Which CSO Metrics Track Overflow Loads? A Benchmark-Based Comparison

CSO control and long-term planning depend on performance metrics that are comparable across systems and meaningful for decision-making. In Europe, a recent policy update introduced a load-based reference for CSO performance assessment. The recast EU Urban Wastewater Treatment Directive (UWWTD, 2024) includes an indicative 2% overflow load objective, defined as overflow load relative to dry-weather load, for combined sewer overflows (CSOs) within Integrated Urban Wastewater Management Plans. This provides a consistent reference point for screening urban service areas where enhanced planning may be warranted and for comparing mitigation strategies. At the same time, CSO assessment practice has relied on a wide range of national indicators, raising a practical question: which established CSO indicators best reflect a load-based reference such as the UWWTD 2% objective?

We performed a systematic screening of European regulatory and guideline documents and compiled 96 CSO indicators and variants, grouped into four classes: load-based, volume-based, event-based, and efficiency-based. Indicator behavior was assessed using a benchmark model suite of 18 real, approved combined sewer systems, simulated within a consistent modeling framework with harmonized assumptions. For each indicator (including normalized forms), Pearson correlation coefficients were computed against the UWWTD load-based reference.

Clear class-level patterns emerged. Load-based indicators show strong, stable positive correlations with the UWWTD load-based reference across systems, consistent with their direct connection to pollutant emissions. Efficiency-based indicators (e.g., forwarding efficiencies) exhibit a strong inverse relationship, providing system-integrative information that complements load metrics. Volume-based indicators correlate moderately to strongly but with pronounced system dependence. In contrast, event-based indicators (e.g., number of CSO events per year) show no meaningful relationship, highlighting their limited ability to represent overflow magnitude and intensity.

This contribution is intended as a toolkit rather than a prescription: it characterizes how commonly used indicator types relate to a load-based reference under controlled benchmarking, supporting informed metric selection within locally defined objectives and regulatory contexts. The indicator-class perspective also helps structure integrated planning portfolios spanning blue-green infrastructure, real-time control (RTC), and climate adaptation, without implying a one-size-fits-all metric.