Assessing Eco‐Efficiency and the Cost of Carbon Abatement in Wastewater Treatment: Evidence From Spain

ABSTRACT Wastewater treatment plants (WWTPs) play a critical role in protecting public health and the environment but are also contributors to carbon emissions. Improving the eco‐efficiency of WWTPs is essential for advancing sustainability in the sector. This study proposes a parametric approach using stochastic frontier analysis to evaluate the eco‐efficiency of 108 WWTPs in Spain and estimate the marginal cost of reducing GHG emissions. The eco‐efficiency index (EcoEI) integrates operational costs, pollutant removal efficiency, and indirect carbon emissions. Results show an average EcoEI of 0.638, suggesting that WWTPs could improve performance by approximately 36%. The estimated marginal abatement cost of carbon emissions ranges from €0.010 to €1.240 per kg CO 2 eq, with an average of €0.309/kg CO 2 eq. Significant differences in marginal costs were found across secondary treatment technologies, though not in eco‐efficiency scores. These findings have important implications for utilities and regulators aiming to enhance sustainability and meet the targets set by EU Directive 2024/3019.