Carbon Emission Accounting and Reduction Pathways for a Municipal Wastewater Treatment Plant in Lanzhou
Under the national carbon peak and carbon neutrality goals, carbon reduction in municipal wastewater treatment plants (WWTPs) has been largely overlooked, yet accurate accounting is the first step toward mitigation. This study establishes a carbon emission accounting method for a municipal WWTP in Lanzhou, covering the operation and maintenance phase, to identify key emission sources and propose feasible reduction pathways. The results show that the total annual carbon emission in 2023 was 61,399.80 t CO2-eq, with an emission intensity of 0.71 kg CO2-eq per tonne of wastewater treated. Monthly emissions were relatively stable, with a coefficient of variation of 3.46%. Direct emissions accounted for 47.47% of the total, with N2O being the dominant contributor (61.89% of direct emissions), followed by CO2 (30.88%) and CH4 (7.23%). Indirect emissions accounted for 52.53%, dominated by electricity consumption (95.15% of indirect emissions). Pearson correlation analysis revealed that direct carbon emissions per tonne were significantly correlated with influent BOD5 concentration, influent TN concentration, BOD5 removal rate, and TN removal rate (P < 0.01). Sensitivity analysis identified sewer retention time, fossil carbon fraction in influent, and solids retention time as the most influential parameters, with sensitivity coefficients of 0.42, 0.35, and 0.28, respectively. Considering uncertainties in emission factors and monitoring errors, the 95% confidence interval for annual total emissions was 55,200–67,600 t CO2-eq, corresponding to an emission intensity of 0.64–0.79 kg CO2-eq per tonne. Recommendations focus on three synergistic reduction strategies: reducing source emissions, lowering energy consumption, and enhancing carbon compensation.