rates, concentrations of feed components, and the time variation of these quantities. For the purposes of simulating a given system, feed rate may be based on hydraulic detention time; substrate concentration (as acetic acid) may be based on the known yield of methane, and other quantities may be chosen to represent realistic levels of cations, ammonia and alkalinity in the effluent. Model ralidation

The effect of reactor temperature on anaerobic digestion was investigated by Golueke (1958) who operated eight bench scale digesters at temperatures from 30 to 65°C in intervals of 5°C. These digesters were allowed to build up an acclimated bacterial population, and were then fed raw settled sewage

sludge once/day, on the basis of a 30-day detention period. The quasi-steady state results obtained in this study are shown by the data points in Fig. 3. Gas production rate exhibited a moderate variation over the range 30-60-'C, with a sharp drop at 6YC. Total volatile acids concentration exhibited a minimum at 35°C and then increased with temperature to relatively high values. Simulations using the digester model were conducted to approximate the once/day feeding used by Golueke. The input consisted of a 1-h withdrawal and l-h feed cycle, to give an average detention time of 30 days. Feed concentrations of volatile acids, ammonia and cations were selected on the basis of the average values of gas production and alkalinity reported by Golueke. A typical daily cycle resulting from this simulation is shown in Fig. 4, for a digester

138

H.O. BUI-tRand J. F. A.~DItEWS

/ I ! // It

/

^

/

I

s ~"

T

l.O

Note change of scale

0.8

¢"

Net growth rote ^

#.

/

I

l #

;

i!

/ I

0.6

/j //!

0.4

,///

0.2

20

so

40 Temp,, °C

50

60

7o

Fig. 2. Variation of growth rate with temperature. i "o

1

4

~

J 12 o

P

I0

o

o

o

i .c_

8

operating at 50~C. As feed enters the digester, volatile acids concentration rises sharply to t790 mg 1-~, and then falls ~adually during the course of the day, to 670 rag l-t. Gas production shows a sharp peak during the feeding period--this is at least partially due to the release of CO: occasioned by a drop in pH which is, in turn, caused by the pulse feed of volatile acids. Cumulative gas production gradually increases during the day, in accordance with the destruction of volatile acids. The figure clearly contrasts the cyclic nature of a digester fed only once day with the conconstant values of volatile acids and gas production rate that would be expected from a digester fed on a continuous basis. If digester operation is monitored by analyzing the liquor withdrawn at the end of a daily cycle, as is often done in conducting bench scale experiments, then it is clear from Fig. 4 that the value obtained for volatile acids, for example, would represent a minimum, rather than a daily average. These results also suggest that it might be possible to improve the quality of the supernatant from a multiple digester installation by permitting a longer period of "rest" prior to the withdrawal of sludge for further processing. Figure 3 compares Golueke's experimental results with the daily minimum values for volatile acids concentration, and the cumulative gas production rate obtained from dynamic simulations over the temperature range involved. The predicted gas production rate is approximately constant with temperature, and then drops sharply as the maximum gowth temperature is approached. Since the model has been simplified to consider the feed as volatile acids, and does not include a volatile solids-to-acids conversion step, it will be recognised that no provision is made for a possible variation in total volatile solids destruction, which may be partly responsible for the variation in gas rate shown by the Golueke data points.

2SO

,70

Cumulative

doily gas production

o,'/,

~ ~o

E 1500 /~procluction

I//1

,/////

g

rote

_?

Net feed rote

Temp..

°C

Fig. 3. Comparison of model predictions with experimental results. O Experimental data of Golueke (1958). - Prediction with pulsed feed. - - - - Prediction with continuous feed.

_~

(Input-output)

0

6

12 Time ,

IS

24

hr

Fig. 4. Cyclic behavior of digester with pulsed feed.

The thermophilic anaerobic digestion process The model successfully predicts both the initial reduction in volatile acids concentration and the sharp increase with temperature. Such an increase in volatile acids with temperature would be expected from the fact that the concentration of viable bacteria decreases as the maximum growth temperature is approached, and a hi~aer substrate concentration will accordingly be required to maintain the required total growth rate. A contributory factor is the decreased solubility of CO: at the higher temperatures, which results in increased pH and lower unionized acid concentrations. This effect would account for a 100'), increase in the total volatile acids concentration required to maintain a given growth rate at 60~C as compared to 30~C. It has been noted that a steady state simulation, which assumes that the pulse feeding strategy adopted here may be approximated by a constant feed rate, would be subject to considerable error. The values for volatile acids that would be obtained from such a "steady state" simplification are shown, for comparison, as a broken line in Fig. 3. From the foregoing it appears that the model is capable of predicting qualitatively the effect of temperature on digestion. Quantitative results, however, will be quite sensitive to the biological parameters chosen. In particular, the maximum growth temperature will depend on the activation energy of the degradation process. Since a variety of growth temperature maxima are found in practice, it may be argued that the degradation activation energy depends on the heat stability of the culture. This in turn would depend on the growth conditions, previous history, and the temperature to which the bacterial population has been acclimated. Variations in temperature sensitivity are therefore likely to be encountered between different installations, and any simulation based on data for a specific case may only be generally applicable on a semi-quantitative basis. For the purposes of the simulations presented here a value of the degradation activation energy was chosen to give good agreement with Golueke's results. The resulting growth curves are shown in Fig. 2, giving a maximum growth temperature of approximately 60-C.

Operating conditions For the simulations that follow, the concentrations of the various components in the feed stream were chosen so as to represent a "typical" feed sludge. rather than to represent specifically the feed used by Golueke. The standard feed and other operating conditions are shown in Table 2, together with the values used in the Golueke simulations. Differences in nitrogen concentrations arise from the choice of a lower alkalinity for the standard case (2000 mg 1-1 as CaCO 3) than was present in Golueke's !1958) experiments (5000 mg 1-t}. Similarly. a value of 1.3 atm represents the mean pressure with which the liquid will be in contact for a typical 6 m deep digester, particularly where gas recirculation is practised, while the pressure in Golueke's bench scale experiments was 1 atm. Feed substrate concentration may be roughly related to an equivalent sludge volatile solids by noting that, according to the stoichiometric coefficients in Table t, one mole of acetic acid reacting will yield 16.6g of CH,~ while in the experiments carried out by Golueke (1958) 1 g of volatile solids destroyed gave an average yield of 0.432 g CH.e. Thus l mole of substrate (acetic acid) reacted would be equivalent to 38 g of volatile solids destroyed, or, assuming 50°0 destruction, to 76 g volatile solids fed. The influent substrate concentration of 0.43 mol l - t selected for the standard feed is thus approximately equivalent to a sludge containing 3.3°,.,0volatiles; depending on the percentage of volatile solids in the feed, this would mean t 5°.. a total solids concentration of 4z/o.

Steady state operating characteristics Figure 3 shows that the volatile acids curve exhibits a minimum, for the 30-day detention time used by Golueke. Such a minimum would represent a condition of maximum stability and would be a preferred operating point. It is therefore of interest to determine the shape of the volatile acids vs temperature curve for a number of loading conditions. Steady state calculations were done for continuous feeding, using the standard feed defined in Table 2, at detention times from 30 down to 2.5 days. These values would rep-

Table 2. Operating conditions used in simulations Feed components Total substrate, St0 Total nitrogen. Nr, Net cations-anions, Zo Total carbonic species, Cr, T~,~= 0; X. = 0

Standard

Golueke simulations

0.43 0.053 0.005 0.005

0.60 0.14 0.005 0.005

mole 1-~ acetic acid mole 1-1 NH.~ Equivalents I- t mole 1- t HCOj-

1.3 6.7~-~;

1.0 2 1~-~,

atm

Other Mean gas phase pressure. Pr Relative gas volume, Vail,"

139

140

H.O. BL'I~ and J. F. ANDREWS !~OC

E

.

.

.

.

:

,

,

the actual numerical values of temperature and concentration presented. Figure 5 does, however, serve to illustrate the trends predicted by the model. The most notable feature is that, for a ~ven detention time. there appears to exist an optimum operating temperature at which volatile acids concentration is a minimum. For higher loading rates the optimum lies at a higher temperature. At the same time, the minimum achievable volatile acids concentration rises with decreasing detention time. Figure 5 also indicates that, t'or each detention time, there is a limited temperature range over which stable operation is possible. This range is bracketed by steeply rising volatile acids concentrations. In each case this may be related to a reduction of viable bacteria in the reactor. As operating temperature is lowered, the rate of growth drops, until the rate at which cells are washed out of the reactor by the effluent exceeds the rate of new growth. On the other hand, an upper temperature limit is reached when the increasing rate of decay overtakes the rate of new cell production. Depending on the operating temperature and the loading rate, the figure thus demonstrates that relatively small temperature changes may cause failure of an operating digester, by pushing operation beyond the stable range. Oviously the risk of failure will be minimized by operating at the optimum temperature for that detention time.

~."OC

-

5.5

E lO00

u o

80C

>

60C

~-

40C

2O(?

L ,

2o

ro

Detention i

time,

i

3'0 .o Temp, *C

days

i

~

i

go

do

f

;'o

Fig. 5. Variation of volatile acids concentration with temperature under steady-state, continuous-feed conditions.

resent loading rates of 1.1-13 kg volatile solids m3d - t (0.07-0.8 lb f t - 3 d - i). The resultant variation of volatile acids concentration with temperature is given in Fig. 5 for various detention times. Since the biological parameters used in these calculations are no more than first estimates, quantitative significance should not be attached to

Dynamic response The dynamic model may be used to illustrate the effect of changes in operating conditions on the stability of a thermophilic digester. A digester, operating

60

"~

4 / J

,¢.

i

50 o. E

t

2 t i

40 i

0

0

i

i

I-"

i

I

I

3F

7.0

/

r~

1

iI J

2

6.5 i

o.

F +-0 0 >

I

6.0

J 0

i

,

~

i

~ Time

4 ~

days

~,

i

6

i

~

0

i

,

~

3 Time,

5.5

i

4

6

7

8

days

Fig. 6. Digester response at 50:C to step increase in feed rate from 0.1 l,ld. - to 0.205 l/ld.

to 0.200 Lld. - - - -

The thermophilic anaerobic digestion process at 50 C with a feed rate of 0.1 l d - ' / l of reactor volume (detention time of 10 daysi, was chosen as the base case and in each instance two step disturbances were simulated: one which permits a return to stable operation, and another which just causes failure. Figures 6 and 7 illustrate the responses to step increases in feed rate and feed concentration, respectively. As soon as the load on the digester is increased, volatile acids increase and pH is depressed. The drop in pH will release carbon dioxide from the liquid, causing a sharp increase in the rate of gas production and the CO_,/CH,~ ratio. Higher bacterial activity due to the higher substrate concentration further increases the gas production rate. As the bacterial population builds up, system ~ariables are slowly brought to their new steady state values. In each case a higher final gas production rate reflects the increased load. For the step change in feed rate, volatile acids settles to a different steady state value, whereas the new steady state is the same as the original for the case where only concentration is changed, but feed rate kept constant. The new value for pH has a complex dependence on the volatile acids concentration, the C O : generation rate. and the relative components in the feed. so that the final value of pH is different from the initial value in each case. Similar considerations apply in the case of the percentage of CO,, in the gas, which settles to about the same value for a change in feed rate only. The cases which lead to failure, in 2 or 3 days after the step increases in feed or concentration, are characterised by an uncontrolled drop in pH, a rise in vola-

141

tile acids and a cessation of gas production. The percent CO_, in the gas phase increases in line with the drop in pH. Figure 8 illustrates the effect of a sudden drop in temperature to 43:C and to 40:C for a digester operating stably at 50:C and a detention time of 6 days. A drop in temperature would normally be due to a breakdown in heating facilities, or an abnormal spell of cold weather, which would cause a ~ a d u a l decrease in operating temperature over a number of days. The sudden drop simulated here is therefore much more severe than would be obtained in practice. Each case shows a sharp initial drop in gas rate and percent C O , . due to the higher solubility of CO,, at the lower temperature. This is followed almost immediately by an increase in COz release, as the pH drops. For the stable case, volatile acids initially increase due to the lower activity of the bacteria, but then the system is slowly brought back into control as the population builds up. The apparent agreement between initial and final steady states is merely coincidental, as may be seen by reference to the steady state data in Fig. 5. Figure 5 also indicates that a stable steady state should be possible at a 6-day detention time and 40-C. However, in executing a sudden j u m p from 5 0 C to 40~C, volatile acids reach inhibitory concentrations before a sufficient increase in bacterial concentration has occurred, and the system fails. The response to an increu.se in operating temperature may readily be inferred from Fig. 5. A sharp increase in volatile acids would be expected, leading to failure as a temperature near 60~C is approached. A digester operating on a steep portion of the volatile

60 I a

J

/ .50

2

r~

o~ t.) i

¢o

t

I

I

I

I

I

I

I

'.O

3L

: r

~

!

im

•u o

8.6

i er i

is t

-r e~

1 i i i i i

g

8.0

I i

o

J

~

~ Time

4 ,

~ doys

~

->

o Time~

s doys

6

~

8

,5.5

Fig. 7. Digester response at 50C to step increase in feed concentration from 0.43 molel-t acetic acid. - to 0.90 mole I- 1 ___= to 0.95 mole 1- *. (Detention time = 10 days).

1-,..

H . O . Bt;Ha and J. F. A.~REWS

"0

-t

t~

/

-% .-J

i/ ,/

iS0

c. C

.J

4o 0

?.O

I

t~

I

t

2

6.5

/

"0 /

u 0

/

t

6.0

0 0 >

o

I

I

i

2

I

3 Time

L

I

4 ,

5 doys

6

7

0

4 Time

Fig. 8. Digester response to step decrease in temperature from 50~C. - (Detention time = 6 days).

acids curve would exhibit an extreme sensitivity to small increases in temperature, as was experienced by Garber et al. (1975), albeit at a somewhat lower temperature (52°C).

5-5

I

;

5

6

7

8

doys

to 43'~C. - - - -

to 40~C.

Acknowledgement--Financial assistance in the form of an Overseas Research Bursary. from the South African Council for Scientific and Industrial Research is gratefully acknowledged. REFERENCES

CONCLUSIONS

Bench as well as plant scale investigations of thermophilic anaerobic digestion have shown that the process is capable of higher reaction rates than the mesophilic process and that a sludge with improved dewatering characteristics and lower concentrations of pathogenic organisms is produced. Conversely, a lower quality supernatant is obtained and requirements for control are more stringent. A dynamic model, which gives a qualitative description of the behaviour of the process under varying operating conditions, is proposed. In addition to predicting that process failure can be caused by sudden changes in temperature, the model also indicates that there is an optimum operating temperature for maximum volatile acids reduction and maximum stability at a given detention time. This optimum temperature increases as detention time decreases. The numerical values used for the biological parameters in the model must be considered as estimates and accordingly the results presented are only semiquantitative in nature. More complete information on the variation of these parameters with temperature will permit the use of the model for evaluation of control strategies and assessment of the limits of operatability for the process.

Allen M. B. (1950) The dynamic nature of thermophily J. gen. Physiol. 33, 205. Andrews "J. F. (1969) Dynamic model of the anaerobic digestion process. J. sanit. Engng Dit,.. Am. Soc. cir. Engrs 95, SAI, 95. Andrews J. F. and Graef S. P. (1971) Dynamic modelling and simulation of the anaerobic digestion process. Anaerobic Biological Treatment Processes, Adv. Chem. Ser. 105, (Edited by Gould R. F.), p. 126. American Chemical Society, New York. Basu A. K. and Leclerc E. (1975) Comparative studies on treatment of beet molasses distillery waste by thermophilic and mesophilic digestion. Water Res. 9, 103. Brock T. D. (1967) Life at high temperatures. Science 158, 1012. Buraczewski G. (19641 Methane fermentation of sewage sludge. Acta Microbiologica Polonica 13, 321. Butler J. N. (1964) Ionic Equilibrium, a Mathematical Approach. Addison-Wesley, Reading, Mass. Coultate T. P. and Sundaram T. K. (1974) Energetics of Ba.cillus Stearothermophilus growth: molar yield and temperature effects on growth efficiency. J. Bacteriol. 121, 55. Fair G. M. and Moore E. W. (1932) Heat and energy relations in the digestion of sewage solids--Ill. Effect of temperature of incubation upon the course of digestion. Sewage Wks J. 4, 589. Fair G. M. and Moore E. W. (1937) Observations on the digestion of a sewage sludge over a wide range of temperatures. Sewage Wks J. 9, 3. Fermentation Research Institute (undated) Biological Treatment of Imlustrial Waste. Pamphlet distributed by FRI, Agency of Industrial Science & Technology, Japan.

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