The role of process configuration in the performance of anaerobic systems

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Pergamon

Waf. Sci. Tech. Vol. 36, No. 6-7, pp. 539-547,1997. © 1997 lAWQ. Published by Elsevier Science Ltd Printed in Great Britain. 0273-1223/97 $17'00 + 0·00

PH: S0273-1223(97)00566-0

THE ROLE OF PROCESS CONFIGURATION IN THE PERFORMANCE OF ANAEROBIC SYSTEMS Richard E. Speece*, Metin Duran*, Goksel Demirer**, Henry Zhang* and Thomas DiStefano*** * Environmental Engineering, Vanderbilt University, Box 6012-Station B, Nashville, TN 37235, USA ** Middle East Technical University, Ankara, Turkey *** Bucknell University, Lewisburg, PA, USA ABSTRACT Many studies on biological systems have been conducted. These studies have demonstrated that subtle differences in process configuration such as phasing, staging, biofilms, granules, gas phase management, and combinations thereof can profoundly impact anaerobic process performance. Possible reasons for these differences as well as examples of various studies are presented. © 1997 IAWQ. Published by Elsevier Science Ltd

KEYWORDS <\naerobic processes; process configuration; process performance. INTRODUCTION fhere appear to be profound differences in anaerobic process performance caused by variations in the :onfigurations of given biomasses, i.e., dependent upon whether two phased/staging or one phased/staging is :hosen, CSTR or plug flow is used and whether dispersed growth or biofilm/granules will be selected. If the nicroorganisms are configured as dispersed growth, they will exist as single or small aggregates of cells 'lith no "self-shielding" occurring and therefore no diffusion gradients. If the chosen configuration of the liomass is dispersed growth, all microorganisms in the reactor would be exposed to the same environmental :onditions, toxicants and substrate concentrations as would be found in the bulk liquid. :mith and McCarty (1989) have indicated that dense packing of microorganisms such as is found in granules nd biofilms grants them significant advantages. Due to the very close proximity of the H 2-producing and Irconsumirig microorganisms a few microorganism widths from each other, the H 2 produced is not nmediately diluted to the bulk liquid concentration but instead is consumed at a relatively high H 2 oncentration. Since the rate of diffusion is proportional to the difference in concentration divided by the istance, the resulting synergy of higher concentrations of H 2 and the reduced distances between H2 roducers and consumers markedly improves the energetics. 539

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. t and Hz metabolism, It would appear that such advantages would be manifested most obviously in propIOna e using propionate . .. . . th liquid phase, ca SInce these are both dIrectly Impacted by Hz concentratIOns In e . However literature concentrations elevated above 20 to 100 mgll to be commonly reported in the lItera~ufre. uent Ev~n in what . t rs are In req . b 10-4 atmosphere reports of excess Hz (> 10-4 atmosphere) being found in methanogemc reac 0 are considered unbalanced reactors, Hz concentrations are most commonly found to e < . Phasing/staging f . ue biomass in each reactor. Staging For purposes of this paper, phasing will r~fer to the development 0 umq e sta in of a microbial process is defined as the recycle of a common bIOmass between two reactors. Th g ~. (h H . . ' mental condItIons suc as P or involves exposing the microbes to dIfferent and varyIng enVIron t t' d . (such as substrate concen ra IOns an temperature) as well as to diverse substrate types and concentratIOns . all . h metabolic intermediate types and concentrations). Staging can be accomplishe~ by ph~SIC Y separatmg t e l . .connectmg , reactors or be accompl'IS hedby pac ki ng the bIOmass . bIOmass mto dIfferent . mto dense . d' granu 'd al es or biofilms, the dimensions of which are many orders of magnitude gn~ater than the SlZ~ of an m IVI u cell. In such a case diffusion plays a prominent role in causing the various microorgamsms to be expose~ to different environmental conditions, substrate types and concentrations. The fact that under. varIOUS conditions microorganisms will grow as granules or biofilms suggests that there are some mherent advantages to be gained thereby. It has been demonstrated in a number of cases that careful phasing/staging of the process can profoundly improve the performance of a microbial process. It is possible that either the liquid and the biomass will move together as is the case with a dispersed growth CSTR or uncoupling may occur. The consortia of microorganisms can also be coupled or uncoupled by phase separation and the Food to Microorganism ratio (FIM) and environmental conditions can be coupled or uncoupled for the liquid and/or biomass. In some cases, without staging, it has been impossible to achieve the desired microbial transformation. Examples of improved performance due to phasing/staging: 1. Two Phase Anaerobic Digestion. Anaerobic digestion involves an acid formation phase with complex substrates and a terminal methane formation phase for all substrates. Massey and Pohland (1978), Ghosh and Klass (1978) and Cohen et ai. (1980) have demonstrated improved process performance caused by separate phasing. Separate phasing optimizes environmental conditions for each phase/stage because in single phase/stage processes both classes of organisms are forced to operate in a common environment. 2. Staging of Gas Phase. Harper and Pohland (1987) demonstrated that staging of the gas phase in anaerobic treatment effectively alters the HZ concentration and enables improved utilization of propionate. 3. Low Substrate Affinity of Microbiota. Some microbial processes, especially anaerobic processes, manifest high Ks values (> 2000 mgll) which may be further exacerbated by reduced temperature. Thus, excessively large single stage reactors are required to achieve low effluent substrate concentrations. Staging or plug flow configurations can reduce reactor volume requirements by an order of magnitude when compared to one stage CSTR's while achieving the same effluent concentrations. 4. Methanogen Predominance. Two broad classes of methanogens are responsible for conversion of acetate -30 mgII to methane under anaerobic conditions. These classes are Methanosaeta (Methanothrix) with K and k ~ -2 day-I, and Methanosarcina with K s = -300 mgll and k = -6 day-I. Methanosdeta tends to predommate at low. substrate .concentrations, with Methanosarcina tending to predominate at higher substrate concentratIons.. Sta~mg enables Methanosarcina to predominate in the first stage and Methanosaeta to predommate m the second stage thus enabling high quality, low effluent concentrations with a minimum of total reactor volumes. This has also been observed in the structure of granules (Guiot et ai., 1992).

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5. Aerobic Activated Sludge and Anaerobic Suspended Growth Reactors Selector Stage. Provision of a relatively short HRT stage (selector) at the head of an activated sludge process maintains the return sludge in contact with a relatively high substrate concentration. Consequently a shift in predominance of microbial population from filamentous (poor settling) to flocculent (good settling) can be realized whereas a one-stage CSTR favors the predominance of filamentous biomass. Similar improvements in biomass settleability have been noted when a selector state wa5 applied to a CSTR anaerobic suspended growth process.

Examples of microbial processes affected only with staging

1. Biological Phosphorus Removal. The process whereby phosphorus is removed from wastewater by a process termed luxury uptake depends upon the development of a microbial population which is able to store polyphosphate granules. Such poly-P microorganisms can only develop in significant numbers by provision of a deeply anaerobic stage followed by an aerobic stage wherein phosphate uptake and poly phosphate granule accumulation occurs. 2. Formation of UASB Granules. The unusually dense microbial granules which cause the success of the upflow anaerobic sludge blanket (UASB) process (-10% volatile solids) can only be developed in a staged process, usually pseudo plug flow. According to the University of Capetown hypothesis, (Sam-Soon et ai., 1987) one of the essential conditions for granule formation is the existence of a zone of high H 2 partial pressure which occurs within a plug flow reactor. Under the same conditions, with the exception of a one stage CSTR reactor, the Methanothrix would grow as dispersed short rods having very poor settling characteristics. 3. Highly Chlorinated Compounds Biotransformation. DDT and PCB's persist for decades if held in a constant environment because complete biotransformation of these compounds is possible only by a staged sequential anaerobic/aerobic environment. 4. NitrificationlDenitrification. The common practice of nitrogen removal from wastewaters is biologically accomplished by providing an aerobic stage for the oxidation of ammonia to nitrate followed by an anoxic stage for biotransformation of nitrate to nitrogen gas. This staging can be accomplished within the same reactor or subsequent reactors by supplying or limiting oxygen transfer. It can also be accomplished in biofilms. POTENTIAL ADVANTAGES OF PHASE SEPARATION AND/OR STAGING Optimized environmental conditions for both acidogens and methanogens. Altered intermediate product formation. Minimized H 2 concentration and maximized free energy for propionate conversion. Minimized inhibitory acetate concentrations in the presence of propionate degraders as well as vice versa. Increased potential of the organic loading rate (OLR). Minimized residual volatile fatty acids (VFA) in the effluent. Altered energy yield to various classes of microorganisms. Shielded later biotransformations from biodegradable toxicity in the influent (e.g., propionate). POSSIBLE CONFIGURATIONS THAT AFFECT PROCESS PERFORMANCE (SEE FIG. 1) 'Ii;,,",

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J. Two or more staged CSTR gas passing on - comm on bioma ss in each stage H. Plug flow without gas take off l. Plug flow with gas take off J. Two staged granules in CSTR K. Two staged granules in plug flow . . L. CSTR acid phase in plug flow fluidized bed, UASB , or anaero bIc flIter ~1 Staging withou t bioma ss recycle N. Staging with biomass recycle

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LITER ATUR E REPO RTS OF FAVO RABL E SEPA RATE STAG ED ANAE ROBIC CONF IGUR ATIO NS Two phase digestion Massey and Pohlan d (1978) and Ghosh and Klass (1978) report ed the advant ages of two phase r Anderson et al. (1994) consid ered that the impor tance of reacto r config uration in microb ial select' well known, studie d the changes in microbial popula tions in two phased digest ion, and derr two phased system s have several advantages over one stage proces ses: 1) selecti on anrl different bacteria in each reactor, 2) increa sed proces s stability, 3) the metha nogen ic pha' prior acid phase.

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Cohen et ai. (1980) operated single and two phase CSTR's under conditions of shock loading and found that while both systems accumulated volatile acids, the two phase system returned to typical operating conditions in 1/5 of the time required by the single phase system. Two stage digestion Wiegant et ai. (1986) studied the staging of propionate degradation in thermophilic anaerobic treatment, frequently a persistent problem. They compared single and two stage UASB reactors and found significantly better results in the two stage process, i.e. 10-13% better treatment efficiency. They concluded that the improvement was due to removal of the biogas evolved in the first stage. Propionate-COD accounted for 80% of the VFA-COD in the effluent of the first stage reactor in their study and is often considered to be the limiting step in the methanogenesis of acidified wastewaters. Acetate is known to be inhibitory to propionate degradation (Kus and Weismann, 1995). Mawson et ai. (1991) found that 1000 to 2000 mg/1 of acetate significantly inhibited degradation of 500 mg/l propionic acid. Wiegant et ai. (1989) hypothesized that staging would not improve performance if toxicity was a problem. van Lier et ai. (1994) studied thermophilic anaerobic treatment in compartmentalized upflow reactors. They reported superior performance of the plug flow configuration as evidenced by very low effluent volatile acids concentrations, high degree of biomass retention and stable reactor performance. One group used either sucrose or a mixture of volatile fatty acids as the feed, with the result that sucrose fostered granule development whereas VFA substrate resulted in almost no granule formation. Sucrose was converted in the first compartment, with butyrate and acetate conversion in subsequent compartments. Propionate was the most difficult intermediate to degrade, but in the last compartment even propionate was degraded almost completely. Staging of the thermophilic process was able to reduce the high steady state VFA concentrations typically found in thermophilic CSTR. It has been established that propionate conversion limits thermophilic CSTR processes. Propionate conversion is thermodynamically the most difficult reaction in the whole digestion process, but van Lier et ai. (1994) showed that unfavorable conditions for propionate conversion characterized only the first stage. In the staged reactor, H 2 was effectively removed from each compartment by venting, precluding its passage to subsequent stages. Since both acetate and H 2 have a detrimental effect on propionate conversion, CSTR configurations cannot accommodate high OLR's for prolonged periods without an increase in VFA concentration in the effluent. However, when using staged processes, low effluent VFA concentrations can be maintained at very high OLR. Gas phase management Harper and Pohland (1987) proposed gas phase management in a staged process configuration. Venting of the gas phase from each stage resulted in more efficient and stable performance when compared to a system whereby the gas from each stage was passed through the subsequent stage. Anaerobic baffled reactor The anaerobic baffled reactor (ABR) offers several potential advantageous features: gas venting from each stage, staging configuration provision and enhanced biomass retention. Grobicki and Stuckey (1991) demonstrated that an ABR could operate stably at high OLR with low effluent VFA.

<\hring (1995) noted that disintegration of granules decreased the rates of propionate and -ations by 20% and 34%, respectively. Compared to the rates of systems with whole granules, ;tance between cells in intact granules was approximately 2 to 3 /lm vs. 10 Jlm after 'mith (1986) indicated that a significant decrease in activity was predictable when the 5 /lm. Morvai et ai. (1992) compared dispersed and granular biomass for sensitivity to '1 and found granular biomass (mainly Methanothrix) to be less sensitive to high substrate 'tion. In addition they found K s increased during granulation due to increased mass 'he methanogenic biologies.

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y1etabolic intermediates Pipyn and Verstraete (1981) showed that the formation of lactate and ethanol from glucose in the acid stage was preferable to volatile acid production since this process distributes a greater proportion of the free energy change of anaerobic degradation to the methanogens. Synthesis was found to be 40% less if the fermentation in the acid phase was directed to ethanol and lactate vs. volatile fatty acids. This was attributable to the differences in the free energy change between the two possible metabolic pathways. Specific degradation rates for acetate and propionate were 2.08 and 1.96 times higher, respectively, in the separated phase vs. single phase systems. Bull er al. (1984) compared single and separated phase anaerobic processes receiving glucose feed and found that the separated phase system consistently gave superior performance. Under variable process conditions, the separated phase system was inherently more stable with shock loads and the pH in the acidification phase was typically 3.5. Metabolic intermediate formation in the acidification phase was correlated with pH. Ethanol was the major intermediate in the acid phase when the pH was 3 to 5. Butyrate was reported to be the major intermediate in the acid phase at pH = 5.7. Little COD reduction was achieved in the acid phase (8% max) with H 2 evolution accounting for most. (The group noted that the inherent stability of the single phase fluidized bed was very good, however.) Substrate composition affected effluent quality from the separated phase reactors. Smith and McCarty (1989) noted that four to seven-carbon n-carboxylic acids were formed by back reactions from acetate and propionate during periods of elevated H 2 partial pressure. The reactions reversed when H 2 levels fell. They found that a correlation was noted between the initial decrease in propionate concentration and an increase in n-propanol accumulation. After a perturbation of the feed with ethanol and propionate, the propionate levels remained elevated even though the H 2 level was below 10- 4 atmosphere. The elevated propionate persisted until the disappearance of the longer chain carboxylic acids. Propionate was reported in our laboratory to be a major metabolic intermediate in the first phase of a two phased systerJ1 while butyrate was the major metabolic intermediate in a two staged system. H 2 partial pressure Smith (1986) measured steady state H 2 partial pressure under 16 different conditions with a series of reactors operated with ethanol, propionate, acetone, isopropanol, primary and secondary sludge, and pulp evaporator condensate as feed. The reactor configurations used were: CSTR, anaerobic baffled reactor and anaerobic filter. Measured H2 partial pressures were less than 2 x 10- 4 atmosphere in all reactors. Perturbation of the feed rate was studied for its effect on the H 2 partial pressure, and it was established that ethanol perturbation resulted in increases of the H 2 partial pressure to over 10- 3 atmosphere while it remained below 10- 4 with propionate perturbation. Biological processes fostering biofilm growth had higher H 2 turnover rates for a given H 2 partial pressure than dispersed growth systems according to Smith (1986). He concluded that biofilm processes can offer greater stability of operation for treating H 2 producing substrates than dispersed growth processes. The K s for H 2 utilization is reported to be 5 to 15 JlM (7.5 JlM is equal to 102 atmosphere). At steady state the H 2 utilizers work at only 1 to 10% of their maximum capacity. Since they work at only 1-10% of their maximum capacity, tremendous reserve capacity is available if the process is configured properly. Smith (1986) estimated the steady state H 2 turnover rates for various laboratory reactors and feeds. The turnover rates in reactors which favored biofilm growth were approximately 10 times greater than for CSTR reac,t' PRELIMINARY RESULTS

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Anaerobic microbial processes have been demonstrated to effectively mineralize 0 biotransformation to convert the volatile fatty acid intermediates of the organics all the \\0 often difficult. Preliminary studies in our laboratory have demonstrated that staging of the significantly enhance both the rate and completeness of the biotransformation process. T that during operation of a glucose fed reactor as a one-stage completely stirred tank r effluent chemical oxygen demand (COD) and volatile fatty acids (VFA) concentration

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and limited the allowable loading rate and gas production in the reactor. When the operation of the reactor was changed on Day 135 to two-stage digestion, by providing a 24-hour contact of the feed with a small fraction (5-10%) of the biomass from the main reactor in a first stage prior to transfer to the main reactor, the performance of the reactor rapidly improved. The volatile fatty acids in the main reactor decreased and the allowable loading rate and gas production increased. Conversely on Day 175 when the reactor operation was changed back to single stage from two-stage, the performance again abruptly declined. 8000 - , - - - - - - - - - - - - - - - - - - . . . . - - - - - , - 2.8 C' -..

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When propionate was fed directly to a one stage CSTR, in another exploratory study (see Fig. 3), the propionate was, efficiently utilized at a reasonable loading rate indicative of H 2 concentration below 10-4 atmosphere. However, when glucose (a high energy substrate) was added, which produces propionate as an intermediate, the propionate concentration increased abruptly at a rate of approximately 10% of the equiv~-".~ ""0D rate of glucose being fed. This result is possibly indicative of a change in degradation pathway, e.g., perhaps lactate or ethanol accumulation. The major intermediate still may have been propionate but the H 2 concentration increased toward 10-4 atmospheres, slowing down the rate and completeness of propionate conversion to H 2 and acetate. From Fig. 3 it is noted that both propionate and acetate concentrations increased.

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Acrylic acid is a hazardous chemical which can be effecti vely biotra nsform ed under anaero bic condit ions with proper acclimation but is quite toxic at concen tration s above 100 mgll. Acrylic acid at a feed concen tration of 3000 mgll can very effectively be conve rted to propio nate and acetate in a one stage VASB , but these intermediates persist in the effluent even when the aqylic acid loadin g rate is reduce d from 12 to 2 gIL-day. But when the effluent from the lowly loaded first stage VASB was passed throug h a separate second stage VASB , the propionat:? and acetate concentrations decrea sed from approx imatel y 1000 mg/l in the effluent of the first stage to below 100 mg/l in the effluent of the second stage VASB . A mixture of acrylic and acetic acid was fed by compu ter control to feed autom aticall y upon deman d to a single stage CSTR and the process manifested instability, eviden ced by elevat ed acrylate and propionate concen tration s in the effluent and erratic feeding demand. When the process was conve rted to two stage configuration, the residual acrylate concentration dropped from about 120 mgll in the single stage process to about 40 mgll in the two stage. Propionate similarly declined from about 1000 to about 300-5 00 mgll. Of greate r significance was the fact that the one stage CSTR failed by Day ISS, where as the two stage CSTR contin ued to operate stably. In another experiment when acrylate was fed to a 30-day HRT single stage CSTR , the acetate and propionate concentrations in the effluen t were chroni cally elevat ed. On the other hand, when the effluent were passed through a I-day HRT VASB , these volatile acids were reduce d to less than 100 mg/1. Similar advantages of two stage over one stage VASB operation were noted for n-propanol as had been reported previously for acrylate. 8

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An anaerobic culture was maintained in our laboratory which was capable of biotra nsform ing tetrachloroethylene to trichloroethylene (TCE) and then to dichlo roethy lene (DCE) using lactate as the primar y substrate. Subsequently, the dechlorinating activity of the culture had declin ed due to unknown ca·uses. An assay was then conducted with this culture to compa re perfor mance under single and two stage process configuration. As shown in Fig. 4, when 1/16 of the bioma ss from the second stage main reacto r contac ted the entire daily feed in a first stage reactor for 24 hours (at which time the entire conten ts of the first stage reactor were then. transferred to the second stage main reacto r with a detent ion time of 20 days) ~he PCE decreased to zero In about 100 hours. In a compa rable single stage config uratio n with identical Inoculum, the PCE had only decreased to 50% of the initial concen tration by 170 hours. The 'T('E was essentially converted to DCE in the two stage config uration by the end of 100 hours, whr r..:as only about 20% of the TCE had been converted to DCE in a one stage reacto r by 170 hours. Sir lilar resl:,!ts were repeated in a later experiment.

Process configuration

547

CONCLUSIONS The microbial ecology and metabolic complexity of the anaerobic process dictate careful consideration of the selection of process configuration. Metabolic pathways impact propionate conversion and process failure has been shown to be remedied by a proper change in process configuration. Enlightened appreciation of the crucial role of process configuration is a prerequisite for the exploitation of the profound advantages inherent in anaerobic processes. REFERENCES Anderson, G. K., Kasapgil, B. and Ince, O. (1994). Microbiological study of two stage anaerobic digestion start-up. Wat. Res., 28, 2383-2392. Bull, M. A., Sterritt, R. M. and Lester, J. N. (1984). An evaluation of single and separated phase anaerobic industrial wastewater treatment in fluidized bed reactors. Biotechnol. Bioeng., 26, 1054-1065. Cohen, A., Breure, A. M., van Andel, J. G. and van Deursen, A. (1980). Influence of phase separation on the anaerobic digestion of glucose - 1. Maximum COD-turnover rate during continuous operation. Wat. Res., 14, 1439. Ghosh, S. and Klass, D. L. (1978). Two-phase anaerobic digestion. Proc. Biochem., 15,2. Grobicki, A. and Stuckey, D. C. (1991). Performance of the anaerobic baffled reactor under steady-state and shock loading conditions. Biotechnol. Bioeng., 37, 344-355. Guiot, S. R, Pauss, A. and Costerton, 1. W. (1992). A structured model of the anaerobic granule consortium. Wat. Sci. Tech., 25, 1-10. Harper, S. R. and Pohland, F. G. (1987). Enhancement of anaerobic treatment efficiency through process modification. J. Wat. Poll. Control Fed. 59, 152-161. Kus, F. and Weismann, U. (1995). Degradation kinetics of acetate and propionate by immobilized anaerobic mixed cultures. Wat. Res., 29,1437-1443. Massey, M. L. and Pohland, E G. (1978). Phase separation of anaerobic stabilization by kinetic controls. 1. Wat. Poll. Control Fed.. 50,2204-2222. Mawson, A. 1., Earle, R L. and Larson, V. E (1991). Degradation of acetic and propionic acids in the methane fermentation. Wat. Res., 25, 1549-1554. Morvai, L., Mihaltz. P. and Hollo, J. (1992). Comparison of the kinetics of acetate biomethanation by raw and granular sludges. Appl. Microbiol. Biotechnol., 36,561-567. Pipyn, P. and Verstraete, W. (1981). Lactate and ethanol as intermediates in two-phase anaerobic digestion. Biotech. Bioeng., 23, 1145-1154. Sam-Soon, P., Loewenthal, R E. and Marais, G. R (1987). Hypothesis for pelletization in the upflow anaerobic sludge bed reactor. Water SA, 13, 69-80. Schmidt, J. E. and Ahring. B. K. (1995). Interspecies electron transfer during propionate and butyrate degradation in mesophilic, granular sludge. Appl. Environ. Microbiol. 61,2765-2767. Smith. D. P. and McCarty, P. L. (1989). Energetics and rate effects on methanogenesis of ethanol and propionate in perturbed CSTR's. Biotechnol. Bioeng., 34, 39-54. Smith, D. P. (1986). Hydrogenotrophic control in methanogenic processes. Ph.D. Dissertation, Stanford Univ. van Lier, J. B., Boersma, E, Debets, M. M. and Lettinga, G. (1994). High rate thermophilic anaerobic wastewater treatment in compartmentalized upflow reactors. Wat. Sci. Tech., 30,251-261. Wiegant, W. M., Hennink, M. and Lettinga, G. (1986). Separation of the propionate degradation to improve the efficiency of thermophilic anaerobic treatment of acidified wastewaters. Wat. Res., 20, 517-524.