An integrated modelling framework for predicting wet-weather discharges pollution

Wet-weather discharged from urban areas by a combined wastewater system represent a threat for surface waters: when the system capacity is reached during medium/big rain events, an untreated mixture of stormwater and wastewater is discharged through combined sewer overflows (CSOs) or bypass (BP) of wastewater treatment plants (WWTP) to a receiving water body. There is currently a lack of knowledge on the actual activation of CSOs and the actual volume and pollution discharged by each system, due to lack of proper monitoring and modelling tools. However, the proposal for the revision of the Urban Wastewater Treatment Directive (UWWTD) proposes to assess the pollution and volumes discharged and limit the laods discharged to a small percentage of the collected wastewater during dry-weather (1% to 3%). It is clear that to be able to perfrom this assessment, proper modelling tools are needed. An integrated modelling framework was applied to a specific case study. The system considers not only the sewer sytem with all CSOs, but also the WWTP with the bypass. Data available on hydraulics of the sewer system were limited to few flowmeters and info on activation and deactivation of CSOs in time. Many information were present on the WWTP and on the bypass, including flowrate. Monitoring campaigns were performed at the WWTP, both in wet-weather and in dry-weather. Moreover, samples were collected of the WWTP bypass at different timeframes for 7 events in 2023. A conceptual model of the sysem was developed and implemented in WEST (DHI A/S). Both the hydraulic model and the quality model were calibrated in dry-weather and in wet-weather. It was possible to predict the activations of different CSOs structures, the volumes and the pollution discharged. Moreover, a comparison between the loads contribution of CSOs, WWTP bypass, and effluent was performed, together wth a comprehensive risk assessment. This study will allow for a comprehensive understanding of the loads discharged by an entire catchment, including combined sewer system, CSOs, WWTP bypass, and WWTP effluent in dry- and wet-weather conditions. Based on this findings, proper interventions on the system can be planned and the efficacy of the implementation can be evaluated in advance.