Optimizing combined sewer overflow storage volume and location using a SWMM environmental effectiveness framework in Jiujiang

Urban water systems are increasingly upgraded to withstand extreme weather events; however, existing studies often insufficiently address the necessity of localized pre-screening and the marginal utility of storage sizing. This study proposed an optimization framework to determine the optimal location and volume ( V ) of storage facilities based on long-term environmental benefits ( E ). Specifically, E was quantified as the reduction in downstream Non-Compliance Duration (NCD)—defined as the cumulative time during which Chemical Oxygen Demand (COD) concentrations exceed the regulatory limit of 30 mg/L. Using a coupled drainage–river model for a case study in Jiujiang, China, initial scenarios demonstrated that situating storage near outlets with the highest pollutant loads yielded the greatest E , validating the pre-screening step. Furthermore, marginal utility analysis revealed a non-linear E - V relationship: the incremental volume effectiveness ratio (IVER, intuitively reflecting the days of compliance gained per cubic meter of added storage) peaked at a site-specific critical volume threshold before diminishing returns set in. Rather than relying on arbitrary sizing, the study demonstrated that by intersecting a targeted minimum effectiveness constraint with a marginal efficiency threshold (e.g., 80% of maximum IVER), an optimal facility volume can be systematically derived. Applying this framework to the Jiujiang case analytically identified an optimal storage volume of 1800 m 3 near outlet 228WS52, yielding a substantial 5-day reduction in NCD. This practical volume-effectiveness approach shifts the design focus from conventional hydraulic interception to direct water quality improvement, providing a globally transferable methodology for optimizing investments in diverse urban drainage systems.