This work aims at optimizing performance of the multimode anaerobic/anoxic/aerobic (AAO) system, which could be flexibly operated under AAO, reversed AAO (RAAO) and anoxic/aerobic (AO) modes, based on effluent quality and operating costs at low temperature of 12.5 degrees C. Influences of sludge retention time (SRT) on effluent quality, functional biomass and operating cost were evaluated for each mode. The AAO has a better performance than RAAO and AO at SRT of 5-25 d under the same operating cost for its higher pollutant removal efficiency and concentration of phosphorus-accumulating organisms. Furthermore, an operating region conforming to required discharge standards was established for the AAO mode by regulating returned activated sludge (RAS) and mixed liquor recirculation (MLR). Optimal condition was obtained using MLR ratio of 100%, RAS ratio of 50%, and SRT of 16 d. Dynamic simulation results showed that multi-criteria modeling was a useful tool in process optimization. (C) 2015 Elsevier B.V. All rights reserved.