The flocculating ability of activated sludge and adhesion of dispersed cell or fine flocs to large floc surfaces influence the level of effluent suspended solids (ESS), or non-settleable fine particles, in the final effluent of biologically treated wastewaters.1-8 Proposed mechanisms for floc formation, including charge neutralization, and polymer- and salt-bridging emphasize the importance of surface properties in floc interactions.2-5,9-12 Increasing attention has been given to the hydrophobic nature of sludge floc and its role in bioflocculation. A more hydrophobic surface has been related to a lower level of ESS.6,13-16 The composition and the properties of extracellular polymeric substances (EPS), particularly proteins, have been shown to be major determinants of the physicochemical properties of flocs, including the hydrophobicity.12,15,17,18

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