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Pollution Prevention and Control Technologies for Plating Operations

Section 6 - Wastewater Treatment


6.2.6 Sludge Dehydration

The solids content of the sludge dewatered on a filter press is usually in the range of 25 to 60 percent. This represents an approximately 20 to 1 volume reduction from the original sludge volume discharged from the clarifier/thickener. Additional dewatering can be accomplished with sludge dehydration equipment that can produce a waste material with a solids content of approximately 90%. This represents an approximately 4 to 1 waste volume reduction above that achieved by the filter press. Sludge dehydration equipment is available in both batch and continuous operating modes. Batch units are typically designed with a heated sludge chamber with rotating blades that consistently break up and blend the sludge cake, thereby exposing it to the heat source. Continuous units have a feed hopper and employ an auger or conveyor system that moves a thin layer of sludge through a drying region and discharges it into a hopper. Some continuous units can be located such that the sludge cake discharge from a filter press drops into the feed hopper of the dehydration unit, making the overall dewatering process more automated. System capacities range from less than 1 ft3/hr to more than 20 ft3/hr of feed. Various heat sources are used for sludge drying, including: electric, electric infrared, steam and gas. Energy requirements are mostly dependent on the water content of the feed stock and the efficiency of a given unit. Sludge dehydration equipment will require an air exhaust system.

During the past five years, sludge dehydration equipment has been one of the most frequently purchased of all pollution prevention and control devices. The Users Survey data show that 29 percent of the plating shops have installed this technology and that approximately 80 percent of these units were purchased between 1988 and 1993.

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