Marginal abatement costs of direct carbon emissions from municipal wastewater treatment plants: Insights for prioritizing pollution removal or carbon mitigation in collaborative control strategies

Municipal wastewater treatment plants (MWWTPs) possess potential on collaborative carbon-pollutant control, as they directly emit methane and nitrous oxide during biochemical treatment apart from indirect emissions. This study quantified marginal abatement costs (MACs) of direct emissions to characterize MWWTPs’ carbon-pollutant synergy degree and calculated carbon quota involving compliance state with China’s national discharge standard (GB 18918-2002) to identify priority on pollutant removal or carbon mitigation. Quantified from direct emissions measured in carbon dioxide equivalents (CO₂-eq) and total oxygen demand (TOD) removal amount, 4594 MWWTPs’ MACs showed variation (1.292∼2.612 kg TOD/kg CO₂-eq) with scale as the dominant factor. In quota, carbon reduction potential (5.89 million kg CO₂-eq) prominently outweighed pollutant removal demand (0.10 million kg CO₂-eq). Policy implications were proposed based on effects of scale economies and regional socioeconomic conditions. The evaluation framework of MACs and quota might also suit other countries’ MWWTPs to manage carbon footprint and guide low-carbon transformations.