What are Combined Sewer Overflows (CSOs)?

Combined sewer overflows, or CSOs, were built as part of sewer collection systems that were designed to carry both sewage and stormwater in the same pipe. When there is not a lot of stormwater, this mix is transported to a wastewater treatment plant where it is processed. However, after heavy rainfall or snowmelt, stormwater and sewage overload the system. Without CSOs, this mix would back up into homes, businesses, and public streets.

Combined sewer systems have regulator structures that allow overloaded systems to discharge into rivers, lakes and coastal areas subjecting them to higher pollutant loads. This can compromise a water body's uses and lead to water quality violations in the receiving waters.

What communities have CSOs, and to what bodies of water do they discharge?

Massachusetts has 24 CSO permittees. They include most of the older urbanized communities across the state, such as Boston, New Bedford, Worcester, and Springfield. The following is a table of permittees and the bodies of water CSOs discharge to.

Combined Sewer Overflow Permittees in Massachusetts pdf format of Combined Sewer Overflow Permittees in Massachusetts   

What happens in a combined sewer during dry weather?

The combined sewer carries all domestic sewage to the wastewater treatment plant, and sewage is not discharged by CSO outfalls.

diagram of combined sewer in dry weather

What happens in a combined sewer during wet weather without CSO control?

Stormwater flows into roof and storm drains and into combined sewers, mixing with domestic sewage. The added volume of the precipitation overloads sewer capacity triggering a combined sewer overflow.

diagram of combined sewer in wet weather without Combined Sewer Overflow control

What is being done to control or eliminate CSOs in Massachusetts?

CSO discharges are regulated by MassDEP and EPA in accordance with state and federal CSO policies and the State Water Quality Standards (WQS). Massachusetts has 24 CSO permittees that have National Pollutant Discharge Elimination System (NPDES) permits issued by EPA Region 1 and MassDEP's Surface Water Discharge Permitting Program. Communities with CSOs include most of the older urbanized communities across the state, such as Boston, New Bedford, Worcester, and Springfield.

Each CSO permittee must implement system controls known as the Nine Minimum Controls. The purpose of these controls is to maximize the efficiency of existing facilities in order to limit the duration and impact of CSO discharges.

The Nine Minimum Controls are:

  1. Proper operation and regular maintenance programs for the sewer system and CSO outfalls.
  2. Maximum use of the collection system for storage.
  3. Review and modification of pretreatment requirements to ensure that CSO impacts are minimized.
  4. Maximization of flow to the POTW for treatment.
  5. Elimination of CSOs during dry weather.
  6. Control of solid and floatable materials in CSOs.
  7. Pollution prevention programs to reduce containments in CSOs.
  8. Public notification to ensure that the public receives adequate notification of CSO occurrences and CSO impacts.
  9. Monitoring to effectively characterize CSO impacts and the efficacy of CSO controls.

A more comprehensive explanation is provided in the EPA guidance (1995) document, Combined Sewer Overflows, Guidance for Nine Minimum Controls.

Facilities must also develop and implement a Long-Term CSO Control Plan, which must demonstrate compliance with the WQS. More information is available in the EPA Guidance (1995) document, Combined Sewer Overflows, Guidance for Long-Term Control Plan.