Deer Island Treatment Plant

All wastewater collected by the Commission’s facilities is conveyed to the MWRA's Deer Island Treatment Plant. The wastewater influent arrives through four underground tunnels. Here, the influent is treated in several stages. The primary stage includes routing the wastewater to primary treatment clarifiers where about half of the pollutants in the wastewater are removed. 

Secondary treatment includes using biological and gravity treatments to bring the level of pollutants removed to over 85%. This meets all discharge permits issued by the EPA and DEP, and allows the treated wastewater to be released to the marine environment. This occurs through a 9.5-mile 24-foot-diameter outfall tunnel that transports effluent into the 100-foot deep waters of Massachusetts Bay, which is constantly monitored for the protection of the environment.