Two full-scale systems operated in parallel,a conventional A2/O system consisting of anaerobic,anoxic and oxic compartments in succession and an inverted system consisting of anoxic,anaerobic and oxic compartments without internal recycle,were compared in terms of their phosphorus removal performance,with an emphasis on phosphate (P) release behaviors,using both operational data and simulation results.The inverted system exhibited better long-term phosphorus removal performance (0.2 ± 0.3 vs.0.7 + 0.7 mg/L),which should be attributed to the higher P release rate (0.79 vs.0.60 kg P/(kg MLSS.day)) in the non-aerated compartments.The P release occurred in both the anoxic and anaerobic compartments of the inverted system,resulting in more efficient P release.Although the abundances of the Candidatus Accumulibacter phosphatis population in the two systems were quite similar ((19.1 ± 3.27)％ and (18.4 ± 4.15)％ of the total microbe (DAPI stained particles) population in the inverted and conventional systems,respectively,by fluorescence in situ hybridization (FISH)),the high-concentration DAPI staining results show that the abundances of the whole polyphosphate accumulating organisms (PAOs) in the aerobic ends were quite different (the average ratios of the poly-P granules to total microbes (DAPI stained particles) were (45 ± 4.18)％ and (35 ± 5.39)％,respectively).Both the operational data and simulation results showed that the inverted system retained more abundant PAO populations due to its special configuration,which permitted efficient P release in the non-aerated compartment and better P removal.