top of page


You may not think about it after you flush, but treating wastewater so it's clean enough to return to an ecosystem takes a lot of work!

Wastewater is used water. It comes from sinks, showers, baths, toilets, washing machines, and dishwashers. Chemicals and debris get into wastewater from everyday household activities, like food scraps, oils, soaps, human waste, and cleaning supplies. In many places, stormwater (rain that falls along paved surfaces) is treated together with sewage. 

What Are Sewers?

In most homes and businesses in urban or suburban areas, wastewater enters an underground sewer. There are three types of sewers: sanitary sewers, combined sewers and storm sewers. A sanitary sewer is designed to carry waste away from homes and businesses to wastewater treatment plants. From wastewater treatment plants, the waste is treated before being discharged into a nearby waterway.


Combined sewers carry a mixture of sanitary waste and stormwater to the wastewater treatment plant. Heavy rains can overflow combined sewers, sending untreated (or partially-treated) wastewater to Combined Sewer Overflows (CSO), which discharge that raw mixture directly into nearby waterways.


Storm sewers, also known as surface or runoff sewers, are designed to collect and carry rainwater, snowmelt, and irrigation runoff into storm drains located in parking lots, streets, and gutters. These drains are connected through a series of underground pipes that carry this water directly to rivers, lakes, and other bodies of water without having been treated at a treatment plant.  This runoff is called stormwater, and is the number one source of water pollution in Massachusetts. 


Most cities use a combination of these types of sewers to manage their water. For example, Boston uses both combined and separated sewer systems.

From Start to Finish

Before returning to the ecosystem, sewage from residential, commercial, and industrial sources is treated at wastewater treatment plants. Water for domestic and/or commercial uses is flushed through the building's pipes until it reaches the local sewers, which are typically owned and maintained by the municipal sewer departments. For about half of Massachusetts residents, their sewage is sent to MWRA's treatment plants near Boston Harbor.

A total of 5,100 miles of local sewers deliver wastewater to MWRA interceptor sewers, which then take it to treatment facilities. The Deer Island Treatment Plant receives primary and secondary wastewater treatment from all other MWRA plants. After treatment, any remaining effluent (outflowing wastewater) is disinfected before being released into Boston Harbor.

The Town of Palmer's wastewater treatment facility, sited along the Ware River, provides a more compact example. This facility discharges treated wastewater directly to the Chicopee River. 


The goal of both primary and secondary wastewater treatment is to ensure that the water released into the bay is clean and safe for both the environment and humans.   

Who is Responsible for Maintaining the Infrastructure?

The EPA's National Pollutant Discharge Elimination System (NPDES) regulates wastewater systems. EPA's stormwater permit regulations apply to water discharges to the Boston Water and Sewer Commission's (BWSC) municipal storm sewer systems and all large water withdrawals. BWSC owns and operates the city's wastewater and storm drainage collection and transportation system. This system serves about 20,500 acres or about 70% of the total land area in Boston.


The Operations Division of the BWSC oversees smaller sewer and drain repair, maintenance, and cleaning jobs. The BWSC cleans debris from 11 brook inlets and outlets throughout Boston as part of storm drain and catch basin cleaning initiatives. Local (and/or regional) municipal wastewater utilities like BWSC are responsible for maintaining this infrastructure throughout the state.  

water pollution outfall picture

Effects on the Water Cycle

When rain hits impermeable surfaces like buildings, roads, and parking lots, that water cannot filter into the ground, nurture plants, and rejoin the water cycle. Instead, that water moves across pavement, picking up debris & chemicals as it flows to a nearby waterbody. This water is called stormwater runoff.

Stormwater pollutes local waterways, where the contaminants endanger wildlife and damage water quality before flowing into larger rivers and the ocean. Traditional stormwater management systems including catch basins, storm drains, and pipes that transport wastewater can become overwhelmed during larger rain storms (in the old cities of Massachusetts, many of these were built in the 1800s). Climate change delivers more frequent and extreme precipitation events at the same time that our state's population is growing. Our existing water infrastructure is literally crumbling in the face of these dual challenges. When lots of stormwater comes all at once (like in a big storm), it moves faster and transports more contaminants at higher flow rates. Old drainage systems aren't equipped to handle that quantity of water, and create localized floods, resulting in erosion and public safety risks. 


To be resilient to climate change and accommodate our growing development needs, we must support infrastructure upgrades statewide. 

Green infrastructure filters air and purifies water naturally

Sustainable Options

Green infrastructure is a low-cost alternative to traditional infrastructure with many extra benefits. These projects use plants, soil, and permeable surfaces to store and filter rain, allowing the water to soak back into the ground where it lands. Green infrastructure reduces the amount of runoff that enters sewage systems and our waterways, making our pipes last longer (meaning fewer expensive upgrades).

Green infrastructure filters and reduces excess nutrients and bacteria in stormwater runoff. These projects also create green space that reduces urban heat, improves wildlife habitat (especially for pollinators!) and naturally protects people from flooding, all by harnessing the power of nature. 


The Boston Water and Sewer Commission has undertaken several projects to help demonstrate “green” alternatives to traditional “gray” stormwater management. These projects include:  

  • Central Square in East Boston  

  • Audubon Circle  

  • Green Infrastructure at 5 Boston Public Schools  

  • Rain Garden at South Street + Bussey Street  

  • Tree Trenches on Harrison Ave  

  • Codman Square  


For another example, the town of Arlington is constructing a bioretention basin (with the help of the Mysic River Watershed Association). The basin will help collect stormwater runoff and reduce pollutants discharged into nearby Alewife Brook, keeping that waterway cleaner. The basin is capped with native plants, and each year will treat and remove 20%-70% of a range of pollutants. The bioretention basins are sized with the "first flush" in mind (the build-up of pollutants that flow from pavement into storm drains at the beginning of a storm). The first flush is especially dirty after a drought - more time for polluants to build up! We know climate change will bring us more storms AND more droughts, so we must design structures with those patterns in mind. 

bottom of page