A tributary is a small stream that flows into a larger stream.
Boston Water and Sewer Commission (BWSC) owns and operates a system for the collection and transport of wastewater and storm drainage in the City of Boston. The system serves approximately 20,500 acres, 70 percent of the total land area of Boston. The remaining 30% is comprised of parks, cemeteries and undeveloped land.
The City is served by two types of wastewater collection systems: separate and combined. A separate sewer system is comprised of sanitary sewers and storm drains. Sanitary sewers are designed to transport only sanitary flow, which includes a certain amount of groundwater infiltration. Storm drains are designed to transport stormwater flows. A combined system performs the dual function of transporting sanitary flow as well as storm water runoff in one conduit identified as a combined sewer. This type of system is common in older cities. Approximately 80 percent of the sewered portion of the City, roughly 16,500 acres, is served by the separate sewers and 20 percent of the sewered portion of the City, approximately 4,000 acres, particularly the older sections of the city, is served by combined sewers. Currently, BWSC has a program to replace combined sewers with sanitary sewers and storm drains.
The spine of the sewer system consists of two major interceptors: The New Boston Main Interceptor and the New East Side Interceptor. Completed in 1988, these interceptors serve the sewer needs of downtown Boston, the South End, Roxbury, Dorchester and South Boston. They provide increased system capacity, which reduces wet weather combined sewer overflow discharges and virtually eliminates dry weather discharge into Boston Harbor and its tributary waters. Other collection facilities provide sewer services to the remainder of the City.
By far, the predominant element of the BWSC wastewater collection system is the sewers. Totaling approximate 1,455 linear miles, the sewer system consists of: 622 miles of sanitary, 595 miles of storm drain, 235 miles of combined sewer, and 3 miles of combined sewer overflow. The sewers are made of stone, brick, vitrified clay, concrete, iron, and cast iron. Vitrified clay is the most prevalent type of sewer material in the system because it has been used for some time as the standard material for the smaller diameter piping which is predominant in the system. Brick is the most common material for the larger pipes in the system because it was the principal building material used for large pipe in the late 1800's. Most new sewers are made of concrete, polyvinyl chloride (PVC) plastic or ductile iron.
Sewers come in many shapes including circular, rectangular, oval, egg, horseshoe and U-shaped. Most sewers are circular, the other shapes are characteristic of the large brick sewers in the system. The circular sewers in the system range in size from six inches in diameter to 126 inches in diameter for the Boston Main Interceptor. The largest noncircular pipe in the system is 156 inches by 108 inches horseshoe-shaped brick sewer.
In addition to the sewers, the system also consists of: 8 pumping stations, 35,934 catch basins, 430 outfalls, 174 regulators, 47,413 manholes and 202 tide gates.
|Deer Island Treatment Plant|
BWSC has in place a renewal and replacement program that involves the trenchless rehabilitation or replacement of sewers or storm drains in response to persistent malfunction, structural deterioration, excessive emergency repairs and other operation and maintenance problems. BWSC identifies sewer and drain lines that require this work through television inspections, sewer system evaluations, surveys and routine maintenance activities.
All wastewater collected by BWSC facilities are conveyed to the Massachusetts Water Resources Authority's (MWRA) Deer Island treatment plant where, after treatment, it is discharged 9.5 miles out into Massachusetts Bay. Visit our Sewer System History page to learn more.