that would allow that to happen are corroded or underwater. The MWRA is planning to use a tunnel boring machine to dig two new 10-foot-diameter tunnels from Shaft 55A in Weston. The new Northern Tunnel would go to the Waltham-Belmont border, and together with smaller service mains in Belmont, Arlington, and Medford, would form a loop with the City Extension Tunnel and City Tunnel. The new Southern Tunnel would go to Shaft 7C on the Dorchester Tunnel in Boston, forming a southern loop. A smaller service main in Boston would provide redundancy for the remainder of the Dorchester Tunnel. As of 2018, the project is expected to take 17 to 23 years to design and construct. Various other "Metropolitan Redundancy Interim Improvements" would increase reliability in the short term and in the long term help eliminate any single point of failure that would necessitate a boil-water order or cause a complete water outage in any given area. This includes making a new connection for the Commonwealth Avenue Pumping Station in Newtown to low service lines, to allow the city to continue to receive water in case the City Tunnel goes out of service. MWRA also has a program to finance replacement or lining of local water mains, to maintain quality for consumers. Sewage system history In 1884, the Boston Main Drainage System was completed, carrying sewage from 18 towns to Moon Island to be held for an outgoing tide. In the early 1900s, the sewage was pumped directly into Boston Harbor. The Metropolitan District Commission built a sewage treatment plant at Nut Island in 1952, and another at Deer Island in 1968. The Clean Water Act of 1972 imposed more stringent requirements. Dumping of sludge into Boston Harbor ended in 1991 by using a facility in Quincy to convert it into fertilizer. Sewage processing was improved and consolidated at Deer Island in the 1990s, with a deep-water discharge system finished in 2000. Federally mandated projects to reduce combined sewer outflow events into Boston Harbor and local rivers were ongoing as of 2004. The South Boston CSO Storage Tunnel was completed in 2011. Water and sewage statistics MWRA Total Water Demand and Wastewater Generation * Total withdrawals and water sales included an additional (temporary) demand from Cambridge while it rebuilt its own water treatment plant. For calendar year 1999, 15 mgd; calendar year 2000, 14 mgd; and calendar year 2001, 6