| The secondary sedimentation tanks of the Blue Plains Wastewater Treatment Plant during it's regular plant operations in Washington, Friday, May 25, 2007. No one can say exactly what is in sludge. It's a constantly changing brew of human, commercial, hospital and industrial wastes. The primary organic ingredient is human excrement, which proponents say makes sludge a useful fertilizer. Critics worry about the metals and pathogens that remain (AP Photo/Lawrence Jackson) |