Phosphorus is an essential element for every living organism, but when it exceeds certain limit in water bodies, it will cause serious environmental issues, such as eutrophication. P accumulation in water bodies can be caused by non-point sources, such as agriculture land runoff, as well as from point sources such as wastewater effluent discharges. In order to avoid eutrophication of water bodies the regulatory agency have traditionally pushed for the removal of phosphorus at wastewater treatment plants rather than regulating the non-point sources; this has been done imposing strict limits on wastewater effluents to natural watershed. At the same time, phosphorus stock on the earth is a limited resource, and its quantity is decreasing steadily, due to its use in the fertilizers production. In order to support the population growth on the earth, phosphorus recovery, especially from wastewater streams has to be considered. Biological processes that remove phosphorus from wastewater, are called Enhanced Biological Phosphorus Removal (EBPR in short) processes, and produce a P rich sludge that can be treated to recover phosphorus in forms that could be used as fertilizers. Several processes have been developed to recover P from EBPR plants, such as Waste Activated Sludge Stripping to Remove Internal Phosphorus (WASSTRIP) and the PhoStrip processes. P recovery within EBPR plant often involved an anaerobic holding tank, where P is released due to PAO activity as well as bacteria decay. However, the impact of EBPR process operating conditions on P-release capacity and kinetics are not fully understood. In addition, it is largely unclear how the anaerobic digestion process of the P-recovery process affects the microbial population, and therefore the EBPR activity in the mainstream, in system where the sludge is recirculated back to the mainstream. In this study, P-release capacity and kinetics were studied by conducting day long endogenous anaerobic digestion tests on activated sludge withdrawn from lab scale sequencing batch reactors operating under different conditions (COD/P and SRT). P-release mechanisms during the digestion test were investigated by Live/Dead analysis, as well as soluble metal ion concentration measurements, which are usually associated with EBPR activity. In addition, PAO activities changes were explored by the microbial population quantification, combined with P-release rate in the present of VFA. Considering the microbial populations, in the acetate fed SBRs are different in quantity and possibly composition, from the population (especially PAOs) in full-scale samples, the same anaerobic test and measurements were performed on a full-scale EBPR WWTP WAS samples. Under anaerobic starvation conditions, it was observed that low COD:P ratio with 10-20 days-SRT had better P recovery potential than other operation conditions involved in this study, in terms of higher amount ortho-P released and faster releasing rate. Among the released ortho-P during the digestion test, majority of it was found to be due by poly-P depletion. In addition, because of the reducing intercellular polymer storage, PAO activity also decreased significantly during the anaerobic digestion test. However, with different population and composition, PAO activity in full scale WAS samples actually increased after the test.