This report documents the results of a project initiated to evaluate the handling and disposal of combined sewer overflow (CSO) treatment residuals. Bench-scale thickening and pilotand full-scale centrifugation dewatering tests were performed at dry-weather and CSO treatment sites in Kenosha, Racine, and Milwaukee, Wisconsin. CSO sludge at Kenosha is biologically generated; that at Milwaukee is physical in nature; and the Racine CSO residuals are of physical-chemical origin. In addition, bench-scale anaerobic digestion studies were conducted to determine the effect of CSO sludges on the anaerobic digestion stabilization process.
The results obtained from this project indicated that the dewatering of CSO sludges appears feasible when the sludges are first degritted, where required, and thickened prior to centrifugation. Under optimum centrifuge-operating conditions, thickened sludges were dewatered to cake concentrations varying from 14.0 percent to 32 percent solids with solids recoveries ranging from 80 percent to 99 percent. Similarly, the dry-weather sludges for the test sites were dewatered to haulable cakes. Moreover, at Kenosha, the dewatering characteristics of wet-dry weather sludge mixtures were similar to those for CSO sludge alone. The bench-scale anaerobic digestion studies showed that no significant adverse effect was realized by adding CSO-generated sludges to dry-weather digesters at feed rates similar to that expected from a typical storm event.
Preliminary economic estimates indicate that first investment capital costs for thickeningcentrifugation of CSO sludges ranged from $0.31 to 2.92 million, with annual costs of $49,500 to $659,300 per year when handling 4.0 to 36.5 tons dry sludge per day. These cost ranges were developed respectively for the cities of Racine, Wisconsin (population 90,700; CSO area 702 acres) and Milwaukee, Wisconsin (population 670,00; CSO area 16,800 acres).
The report recommends that a full-scale CSO sludge dewatering facility employing degritting, thickening, and centrifugation should be developed as a demonstration site for further evaluation of the treatment of CSO residuals.
This report was submitted in fulfillment of Contract No. 68-03-0242 by the Environmental Sciences Division of Envirex, Inc. under the sponsorship of the U.S. Environmental Protection Agency. Work for this report covers the period from August 1975 to September 1976.
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