According to the thermodynamic theory, microbial granulation is a creation process for a new granule-liquid interface by disrupting preexisting individual bacteria-liquid interface, and a molecular contact between the two adhering bacterial surfaces is involved. The free energy of adhesion
(AGadh) can be expressed as follows (Rouxhet and Mozes, 1990): AGadh = 20^ - rl1/2)(rl1/2 - r^2)
where, rc is the surface free energy of bacteria, rl is the surface free energy of liquid, and rs is the surface free energy of inert particle.
This equation shows that if the surface free energy of bacteria is lower that that of the liquid, the free energy of aggregation decreases and aggregation is favored with decreasing surface energy of the inert carrier. The opposite trend would occur if the surface energy of bacteria is higher than that of the liquid. In fact, the above thermodynamic equation is a theoretical basis of the surface tension model. It was found in a UASB reactor that aggregation of hydrophilic cells was enhanced at low liquid surface tension, while the opposite was true for hydrophobic cells (Thaveesri et al., 1995). Depending on the liquid surface tension (y) in the UASB reactor, bacteria may grow in rather loose associations, in multilayered granules (y < 50mN/m), or in mixed conglomerates (y > 56mN/m) (Thaveesri et al., 1995; Grootaerd et al., 1997).
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