MWRA online - home
Water System
Sewer System
Harbor and Bay
School Program
About MWRA
Doing Business with MWRA
Contact MWRA

Boston Harbor and Massachusetts Bay
MWRA Environmental Quality Department

The Massachusetts Bay Outfall

Tunnel Schematic
Outfall Tunnel Schematic
(View Larger Image)

Diffuser Schematic
Diffuser Cap: Initial dilution of the effluent from the new diffusers is about 1 part treated effluent to 100 parts seawater.



The outfall for the Deer Island Treatment Plant is an important part of greater Boston's wastewater management program. This outfall, which started up in September of 2000, discharges treated sewage, called effluent, into Massachusetts Bay instead of into the shallower waters of the Harbor, which has helped the Harbor's recovery.

The outfall begins with a deep rock tunnel extending under Massachusetts Bay to a point about 9.5 miles east of Deer Island. Although there was much consensus on the need for a deep-water discharge of Deer Island effluent, questions remained about whether Massachusetts and Cape Cod Bays could, like Boston Harbor, become degraded by sewage effluent. The outfall site was chosen only after considerable scientific and technical study and extensive public participation.


Deer Island effluent quality has been vastly improved, as a result of source reduction and secondary treatment. With secondary treatment, 85% of suspended solids, 85% of oxygen consuming material (BOD), and up to 90% of toxic contaminants are removed from the treated wastewater, or effluent, before discharge. Disease-causing microorganisms are also better reduced by secondary treatment because disinfection is more effective with less solids in the wastewater.


The initial dilution of the effluent in the bay is about 100 to 1, far greater than the 14-to-1 dilution achieved at the old 30-foot-deep outfall in Boston Harbor. The impacts of nutrients, which are not targeted for removal by secondary treatment, should be minimized by effective dilution in the Bay. The bay discharge is through a diffuser, which consists of more than fifty pipes that rise to the seafloor over the last 6,600 feet of the tunnel's length. Each pipe connects to a diffuser cap which splits the flow into several streams, each issuing from a small port. More than 400 diffuser ports disperse the effluent into the 100-foot-deep waters in the Bay, where it is diluted in large volumes of seawater.

To ensure that MWRA's effluent discharge does not have major effects on Massachusetts Bay, MWRA's discharge permit requires detailed reporting to both regulators and the public. In addition, the National Marine Fisheries Service issued a biological opinion that the outfall, more than 16 miles away from identified North Atlantic right whale habitat, was not likely to jeopardize the right whale or other endangered species.


The links below from an older report on the outfall are still helpful in understanding the background of this important project. (These PDF files require free PDF reader available from Adobe Systems.)

1. How the Massachusetts Bay outfall site was chosen


Using a water quality model, scientists chose three possible sites for the new outfall. After regulatory review and public comment, the current site was chosen because it is an area where circulation is greater and more variable than in the Harbor, the water is sufficiently deep, and construction was feasible.

2. How the outfall benefits water quality
3. The quality of the discharge into Mass. Bay
4. The health of the marine ecosystem of the Bays
5. How the discharge was expected to affect the Bays
6. What about the right whale?
7. How we will know if the outfall is causing a problem
8. What will be done if a problem occurs