Microbial adhesion or self-immobilization is regarded as the onset of anaerobic granulation process, and can be defined in terms of the energy involved in the interaction of bacterium-to-bacterium or bacterium-to-solid surface. In a thermodynamic sense, when one bacterium approaches another, the interactions involve repulsive electrostatic force, attractive van de Waals force, and repulsive hydration interaction. Some authors analyzed the granulation mechanism in terms of energy involved in the adhesion itself, due to the physico-chemical interactions between cells walls or between cells walls and alien surfaces. Factors like hydropho-bicity and electrophoretic mobility are objectively taken into account. Based on the thermodynamics, some physico-chemical models for anaerobic granulation have been developed, those include inert nuclei model, selection pressure model, multivalence positive ion-bonding model, ECP bonding model, synthetic and natural polymer-bonding model, secondary minimum adhesion model, local dehydration and hydrophobic interaction model, and surface tension model.
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