The South Boston CSO Storage Tunnel, also known as the North Dorchester Bay CSO Storage Tunnel, is a large underground facility designed to reduce untreated sewage discharges into Boston Harbor from the Massachusetts Water Resources Authority combined sewer and stormwater system.
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.
[citation needed] Combined sewers are problematic because during heavy storms, they are forced by a high volume of rainwater from storm drains to carry untreated sanitary 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] After the storm is over, the tunnel is "dewatered" back into the network at a rate the Deer Island Waste Water Treatment Plant can handle.