Sewer System

In 1833, sanitary waste was allowed into the storm sewer system. Using the storm sewer to drain sanitary waste was the first appearance of a combined sewer system in Boston. The practice proliferated throughout the city and just one year later, the city encouraged adding rainwater from roofs to the system to assist in flushing the sewers of sanitary waste. This flushing did not solve the problem. Health issues related to water contamination, such as cholera, typhoid, and dysentery, began to increase among Bostonians.

Moon Island Reservoirs
In 1875, a study was initiated to address the public health problem, which led to the construction of the Boston Main Drainage System (BMDS). The BMDS was the original backbone of Boston's early sewer system. The BMDS was constructed from 1877 to 1884 under the direction of a special committee established by the City. The purpose of the BMDS was to intercept local sewers and carry the sanitary waste and stormwater runoff to an offshore disposal point. The system included: 25 miles of intercepting sewers, the Calf Pasture pumping station in Dorchester, the Dorchester Bay Tunnel, and an outfall pipe at Moon Island in Boston Harbor. Neither the pumping station nor the tunnel is now in use.  

As Boston's population grew and its city limits expanded due to annexation and land reclamation, the drainage works were subsequently enlarged to accommodate this growth. 

The BMDS disposed of a substantial portion of the City's waste; however, there were parts of Boston lying outside the service area. To address this, the Metropolitan Sewerage System was formed in 1889. The first of its kind in the country, the Metropolitan Sewerage District included: the North Metropolitan Sewer District, the Charles River Valley Sewer System, and the South Metropolitan Sewer District.

In 1919, to oversee the complex Metropolitan Sewerage District, the Massachusetts Legislature created the Metropolitan District Commission (MDC), now the Department of Conservation and Recreation. The Boston Main Drainage System, however, continued to be owned by the City. Up to this point, wastewater was merely collected and deposited into Boston Harbor. Due to worsening pollution, city planners recommended the construction of treatment plants before depositing the wastewater into the harbor. 

In response, the MDC constructed two primary wastewater treatment plants: the Nut Island Wastewater Treatment Plant in 1952 and the Deer Island Wastewater Treatment Plant in 1968. The outlet facilities at Calf Pasture and Moon Island were maintained as a backup for the Deer Island Plant during wet weather. 

In 1977, ownership and operating responsibility for the sewer system was transferred from the City to the newly formed BWSC. In 1985, Massachusetts legislation was enacted that transferred the possession, control, and operation of the MDC Water and Sewerage Divisions to the newly created Massachusetts Water Resources Authority (MWRA).

Deer Island Treatment Plant  
Once in control of the system, BWSC began system modifications and improvements. In 1988, construction of the New Boston Main Interceptor and the New East Side Interceptor were completed, replacing portions of the original BMDS. 

In addition, MWRA was also incorporating improvements to the system. Between 1997 and 2000, the MWRA upgraded the Deer Island Wastewater Treatment Plant from primary to secondary treatment, opened the Inter-Island Tunnel connecting Nut Island Wastewater Treatment Plant to Deer Island, thereby eliminating primary discharge from Nut Island, and completed the 9.5 mile Outfall Tunnel to move discharge from the confined waters of Boston Harbor to the deep waters of Massachusetts Bay.

Today, all wastewater collected by BWSC facilities is conveyed to the MWRA's Deer Island Treatment Facility for secondary treatment. BWSC's wastewater collection system serves approximately 20,500 acres. 

The City is served by two types of wastewater collection systems: separated and combined.

• A separated sewer system is comprised of sanitary sewers and storm drains. Sanitary sewers are designed to transport only sanitary flow, whereas storm drains are designed to transport stormwater flows
• A combined system performs the dual function of transporting sanitary flow as well as stormwater runoff in one conduit identified as a combined sewer. This type of system is common in older cities

Currently, BWSC’s Engineering Department replaces combined sewers with sanitary sewers and storm drains each year to reduce the amount of combined sewer pipe in the system.

The BWSC wastewater collection system is comprised of approximately 1,538 linear miles. The system consists of: 710 miles of sanitary sewer, 670 miles of storm drain, 155 miles of combined sewer, and 3 miles of combined sewer overflow pipe. Existing sewers are made of various materials including, but not limited to, stone, brick, slate, wood, vitrified clay, concrete, ductile iron, and cast iron. New sewers and drains are made of reinforced concrete, polyvinyl chloride (PVC) plastic, or ductile iron. Sewers also come in various shapes including, but not limited to circular, rectangular, oval, egg, horseshoe, and U-shaped.

In addition to the wastewater pipes, the system also consists of: 9 pumping stations, 30,327 catch basins, 267 outfalls, 149 regulators, 49,746 manholes, and 201 tide gates.

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 eliminates dry weather discharge into Boston Harbor and its tributary waters. Other collection facilities provide sewer services to the remainder of the City.

BWSC has 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 is conveyed to the MWRA's Deer Island treatment plant where, after treatment, it is discharged 9.5 miles out into Massachusetts Bay.

All wastewater collected by the Boston Water and Sewer BWSC facilities is conveyed to the MWRA Deer Island Treatment Plant in Boston Harbor.