ABSTRACT
Waste stabilization ponds (WSPs) are large, shallow basins in which raw sewage is treated by natural processes involving both algae and bacteria. They are mostly used for sewage treatment in temperate and tropical climates. WSP systems comprise a single string of anaerobic, facultative, and maturation ponds in series, or several such series in parallel. In essence, anaerobic and facultative ponds are designed for removal of biochemical oxygen demand (BOD), and maturation ponds are designed mainly for pathogen removal. In most cases, only anaerobic and facultative ponds will be needed for BOD removal when the effluent is to be used for restricted crop irrigation and fishpond fertilization, as well as when weak sewage is to be treated prior to its discharge to surface waters. Maturation ponds are only required when the effluent is to be used for unrestricted irrigation, thereby having to comply with the World Health Organization guideline of >1000 fecal coliform bacteria/100 mL. It is possible to obtain close to 90% removal of BOD5 (i.e., a BOD5 of about 10–20 mg/L), a phosphorus removal of about 20%–35%, and a total nitrogen removal of 20%–40%. Usually, nitrate is removed very effectively. The organic nitrogen is easily oxidized to ammonia but ammonia is usually removed by a low efficiency due to insufficient oxygen concentrations in the ponds. By aeration—even rather moderate aeration—it is possible to increase the nitrogen removal to almost 80% by oxidation of ammonium to nitrate, which, as indicated above, is removed usually with high efficiency. By careful design of the maturation ponds, the removal of coliform bacteria will be satisfactory.
