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|CSO Outfalls with Tide Gates at Fort Point Channel|
The City of Boston 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 and storm drains are designed to transport stormwater flows. However, a combined system performs the dual function of transporting sanitary flow as well as storm water runoff in one pipe identified as a combined sewer overflow (CSO). This type of system is common in older cities. During a storm event, the combined sewer can reach capacity; and when this happens, it discharges its flow which consists of both sanitary and rain water into receiving waters.
Currently, Boston has approximately 235 miles of combined sewers and 37 active CSO outfalls. These CSO outfalls are necessary to provide relief only when capacity has been reached. Without this relief, the combined sewage and stormwater would back up into homes and businesses. Generally, overflows do not occur and all wastewater is conveyed to the Massachusetts Water Resources Authority's (MWRA) Deer Island treatment plant. According to studies conducted by the MWRA, the potential for a CSO discharge increases when the amount of rainfall exceeds 0.5 inches. Discharges of combined flows depend upon the rainfall amount, its intensity and duration. When CSO discharges occur, the Massachusetts Water Quality Standards for swimming and contact recreation can be violated.
The 37 active combined sewer overflows are permitted to discharge combined sewage during wet weather events under the Clean Water Act through a National Pollutant Discharge Elimination System (NPDES) Permit issued jointly by the Massachusetts Department of Environmental Protection (DEP) and the U.S. Environmental Protection Agency (EPA). Beginning in 1990, BWSC has submitted annual readings that estimate the frequency and volume of CSO discharge. The CSO outfall pipes are located along Boston Harbor, Dorchester Bay and the Charles River in the Lower Charles River Basin. BWSC conducts monthly inspections of the critical structures upstream of these outfalls.
To correct this situation, over the years, Boston Water and Sewer Commission (BWSC) has undertaken a number of easy-to-install improvements as well as more costly improvements such as sewer separation projects. Six sewer separation projects have been completed, for a total of approximately 43 miles of new storm drains. Presently, there are other sewer separation projects in various stages of construction and design.
As part of BWSC's Public Notification Program, the following maps showing CSO outfall pipes were developed for public review. The maps, when applicable, also show outfall pipes owned by the Massachusetts Water Resources Authority (MWRA) as well as the Cities of Chelsea and Cambridge.