Sewage sludge is an inevitable by-product of wastewater treatment processes. Being the concentrate of impurities removed from the spent water, it contains compounds of agricultural value that mainly include nitrogen, phosphorus, and organic matter, and potential harmful substances that include trace elements (for example, Cd and Pb), organic chemicals (for example, pesticides, polychlorinated biphenyls (PCBs)) and pathogens. In the urban settings and from the environmental sanitation perspective, sewage sludge is a 'waste'. Its final disposal, however, has been problematic as it lacks appropriate outlets and there is not an entirely satisfactory resolution. Land application is by far the most common approach employed by treatment works around the world.
'Biosolids', in the United States, refers to municipal sewage sludge that has been treated to meet product quality standards for environmentally safe land applications. The term biosolids is employed to emphasize the beneficial nature of the products. The United States Environmental Protection Agency (USEPA) defines biosolids as 'the nutrient-rich organic materials resulting from the treatment of domestic sewage in a treatment facility' that may
Trace Elements in Soils