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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 it does so at considerable cost to local water quality.
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.
WHY CSOs MUST BE CONTROLLED
CSOs occasionally discharge a combination of rainwater and sewage into Boston Harbor, the Charles, the Mystic and the Neponset Rivers.
WHAT IS BEING DONE ABOUT CSOs?
As of December 31, 2007: |
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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.
As of December 31, 2007: |
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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 at the Deer Island treatment plant. Any treated wastewater that exceeds a facility's storage capacity is be 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.
IS MWRA CLEANING UP THE CHARLES RIVER?
The MWRA has already achieved significant reduction of CSO related impacts to the Charles River. Wastewater system improvements already completed have dramatically reduced combined sewer overflows to the Charles River by transporting much more flow to MWRA’s new Deer Island Treatment Plant. Also, the 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.
Evidence of a cleaner Charles River is emerging:
- The river is now generally safe for recreational boating.
- Dry weather bacteria counts have been decreasing. In 1998, 90% of water quality samples taken by MWRA met the boating standard.
- Healthy populations of river herring in the Charles provide the Division of Marine Fisheries with fish to stock other Massachusetts Rivers.
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 is also an active participant in the U.S. Geological Survey’s Lower Charles River Project.
WHAT
WILL THE 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 $650
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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CSO PROJECTS BY COMMUNITY
Boston
- Fox Point CSO Facility Upgrade
- Commercial Point CSO Facility Upgrade
- Neponset Sewer Separation
- Constitution Beach Sewer Separation
- East Boston Branch Relief Sewer
- Union Park Detention/Treatment Facility
- North Dorchester/Reserved Channel Conduits and Treatment
- Fort Point Channel CSO Storage
- South Dorchester Bay Sewer Separation
- Stony Brook Sewer Separation
- Floatables Control
- Localized hydraulics relief project
Cambridge
- Alewife Brook Sewer Separation and Hydraulic Relief
- Floatables Control
- Cottage Farm CSO Facility Upgrades
- Prison Point CSO Facility Upgrade
- Localized hydraulics relief projects
- Go to City of Cambridge Department of Public Works Website
Somerville
- Somerville Marginal CSO Facility Upgrade
- Localized sewer separation projects
- Floatables Control
Chelsea
- City of Chelsea Trunk Sewer Relief
- MWRA Chelsea Branch Sewer Relief
- Chelsea Outfall Rehabilitation
- Floatables Control
Rev. 13-Aug-2008 5:27 PM