Anoxic–Oxic+Subsurface Flow Constructed Wetland+Surface Flow Constructed Wetland Combined Process Optimizes Urban Secondary Effluent N/P to Cultivate Nontoxic Green Algae

High concentration of nitrogen and phosphorus (above 20 and 1.5 mg/L, respectively, for TN and TP) and low ratio of nitrogen to phosphorus (10–15 for N/P) in secondary effluent of cities in China are easy to stimulate the malignant proliferation of toxic microalgae in receiving water. Accordingly, this study proposes a combination process of anoxic–oxic (A/O)+subsurface flow constructed wetland (SSFCW)+surface flow constructed wetland (SFCW) with N/P regulation as the core. The combination process parameters are optimized through the feedback regulation mechanism of the N/P ratio and the level of the terminal effluent, so that the N/P ratio and level of the terminal effluent are conducive to restraining eutrophication and stimulating the growth of nontoxic green algae. Compared with toxic green algae, nontoxic green algae had more growth advantages when N/P was 15 (mg/L):0.3 (mg/L). Based on technical and economic indicators, sludge retention time (SRT) and hydraulic retention time (HRT) of A/O system were 25 days and 15 h, respectively, and TN and TP concentrations of effluent treated by A/O system were 15–20 and 0.5–1.0 mg/L, respectively. Based on the effluent quality of A/O and the target N/P and level required by the terminal effluent of combined process, the quantitative phosphorus removal and N/P regulation of effluent were realized by the quantitative mixing of phosphorus removal functional filler steel slag and common filler ceramsite in SSFCW. When HRT of SSFCW is 12 h and surface hydraulic load is 0.69 m3/(m2·days), the effluent TN and TP concentrations are 15 and 0.3 mg/L, respectively. In this study, through the dual control of target water quality feedback regulation and technical and economic indicators, the optimal treatment process of urban sewage for the purpose of ecological resources was determined.