Based on the microscopic observations, a multilayer model for anaerobic granulation was initially proposed by MacLeod et al. (1990); Guiot et al. (1992). According to this model, the microbiological composition of granules is different in each layer. The inner layer mainly consists of methanogens that may act as nucleation centers necessary for the initiation of granule development. H2-producing and H2-utilizing bacteria are dominant species in the middle layer, and a mixed species including rods, cocci, and filamentous bacteria takes predominant position in the outermost layer (Fig. 1.2). To convert a target organic to methane, the spatial
organizations of methanogens and other species in UASB granules are essential.
The layered structure of UASB granules is supported by the works of Ahring et al. (1993); Lens et al. (1995) with immunological and histologic methods, with a dynamic model (Arcand et al., 1994), with microelectrodes (Santegoeds et al., 1999), and with fluorescence in situ hybridization using 16S rRNA-targeted oligonucleotides (Sekiguchi et al., 1998, 1999; Tagawa et al., 2000). A distinct layered structure was also found in the methanogenic-sulfidogenic aggregates, with sulfate-reducing bacteria in the outer 50-100 |xm and methanogens in the inner layers (Santegoeds et al., 1999). Unlike the initial multilayer model proposed by MacLeod et al. (1990), recent research showed that UASB granules had large dark non-staining centers, in which neither archaeal nor bacterial signals could be found (Rocheleau et al., 1999). In fact, the non-staining center in the UASB granules might be formed as a result of the accumulation of metabolically inactive, decaying biomass, and inorganic materials (Sekiguchi et al., 1999).
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