Abstract: Simultaneous biological phosphorus and nitrogen removal by enhanced anoxic phosphate uptake was investigated in an anaerobic-aerobic-anoxic-aerobic sequencing batch reactor(modelⅡ).The ratio of the anoxic phosphate uptake to the aerobic phosphate uptake capacity increased from 28.2 % to 68.3 % by insert an anoxic phase in an anaerobic-aerobic SBR(modelⅠ).The modelⅡSBR system showed stable phosphorus and nitrogen removal performance.Average removal efficiencies ofρ(COD),ρ(PO₄^3--P),andρ(TN) were 92.0%,98.0%,and 81.5%,respectively.Through batch experiment,It was found thatρ(NO₂^--N) (up to 30 mg/L)was not detrimental to the anoxic phosphate uptake and could serve as an electron acceptor likeρ(NO₃^-N).In fact,the phosphate uptake rate was even faster in the presence ofρ(NO₂^--N)as an electron acceptor compared to the presence ofρ(NO₃^--N).It was found that on-line sensor values of pH and E_ORP were somehow related with the dynamic behaviors of nutrient concentrations(ρ(COD),ρ(NH₄⁺-N),ρ(NO₃^--N),andρ(PO₄^3--P)) in the SBR.The inflexion of E_ORP profile corresponding to the end of phosphorus released in anaerobic phase;the inflexion of E_ORP and pH profiles corresponding to the end of nitrification in aerobic phaseⅠ;the inflexion of E_ORP and pH profiles corresponding to the end of denitrification in anoxic phase;the inflexion of E_ORP and pH profiles corresponding to the end ofρ(COD)exhaust and phosphorus uptake in aerobic phaseⅡ.These pH and E_ORP profile characteristic point could be used as real-time control parameters to adjust the duration of each operational phase in the modelⅡSBR,thus could enhance the nitrogen and phosphorus removal efficiency.