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Boston Harbor and Massachusetts Bay
Massachusetts Water Resources Authority
       

MWRA Blending Notifications

Every gallon of wastewater discharged from the Deer Island Treatment Plant into Massachusetts Bay receives treatment. In fact, MWRA has consistently met strict water quality standards for its effluent, and the quality of the discharge has been even better than predicted when planning the bay outfall. The three main phases of sewage treatment at Deer Island are:

1. Primary Treatment: For all flow, removes floating litter and grit, and solids settle out in a tank.
2. Secondary Treatment: Microbes break down more solids and contaminants that are removed.
3. Disinfection: For all flow, chlorine is added to kill bacteria and viruses before discharge.

What is blending? MWRA's goal is to apply three phases of treatment to all wastewater. However, in very wet weather, flow can exceed the amount that secondary treatment is designed to handle; at these times, excess primary-treated flow is diverted around the secondary process and then blended with the secondary effluent before being disinfected and discharged miles out to sea.
Subscribe to blending notifications.

Reasons for blending: Blending allows more water to be processed by the treatment plant during heavy rain, while relieving the pressure of high flow on the secondary (biological) treatment process. This prevents street flooding and sewage backups into homes and businesses, and reduces combined sewer overflows which pollute local waterways. Blending also prevents the washout of beneficial bacteria that are cultivated to treat flows to secondary levels, which could reduce the treatment plant's ability to provide secondary treatment for weeks. A blending event lasts only a few hours, and blended flow still meets the strict water quality limits of MWRA's NPDES discharge permit, as 99% of this flow has received full secondary treatment. Nevertheless, the Massachusetts Department of Environmental Protection requires MWRA to notify regulators and the public when blending occurs. Jump to blending notification table.

In 2023 blended flows were only 1.1% of total discharges, on average



Monthly discharges: full-secondary-treated (blue)
and primary-treated/blended (orange) flows.

In communities with older, combined sewer systems that mix stormwater and sewage, blending can prevent combined sewer overflows (CSOs) polluting Boston Harbor, and the Mystic & Charles rivers. The U.S. Environmental Protection Agency confirmed this in EPA’s 1994 CSO Control Policy and the Nine Minimum Controls. All treated flow is discharged through a long outfall into Massachusetts Bay.

MWRA's outfall T01 in Massachusetts Bay

The Mass. Bay outfall is 9.5 miles long and discharges over 5 miles from any shoreline.
MWRA monitors a wide area around the discharge to protect ocean life.

Public health officials recommend avoiding contact with water bodies during rainstorms and for 48 hours afterwards, as there may be increased health risks due to bacteria or other pollutants associated with urban stormwater runoff and CSO discharges. The discharge area is several miles offshore in a shipping lane, where there are no recreational swimmers or boaters. Blended discharge consists of partially treated sewage and waste. The Massachusetts Department of Environmental Protection considers primary treated wastewater to be partially treated and requires MWRA to notify regulators and the public when blending occurs.

Subscribe to MWRA discharge notifications

Previous MWRA blending events (Excel)

For more information about the 1994 CSO Policy and the Nine Minimum Controls visit https://www.epa.gov/npdes/combined-sewer-overflows-csos

 

The blended discharge notification table below provides additional details. Discharge duration and volume are estimates. Data is preliminary and subject to change.

 

 

 

 

 

 

 

 

 

 

 

Event rainfall is from the National Weather Service gauge at Logan Airport, which may vary substantially from local rainfall conditions.