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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority


The Massachusetts Bay Outfall

Schematic of outfall tunnel and risers
(Click on image to enlarge)

Outfall diffuser: Initial dilution of the effluent from the diffusers is about 1 part 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.


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.

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. An independent panel of scientists, The Outfall Monitoring Science Advisory Panel (OMSAP), reviews monitoring data and provides advice on scientific issues related to the discharge permit and related monitoring. 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.

Outfall Monitoring Overview reports
Questions and Answers before building the Mass. Bay outfall
Ambient water quality monitoring near the bay outfall
Water quality changes after the bay outfall started up
The State of Boston Harbor: About the Outfall