Secondary minimum adhesion model is based on the DLVO theory for colloidal particles, which proposes that reversible adhesion takes place in the secondary minimum of the DLVO free energy curve. The Gibbs energy of the reversible adhesion is relatively small, and there is always a separation distance between the two adhering bacteria. Thus, the reversible adhesion can change to irreversible adhesion at the primary minimum by overcoming the energy barrier or by protruding fibrils or fimbriae, which bridge the gap between bacteria (Rouxhet and Mozes, 1990). The secondary minimum adhesion model accounts for both the surface charge and the surface energy or hydrophobicity, which are relevant to long- and short-range forces. It appears from this model that anaerobic granulation would start from the self-immobilization of bacteria through reversible and followed by irreversible microbial interaction. It should be realized that the secondary minimum adhesion model merely looks into the thermody-namic aspects of bacterial interaction, thus the real meaning of this model is somewhat limited with respect to a biologically defined engineering rector.
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