Synergistic strategies for pollution and carbon emission reduction in China's wastewater treatment: A comprehensive tiered assessment and benchmarking framework.

China's dual-carbon goals challenges wastewater treatment plants (WWTPs), requiring integrated pollution control and carbon emission reduction. Critical gaps hinder China's strategies for WWTPs, particularly in identifying upgrade needs, optimizing performance, and assessing the local benchmark plants as references. This study addresses these gaps through greenhouse gas (GHG) emission accounting and impact factor analysis, using monthly data from 2232 WWTPs across China. A tiered assessment methodology was developed to evaluate the synergy between pollution reduction and carbon mitigation, including indicators, methodologies, and assessment criteria. Results indicate that indirect emissions from electricity and chemical consumption accounted for 59.9% of total GHG emissions. Key factors influencing these indirect emissions included plant scale, treatment processes, geographic area, operational load, electricity consumption, and influent quality. Through the tiered assessment, WWTPs were classified into three categories: priority control (861 plants), general control (730 plants), and maintenance (641 plants). Furthermore, 222 benchmark plants were identified as exhibiting optimal synergy between pollution control and carbon reduction. For 80% of the benchmark plants, the ranges for carbon emission intensity, influent COD, influent C/N ratio, electricity consumption intensity, and operating load were 0.258-0.482 ​kg CO