Consideration on the Physicochemical Models

The discussion so far seems to suggest that each physico-chemical model accounts for contribution of only one or two factors to the initial granulation process in UASB reactor. As these factors exert their influences under specific environmental conditions and in specific steps during the entire granulation process, the physico-chemical models provide only simple descriptions on anaerobic granulation.

The inert nuclei model can be easily understood with a hypothesis that the formation of UASB granules is favored by the presence of inert particles in the reactor. However, there was evidence that anaerobic granules could be developed even without adding any inert materials (Thiele et al., 1990). It should be realized that besides attachment on solid surfaces, self-immobilization of bacteria can also lead to formation of microbial aggregates.

With respect to the multivalence positive ion-bonding model, some studies had shown that calcium ion did not contribute to sludge granulation (Guiot et al., 1988) and that a high concentration of magnesium ion caused disintegration of granules (Schmidt and Ahring, 1993). A research in membrane fusion indeed indicated that Ca2+ might cause conforma-tional changes of some surface proteins or polypeptide groups that could interact with two surfaces and bridge them together (Papahadjopoulos et al., 1990). On the other hand, it had been proposed that the beneficial effect of calcium addition on anaerobic granulation was probably due to the calcium-induced dehydration and fusion of bacterial surfaces (Teo et al., 2000). The calcium-induced cell fusion might initiate the formation of cell cluster, which acts as microbial nuclei of anaerobic granulation.

In the secondary minimum adhesion model, the DLVO theory is unable to make predictions at short distances due to breakdown of the computation of electrical interactions. It also neglects the forces which are important at short distances, such as hydrogen bonding and other effects involved in solution and hydrophobic bonding (Rouxhet and Mozes, 1990). While in the local dehydration and surface tension models, bacterial granulation is oversimplified to a purely thermodynamic process. Such a simple description is usually inadequate, as microbial aggregation is a very complex biological phenomenon and many unidentified factors are believed to be involved. It seems impossible to develop a pure thermodynamic model with satisfactory confidence level. The fact that bacteria cannot be simply treated as physically defined dead colloidal particles, and bacteria indeed have no well-defined surface boundary, simple geometry, or uniform molecular surface composition, the physico-chemical forces alone are not able to completely explain the entire microbial granulation process. It is thus suggested that the physico-chemical phenomena involved in microbial granulation ought to be related to the biological triggers controlling the granulation.

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