Data-driven threshold identification for WWTP carbon emission: a 5-year monitoring from Shanghai’s typical wastewater infrastructure

Identifying the carbon emission characteristics of wastewater treatment plants (WWTPs) is fundamental for taking effective energy-saving and carbon reduction measures. This study conducted a comprehensive carbon emission analysis of a representative sludge-water co-treatment facility in Shanghai through establishing an integrated full-process accounting framework that encompasses both wastewater and sludge treatment lines, utilizing five-year operational data from 2020 to 2024. Carbon emissions were quantified by using the emission factor methodology, and their characteristics including total emissions, intensity, and driving factors were analyzed using statistical methods, notably power-law regression. Results show that while the wastewater treatment section contributed 60.8% of total emissions, the sludge treatment system demonstrated substantial emission potential by exceeding 30% contribution even when processing internally generated sludge, with its proportional impact amplifying progressively as treatment capacity increased, thereby highlighting the necessity of holistic system boundaries. The plant’s overall carbon emission intensity peaked at 0.904 kg CO 2 -eq/m 3 , which exceeded Shanghai’s average level of 0.75 kg CO 2 /m 3 . Carbon emission intensity exhibited significant negative power-law correlations and scale effects with wastewater treatment volume, CODcr removal, and sludge throughput. Crucially, critical operational thresholds emerged in these nonlinear relationships, exemplified by the approximate 70% utilization rate of sludge treatment design capacity, where sub-threshold operations triggered exponential growth in emission intensity whereas supra-threshold expansions yielded progressively diminishing marginal returns. Energy consumption dominated emission sources at 42.3% of the total, with electricity demand constituting 73.3% of energy-related emissions. Notably, comparative analysis suggests that applying default IPCC N 2 O emission factors could overestimate direct emissions by 18-22%, underscoring the imperative for developing region-specific emission parameters. These findings demonstrate that coordinated control of treatment scale around critical thresholds, coupled with process optimization, is crucial for achieving low-carbon operations in sludge-water co-treatment systems and provide scientific support for targeted carbon reduction strategies.