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Explain the chemical reactions involved in enhanced biological phosphorus removal (EBPR) and the role of polyphosphate-accumulating organisms (PAOs).



Enhanced Biological Phosphorus Removal (EBPR) is a biological process used in wastewater treatment to remove phosphorus from wastewater using specific microorganisms called polyphosphate-accumulating organisms (PAOs). The process involves two key phases: an anaerobic phase and an aerobic phase. In the anaerobic phase, the PAOs are subjected to conditions without oxygen and in the presence of readily biodegradable organic carbon, typically in the form of volatile fatty acids (VFAs) like acetate. Under these conditions, PAOs uptake the VFAs. To gain the energy needed for this uptake, they break down their internal stores of polyphosphate (poly-P). Polyphosphate is a long chain of phosphate molecules linked together. The breakdown of polyphosphate releases phosphate (PO4^3-) into the surrounding water. Simultaneously, the PAOs store the VFAs as polyhydroxyalkanoates (PHAs), such as polyhydroxybutyrate (PHB), within their cells. Essentially, they "trade" phosphate for carbon. This release of phosphate in the anaerobic zone is essential for the overall process. In the aerobic phase, the PAOs are exposed to oxygen. Under these conditions, they use the stored PHAs as an energy source to grow and, most importantly, to take up phosphate from the surrounding water. This uptake of phosphate is far greater than what they released in the anaerobic phase. The phosphate is then used to replenish their polyphosphate stores. The chemical reactions can be summarized as follows: Anaerobic Phase: VFAs uptake + Poly-P breakdown -> PHA storage + Phosphate release. Aerobic Phase: PHA utilization + Phosphate uptake -> Poly-P storage + Biomass growth. The PAOs are then removed from the system through sludge wasting, effectively removing the excess phosphorus that they have accumulated. The key to EBPR is the cyclical exposure of the PAOs to anaerobic and aerobic conditions, which selects for their growth and allows them to accumulate far more phosphorus than other microorganisms. Without the anaerobic phase, PAOs would not have the selective advantage needed to dominate the microbial community and effectively remove phosphorus. For example, a common operating strategy is to direct the influent wastewater, containing high concentrations of readily biodegradable carbon, directly into an anaerobic selector tank to promote VFA production and subsequent PAO activity. Therefore, the chemical reactions and the unique metabolic capabilities of PAOs are central to the success of EBPR in wastewater treatment.