Approximately 6.9 million ton of biosolids were generated in the United States in 1998, and about 60% of it was used beneficially in land applications, composting, and landfill cover. It is estimated that, by 2010, 8.2 million tons will be generated, and 70% of the biosolids is expected to be used beneficially (USEPA, 1999). Recycling options are described in various documents (Crites and Tchobanoglous, 1998; Crites et al., 2000; USEPA, 1994a, 1995a,c). Sludges are a common by-product from all waste treatment systems, including some of the natural processes described in previous chapters. Sludges are also produced by water treatment operations and by many industrial and commercial activities. The economics and safety of disposal or reuse options are strongly influenced by the water content of the sludge and the degree of stabilization with respect to pathogens, organic content, metals content, and other contaminants. This chapter describes several natural methods for sludge treatment and reuse. In-plant sludge processing methods, such as thickening, digestion, and mechanical methods for conditioning and dewatering, are not included in this text; instead, Grady et al. (1999), ICE (2002), Metcalf & Eddy (2003), Reynolds and Richards (1996), and USEPA (1979, 1982) are recommended for that purpose.
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