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Microbial flocs are naturally hydrated, due to the presence of large numbers of hydroxyl, carboxyl, and phosphate groups. Side chains in amino acids, the methyl groups in polysaccharides, and the long-chain carbon groups in lipids all contribute to the hydrophobic properties of sludge flocs. Flocs are negatively charged under neutral pH conditions. The presence of ionizable groups such as carboxyl, phosphate, and amino groups in the EPS and cell surfaces is responsible for the density of surface charge. The zeta potential of sludge flocs is usually in the range of -10 to -30 mV.2,13,19 Simple measures of the physicochemical properties, including hydro-phobicity and surface charge, may be reliable indicators for predicting bioflocculation in the operation of biological wastewater treatment processes.15

The purposes of this study were to evaluate the surface tension of sludge flocs by using contact angle measurement and Neumann's equation-of-state, to investigate the influence of sludge retention time (SRT) on the surface tension of sludge flocs, and to test the feasibility of using surface thermodynamic concept to predict bioflocculation.

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