ABSTRACT
The purpose of wastewater treatment is to remove the suspended solids, dissolved chemicals, pathogens, and other biological organisms from municipal sewage and contaminated industrial process waters, to yield an effluent that can legally and safely be discharged into the environment. In the preliminary treatment stage, coarse solids and grit are removed through a combination of screening and settling by gravity. The preliminary and primary treatment stages rely primarily on physical processes such as screening and gravity settling. Chemicals may be added to promote the settling of suspended solids. Secondary treatment commonly employs biological processes which are especially effective in the removal of soluble organic material (BOD) from a wastewater. The activated sludge process is effective in the removal of soluble BOD from wastewater but it can also be designed and operated in a manner which permits nitrogen removal in selected parts of the system.
