A tributary is a small stream that flows into a larger stream.
|Egg Digesters Deer Island|
All wastewater collected by the Boston Water and Sewer Commission's (BWSC) facilities is conveyed to the Massachusetts Water Resources Authority's (MWRA) Deer Island Treatment Plant in Boston Harbor. The MWRA sewer system maintains and operates a 240 mile network of large interceptor sewers for wastewater collection, eleven pumping stations serving low-lying areas, five headworks where wastewater is pre-treated, five combined sewer overflow facilities and two wastewater treatment plants serving 43 communities, including the City of Boston.
The wastewater influent arrives at Deer Island through four underground tunnels. Here, the wastewater is treated in two stages: primary and secondary. The primary stage includes removing grit for disposal in an off-island landfill and 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 percent. The biological process is a pure oxygen-activated sludge system, using microorganisms to consume organic matter that remains in the wastewater. Deer Island Treatment Plant generates 130-220 tons of pure oxygen per day to support the secondary treatment process.
Finally, after passing through the primary and secondary treatment processes, wastewater is disinfected with sodium hypochlorite to kill bacteria. After disinfection and dechlorination, the effluent is ready to be discharged. These processes meet all discharge permits issued by the Environmental Protection Agency (EPA) and the Massachusetts Department of Environmental Protection (DEP) and allow the treated wastewater to be released into the marine environment, which is constantly monitored. The effluent wastewater is then discharged into the 100 foot deep Massachusetts Bay through a series of diffusers located at the end of a 9.5 mile, 24 foot diameter outfall tunnel, which was completed in 2000.
Sludge resulting from the treatment processes at Deer Island is anaerobically digested in 12 distinctive egg-shaped anaerobic digesters, each 90 feet in diameter and approximately 130 feel tall. Digested sludge is then transported by barge to be processed at the dewatering and pelletization facility located at the Fore River Shipyard in Quincy where it is further processed into a fertilizer product.
Air scrubbers and carbon absorbers remove odors and volatile organic compounds from treatment process "off-gases". Odor control is used for primary and secondary treatment process facilities, as well as for the sludge processing.