The bioconversion of organics into methane proceeds through a series of complex biochemical changes, and little is known about the individual
steps involved due to the many pathways available for an anaerobic community. Figure 1.1 illustrates simplified pathways of methane fermentation of complex wastes by various routes. The microbial species including methanogens and acidogens form a syntrophic relationship in which each bacteria group constitutes a significant link in a complex chain of bioconversion. The syntrophic microcolony model suggests that the syntrophic relationship eventually lead to the formation of stable microcolonies or consortia, viz initial granules (Hirsh, 1984). Anaerobic granule indeed can be regarded as the congregation of cells to form fairly stable, contiguous, multicellular associations under physiological conditions in a defined biological system. The close packing of bacteria in granule architecture inherently facilitates the exchange of metabolites.
In UASB granules, different groups of bacteria carry out sequential metabolic processes, and interspecies syntrophic reactions are energetically beneficial. Because of the need for such close proximity, random cell-to-cell association in UASB granules would not enhance metabolic reactions. As pointed out by Fang (2000), "biogranules are developed through evolution instead of random aggregation of suspended microbes". In order to maintain high metabolic efficiency, the granule-associated cells would present in an organized structure, and signaling mechanisms in organizing the syntrophic species can be predicted (Shapiro, 1998). Therefore, it appears from the syntrophic microcolony model that the driving force for sludge granulation should be a result of the needs for bacterial survival or balance and for optimal combination of different biochemical functions of multiple species under the culture conditions.
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