Sewage sludge is a concentration of filtered or discareded wastes from sewer systems. It has been manufactured to be used as a fertilizer for farming.

Sewage sludge contains a variety of substances that may cause environmental and public health problems. These include: toxic metals (such as Hg, Cu, Ni, Pb, Zn, As, Cd, Se, Mo, Co, Cr and Ag), nitrogen (predominantly in the form of ammonium and organic-N), pathogens, phosphate, greenhouse and acid gases, and organic chemicals (most notably PCBs, organochlorine pesticides, PAHs, phthalate esters, surfactants, furans, chlorobenzenes, solvents, phenols and dioxins).

The concentration and extent to which the above chemicals are present in the sludge are dependant on its source. Sludge from industrialized, urban areas, not surprisingly have the highest concentrations of these chemicals.

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