More about:

Guide Sanitary Sewer Systems & Combined Sewer Overflows

How MassDEP regulates combined sewer overflows and sewer systems in Massachusetts.

Sanitary Sewer Systems & Combined Sewer Overflows

Systems of pipes and pumps that transport wastewater to wastewater treatment plants are called sanitary sewers. Water used in homes or industry is flushed through their pipes until it reaches local sewer mains owned and operated by municipal or regional sewer departments.

Sewer mains flow into progressively larger pipes until they reach the wastewater treatment plant. In the ideal case, a sewer system is completely gravity-powered. In situations where gravity cannot do all the work, the sewer system includes grinder-pumps or lift stations to move the wastewater to the treatment plant. Manholes allow access to the sewers by means of vertical openings and covers.

MassDEP has responsibility for ensuring that sanitary sewer systems are in compliance with the requirements of the Massachusetts Clean Waters Act and the regulations adopted under 314 CMR 1.00 through 9.00.

Proper operation and maintenance of sanitary sewers is critical to public and environmental health. This website has guidance for owners, managers and operators seeking to optimize operation of their systems as well as information about notifying MassDEP about emergency overflows, bypasses, and sewage backups.

Infiltration and inflow (I/I) is groundwater, rainwater and snow melt that enter sewer systems through defects in sewers or illegal connections. I/I reduces the capacity of sewer systems and treatment facilities to transport and treat wastewater. During periods of high groundwater and large or sudden storm events, I/I entering sanitary sewers may cause surcharging, wastewater backups into homes and businesses, and inadequate treatment. The policy available on this website is intended to help municipalities manage infiltration and inflow. In rare instances, property owners may experience flooding and sewage backups. 

Newer sewer systems were built with separate systems for sanitary and stormwater flows. However, older cities across the state may still have combined systems designed to carry both sanitary sewage and stormwater in the same pipes. 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. Combined sewer systems have regulator structures that allow overloaded systems to discharge into rivers, lakes and coastal areas subjecting them to higher pollutant loads. 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. This can compromise a water body's uses and lead to water quality violations in the receiving waters. Without CSOs, this mix would back up into homes, businesses, and public streets.  CSO discharges are regulated by MassDEP and US EPA in accordance with state and federal CSO policies and the State Water Quality Standards. 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.

Combined Sewer Overflows: Frequently Asked Questions

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

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.

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.

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 EPA's guidance 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 Water Quality Standards. More information is available in EPA's guidance document, "Combined Sewer Overflows Guidance for Long-Term Control Plan." 

Additional Resources for Combined Sewer Overflows: Frequently Asked Questions

Massachusetts Water Resources Authority Combined Sewer Overflow Variances

Listed below are final determinations for CSO Variances for the Lower Charles River/Charles Basin and the Alewife Brook/Upper Mystic River, and MWRA's Combined Sewer Overflow post-construction compliance monitoring program scope of work.

These variances authorize limited combined-sewer-overflow discharges into the respective waterbodies, as permitted under 314 CMR 4.00, to provide for the completion of long-term CSO controls. Updated May 2017.

Additional Resources for Massachusetts Water Resources Authority Combined Sewer Overflow Variances

Image credits:  Sandra Rabb

Feedback

Did you find the information you were looking for on this page? * required
We use your feedback to help us improve this site but we are not able to respond directly. Please do not include personal or contact information. If you need a response, please locate contact information elsewhere on this page or in the footer.
Tell us what you think