The state of knowledge on the microbiological aspects of granular methanogenic sludge in upflow anaerobic sludge blanket (UASB) reactors is reviewed. Granular methanogenic sludge has been observed in a variety of upflow reactors, which is in agreement with the hypothesis that these systems select for well settling sludges. In methanogenic UASB reactors various types of well settling sludges have been observed. Microscopic and kinetic evidence suggests that acetoclastic Methanothrix-like organisms play an important role in determining which type of sludge will develop under methanogenic conditions. The mechanism of granulation still remains obscure. Extracellular material indoubtedly plays a major role as a matrix which keeps the cells together. Scanning electron microscopic evidence for the presence of extracellular material is available, but transmission electron microscopy has failed to validate the hypothesis that exopolysaccharides are the dominant components in this matrix.

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