Impact of Extreme Rainfall and Tide on the Combined Sewer Overflows in a Low-Lying Coastal City

A combined sewer system (CSS) simultaneously transports stormwater and sewage with insufficient drainage capacity will induce combined sewer overflows (CSOs) pollution, deteriorating the aquatic ecosystems. However, insufficient studies have investigated the combined impacts of spring tide and heavy rainfall on CSOs. To fill this gap, in this study, the combined impact of extreme rainfall and spring tide on CSOs pollution, non-point source (NPS) pollution, and a wastewater treatment plant (WWTP) is quantified by simulating hydro-hydraulic and water quality processes. A low-lying coastal city, Macao Peninsula, is selected as the study area where the outfalls along the coast are affected by tidal fluctuation. In Macao, the spatial distribution of CSOs volume is influenced by the catchment size and the adjacent CSO structures. CSOs demonstrate a longer duration and lower initial concentration during the high tide due to the tidal blocking effect. The total pollutant load during the high tide is greater than that during the low tide, where fluctuations owing to tidal effect could reach 38%. From a city-wide perspective, the contributions of both NPS and CSO pollutions are nearly 50% in the 2-year rainfall return period. The contribution increment of CSOs between the most optimistic and the worst scenarios is about 16% for TN (total nitrogen) and TP (total phosphorus), which may exacerbate eutrophication. Furthermore, CSOs dramatically affect the quantity and quality of WWTP influents, bringing a more significant pollutant load with an increment of 42% for TSS and 69% for other pollutants in the 2-year rainfall return period. This study proposes a new insight into the synergistic impact of extreme rainfall and spring tide on CSOs, which is constructive to decision-making for controlling CSO pollution in low-lying tidal cities