Combined sewers cause serious water pollution problems during combined sewer overflow (CSO) events. This occurs when combined sewage and surface runoff flows exceed the capacity of the wastewater treatment plant or the maximum flow rate of the system which transmits the combined sources. The overflow is often discharged to a receiving body of water that combines a variable mixture of untreated human and industrial waste, polluted runoff, debris and scoured materials that build up in the collection system during dry weather periods. Discharge contains a variety of contaminants such as chemicals, pathogenic microorganisms, viruses and cysts that impair water quality and impact human health and wildlife. Combined sewer overflows are the leading source of water pollution in the United States, with approximately 772 communities being susceptible as of 20171.
Combined pump stations
To manage these variable wet-weather flows within sewer networks, combined pump stations are commonly used. Combined pump stations are designed to handle daily wastewater flows as well as stormwater and other surface run-off and are common in many wastewater networks. When designing a combined station, you should consider the normal fluctuations from the sewage network as well as the maximum inflow resulting from storm events. Both these inflow rates can differ widely from each other, and the maximum inflow can be much greater than the normal flow.
Disinfection for combined sewer overflow
In situations where overflow still occurs, additional treatment such as disinfection may be required to reduce environmental impacts. Factors to consider:
- Quality of the wastewater being discharged
- Potential toxic effects
- Ease of operation and maintenance
- Regulations governing residual standards
Combined sewer overflow events occur intermittently and vary in flow rate, this makes it difficult to add and regulate a chemical disinfectant. Additionally, many CSO outlets are located in areas that are not readily accessible, leading to the requirement of an automatic system.
CSO monitoring and reduction
Alongside physical infrastructure and treatment approaches, monitoring and predictive control can further help reduce CSO impacts.
Xylem Vue’s Sewer Management application provides real-time visibility of network flows and predictive flow forecasting, helping utilities maximize the capacity of existing infrastructure while reducing the need for costly capital improvements and new investments.
The solution helps utilities reduce the frequency and magnitude of CSO events by optimizing the use of available network capacity, supporting compliance with regulatory requirements. It also reduces the likelihood and impact of flooding in both combined and separate sewer systems, enhancing service levels and customer satisfaction through greater operational visibility and control.
