Consideration on the Structural Models

Capetown model postulates that anaerobic granulation would not take place in UASB reactors treating acetate, propionate, or butyrate because of inadequate hydrogen partial pressure. However, there was experimental evidence that anaerobic granules could be formed in UASB systems fed with the substrates mentioned above (Ahring et al., 1993; van Lier et al.,

1995). On the other hand, high hydrogen partial pressure is not desirable with respect to the granule-associated bacteria, because the partial pressure of hydrogen must be maintained at low level to ensure efficient fermentation of the volatile fatty acids. This may imply that the Capetown model is applicable only under some specific conditions. The importance of ECP in anaerobic granulation has been evidenced (Schmidt and Ahring, 1994,

1996). It seems that ECP may play an important role in building spatial structure and maintaining the stability of granules. However, the contribution ECP to the initiation of anaerobic granulation remains debatable. In addition, a high amount of ECP seems unnecessary for forming active granules. Instead, it has been found that too much ECP may deteriorate floc formation (Schmidt and Ahring, 1996).

In the syntrophic microcolony model, a close synergistic relationship among different microbial groups is essential for breaking down the complex organic wastes. Syntrophic microcolonies provide the kinetic and thermodynamic requirements for intermediate transference and therefore efficient substrate conversion (Schink and Thauer, 1988). It seems certain that the synergistic requirements provide a driving force for bacteria to form granules, in which different microbial species function in a synergistic way to increase the chance of survival.

Contrary to the multilayer model, anaerobic granules with non-layered structure were also reported (Grotenhuis et al., 1991; Fang et al., 1995; Wu et al., 2001). There was evidence that a layered and non-layered microstructure of the UASB granules may be developed with carbohydrates and substrates having a rate-limiting hydrolytic or fermentative step (e.g. proteins), respectively (Fang et al., 1995; Fang, 2000). This is probably due to different initial steps in the carbohydrate and protein degradation. The initial carbohydrate degradation to small molecules is faster than its subsequent degradation of the intermediates, whereas the initial step in the protein degradation is a rate-limiting step. Results from fluorescence in situ hybridization combined with confocal scanning laser microscopy showed that protein-fed granules possesses non-layered structure with a random distribution of Methanosaeta concilii (Rocheleau et al., 1999). However, different types of granules may also form on the same substrate (Daffonchio et al., 1995; Schmidt and Ahring, 1996). Based on microscopic examination of the UASB granules, Fang (2000) proposed that the microbial distribution of the UASB granules strongly depends on the degradation thermodynamics and kinetics of individual substrates. Therefore, it appears that different dominating catabolic pathways may give rise to different structural granules.

So far, none of the structural models could explain a spontaneous and sudden washout of the established granular sludge bed as a result of a change in wastewater composition, which is commonly encountered in the operation of UASB systems. The point is, if a factor that is independent of the wastewater composition can initiate the formation of UASB granules, a change in the wastewater composition should not lead to the washout of the entire granular sludge bed. Thus, it is a reasonable speculation that there should be a substrate composition-associated factor that could also contribute to the formation of UASB granules. However, this proposition is yet to be included in the current structural models discussed previously.

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