Understanding the impacts of decentralized circular water strategies for combined sewer overflow and urban flooding

Combined sewer overflows (CSOs) and urban flooding remain critical challenges in many urban areas due to undersized, aging sewer infrastructure and increased runoff, and are projected to intensify with urbanization, population growth, and climate change. This study evaluates how decentralized, household-scale circular-water strategies influence sewer performance and assesses their potential to reduce CSO discharges and flooding under present and future climate conditions. A calibrated 1D Personal Computer Strom Water Management Model (PCSWMM) was used to simulate and evaluate 16 circular-water configurations, along with climate stress tests incorporating precipitation intensification (+10%, +20%, +30%) and Sea Level Rise (SLR) (+0.3 m, +0.9 m, +1.8 m) scenarios. Results show that combined strategies (greywater reuse, rainwater harvesting, and efficient fixtures) achieved the greatest reduction in CSO and flood volumes by 11% relative to the baseline. Under climate stress, CSO discharges increased with precipitation intensification but declined under high sea level rise due to outfall submergence, while flooding increased as elevated tidal levels restricted outfall capacity. When circular-water strategies were applied under the same climate projections, the system exhibited resilience, with combined strategies reducing CSO by 11–12% and flood volumes by 11–13% relative to each climate-stressed baseline. Although decentralized interventions alleviated hydraulic stress, projected climate extremes indicate the need for supplemental centralized infrastructure to fully manage overflow and flooding risks. Overall, findings demonstrate that these strategies can meaningfully reduce sewer pressures, enhance resilience under compound climate stressors, and support integrated adaptation planning for communities. • Decentralized circular water strategies reduce CSO volumes and pluvial flooding. • Rainwater harvesting, greywater reuse and smart fixtures yielded largest reduction. • CSO and flooding worsen with precipitation intensification and sea level rise. • Circular water interventions show resilience under changing climate conditions.