ABSTRACT
Assisted by themassive construction grant programof theWater PollutionControlAct, a large number of primary and secondary municipal and industrial wastewater treatment plants were brought on-line in the United States during the period 1970-1990. The plants generate large quantities of waste solids [402]. These “biosolids” include several waste streams:
“Primary” sludge (produced by gravity settling in the first stage of treatment). “Biological” (“secondary”) sludge produced in the activated sludge process. “Advanced” wastewater treatment sludge from the tertiary treatment process. Other treatment plant solids such as the “screenings” recovered by themechanical removal of
large-dimension solids, “grit” recovered by settling sand and other coarse granular solids, and “scum” or “skimmings” recovered as the floatable solids skimmed from clarifiers.
