The cohesion of activated sludge biological flocs is controlled by the ratio between hydrophilic exopolymers in which bacteria are embedded and hydrophobic interactions. As shown in this study highly hydrophobic bacteria coexist in the flocs with highly hydrophilic bacteria which seem to release large quantities of exopolymers (polysaccharides, DNA, proteins…). Addition to the activated sludge of easily assimilable nutrients such as glucose does not change the hydrophobicity of bacteria but increases both exopolymer production and the sludge volume index. The environmental conditions and the bioavailability of nutrients may considerably shift the hydrophilic/hydrophobic balance. Moreover experiments carried out with four different strains to fourteen isolated from activated sludges have shown that a few strains (variable types) have quite considerable possibilities of hydrophobic property variations in the course of growth.

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