EFFECT OF TEMPERATURE DECREASE ON THE MICROBIAL POPULATION AND PROCESS PERFORMANCE OF A MESOPHILIC ANAEROBIC BIOREACTOR

The effect of a temperature decrease from 33°C to 12°C was investigated for anaerobic digestion of crop residues. A laboratory-scale reactor (R0) was inoculated with mesophilic sludge and operated as continuously stirred fed-batch system at temperatures of 12°C, 18°C and 33°C. Changes in the microbial populations of the sludge were followed by means of fluorescence in situ hybridization analysis. Methane was produced in R0 at all temperatures. Stable long-term operation at 18°C was achieved yielding 151 mlCH4 gVSadded -1 at a rate of 108 mlCH4 lR -1d-1 once the microbial populations of the sludge had adapted to this temperature. After operation at 18°C, the contents of R0 was mixed and distributed into three smaller reactors, which were operated at 18°C (R18), 25°C (R25) and 37°C (R37), respectively. Methane production rates for R37 and R25 were 366 and 310 mlCH4 lR -1d-1, respectively, which were higher than the 215 mlCH4 lR -1d-1 obtained in R0 when this was operated at 33°C. Hydrolysis was found to decrease when temperature was decreased and especially below 25°C. At temperatures below 16°C, acidogenesis and methanogenesis were the rate-limiting steps. Adaptation of the mesophilic sludge to 18°C was indicated by an increase in the ratio of Bacteria to total prokaryotes (sum of Archaea and Bacteria). This was thought to be caused by enrichment of Bacteria in the sludge, which appeared to be an important adaptation mechanism. During the adaptation, the Methanomicrobiales and Methanosarcinaceae populations increased relative to the total Archaea population whereas the Methanosaeta population decreased. The population changes were reflected by reactor performance.