Dewatering

Dewatering of sludge is a physical unit operation to reduce the moisture of the sludge in preparation for subsequent further treatment processes. As the agricultural use of the sludge and landfilling are increasingly restricted in the United States and elsewhere, drying and incineration are widely implemented. As a result, the costs related to the treatment of sludge have considerably risen and commonly represent 35-50% of the total operating costs of the wastewater treatment. Reducing the amount of sludge produced and improving the dewaterability are of paramount importance. This objective of sludge reduction has stressed the importance of using an extended aeration biology, using a biological phosphorus removal (instead of chemical precipitation), using sludge digesters, etc. A further reduction and an improvement of the dewaterability require advanced sludge treatment technologies, such as the Fenton reaction and peroxide oxidation (Neyens et al., 2002).

A number of techniques of dewatering are used for removing moisture; some rely on natural forces such as evaporation and percolation, and the others are assisted mechanically and thermally during dewatering. The objective of dewatering is moisture content of 60-80%, depending on the disposal method. Dewatering of the sludge is currently achieved through the use of mechanical dewatering and thermal dehydration devices. The most widely used mechanical dewatering device is the filter press. Other technologies employed include vacuum filters, centrifuges, and belt presses. Mechanical dewatering will produce a sludge with an approximately 10-60% solids content (e.g., by using common centrifuges or belt presses, only 20-25% dry solids can be obtained). Sludge dryers are used to further remove moisture from the sludge and are capable of producing a material with 90% solids content. Because all dewatering devices are dependent upon proper sludge conditioning, a carefully designed chemical feed system should be included as part of the dewatering facility.

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