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Combined Sewer Overflows (CSOs)
Massachusetts Water Resources Authority

 

Not all sewer systems are created equal. While modern systems transport rainwater and sewage from homes and businesses through separate pipes, some older systems like Boston's have "combined" sewers that carry both flows together. During normal conditions flows are delivered to treatment plants. During very heavy rains, when flows sometimes double and even triple, these systems become overloaded. Built-in overflows (called combined sewer overflows or "CSOs") must then act as relief points by releasing excess flows into the nearest body of water. This prevents sewage backups into homes and onto area streets, but the discharges can impact water quality. Most of the active CSOs in MWRA's service area receive treatment such as screening, disinfection and dechlorination.

WHY CSOs MUST BE CONTROLLED

CSOs occasionally discharge a combination of rainwater and sewage into Boston Harbor, the Charles and Mystic Rivers, and Alewife Brook.

PUBLIC HEALTH AND ENVIRONMENTAL IMPACTS OF CSOs

Public health officials recommend avoiding contact with active CSO receiving waters during rainstorms and for 48 hours afterwards as there may be increased risks due to bacteria and pollutants associated with urban stormwater runoff and CSOs. See map for the locations of active CSOs in MWRA's service area.

WHERE CSOs ARE LOCATED

Boston, Cambridge, Chelsea and Somerville have combined sewer systems that connect to MWRA's system. In 1987, a total of 84 active, uncontrolled CSOs discharged combined flows into Boston Harbor and into the Charles, Mystic Neponset Rivers. As of May 2011, CSO discharges have been eliminated from 32 of the 84 outfalls and virtually eliminated from 5.

Map of Combined Sewer
Overflow (CSO )Outfalls
and CSO Control Projects

Cso Control Map


Larger image

Updated 04/2012

WHAT IS BEING DONE ABOUT CSOs

Since 1987

32 of the 84 original CSOs have been closed and discharges from 5 outfalls have been virtually eliminated with a 25-year level of CSO control.

Annual CSO volumes have been reduced by 84% overall.

82% of CSO flows have been treated.

CSO CONTROL PLAN

Since MWRA was created in 1985, solving the problems related to CSOs has been a top priority. MWRA developed a CSO Control Plan in 1994. The plan has since undergone environmental review and received federal and state approvals, allowing the projects to move forward on a design and construction schedule mandated by the Federal Court as part of the Boston Harbor Case. The CSO Control Plan been revised over time to reduce cost and to make best use of improving technology.

Since 1987

29 of the 35 original CSO Control Plan projects have been completed.

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HOW CSO FACILITIES WORK

MWRA operates facilities that screen and chlorinate combined wastewater (sewage and storm water) prior to discharge or further treatment.

MWRA's CSO facilities at Union Park (Boston), Cottage Farm and Prison Point (Cambridge) store and pump CSO flows. Screened and chlorinated wastewater can be stored and pumped back into the system for processing at the Deer Island treatment plant. Any treated wastewater that exceeds a facility's storage capacity is discharged through CSO outfalls.

At the Somerville Marginal facility, combined wastewater flows enter and leave the facility by gravity, not pumping. The wastewater is screened, chlorinated and dechlorinated. The disinfected wastewater overflows into its receiving water as quickly as it arrives at the facility.

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COMPLETED CSO CONTROL PROJECTS 1996-2011

Click on links for details about each project, including receiving water, cost, water quality benefits and more, or download a printer-friendly PDF.

1996
  Somerville Baffle Manhole Separation
2000
  Cottage Fam CSO Facility Upgrade
  MWRA Floatables Control and Outfall Closing Projects
  Constitution Beach Sewer Separation
  Hydraulic Relief at Outfall BOS017 (Charlestown)
  Hydraulic Relief at Outfall CAM005 (Cambridge)
  Neponset River Sewer Separation
  Chelsea Trunk Sewer Replacement
2001
  Chelsea Branch Sewer Relief
  CHE008 Floatables Control and Outfall Repair
  Prison Point CSO Facility Upgrade
  Somerville Marginal CSO Facility Upgrade
  Fox Point CSO Facility Upgrade
  Commercial Point CSO Facility Upgrade
2006
  Pleasure Bay Storm Drain Improvements
  Stony Brook Sewer Separation
2007
  South Dorchester Bay Sewer Separation
  Fort Point Channel Sewer Separation
  Region-wide Floatables Controls
  Union Park Detention/Treatment Facility
  BOS019 (Little Mystic Channel) CSO Storage Conduit
2008
  Prison Point Facility Optimization
2009
  Cottage Farm Brookline Connection and Inflow Controls
  Morrissey Boulevard Storm Drain
2010
  Bulfinch Triangle Sewer Separation
  East Boston Branch Sewer Relief
  Alewife Interceptor Connection Relief and Floatables Controls
2011
  CAM400 Manhole Separation
  North Dorchester Bay Storage Tunnel and Related Facilities

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CSO CONTROL PROJECTS IN CONSTRUCTION


CAM004 Stormwater Outfall and Basin, 2013
Brookline Sewer Separation, 2013
Reserved Channel Sewer Separation, 2015
CAM004 Sewer Separation, 2015

CSO CONTROL PROJECTS IN DESIGN PLANNING

Control Gate and Floatables Control at Outfall MWR003 and
MWRA Rindge Ave. Siphon Relief (design start 2012)
Interceptor Connection Relief and Floatables Control at
Outfall SOM01A (design start 2012)

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DETAILS - COMPLETED CSO PROJECTS

(Download printer-friendly PDF)

1. SOMERVILLE BAFFLE MANHOLE SEPARATION
Receiving Water(s): Alewife Brook, Upper Mystic River somerville baffle manhole separation
Year Completed: 1996
Cost: $400,000
Description: Separated common manholes connecting local sewer and storm drain systems. City of Somerville performed design and construction with MWRA financial assistance.
Water Quality Benefit(s) Eliminated CSO discharges at three City of Somerville outfalls: SOM001,SOM006, SOM007
Frequency of Discharge (typcial year)
  Before project: 2
  With project: 0
Annual Discharge Volume (typical year):
  Before project: 0.04 million gallons
  With project: 0 gallons (discharges eliminated)
CSO Reduction by volume: 100%
2. CONSTITUTION BEACH SEWER SEPARATION
Receiving Water(s): Boston Harbor/
Constitution Beach
constitution beach cso project image
Year Completed: 2000
Cost: $3,768,888
Description: Installed approximately 14,000 linear feet of storm drain to separate the combined sewer system, removed stormwater flows from area sewers, and eliminated CSO discharges to Constitution Beach, allowing MWRA to decommission the Constitution Beach CSO treatment facility. BWSC performed design and construction with MWRA financial assistance.
Water Quality Benefit(s) Eliminated CSO discharges to Constitution
Beach to comply with Class B water quality standards. Closed outfalls MWR207(BOS002)
Frequency of Discharge (typcial year)
  Before project: 16 treated
  With project: 0
Annual Discharge Volume (typical year):
  Before project: 1.35 million gallons
With project: 0 gallons (discharges eliminated)
CSO Reduction by volume: 100%
3. HYDRAULIC RELIEF AT OUTFALL CAM005,
4. HYDRAULIC RELIEF AT OUTFALL B0S017
Receiving Water(s): CAM005: Upper Charles River Basin.
BOS017: Mystic River/Chelsea Creek Confluence.
hydraulic relief projects
Year Completed: 2000
Cost: $2,294,549
Description: CAM005: In Cambridge, the 40-foot long, 24-inch diameter dry weather connection between the CAM005 regulator and MWRA’s North Charles Metropolitan Sewer was relieved with a 54-inch additional connection.
BOS017: In Charlestown, 190 feet of 36-inch diameter pipe were installed in Sullivan Square to divert two local (BWSC) combined sewers to a direct connection with MWRA’s Cambridge Branch Sewer. In addition, a 10-foot long restriction between the Charlestown and Cambridge Branch Sewers, adjacent to Sullivan Square, was eliminated.
Water Quality Benefit(s) Minimized CSO discharges to meet
B(cso) water quality standards (>95% compliance with Class B) at CSO Outfalls CAM005, BOS017.
CAM005 - Frequency of Discharge (typcial year)
  Before project: 11
  With project: 3
Annual Discharge Volume (typical year
  Before project: 3.8 million gallons
  With project: 0.84 million gallons
CSO Reduction by volume: 78%
BOS017 - Frequency of Discharge (typcial year)
  Before project: 18
  With project: 1
Annual Discharge Volume (typical year):
  Before project: 2.5 million gallons
  With project: 0.02 million gallons
CSO Reduction by volume: 99%
5. NEPONSET RIVER SEWER SEPARATION
Receiving Water(s): Neponset River neponset river
Year Completed: 2000
Cost: $2,444,394
Description: Installed approximately 8,000 linear feet of
storm drain to separate the combined sewer
system, removed stormwater flows from
area sewers, and closed CSO regulators,
eliminating CSO discharges at the two
remaining CSO outfalls to the Neponset
River. BWSC performed design and
construction with MWRA financial
assistance.
Water Quality Benefit(s) Eliminated CSO discharges from outfalls BOS093 and BOS095 to Neponset
River to comply with Class B water quality
standards and protect South Dorchester Bay
beaches (Tenean Beach.
Frequency of Discharge (typcial year)
  Before project: 17
  With project: 0
Annual Discharge Volume (typical year):
  Before project: 5.8 million gallons
With project: 0 gallons (discharges eliminated)
CSO Reduction by volume: 100%
6. CHELSEA TRUNK SEWER REPLACEMENT
7. CHELSEA BRANCH SEWER RELIEF
8. CHE008 OUTFALL REPAIRS
Receiving Water(s): Mystic River/Chelsea Creek Confluence
Chelsea Creek
chelsea cso projects
Year Completed: 2000-2001
Cost: $29,779,319
Description: Replaced 18-inch diameter city-owned trunk sewer with 30-inch pipe, relieved MWRA’s Chelsea Branch and Revere Extension Sewers with 48-inch to 66-inch diameter pipe, rehabilitated Outfall CHE008, and installed underflow baffles for floatables control at all outfalls.
Water Quality Benefit(s) Minimized CSO discharges to meet B(cso)
water quality standards (>95% compliance
with Class B) at outfalls CHE002, CHE003, CHE004, CHE008.
Frequency of Discharge (typcial year)
  Before project: 8
  With project: 4
Annual Discharge Volume (typical year):
Before project: 9.0 million gallons
  With project: 0.58 million gallons
9. UPGRADE COTTAGE FARM CSO FACILITY
10. UPGRADE PRISON POINT CSO FACILITY
11. UPGRADE SOMERVILLE MARGINAL CSO FACILITY
12. UPGRADE FOX POINT CSO FACILITY
13. UPGRADE COMMERCIAL POINT CSO FACILITY
Receiving Water(s): Lower Charles River Basin, Upper Inner Harbor, Upper Mystic River, Mystic River/Chelsea Creek Confluence, South Dorchester Bay upgrades to cso facilties image
Year Completed: 2001
Cost: $22,261,200  
Description: Upgraded chlorine disinfection systems,
added dechlorination systems, process
control and safety improvements.
Water Quality Benefit(s)

Upgrade treatment to meet Class B water
quality criteria, including residual chlorine
limits for outfalls MWR201, MWR203
MWR205, MWR205A (SOM007A), MWR209 (BOS088/BOS089), MWR211(BOS090)

These projects improved treatment performance, but did not affect treated discharges' frequency or volume.

14. PLEASURE BAY STORM DRAIN IMPROVEMENTS
Receiving Water(s): North Dorchester Bay pleasure bay aerial
Year Completed: 2006
Cost: $3,200,000
Description: Stormwater piping and appurtenances to relocate stormwater dischargers from Pleasure Bay to Reserved Channel.
Water Quality Benefit(s) Eliminated storm water discharges to Pleasure
Bay Beach.
15. STONY BROOK SEWER SEPARATION
Receiving Water(s): Lower Charles River Basin stony brook sewer separation
Year Completed: 2006
Cost: $44,198,623
Description: Installed 74,000 linear feet of new storm drain to remove stormwater runoff from local sewers serving a 609-acre area in Jamaica Plain, Mission Hill and Roxbury, and disconnected an already-separated storm drain system serving an adjacent 548-acre area from the sewer system.
Water Quality Benefit(s) Minimized CSO discharges to meet B(cso)
water quality standards (>95% compliance
with Class B) at MWR023 (Stony Brook Conduit).
Frequency of Discharge (typcial year)
  Before project: 22
  With project: 2
Annual Discharge Volume (typical year):
  Before project: 44.5 million gallons
With project: 0.13 million gallons
CSO Reduction By Volume 99.7%
16. SOUTH DORCHESTER BAY SEWER SEPARATION
Receiving Water(s): South Dorchester Bay south dorcester bay sewer separation
Year Completed: 2007
Cost: $118,394,583
Description: Installed 135,700 linear feet (25.7 miles) of
storm drain to remove stormwater runoff
from local sewers serving a 1,750-acre area
in Dorchester. Closed all CSO regulators,
allowing decommissioning of MWRA’s
Fox Point and Commercial Point CSO
facilities.
Water Quality Benefit(s) Eliminated CSO discharges to Savin Hill,
Malibu and Tenean beaches, in compliance
with Class B water quality standards at outfalls MWR209(BOS088/BOS089), MWR211(BOS090)
Frequency of Discharge (typcial year)
  Before project: 20 (treated)
  With project: 0
Annual Discharge Volume (typical year):
  Before project: 30 million gallons
With project: 0 gallons (discharges eliminated)
CSO Reduction By Volume 100%
17. FORT POINT CHANNEL SEWER SEPARATION
Receiving Water(s): Fort Point Channel fort point channel sewer separation
Year Completed: 2007
Cost: $11,866,775
Description: Removed stormwater runoff from local andMWRA combined sewers by installing 4,550 feet of new storm drains serving 55 acres in the Fort Point Channel area. Also overflow weirs were raised, at BOS072 & BOS073. At both locations new tide gates were installed, and underflow baffles were constructed for floatables control.
Water Quality Benefit(s) Minimized CSO discharges to meet B(cso)
water quality standards (>95% compliance
with Class B).
Frequency of Discharge (typcial year)
  Before project: 9
  With project: 0
Annual Discharge Volume (typical year):
  Before project: 3.0 million gallons
With project: 0 gallons (discharges eliminated)
CSO Reduction By Volume 100%
18. REGIONWIDE FLOATABLES CONTROL
19. MWRA FLOATABLES CONTROL AND OUTFALL CLOSING PROJECTS
Receiving Water(s): Regional Application floatables control image
Year Completed: 2007
Cost: $2,469,970
Description:

Projects involved floatables controls and regulator or outfall closings that are independent of the larger projects. The project affected various outfalls system-wide.

In March 2000, MWRA closed Outfalls MWR021 and MWR022 to CSO discharges.

Water Quality Benefit(s)

Complied with EPA Policy Nine Minimum
Controls requirement to control solid and
floatable material. Eliminated CSO discharges at select outfalls. The floatables controls did not affect CSO discharge frequency or volume.

20. UNION PARK DETENTION/TREATMENT FACILITY
Receiving Water(s): Fort Point Channel union park cso facility
Year Completed: 2007
Cost: $49,583,406
Description: Added CSO treatment facility to existing BWSC Union Park Pumping Station with fine screens, chlorine disinfection, dechlorination, and 2 million gallons of detention storage.
Water Quality Benefit(s) Provided treatment of Union Park pumping station discharges to Fort Point Channel at BOS 070 to meet Class B water quality criteria, including residual chlorine limits, and lowered discharge frequency and volume with on-site detention basins.
Frequency of Discharge (typcial year)
  Before project: 25 (untreated)
  With project: 17 (treated)
Annual Discharge Volume (typical year):
  Before project: 132.0 million gallons
With project: 71.4 million gallons/year
CSO Reduction By Volume 46%
21. BOS019 CSO STORAGE CONDUIT
Receiving Water(s): Upper Inner Harbor
(Little Mystic Channel)
BOS019 CSO Storage Conduit
Year Completed: 2007
Cost: $14,287,800
Description: Installed twin-barrel 10’x17’box conduit to provide 670,000 gallons of off-line storage, between Chelsea St. and the Mystic Tobin Bridge, Charlestown. Included above ground dewatering Pump Station.
Water Quality Benefit(s) Minimized CSO discharges at BOS019 to meet B(cso) water quality standards (>95% compliance
with Class B).
Frequency of Discharge (typcial year)
  Before project: 13
  With project: 2
Annual Discharge Volume (typical year):
  Before project: 4.4 million gallons
With project: 0.6 million gallons
CSO Reduction By Volume 86%
22. PRISON POINT CSO FACILITY OPTIMIZATION
Receiving Water(s): Upper Inner Harbor prison point facility
Year Completed: 2008
Cost: $50,000
Description: Minimized treated CSO discharges at MWR203 to the Inner Harbor by optimizing the operation of existing facility gates and pumps to maximize in-system storage and convey more flow to Deer Island
Water Quality Benefit(s) Reduced treated CSO discharges into the Upper Inner Harbor.
Frequency of Discharge (typcial year)
  Before project: 30 (treated)
  With project: 17 (treated)
Annual Discharge Volume (typical year):
  Before project: 335 million gallons
With project: 243 million gallons
CSO Reduction By Volume 27%
23. COTTAGE FARM BROOKLINE CONNECTION AND INFLOW CONTROLS
Receiving Water(s): Lower Charles River Basin cottage farm image
Year Completed: 2009
Cost: $3,326,823
Description: Optimized the combined conveyance capacity of the two MWRA sewers that carry flows across the Charles River by interconnecting overflow chambers outside the Cottage Farm CSO facility; increased this conveyance capacity by bringing into service a parallel, previously unutilized 54-inch diameter sewer (the “Brookline
Connection”)
Water Quality Benefit(s) Minimized treated CSO discharges to the Charles River Basin at MWR201.
Frequency of Discharge (typcial year)
  Before project: 7 (treated)
  With project: 7 (treated)
Annual Discharge Volume (typical year):
  Before project: 44.5 million gallons
With project: 24 million gallons
CSO Reduction By Volume 46%
24. MORRISSEY BOULEVARD STORM DRAIN
Receiving Water(s): North Dorchester Bay morrissey boulevard
Year Completed: 2009
Cost: $36,405,252
Description: Installed 2,800 linear feet of 12-foot by 12-foot box conduit for stormwater conveyance, with gated connection to North Dorchester Bay CSO Storage Tunnel at upstream end, new outfall to Savin Hill Cove, and pollution prevention measures.
Water Quality Benefit(s) Increased level of stormwater control along
the South Boston beaches by redirecting
some stormwater to Savin Hill Cove in large
storms.
25. BULFINCH TRIANGLE SEWER SEPARATION
Receiving Water(s): Charles River bulfinch triangle sewer separation mwra
Year Completed: 2010
Cost: $9.6 million
Description: Installed 4,500 linear feet of of new storm drain and and special structures. Performing TV inspections and sewer cleaning.
Water Quality Benefit(s) Reduces CSO discharges to the Charles River, especially at MWRA’s Prison Point CSO facility, and allows BWSC to eliminate CSO discharges at Outfall BOS049. When the isolation of the outfall is complete, BOS049 will no longer discharge to the Charles River Basin.
26. EAST BOSTON BRANCH SEWER RELIEF
Receiving Water(s): Boston Harbor and Chelsea Creek East Boston Branch Sewer Relief Project map - MWRA
Year Completed: 2010
Cost: $85.2 million
Description: Upgrade of MWRA’s 115-year-old interceptor system serving most of East Boston, using a combination of construction methods: microtunneling, pipebursting, open-cut excavation and pipe relining.
Water Quality Benefit(s) Reduces CSO discharges to Boston Harbor and Chelsea Creek through outfalls BOS003 and BOS014.
27. INTERCEPTOR CONNECTION RELIEF AND FLOATABLES CONTROL AT CAM002 AND CAM401B AND FLOATABLES CONTROL AT CAM001
Receiving Water(s): Boston Harbor and Chelsea Creek
interceptor connection relief
Year Completed: 2010
Cost: $85.2 million
Description: Upgrade of MWRA’s 115-year-old interceptor system serving most of East Boston, using a combination of construction methods: microtunneling, pipebursting, open-cut excavation and pipe relining.
Water Quality Benefit(s) Reduces CSO discharges to Boston Harbor and Chelsea Creek through outfalls BOS003 and BOS014.
28. CAM400 COMMON MANHOLE SEPARATION
Receiving Water(s): Alewife Brook cam400 map
Year Completed: 2011
Cost: $4.7 million
Description: Replaced common storm drain and sewer manholes with separate manholes and associated piping in the local, mostly residential streets bounded by Alewife Brook Parkway, Massachusetts Avenue, Magoun Street and Whittemore Avenue, as well as a portion of the WR Grace property off Whittemore Avenue.
Water Quality Benefit(s)

Eliminated CSO discharges to Alewife Brook at Outfall CAM400.

29. NORTH DORCHESTER BAY STORAGE TUNNEL AND RELATED FACILITIES
Receiving Water(s): North Dorchester Bay north dorchester bay thumbnail
Year Completed: 2011
Cost: $224.7 million
Description: Constructed a 10,832-ft., 17-ft. diameter soft-ground tunnel, drop shafts and CSO and stormwater diversion structures along outfalls BOS081-BOS087; 15-mgd tunnel dewatering pump station at Massport’s Conley Terminal; 24-inch force main; and below-ground tunnel ventilation and odor control facility at the upstream end of the tunnel.
More information
Water Quality Benefit(s)

Eliminated CSO and separate stormwater discharges up to the 25-year storm and 5-year storm, respectively.

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CSOs AND THE CHARLES RIVER

The Massachusetts Water Resources Authority, Boston Water and Sewer Commission and the City of Cambridge have completed a number of projects to reduce CSOs to the Charles River. These projects have already reduced average annual CSO discharge volume to the Charles River basin by 98% since 1988.

Evidence of a cleaner Charles River is emerging:

  • The river is now generally safe for recreational boating.
  • Healthy populations of river herring in the Charles provide the Division of Marine Fisheries with fish to stock other Massachusetts Rivers.

Other wastewater system improvements have also dramatically reduced CSOs to the Charles River by transporting much more flow to MWRA's Deer Island Treatment Plant. Also, MWRA's Cottage Farm CSO facility on the Charles River at the BU Bridge has been upgraded to optimize treatment system performance and minimize potential harm to aquatic life in the river posed by discharges.

The CSO work is in part guided by the Lower Charles River Basin CSO Variance issued by the Massachusetts Department of Environmental Protection. Each year, The US EPA provides a detailed summary of the progress that has been accomplished toward cleaning up the lower Charles River. The "Charles River Report Card" has been released every year since 1995. Since that time, the Charles River's grade has gone from a "D" to a "B+" for 1998.

charles river poster - 2010

CSOs: Controlling Combined Sewer Overflows in Charles River Basin - CSO location map and more information

jpg | pdf

Besides implementing CSO control projects and conducting its own water quality investigations of the Charles River, MWRA has provided financial support to the Charles River Watershed Association’s studies, and in the U.S. Geological Survey’s Lower Charles River Project.

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HOW MUCH WILL THE CSO CONTROL PLAN COST?

In 1990, MWRA and its engineering consultants recommended a $1.3 billion dollar CSO plan that called for an extensive system of pipes that would drop wet-weather flows into deep, bedrock tunnels where they would be stored temporarily until being pumped to Deer Island for treatment. In 1994, a second look at both CSOs and the entire sewer system brought a different solution. The new plan, with a price tag of approximately $885 million, is in keeping with new state and federal CSO policies and takes into account other MWRA facility improvements that also reduce CSO volumes and impacts. This plan recognizes that sewer system improvements have already reduced CSO volumes significantly and provided treatment for the majority of the remaining flows.

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Updated September 8, 2014