The role of bound water content in defining sludge dewatering characteristics

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Virginia Polytechnic Institute and State University


Several available methods of measuring sludge bound water content in the laboratory were examined. The effect of polymer conditioning on the bound water content of biological sludge samples was measured using the dilatometric method. The effects of mechanical dewatering on the bound water content of biological sludge samples and on chemical sludge samples was measured using the same method. The controlled drying method was used to measure the effect of polymer conditioning and mechanical dewatering on the chemically bound water fraction.

The relationship between bound water content and cake solids concentration was examined, as well as the relationship between cake solids concentration and sludge bulk density. The role of apparent sludge floc density was examined.

The dilatometric method was found to be the most accurate and most convenient method for measuring the chemically bound water fraction. Polymer conditioning was found to release significant volumes of bound water. Further bound water release was produced by mechanical dewatering. The amount of bound water released increased with the degree of mechanical dewatering pressure applied. The chemically bound water fraction was not affected by polymer conditioning or mechanical dewatering.

A reduction in bound water brought about a corresponding increase in cake solids concentration. Sludge bulk density increased with cake solids concentration. Apparent sludge floc density of the unconditioned, underwatered sludge sample was predictive of ultimate dewatering performance in many cases.