By studying the drying characteristic curve, the moisture in sludge is classified into four categories: free moisture, interstitial moisture, surface moisture and bound moisture. The effects of three dewatering procedures: gravity drainage, vacuum filtration, and centri-fugation, as well as chemical conditioning by a cationic polymer, and physical conditioning by freeze-thaw on the moisture distribution, are investigated. It is found that the dewatering procedures studied can only remove part of the free and interstitial moisture; surface and bound moisture are not affected. Part of the free and interstitial moisture also remains with the sludge cake, which can be considered as the inherent inefficiency of the dewatering process. The complete removal of all the free moisture can be considered as the theoretical best performance any mechanical dewatering device can achieve. Polymer addition is found to improve the dewaterability of the sludge by reducing the inherent inefficiency, primarily by reducing the amount of free moisture retained in the cake. Freeze-thaw is found to effect the removal of free, interstitial and surface moisture, bringing about a dramatic increase in dewaterability. The treated sludge cake drains readily and retains no free moisture.

In 1986, the Cities of Hickory, Newton, and Conover, together with Catawba County, formed the Hickory Regional Sludge Consortium (HRSC) to develop a regional approach to manage wastewater residuals and septage in North Carolina. The in-vessel composting approach was selected and the Regional Sludge Composting Facility (RSCF) was subsequently constructed, largely funded by a grant from the U.S. Environmental Protection Agency (USEPA). The RSCF began operating in 1990 to process wastewater residuals and septage produced by the members of the HRSC but was soon shut down due to odor complaints. The RSCF reopened in January 1995 with an upgraded odor control system and has since been operating continuously under contract by Professional Services Group (PSG). PSG manages the receiving, dewatering, composting, curing, and distribution of the finished compost, as well as monitoring and regulatory compliance testing.The RSCF consists of a liquid residuals receiving station, amendment receiving and storage, residuals dewatering by belt filter presses, active composting in four in-vessel tunnel reactors, and an outdoor curing and storage area. A unique feature of the RSCF is that liquid residuals are blended with amendment prior to dewatering. From 1995 to 1997, the RSCF processed an average of 12 DT/d of residuals. A continual deterioration in dewatering performance was observed, primarily due to changes in sludge characteristics. The impacts of lower cake solids content are higher requirements of amendment and longer retention time required for composting and curing. Furthermore, the contract operator has identified limitations in curing capacity. More than half of the compost was not cured and was disposed off site. Compost properly cured was successfully marketed.This paper illustrates the importance of a system approach to designing and operating a residuals composting facility. The proper selection and sizing of dewatering and composting technology as well as provision of adequate odor control through all stages of the process are critical for the success of any residuals composting facility.

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