TheSouth Boston CSO Storage Tunnel, also known as theNorth Dorchester Bay CSO Storage Tunnel, is a large underground facility designed to reduce untreated sewage discharges intoBoston Harbor from theMassachusetts Water Resources Authority combined sewer and stormwater system. It was opened on July 23, 2011, and is part of the federally mandated Boston Harbor Cleanup project.[1][dead link][2] CSO stands forCombined Sewer Overflow.
The main part of the facility is a tunnel 17 feet (5.2 m) in diameter, running 2.5 miles (4.0 km) along the harbor front. The tunnel starts at an Odor Control Building(42°19′21″N71°02′56″W / 42.3225°N 71.0490°W /42.3225; -71.0490 (Odor Control Building)), continues along the harbor front, with a midpoint near42°19′46″N71°02′14″W / 42.3294°N 71.0373°W /42.3294; -71.0373, and ends with a pump station at42°20′19″N71°01′18″W / 42.3385°N 71.0216°W /42.3385; -71.0216 (Pumping station).[citation needed]
Combined sewers are problematic because during heavy storms, they are forced by a high volume of rainwater fromstorm drains to carry untreatedsanitary sewer output into Boston harbor, including dangerous amounts of human waste. In addition to the tunnel project, the MWRA is undertaking costly sewer separation in parts of South Boston near the Reserved Channel, and reconfiguring various drains and outflows.[3][dead link] The tunnel provides a buffer to allow some combined sewers to remain in service. It has sufficient buffer capacity to hold combined sewage and rain water during most storms, helping to eliminate the Combined Sewer Outflow events that polluted nearby beaches on average 20 times per year.[4][dead link] After the storm is over, the tunnel is "dewatered" back into the network at a rate theDeer Island Waste Water Treatment Plant can handle.
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