Optimal strategies evaluated by multi-objective optimization method for improving the performance of a novel cycle operating activated sludge process

• A novel cycle operating activated sludge process was simulated with ASM2d.
• One open loop and three time controller strategies were compared.
• EQ and OC were optimized simultaneously by multi-objective optimization method.
• Step operating time could be determined properly in the time controller strategies.
• Better effluent quality and lower operation cost were achieved.

The aim of this work was to improve performances of effluent quality (EQ) and operation cost (OC) for a novel cycle operating activated sludge process by using multi-objective optimization method. One open-loop and three closed-loop strategies: (i) open loop optimization (OLO); (ii) time controller based on SNO3SNO3 or dSNO3dSNO3 (TCNO3TCNO3); (iii) time controller based on SNH4SNH4 or dSNH4dSNH4 (TCNH4TCNH4) and (iv) time controller based on SPO4SPO4 or dSPO4dSPO4 (TCPO4TCPO4), were evaluated. The conditions of three weather dynamic influents (Dry-2, Rain-2 and Storm-2) were investigated. Results showed that trade-offs between EQ and OC of each strategy could be presented. Four solutions, which have the same EQ as that of default strategy, were separately selected from the optimized four strategies. Comparing OLO with default strategy, 24.13% of OC was reduced with Dry-2, and with Rain-2 and Storm-2, EQs were reduced by 13.94%, 6.83%, and OCs by 22.43%, 23.22% respectively. EQs of TCNO3TCNO3, where step operating time could be controlled properly, were reduced by 6.11% and 4.04% with Rain-2 and Storm-2 respectively compared with OLO, moreover, OCs of TCNO3TCNO3 were all lower than that of OLO under three influents. Therefor this study considered that OLO and TCNO3TCNO3 strategies could be separately used under open-loop and closed-loop conditions to achieve better performances.