IntroductionIn anaerobic digestion, various bacteria are responsiblefor the degradation of organic matter, the conversion ofinitial fermentative products to acetate, carbon dioxideand hydrogen, and the formation of methane in the finaldigestion. Since the numbers and activities of mostbacteria are strongly dependent on the temperature, itis important that the optimal reaction temperature ismaintained.

When the temperature changes, even for a short period,temperature-susceptible bacteria can be shocked so thatthe whole digestion process can be affected. Severalworkers (Topiwala and Sinclair, 1971; Peck et al., 1986;Bull et al., 1983a,b) have found that just after thetemperature changes, the activities of bacteria weresignificantly suppressed. As a result, the more volatilefatty acids were accumulated until the bacteria recov-ered from the temperature shock. If bacteria are notsubjected to a the temperature change, the removal effi-ciency of organic matter can be estimated as a functionof temperature (Cha et al., 1992).

Transient changes in operating parameters affect theintegrity of the conventional anaerobic digestion.However, the effect of temperature drop on acidogensisitself is not clearly understood. Considering that theacidogenesis eventually produces acetate which serves asa starting material to produce approximate 70% ofmethane, any phenomena due to temperature drop

during acidogensis should be carefully investigated toprovide better understanding for the temperature depen-dence of acidogensis. This paper describes the behaviorof bacteria and the degree of substrate degradationduring temperature drop.

Materials and methods A schematic diagram of the anaerobic reactor foracidogenesis is shown in Fig. 1. The starch solutionequivalent to 9040 mg COD/L as a substrate wascontinuously fed at a flow rate of 3.4 L/d into the reactor and the same amount of treated solution wasdrawn from the reactor. The liquid volume in the reactorwas 0.85 L throughout the experiment. The initialtemperature in the reactor was set to 30°C and to drop to 25, 20 and 15°C sequentially. After eachtemperature drop of 5°C over 3 hours, the reactor was maintained at the steady-state condition for about30 days until the next temperature drop. The incomingand outgoing solutions were analyzed for the concen-trations of carbohydrates and volatile fatty acids (VFA).To determine the concentration of carbohydrate, aspectrophotometric measurement based on anthronemethod was used (Herbert et al., 1971). The concen-trations of VFAs were determined by a gas chromatog-raphy and the values of chemical oxygen demand werecalculated from those. The number of bacteria in thereactor were measured based on the most probablenumber (MPN) method established by Chartrain et al. (1987).

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Biotechnology Letters, Vol 19, No 5, May 1997, pp. 461–464

© 1997 Chapman & Hall Biotechnology Letters · Vol 19 · No 5 · 1997 461

Suppression of acidogenic activities due to rapid temperature drop inanaerobic digestionGi Cheol Cha, Hyung Keun Chung and Jae Chun ChungDepartment of Environmental Science and Technology, Yonsei University, 234-Maeji Hungub, Wonju, Kangwon-Do,220Ð710, Korea

A rapid temperature drop during anaerobic acidogenesis in an anaerobic reactor system resulted in the sharpsuppression of carbohydrate decomposition and production of volatile fatty acids. When the temperature was droppedfrom 30¡C to 25, 20, 15¡C sequentially, the numbers of bacteria were slowly reduced without showing temperatureshock. The acidogenesis, however, was dramatically affected after each temperature drop; the removal efÞciency ofcarbohydrate was reduced from 92% to 84%, 72%, 25% with showing the minima of 78%, 52% and 10% due tothe rapid temperature drop respectively. This indicates that the acidogens lose the activities momentarily during the rapid temperature drop and require certain period of time to recover the activities at the adjusted fermentationtemperature.

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Results and discussion The number of bacteria involving anaerobic digestionwould be reduced if the fermentation temperaturedecreases. Figure 2 shows that with 6-hour hydraulicretention time, the number of bacteria was decreasedwith decreasing the temperature from 30 to 15°C viatwo rapid temperature drops by 5°C over 3 hours. Thenumber of acidogens and H2-utilizing methanogensexpressed in terms of logarithm appeared to be decreasedlinearly as the fermentation temperature decreased. Inthe case of the homo-acetogens, the bacterial number

was not reduced when the temperature was droppedfrom 30 to 25°C. This may suggest that the nature ofhomo-acetogens was not significantly varied above25°C, but below 25°C these bacteria were more affectedby the decrease in temperature. In any case, thereduction in these bacterial numbers during the rapid temperature drop was minimal. The number ofacetate-utilizing methanogens, which was counted 8MPN/mL at 30°C, was reduced to 2 MPN/mL or lesswhen the temperature was decreased. This indicates thatthe formation of methane by the acetate-utilizing

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462 Biotechnology Letters · Vol 19 · No 5 · 1997

Figure 1 Schematic diagram of the anaerobic acidogenesis system. GS, gas sampling port; LS, mixed liquid port; MS,magnetic stirrer; P, pump.

Figure 2 Dependence of bacterial population on temper-ature. The samples taken from the reactor were incubatedat 35¡C for 2 months before analysis.

Figure 3 Dependence of the carbohydrate removal efÞ-ciency on temperature.

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methanogens would be insignificant compared to theH2-utilizing methanogenesis.

To evaluate the degree of carbohydrate degradation withdecreasing temperature rapidly, the concentrations ofcarbohydrate before and after the acidogenesis weremeasured to calculate the removal efficiency of carbo-hydrate. Figure 3 shows that with 6-hour hydraulicretention time, the removal efficiency was dramaticallydecreased after each temperature drop, increased to acertain level after about 15 days, and finally was main-tained at that level until the next temperature drop. Asthe number of acidogens decreased due to temperatureshock did not occur (see Fig. 2), the unexpectedly abrupt decrease in the removal efficiency just after thetemperature drop would be due to the significant reduc-tion of the acidogenic bacterial activities. Note that ifthe acidogenesis does not undergo temperature shock,the removal efficiency may be estimated based on the number of acidogens. In contrast to the case with6-hour hydraulic retention time, the same experimentwith 2-day hydraulic retention time showed that the carbohydrate removal efficiencies were ranged95–90% and less than 5% of reduction occurred duringthe rapid temperature drop. This indicates that theeffect of reduction of the acidogens due to the temper-ature shock would be compensated by the effectiveacidogenesis during relatively long hydraulic retentiontime.

The anerobic digestion of carbohydrate was primarilyconverted to two major species, one of which is the gassuch as CO2 and H2 and other is non-gaseous speciessuch as volatile fatty acids and alcohols. Figure 4 shows

how the gas production rate was varied with the temper-ature drop from 30 to 15°C. The data just after eachtemperature drop show abrupt changes of the gasproduction rate due to temperature shock. When thetemperature was dropped from 25 to 20°C and 20 to15°C, the data show similar trend to those shown inFig. 3. This indicates that the amount of gas producedis positively related to the amount of carbohydratedecomposed. However, if the digestion condition ismore favorable to produce non-gaseous species than gasand if a gas-consuming process become important, thetrend for the amount of gas observed would not bedirectly related to the amount of carbohydrate decom-posed. This situation would happen at high tempera-ture. Note that the temperature was dropped from 30to 25°C, the gas production rate was increased by 19%after about 5 days slightly decreased to a stable levelwhich was still 12% greater than at 30°C. Throughoutthe experiment, the gas contained 55–60% CO2,35–40% H2 and less than 1% CH4.

Figure 5 shows that decreasing the temperature from30 to 15°C significantly decreases the concentrations ofmajor volatile fatty acids (C2–C4). The average pHvalues at the stabilized state were 6.3 for 30–25°C, 6.5for 20°C, and 6.7 for 15°C. Upon each temperaturedrop, the transient maxima equivalent to pH 0.1–0.3were observed. The volatile fatty acids over C5 were not showed in Fig. 4 because their existence is limitedto only small quantities. In the temperature rangestudied with 6-hour hydraulic retention time, theconcentration of n-butyrate was the greatest, followedby acetate and propionate. When the temperature wasdropped from 30 to 25°C, any noticeable change due

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Suppression of acidogenic activities due to rapid temperature drop in anaerobic digestion

Biotechnology Letters · Vol 19 · No 5 · 1997 463

Figure 4 Dependence of gas production rate on temper-ature.

Figure 5 Dependence of the concentrations of variousvolatile fatty acids on temperature.

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to the temperature shock was not observed, even thoughthe concentration of n-butyrate was slightly decreasedover about 30 days. The further temperature drop to20 and 15°C caused sharp decrease initially in theconcentrations of fatty acids which were much lowerthan those observed at the finally stabilized condition.This observation was very clear for n-butyrate. Thisindicates that the acidogenesis was unbalanced forcertain period just after the rapid temperature drop andneeded recovery period (about 15 days in this experi-ment) until the acidogenesis were fully stabilized at thelowered temperature. Considering that the number ofbacteria were not greatly reduced during that period,this result confirms that the activities of acetogens weresignificantly degraded due to the temperature shock.

ReferencesBull M.A., Sterritt R.M. and Lester J.N. (1983a). Chem. Tech.

Biotechnol., 33B, 221–230.Bull M.A., Sterritt R.M. and Lester J.N. (1983b). Water Res.,

17, 1563–1568.Cha G.C., Li Y.Y. and Noike T. (1992). Proc. of Environ. & Sani.

Eng. Research, 28, 29–37 (in Japanese).Chartrain M., Bhatbagar L. and Zeikus J.G. (1987). Appl. Environ.

Microbiol., 53, 1447–1456.Herbert D., Phipps P.J. and Strange R.E. (1971). Methods in

Microbiology, 5B, 266–267.Mountfort D.O. and Asher R.A. (1978). Appl. Environ. Microbiol.,

35, 209–232.Peck M.W., Skilton J.M., Hawkes F.R. and Hawkes D.L. (1986).

Water Res., 20, 453–462.Topiwala H. and Sinclair C.G. (1971). Biotechnol Bioeng., 13,

795–813.

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464 Biotechnology Letters · Vol 19 · No 5 · 1997

Received 3 February 1997; Revisions requested 14 February 1997;

Revisions received 21 March 1997; Accepted 24 March 1997

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