Combined Sewer Overflow (CSO)
A combined sewer system collects rainwater runoff (stormwater) and sanitary sewage (wastewater) into one pipe. Normally, it can transport all of the wastewater to a treatment plant. Sometimes, in heavy rain events, the amount of runoff exceeds the capacity of the system. When that happens, untreated or partially treated stormwater and wastewater flows into nearby waterbodies. These events are called combined sewer overflows (CSOs).
How do we deal with CSO in the Blackstone?
Worcester, MA
Worcester constructed a unique CSO Treatment Facility in 1984. This facility usually acts as a regular pump station to Upper Blackstone Clean Water (UBCW), the wastewater treatment facility in Worcester. The water that UBCW discharges to the Blackstone River is cleaner than the river it is entering.
However, during extreme rain events when the infrastructure cannot handle the water volume coming in, the CSO Treatment Facility can hold water until there is more capacity OR partially treat the water and release it into the Blackstone River. In very extreme rain events, the CSO treatment facility may have to also bypass and discharge untreated CSO water directly to the Blackstone, but this is very rare.
The total length of CSO pipe is only about 7% of the total length of sewer pipe in the city. There is only one discharge location in Worcester at 25 Tobias Boland Way (behind Walmart).
Pawtucket, RI
In 2021, the Narragansett Bay Commission began creating a large tunnel that will store Pawtucket’s CSO. The Pawtucket CSO Tunnel Project is designed to have the capacity to store all contributing overflows during a storm event (up to the three-month storm) until it can be treated by the Narragansett Bay Commission. This project requires drilling 30-foot diameter tunnel through 11,600 feet of solid bedrock. The tunnel is just one component of the $1.3 billion 20-year Pahse III of the Narragansett Bay Commission’s CSO Abatement Program.
The climate is changing, and we can expect more frequent and intense rainstorms in the Blackstone.
Typically, the Worcester CSO Facility reports 12-15 CSO events per year.
There were 30 in 2023.
Who is affected by CSO from Worcester?
Communities directly downstream of the CSO outfall in Worcester
Residents Living in CSO Areas in Worcester
Sometimes we have “catastrophic flooding” when the CSO Treatment Facility is essentially underwater. Water comes up from pipes into the street. This water has diluted sewage in it. At this time we do not know if there are health impacts associated with interacting with the floodwater, but we recommend avoiding it if possible!
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Watch New England's Unseen Environmental Threat: Combined Sewer Overflows - A Documentary
Created by Alisha Taha, UMass Amherst student with support from Professor Regine Spector, Professor of Political Science at UMass Amherst
