Carbon Reduction Strategies for Typical Wastewater Treatment Processes (A2/O): Response Surface Optimization, Mechanism, and Application Analysis

With increasing wastewater treatment demands and decarbonization goals, synergistic reduction in pollutants and green house gas (GHG) emissions is crucial. High process emissions like N2O pose significant challenges, yet optimized carbon reduction strategies for conventional plants are lacking. This study developed three mathematical models to quantify the impact of dissolved oxygen (DO), influent salinity, and C/N ratio on direct emissions (CH4, N2O) and indirect emissions. Response Surface Methodology (RSM) optimized these factors to minimize GHG emissions under three accounting scenarios: (1) plants with CH4 reuse systems: salinity = 0.5 g L−1, DO = 3.67 mg L−1, C/N = 12.75; (2) plants focusing solely on direct emissions: salinity = 0.5 g L−1, DO = 3.35 mg L−1, C/N = 3; and (3) plants assessing total emissions: salinity = 0.5 g L−1, DO = 2.5 mg L−1, C/N = 7.18. Key findings indicated that increasing salinity exacerbated greenhouse gas emissions. Elevated DO levels in the aerobic stage reduced N2O emissions but increased indirect emissions in the A2/O process. Higher C/N ratios promoted anaerobic CH4 production, but sufficient carbon reduced N2O by enabling complete heterotrophic denitrification. A 60−day continuous GHG emissions monitoring campaign was conducted at a WWTP to validate the actual emission reductions achievable under the identified optimal control conditions. An analysis and comparison of operational and economic costs were also performed. The findings provide practical insights into sustainable GHG emission management and offer potential solutions to advance the synergistic reduction in GHG emissions and pollutants.