Despite these positive results of soil amendment with biosolids, the dark side of the sludge is evident in the health risks. Sewage sludges also contain components considered to be harmful to the environment. Sewage sludge contains three constituents of environmental concern: (1) heavy metals; (2) organic pollution; (c) pathogenic organism . Generally, sludges contain heavy trace elements such as Cu, As, Cd, Ni, Zn, and Pb in diverse concentration; these originate from a number of different sources, such as industry, commerce, business, domestic household waste, corroding pipes, and runoff from roads and roofs .
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