Three UASB reactors were operated to investigate the population dynamics of anaerobic sludge granulation. It is found that the increase of bacterial population relates to the bacterial status in anaerobic food chain and relates to sludge organic loading rates. In order to form granular sludge, it is necessary to have sufficient amount of different groups of bacteria in the sludge. In our experiment, the population of fermentative bacteria, propionate degraders, butyrate degraders and methanogens is about 107-8, 105-7, 105-7 and 105-7 cells/ml respectively at the appearance of granular sludge. Filamentous methanogenic bacteria are found to be the important species in sludge granulation. Based on the results obtained, a descriptive model is proposed to describe the microbial characteristics of granulation.
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