Lower temperature or polyphosphate limitation is the favorable condition to enrich polyphosphate-accumulating organisms (PAOs) or glycogen accumulating organisms (GAOs) in biological phosphorus removal process.In this study,a sludge highly enriched in PAOs was used to investigate the metabolic pathways and carbon transformation in batch tests under the two conditions for elucidating the contradiction.The results showed that the PAOs are able to take up acetate and store it as polyhydroxyalkanoates (PHA) in anaerobic phase although the poly-P pool is emptied through washing with negligible P release.The stoichiometric ratios of the PHB and PHV production and glycogen degradation to the acetate uptake combined with the Nile blue staining indicated that the PAOs possess the anaerobic metabolism of GAOs using glycogen as the sole energy source,but no transformation of PAOs to GAOs is observed.After reconverting to the normal operation,phosphorus uptake and release are recovered the same as the parent reactor.