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Managing Worcester's Combined Sewer System

Worcester DPW&P Sewer Operations is responsible for the operation and management of sewer infrastructure that has three components: the sanitary sewer system, the surface sewer or stormwater system and the combined sewer system. The sanitary sewer system conveys wastewater (sewage) from homes and businesses to the Upper Blackstone wastewater treatment plant. The surface sewer or stormwater system collects rainfall from City streets and pipes it to the nearest waterway. The combined sewer system collects both sewage and stormwater and conveys it to the Upper Blackstone plant for treatment before it discharges to the Blackstone River. However, during heavy, prolonged rains the amount of stormwater entering a combined sewer system can be overwhelming for the pipes and the treatment plant. This can lead to combined sewer overflows, or CSOs, which pollute rivers and streams to which they flow. Most large cities with old sewer infrastructure have combined sewers and CSOs. Because of this, regulatory agencies like the USEPA and MassDEP have focused attention on the need to minimize and treat CSOs so they do not impair the rivers and waterways of the nation. Worcester has not escaped this regulatory attention.

Worcester's combined sewer system covers some 4 square miles of the City including downtown, Shrewsbury Street, Green Island and parts of Main South. Some of the oldest pipes in the City are combined sewers with many constructed of brick in the mid to late 1800's. Some of these brick combined sewers are enormous with diameters up to 96 inches (8 feet). Decades ago, when heavy rains caused CSOs, the mix of stormwater and sewage would flow untreated to the Blackstone River. In the 1980s, Worcester became one of the first cities in New England to construct a CSO treatment facility designed to provide some level of wastewater treatment to the combined sewer flow before it entered the River.

The Quinsigamond Ave Combined Sewer Overflow Treatment Facility (QACSOTF), located near Crompton Park, functions as a sewer pumping station during dry periods or small rain storms when only sewage or sewage mixed with moderate amounts of runoff is flowing through the combined sewers. The facility pumps this sewage to the Upper Blackstone plant for treatment. When heavy rains occur the facility switches to treatment mode. Treatment at QACSOTF includes:

  • Bar racks to remove large objects like branches, rags and trash which could damage downstream equipment.
  • Disinfection with sodium hypochlorite (chlorine bleach) to kill bacteria.
  • Storage and settling, where the flow moves slowly through 2.5 million gallon contact tanks allowing solids to drop out of the water, maximizing disinfection effectiveness and retaining this large volume for release to the river or later pumping to the Upper Blackstone.
  • De-chlorination to remove any chlorine remaining from the disinfection process.

After treatment the flow moves into the former Mill Brook and eventually discharges into the Blackstone River.

Since the QACSOTF discharges to the Blackstone River it is regulated by the USEPA and MassDEP under the National Pollution Discharge Elimination System (NPDES) permitting program under the Clean Water Act. The permit issued by these agencies sets limits on the amount of various contaminants that the facility can discharge. The most recent permit also expresses the regulators' desire to have the City take steps to reduce the frequency of discharges from QACSOTF to the Blackstone River. Since discharge from the facility is driven by the weather, and DPW&P has yet to master control of the weather, reducing the number of discharges from the facility appears to be a daunting task. However, there are some measures that have and will be taken by DPW&P in an effort to meet the goals of USEPA/MassDEP.

Options to reduce the amount of stormwater entering the combined sewer system were explored and the most cost-effective were identified in the 2004 CSO Long-term Control Plan and are being implemented. For example, overflow from Green Hill Pond in Green Hill Park previously entered the combined sewer system in Belmont Street. Recent changes to the Pond's overflow structure now takes most of that relatively clean pond water and sends it to Lake Quinsigamond via Coal Mine Brook. DPW&P is also completing modifications to the old Mill Brook conduit upstream of the QACSOTF so that greater volumes of combined sewage can be stored underground for slow, controlled release to the QACSOTF. There is also ongoing evaluation and discussion with regulatory agencies relative to installing larger pumps at QACSOTF so that more combined sewage can be pumped to the Upper Blackstone wastewater treatment plant when current upgrades are completed at that facility in 2009. It has yet to be determined whether this is the best alternative and DPW&P awaits a decision from USEPA and MassDEP before moving forward.

While other ways to separate stormwater from sewage may exist many of these options are too costly. Cost-effective combined sewer separation projects will be considered but it is beyond the financial capabilities of the City and its ratepayers to move forward with a wholesale elimination of the combined sewer system.

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