What is the main concern associated with combined sewer systems during heavy rainfall?

During heavy rainfall, combined sewer systems—which collect both wastewater and stormwater—can become overwhelmed. This excess water often exceeds the system's capacity, leading to combined sewer overflows (CSOs). When these overflows occur, untreated or partially treated sewage can be discharged directly into nearby waterways, posing significant risks to public health and the environment. This is the primary concern associated with combined sewer systems during periods of heavy rain.

The overflow situation can contaminate natural bodies of water, creating a hazardous environment for aquatic life and people who may come into contact with the water. Additionally, these events can lead to negative economic impacts on communities, due to increased remediation costs and potential fines from regulatory agencies.

While increased treatment costs and filtration complications can arise from the subsequent demand for more treatment and infrastructure improvements, they stem from the more immediate and severe problem of pollution caused by overflow events. Reduced evaporation rates do not directly relate to the concerns of combined sewer systems in the context of heavy rainfall, as the primary issue lies with water pollution from CSOs.