The effect of certain sludge floc properties on the dewatering characteristics of biological and chemical waste sludges

Abstract

The technical and economical problems associated with sludge handling and disposal have significantly increased in the past years due to: (1) the introduction of more stringent water and air pollution regulations, often requiring the use of processes that produce large quantities of sludge as by-products; and (2) the development of regulations stipulating the means of ultimate disposal of sludge residues. These problems have resulted in a need for producing sludges with both good dewatering characteristics and low water content so as to minimize the ultimate volume of solids which must be disposed of. Recognizing the need for further work in this area of research, the objectives of this study were to examine the fundamental parameters which affect sludge dewatering characteristics. Included in this investigation were an analysis of both sludge dewatering and sludge handling characteristics. Parameters of interest were floe size, shape, and density, sludge viscosity and shear strength, and floc compressibility.

The experimental investigation involved the collection of both water and wastewater sludge samples from full-scale treatment plants in the Commonwealth of Virginia. These samples were analyzed for each of the parameters previously mentioned; in addition, each was subjected to dewatering studies using laboratory-scale vacuum filtration, sanbed, and filter press equipment.

The experimental results showed clearly that particle size distribution was a key parameter for describing the response of sludges to each of the dewatering methods utilized. For biological sludges, other factors such as biopolymer content were felt to have a major effect on dewatering characteristics. Sludge floc density was shown to have a direct correlation to the ultimate solids concentration obtained by vacuum filtration. Sludge handleability was conveniently evaluated through the use of the standard Atterburg liquid limit test procedure. Finally, the use of high-pressure filtration was found to provide much more appropriate information regarding water content than the centrifugation method proposed by Vesilind (1979).