4.2. Onsite treatment of the effluent stream4.2.1. Sludge thickeningUsually, RAS effluents are characterized by a low solid content (<2%) and fluctuate in volume as a result of specific feeding and cleaning regimes. As direct disposal of these effluents is costly, solids thickening and stabilization of the effluent flow is often required before final disposal. Thickening of the sludge through settling of solids in basins or ponds (Bergheim et al., 1993), through solids capture by means of geotextile bags (Schwartz et al., 2004, 2005) or, more recently, by means of belt filters (Timmons and Ebeling, 2007) and membrane reactors (Sharrer et al., 2007) are applied in RAS. The various methods are often used in combinationwith coagulation/flocculation processes to allow a more complete removal of suspended solids as well as phosphorus from the effluent water (Danaher et al., 2011b; Ebeling et al., 2003, 2006; Sharrer et al., 2009). In combination with dewatering, the various methods used for sludge thickening may produce sludge with a solid content of between 5 and 22% (Sharrer et al., 2009).
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