Key Takeaways & Executive Findings
- •• • The WWTP emitted 61,399.80 t CO2-eq in 2023, with an intensity of 0.71 kg CO2-eq/t; monthly variation was low (CV=3.46%), indicating stable operation. • • Direct emissions (47.47%) were dominated by N2O (61.89% of direct), while indirect emissions (52.53%) were dominated by electricity (95.15% of indirect), making electricity the single largest source (49.98% of total). • • Pearson correlation (P<0.01) linked direct emissions to influent BOD5 and TN concentrations and their removal rates, highlighting process control targets. • • Sensitivity analysis showed sewer retention time (0.42), fossil carbon fraction (0.35), and solids retention time (0.28) as key operational levers; uncertainty analysis gave a 95% CI of 0.64–0.79 kg CO2-eq/t.
Abstract
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.
1. Introduction
Municipal wastewater treatment is an energy-intensive process with significant greenhouse gas (GHG) emissions, yet its contribution to national carbon footprints is often underestimated. In China, the sector accounts for 1–2% of total emissions, and with rapid expansion of treatment capacity, emissions are projected to rise. Existing accounting methods often lack system boundaries that include upstream sewer networks and downstream sludge handling, leading to incomplete inventories and missed reduction opportunities.
This study addresses these gaps by applying a comprehensive accounting framework to a large WWTP in Lanzhou, covering the full system from sewer to effluent and sludge. By quantifying direct and indirect emissions, identifying key drivers via correlation and sensitivity analyses, and proposing targeted reduction measures, the work provides a replicable methodology for urban WWTPs to achieve synergistic pollution and carbon reduction.
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XU Wen, YANG Rui, LIU Xiangrui, MAO Yaoru, CHENG Xiuwen (2026). Carbon Emission Accounting and Reduction Pathways for a Municipal Wastewater Treatment Plant in Lanzhou. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511025
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Frequently Asked Questions
What are the main sources of direct and indirect emissions in the WWTP, and how can they be mitigated?
Direct emissions (47.47% of total) are dominated by N2O (61.89% of direct), mainly from biological nitrogen removal; CH4 (7.23%) arises from anaerobic zones and sewers. Indirect emissions (52.53%) are dominated by electricity (95.15% of indirect), used for aeration and pumping. Mitigation includes optimizing aeration control to reduce N2O, improving sewer conditions to limit CH4, and adopting energy-efficient equipment and renewable energy.
How sensitive are the emission estimates to key operational parameters?
Sensitivity analysis identified sewer retention time (sensitivity coefficient 0.42), fossil carbon fraction in influent (0.35), and solids retention time (0.28) as the most influential. This means that controlling these parameters can significantly reduce emissions; for instance, reducing sewer retention time can lower CH4 formation, while optimizing SRT can minimize N2O.
What is the uncertainty range of the reported emission intensity, and what causes it?
The 95% confidence interval for emission intensity is 0.64–0.79 kg CO2-eq/t, with the main uncertainty arising from emission factors, particularly for N2O. The study recommends on-site monitoring of N2O and CH4 to develop local emission factors and improve accuracy.
How does the carbon emission intensity of this plant compare with other Chinese WWTPs?
The reported intensity of 0.71 kg CO2-eq/t is within the typical range for Chinese municipal WWTPs (0.5–1.0 kg CO2-eq/t). The relatively low value may be attributed to the plant's large capacity and efficient processes, but direct comparison requires consistent accounting boundaries.
What are the most effective reduction strategies based on the findings?
The study recommends three synergistic strategies: (1) source reduction by controlling influent BOD5 and TN concentrations to minimize direct emissions; (2) energy efficiency improvements, focusing on aeration and pumping systems; and (3) carbon compensation through on-site renewable energy generation (e.g., solar) and resource recovery from sludge.
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