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Sludge Dewatering. Dewatering is a pysical (mechanical) unit operation used to reduce the moisture content of sludge for one or more of the following reasons:. The costs for trucking sludge to the ultimate disposal site become substantially lower when sludge volume is reduced by dewatering.
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Sludge Dewatering Dewatering is a pysical (mechanical) unit operation used to reduce the moisture content of sludge for one or more of the following reasons: • The costs for trucking sludge to the ultimate disposal site become substantially lower when sludge volume is reduced by dewatering • Dewatered sludge is generally easier to handle than thickened or liquid sludge. In most cases, dewatered sludge may be shoveled, moved about with tractors fitted with buckets and blades, and transported by belt conveyors. • Dewatering is required normally prior to the incineration of the sludge to increase the energy content by removal of excess moisture.
Sludge Dewatering Dewatering is a pysical (mechanical) unit operation used to reduce the moisture content of sludge for one or more of the following reasons: • Dewatering is required before composting to reduce the requirements for supplemental bulking agents or admendments. • In some cases, removal of the excess moisture may be require to render the sludge odorless and nonputresciple. • Sludge dewatering is required prior to landfilling in monofills to reduce leachate production at the landfill site.
Dewatering Processes The selection of the dewatering device is determined by the type of sludge to be dewatered, characteristics of the dewatered product, and the space available. The available dewatering processes include • vacuum filters, • centrifuges, • belt filter presses, • recessed plate filter presses, • drying bends and • lagoons.
Dewatering Processes For smaller plants where land availability is not a problem, drying beds or lagoons are generally used. Conversely, for facilities situated on constricted sites, mechanical dewatering devices are often chosen. Some sludges, particularly aerobically digested sludges, are not amenable to mechanical dewatering. These sludges can be dewatered on sand beds with good results. When a particular sludge must be dewatered mechanically, it is often difficult or impossible to select the optimum dewatering device without conducting bench-scale or pilot studies. Trailer-mounted full-size equipment is available from several manufacturers for field-testing purposes.
Vacuum Filtration Vacuum filtration has been used for municipal sludge dewatering for over 60 years, but its use has declined in the past ten years because of the development of and improvements to alternative mechanical dewatering equipment. Some of the reasons for its decline in popularity are • System complexity, • Need for conditioning chemicals, and • High operating and maintenance costs.
Centrifugation For separating liquids of different density, thickening slurries, or removing solids, the centrifugation process is widely used in the industry. The process is applicable to the dewatering of wastewater sludges and has been used with varying degrees of success in both the United States and Europe. The centrifugal devices used for thickening sludge, solid bowl and basket centrifuges, may also be used for sludge dewatering.
Belt Filter Press They are continuous-feed sludge-dewatering devices that involve the application of chemical conditioning, gravity drainage, and mechanically applied pressure to dewater sludge. The belt filter press was itroduced in the U.S in the early 1970s and has become one of the predominanty sludge-dewatering devices. Belt filter presses have proven to be effective for almost all types of municipal wastewater sludge.
Filter Presses In a filter press, dewatering is achieved by forcing the water from the sludge under high pressure. Various types of filter presses have been used to dewater sludge. The two types used most commanly are; • The Fixed-Volume, Recesses Plate Filter Press, and • The Variable-Volume, Recesses Plate Filter Press
Advantages cited for the filter press include; • High concentrations of cake solids, • Filtrate clarity, and • High solids capture. Disadvantages include; • Mechanical complexity, • High chemical costs, • High labor costs, and • Limitations on filter cloth life
Sludge Drying Beds They are the most widely used method of sludge dewatering in the United States. Sludge drying beds are typically used to dewater digested sludge. After drying, the sludge is removed and either disposed of in a landfill or used as a soil conditioner. The principle advantages of drying beds are; • Low cost, • Infrequent attention required, and • High solids content in the dried product.
Four types of drying beds are used; • Conventional sand, • Paved, • Artificial media, and vacuum-assisted. Because conventional sand drying beds are used most extensively, more detailed discussion is provided for this type of drying bed.
Lagoons Drying lagoons may be used as a substitute for drying beds for the dewatering of digested sludge. Lagoons are not suitable for dewatering untreated sludges, limed sludges, or sludges with a high-strength supernatant because of their odor and nuisance potential. The performance of lagoons, like that of drying beds, is affected by climate; precipitation and low temperatures inhibit dewatering.
Lagoons are most applicable in areas with high evaporation rates. Dewatering by subsurface drainage and percolation is limited by increasingly stringent environmental and groundwater regulations. If a groundwater aquifer used for a potable water supply underlines the lagoon site, it may be necessary to line the lagoon or otherwise restrict significant percolation.
