Several forms of the phosphorus in wastewater were analyzed using a fraction method with a membrane reactor. A primary aim of this study was to differentiate the two main groups, as either "biologically bound" or "physicochemically bound," with the quantification of bound phosphorus in animated sludge. After the four-level extraction treatment with distilled water and various solvents such as bicarbonate-dithionate, NaOH and HCl, the redox-sensitive-P and P bound to Fe(III)-hydroxides were found as main precipitation products in the anaerobic chamber (AN). Whereas the aerobic chamber (AE) contained mainly the precipitates of P bound to metals, such as Al and Fe, sorptive-bound phosphorus on metals and organic compounds etc. With iron precipitation, the levels of Fe-P, Al-P, and Fe-P adsorbed by Fe(OH)(3) were particularly high in the AE and AN. The sum of inorganic-P (physicochemically bound phosphorus) increased with iron precipitation, whereas that of organic-P and poly P (biologically bound phosphorus) decreased. (C) 2009 Elsevier B.V. All rights reserved.