Floc property effects on sludge dewatering characteristics

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Virginia Water Resources Research Center, Virginia Polytechnic Institute and State University

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, which often require the use of processes that produce large quantities of sludge as by-products, and (2) the development of regulations that stipulate the means of ultimate disposal of sludge residues. These regulations have resulted in the need to produce sludges with both good dewatering characteristics and low water content in order to minimize the ultimate volume of solids that require disposal. Thus, the objective of this study was to examine the fundamental parameters that affect sludge dewatering characterics. Included in the study are analyses of both sludge dewatering and sludge handling characteristics. Parameters of interest are floe size, shape, and density; sludge viscosity and shear strength; and floc compressibility.

The investigation involved the collection of both water and wastewater sludge samples from fu II-scale treatment plants in Virginia. These samples were analyzed for each of the parameters mentioned above and were then subjected to dewatering studies using laboratory-scale vacuum filtration, sandbed, and filter press equipment. Sludge handleability was evaluated through the standard Atterburg liquid limit test procedure. Three main conclusions can be drawn from this study:

  1. Particle size distribution is a key parameter for describing the response of sludges to each of the dewatering methods used, although other factors, such as biopolymer content, apparently have a major effect on the dewatering characteristics of biological sludges;
  2. Sludge floe density has a direct correlation to the ultimate solids concentration obtained by vacuum filtration; and
  3. The use of high-pressure filtration provides much more appropriate information on water content than the centrifugation method proposed by Vesilind [1979].