Phosphate release and uptake in enhanced biological phosphorus removal from wastewater

Phosphorus removal from municipal and industrial waste waters is required to protect receiving waters from eutrophication (nutrient enrichment). Many treatment plants have been de signed or upgraded to uptake phosphorus, most often by the addition of chemicals. As an alternative, an enhanced biological phosphorus (bio-P) removal process offers the advantage of not requiring chemical additions and of reducing the volumes of sludge produced. A simple bio-P removal process must include at least two stages: non-aerated and aerated. In the first stage, influent wastewater and return sludge are combined under non aerated conditions. Phosphate release from the biomass and sol uble chemical oxygen demand (COD) removal are observed un der these anaerobic conditions in the absence of both oxygen and nitrate. In the subsequent aerated stage, phosphate removal from solution, COD removal and nitrification take place. Phos phorus-rich sludge is usually wasted from this aerobic reactor. Many processes also include an intermediate denitrification stage (often referred to as "anoxic") in which nitrified mixed liquor is recycled from the aerated reactor.