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Combined Sewer Overflow (CSO)

Impact on Receiving Water and Recreational Activities

What are Combined Sewers?

Combined sewer systems (CSS) are sewers that are designed to collect storm water runoff, domestic sewage, and industrial wastewater in the same pipe. During rain events, when storm water enters the sewers, the capacity of the sewer system may be exceeded and the excess effluent will be discharged directly to the receiving water. A combined sewer overflow (CSO) is the discharge from a combined sewer system that is caused by snow melt or storm water runoff.

Combined sewers are found across New York State (NYS), except on Long Island. However, most CSOs are found in large cities. Most large municipal sewer systems in NYS consist of combined sewers in older downtown urban areas with separate sanitary and storm sewers serving outlying tributary suburban areas.

Inventory of CSO Outfalls

About ten percent of the CSOs in the United States are found in NYS. Each combined sewer system in NYS is required to have a State Pollutant Discharge Elimination System (SPDES) permit, which is issued by the NYS Department of Environmental Conservation (DEC). As early as 1993, DEC had an inventory of approximately 1300 CSO outfalls listed in the permits of Publicly Owned Treatment Works (POTW) with CSOs. As of 2009, this CSO outfall inventory is less than 1000 due to various CSO abatements by the permittees.

Impacts of CSOs

Since CSOs contain mixtures of domestic sewage, storm water runoff, and sometimes, industrial wastewater:

  • They may contain high levels of suspended solids, toxic chemicals, floatable material and other pollutants that can cause the exceedance of NYS water quality standards and pose risks to human and aquatic health.
  • They may preclude, impair, stress or threaten (cause) bathing beach closures, due to floating debris or slicks, shellfish bed closures, algae blooms, and depressed levels of oxygen in the water.

Wet Weather Water Quality Act

In 1994, the United States Environmental Protection Agency (EPA) issued a National CSO Control Policy. The Wet Weather Water Quality Act of 2000 requires combined sewer systems to conform to the requirements in the National CSO Control Policy. The requirements include implementing Nine Minimum Controls (NMC) and a Long-Term Control Plan (LTCP). The NMCs are technology-based controls that can be used to abate CSOs. The LTCP consists of more extensive characterization and monitoring of the combined sewer system and the receiving water, as well as selection and implementation of CSO control alternatives, with the intent of minimizing the impacts of CSOs on water quality.

Best Management Practices

In accordance with EPA's National Control Policy, DEC has the authority to enforce CSO requirements under the Wet Weather WQ Act As a result, DEC developed 15 Best Management Practices (BMPs), which embody the NMCs and require the control of CSO discharges from combined sewer systems. The BMPs are to be included in all SPDES permits for combined sewer systems as applicable.

CSO Long Term Control Plan

Where implementation of the BMPs alone will not result in compliance with the CWA, DEC will also include the long-term control plan requirements in an enforceable mechanism for combined sewer systems. Phase I of the long-term control plan is a comprehensive study of the combined sewer system and the effect of CSOs on the receiving water quality. Once it is determined whether water quality standards in the receiving water are impacted by CSO discharges, if necessary, the permittee will:

  1. Research alternatives for mitigation of the impact of CSO discharges and choose one or more of those alternatives for implementation.
  2. Suggest a schedule of construction or implementation of those alternatives, i.e., Phase II of the long-term control plan, to DEC.
  3. Perform post-construction monitoring, once the control measures are in place, to ensure that the goals of the long-term control plan and the appropriate water quality standards are met.


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