The basic principle of biological phosphorus removal is to expose bacteria to alternating anaerobic and aerobic conditions to promote “luxury uptake” of phosphorus. Under anoxic to anaerobic conditions, phosphorus accumulating organisms (PAO) have the ability to take in organic substrate. In order to obtain the energy to incorporate the organic substrate into the microbial cell under anaerobic conditions, the bacteria releases phosphorus into the wastewater. When aerobic conditions are restored, the organic substrate taken in is converted to energy and cell mass. This allows the bacteria to take in phosphorus then needed to fulfill its nutrient needs.