Sludge granulation in a 15 m3 UASB reactor, with alcoholic stillage from ethanol fermentation broth as influent, has been studied. Based on kinetic data as well as both scanning and transmitting scope observation and identification, a hypothesis for the mechanism of anaerobic sludge granulation is suggested. The first step of granulation is the formation of nuclei. The bacteria involved in the nucleus formation are mainly M. sarcina and M. thrix. The acetic acid concentration and the “selection pressure” play a decisive role for determining the nucleus formation. A wide spectrum of other bacteria, which the methanogens must syntrophically grow with, also play a very important role during the second step, that is the process of a nucleus growing into a granule.

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