Combined Sewage Overflows
Frequently Asked Questions | Beach Closings | Michigan Department of Environmental Quality
Combined Sewage Overflows
Frequently Asked Questions
What are Combined Sewer Overflows (CSOs) and What Causes Them?
Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems were designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, or other water bodies. Historically, CSOs are among the major sources for beach closings and other water quality impairments.
Many combined sewer systems have recently installed treatment facilities (called retention/treatment basins), which are designed to capture the combined sewage and rain water long enough to provide initial treatment and disinfection. This initial treatment often involves allowing solids to settle, the skimming of floatable materials such as oils; and disinfection of disease causing organisms, often accomplished through the addition of chlorine. This combined rainwater and sewage wastewater, with chlorine disinfection is the typical treated CSO discharge in the state of Michigan; therefore, many CSO releases are considered partially treated sewage. The treatment provided significantly reduces the amount of pollutants discharged and the associated public health risk.
Who Will Let Me Know Whether the Water Is Safe for Swimming, Fishing or Canoeing?
When raw or partially treated sewage is released into a river, lake or stream the responsible city is required to notify the health department. The OCHD may sample, or require the responsible municipality to sample, the water-body that received the sewage discharge. If the discharge poses a public health threat, then the health department will issue a public health advisory to notify people of the dangers associated with river or lake water contact.
How are CSOs Addressed?
CSOs are a problem nationwide. Michigan initiated a CSO control program in 1988, and in 1994 the federal government developed a nationwide CSO Control Policy. This policy suggested that states use an enforceable mechanism, preferably the permit program that was initiated by the federal Clean Water Act (called the National Pollutant Discharge Elimination System) to require CSO communities to implement interim measures referred to as nine minimum controls by January 1, 1997, and to develop CSO Long- Term Control Plans (LTCPs). The nine minimum controls basically included interim measures that could be undertaken to improve the quality of the combined wastewater before major sewer system construction activities would be undertaken as part of the LTCP. Once the state and the community reached agreement on the LTCP, the community would then implement the CSO controls as soon as practicable. In Michigan, these LTCPs are contained in various legal documents including state issued NPDES permits, Administrative Consent Orders, Abatement Orders, and other types of court orders. In Michigan, all municipalities with CSOs have completed the necessary interim control measures and have developed LTCPs.
What is the Main Challenge for Communities to Address in Controlling CSOs?
Several challenges exist in controlling CSOs, but the main challenges are the costs associated with mounting water and wastewater infrastructure improvements and the financial resource-intensive nature of CSO controls. CSO LTCPs typically involve major infrastructure investments that must compete with other community needs.
