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1. 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.

The City of Taunton has one EPA Region 1 and MassDEP permitted CSO located on West Water Street that has an outfall to the Taunton River.

2. What happens in to the combined sewer during wet weather?

The City of Taunton has an EPA Region 1 and MassDEP permitted CSO located on West Water Street that outfalls into the Taunton River.  The City of Taunton’s Wastewater Treatment Facility (WWTF) has a SCADA system that monitors the City’s one permitted, active CSO.  When the SCADA system registers a CSO event an alarm is triggered. The flow meter at the CSO outfall records the flow rate and totals and trending on SCADA reports the start and stop time of the overflow event.

When the SCADA system registers an alarm, WWTF staff are required to go out on site to the CSO outfall to further verify that the CSO is active. From there, the final Combined Sewer Overflow Notification Plan will take effect.  Please see the City's Final Combined Sewer Overflow Notification Plan.  WWTF are required to file a report with MassDEP, https://www.mass.gov/how-to/sewage-notification-system

3. 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 previously had 24 CSO permittees, but there has been a reduction to 19 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:

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

4. 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.