Operational cost impacts of tertiary treatment in wastewater treatment plants: An inverse optimization approach

Wastewater treatment plants (WWTPs) are critical infrastructure in urban water management. However, their operation entails substantial costs that must be accurately assessed to support informed policy and regulatory decision-making. Directive (EU) 2024/3019 introduces more stringent nitrogen (N) and phosphorus (P) removal requirements for WWTPs, with potentially significant economic implications. This study develops and applies a novel methodological framework to estimate the increase in operational expenditure (OPEX) required for WWTPs to comply with these new standards while preserving their existing techno-economic efficiency. The proposed framework is based on inverse data envelopment analysis and enables the quantification of plant-specific compliance costs. An empirical application to 74 Spanish WWTPs reveals substantial heterogeneity in both current efficiency levels and the additional OPEX required to meet regulatory targets. On average, compliance with Directive (EU) 2024/3019 increases total OPEX by 25.9%, corresponding to an average increase of approximately 0.134 €/m 3 , although cost impacts vary widely across facilities and are not systematically related to plant size or baseline efficiency. While most WWTPs can achieve the required N and P removal levels without efficiency losses, the largest fully efficient plants face structural constraints that limit further performance improvements. Overall, the proposed framework provides a transferable, plant-level tool to support ex-ante regulatory impact assessments and informed investment and tariff decisions under evolving environmental policy frameworks.