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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority


Boston Harbor Project Timeline

Aerial photo of discharge from Deer Island into Boston Harbor, circa 1989
Aerial photo of Deer Island without discharge, June 2002
Deer Island Treatment Plant (DITP) discharges were visible in the President Roads navigation channel of Boston Harbor.
Deer Island and President Roads after DITP discharges were diverted to Massachusetts Bay.

The Massachusetts Water Resources Authority (MWRA) assumed responsibility for Greater Boston's water and sewer systems following the passage of its enabling act in 1984.

Judge A. David Mazzone ordered a 13-year schedule to construct the new Deer Island Treatment Plant (DITP) and related facilities. Until 2004, Judge Mazzone oversaw the design and construction of MWRA's Deer Island Treatment Plant and other major sewer facilities on behalf of the federal court.

MWRA started disposing of sewage scum in a landfill instead of discharging it into the harbor. Scum removal left less oil, grease, and floating matter in effluent. MWRA made interim repairs and upgrades to the old DITP including a more reliable disinfection system, thus discharging fewer bacteria in the effluent. Read more about the history of the Deer Island Treatment Plant (pdf).

Pumping capacity was about 700 million gallons per day (mgd) in 1989. This eventually increased to 900 mgd in 1998. More wastewater was treated and combined sewer overflow (CSO) discharges were reduced.

MWRA stopped discharging sludge into Boston Harbor from the old Deer Island and Nut Island treatment plants, and began recycling sludge into fertilizer at the Fore River pelletizing plant. Solids discharged to the harbor decreased by 40 dry tons per day, and there were other corresponding improvements in water quality.

The new DITP put into service the first stages of improved primary treatment.There was a drop in total solids discharged, as well as further decreases in bacteria, Biochemical Oxygen Demand (BOD), Nitrogen and Phosphorous. MWRA releases reports on Boston Harbor watersheds (pdf) and the new Deer Island Treatment Plant (pdf) for the public.

The first of three batteries of secondary treatment started up at DITP. The beginning of secondary treatment marked a dramatic decrease in BOD and a continued improvement in water quality.

The second battery of secondary treatment started up at DITP. MWRA completed construction of the Inter-island tunnel, which transferred South System flows to DITP from the Nut Island Treatment Plant (NITP) allowing most flow to receive secondary treatment. This ended effluent discharges from NITP to the south Harbor. Today, the Nut Island Headworks facility and a park have replaced the old treatment plant. MWRA released a public report on the Boston Harbor Project.

DITP discharges were diverted to the new Bay outfall, ending discharges into Boston Harbor. Better-treated effluent is now dispersed through an undersea tunnel and diffusers 9.5-miles out into the deeper waters of Massachusetts Bay, where it is well diluted. MWRA released a public report on the new Massachusetts Bay outfall. (pdf)

All batteries of secondary treatment are operating at DITP, optimizing treatment. Secondary facilities are improved. Biosolids are sent to the pelletizer plant to convert to fertilizer. The effluent discharged consistently meets expected standards. Bacteria levels and other pollution indicators have lowered at the previous outfall sites in Boston Harbor.
As part of the Boston Harbor Project, MWRA created a long-term plan to reduce combined sewer overflows (CSOs), which discharge a mixture of stormwater runoff and sewage directly into the harbor during heavy rainstorms. In the 1980s, CSOs annually discharged about 3.3 billion gallons of partially treated or raw combined sewage into the harbor and its tributary rivers. At the completion of the CSO Plan,
annual CSO volumes were reduced by over 2.8 billion gallons a year.
Combined sewer overflow
Combined Sewer Overflow (CSO)


More about Boston Harbor