The upflow anaerobic sludge blanket (UASB) reactor is increasingly gaining popularity for high strength organic wastewater treatment because of its high biomass concentration and rich microbial diversity (Lettinga et al., 1980; Hulshoff Pol et al., 1988; Fang et al., 1995; Schmidt and Ahring, 1996; Wu et al., 2001). High biomass concentration and rich microbial diversity give rise to rapid contaminant degradation, implying that highly concentrated or large volumes of organic waste can be treated in compact UASB reactors. Comparing to other anaerobic technologies, such as anaerobic filter, anaerobic sequencing batch reactor, anaerobic expanded bed, and fluidized bed reactors, a unique feature of the UASB system is its dependence on biogranulation process. It appears that anaerobic granular sludge is a core component of a UASB reactor. The granules are generally dense and enriched with multispecies microbial communities. None of the individual species in the granular ecosystem is capable of degrading complex organic wastes separately.
One major drawback of UASB reactors is its extremely long start-up period, which generally requires between 2 and 8 months for successful development of granular sludge. To reduce the space-time requirements and leading to a cheaper treatment of high strength wastes, strategies for expediting granules development are highly desirable for UASB systems. In achieving such a purpose, a thorough understanding of the mechanisms for anaerobic granulation is essential. This chapter attempts to review the existing mechanisms and models for anaerobic granulation in UASB systems, and also tries to build up a general model for anaerobic granulation.
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