System-Level Coordinated Optimization of Coupled Wastewater Treatment Plant–Constructed Wetland Systems for Enhanced Pollutant Removal and Low-Carbon Operation

Coupling municipal wastewater treatment plants (WWTPs) with constructed wetlands (CWs) has become an effective strategy for improving effluent quality while providing potential benefits in resource efficiency and environmental sustainability. However, previous studies have predominantly optimized WWTPs and CWs as independent treatment units, whereas coordinated operation of integrated WWTP–CW systems has received comparatively limited attention. This study evaluated a full-scale coupled WWTP–CW system using long-term operational data collected during 2021–2022. Environmental impacts were quantified using scenario-based calculations incorporating electricity consumption, chemical consumption, sludge production, and IPCC-based greenhouse gas accounting. Pollutant removal performance, electricity consumption, chemical consumption, and greenhouse gas (GHG) emissions were comprehensively assessed, and operational scenarios were developed to investigate the environmental benefits of coordinated optimization. The results showed that the WWTP consistently satisfied the Chinese Class 1A discharge standard, whereas the ancillary CW achieved effective polishing for COD and NH3-N but exhibited relatively low removal efficiencies for TN and TP because of insufficient biodegradable carbon in the influent. Scenario-based simulation demonstrated that moderately relaxing pollutant removal within the WWTP while maintaining compliant effluent quality substantially enhanced the treatment performance of the downstream CW. Under the optimized scenario, the average removal efficiencies of COD, TN, and TP in the CW increased by 28.3%, 17.4%, and 19.8%, respectively. Meanwhile, electricity consumption, sludge production, and dewatering-agent consumption in the WWTP were reduced by approximately 25%. When compared on a harmonized water-volume basis, unit external carbon-source consumption likewise decreased under the optimized scenario. Consequently, overall greenhouse gas emissions decreased by 40.3%, from 527.9 to 315.3 g CO2-eq·m−3 of treated wastewater. These findings demonstrate that coordinated pollutant allocation between engineered and ecological treatment units can substantially improve the environmental sustainability of existing WWTP–CW systems, providing a practical, low-carbon operational strategy without infrastructure expansion.