The effects of the rapid change in temperature and HRT on the characteristics of substrate degradation and bacterial population levels in anaerobic acidogenesis were investigated by using the laboratory scale anaerobic reactor. The degradation efficiency of substrate in acidogenesis was significantly affected by the rapid drop in temperature particularly at short hydraulic retention time and low temperature. The n-butyrate concentration was increased or decreased for the same change in temperature when the HRT conditions were different from each other. The numbers of acidogens, homo-acetogens and H2-utilizing methanogens were not affected in the first 3 days after temperature was rapidly dropped at all the temperature conditions. At 15°C, the complete washout of acetate-utilizing methanogens was observed. The bacterial forms varied according to the changes in the hydraulic retention time and temperature.

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