Effect of temperature decrease on the microbial population and process performance of a mesophilic anaerobic bioreactor

The effect of a temperature decrease from 33 degrees C to 12 degrees C was investigated for anaerobic digestion of crop residues. A laboratory-scale reactor (R,) was inoculated with mesophilic sludge and operated as continuously stirred fed-batch system at temperatures of 12 degrees C, 18 degrees C and 33 degrees C. Changes in the microbial populations of the sludge were followed by means of fluorescence in situ hybridization analysis. Methane was produced in R, at all temperatures. Stable long-term operation at 18 degrees C was achieved yielding 151 mlCH(4) gVS(added)(-1) at a rate of 108 mlCH(4) l(R)(-1)d(-1) once the microbial populations of the sludge had adapted to this temperature. After operation at 18 degrees C, the contents of R-0, was mixed and distributed into three smaller reactors, which were operated at 18 degrees C (R-18), 25 degrees C (R-25) and 37 degrees C (R-37) respectively. Methane production rates for R-37 and R-25 were 366 and 310 mlCH(4) l(R)(-1)d(-1), respectively, which were higher than the 215 mlCH(4) l(R)(-1)d(-1) obtained in R-0 when this was operated at 33 degrees C. Hydrolysis was found to decrease when temperature was decreased and especially below 25 degrees C. At temperatures below 16 degrees C, acidogenesis and methanogenesis were the rate-limiting steps. Adaptation of the mesophilic sludge to 18 degrees 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.