Before September 2000
The treated wastewater (effluent) was discharged near shore within the Boston Harbor. The strong tidal currents would eventually move the effluent into Massachusetts Bay but some effluent would come back to the Harbor with each incoming tide. This caused serious pollution in the Harbor before secondary treatment was implemented at the treatment plant in 1998.

Treated wastewater was discharged near the primary deep-water entrance to Boston Harbor. Image credit: MWRA 1996

Today
A 9.5 mile, 24-foot diameter outfall tunnel releases the treated effluent into the deeper waters of Massachusetts Bay. This tunnel was drilled through the bedrock (the base, foundation) of the bay. The effluent is discharged through a diffuser system, consisting of more than 50 pipes. Each pipe has a diffuser cap with ports that sits on the seafloor (sediment). These diffuser caps disperse the effluent into the Massachusetts Bay. This new outfall allows for better dilution of nutrients in the effluent. See the section "Why Treat Wastewater" to learn more about nutrients.

Diffuser cap on land.

The effluent mixes with sea water which dilutes the effluent, lowering the concentration of
contaminants that may have been left over from secondary treatment.

To learn more about the outfall, visit http://www.mwra.state.ma.us/harbor/html/outfall_update.htm