New York City has activated the final stage of Water Tunnel No. 3's Manhattan leg, turning on a new spigot for the borough for the first time in more than 90 years.

The activation of this 8.5-mile stretch of the $6-billion tunnel—one of the largest infrastructure projects in the city's history—"is a historic milestone," Mayor Michael Bloomberg said in an Oct. 16 statement announcing the event.

Meanwhile, the city's trunk and water main installation work that connects the tunnel to the water distribution system is ongoing.

Embedded 400 to 600 feet deep, Tunnel No. 3 will add redundancy to the city's water supply. Tunnel No. 1 has served Manhattan since 1917, and activation of the new tunnel means the older one can be shut off for repairs. Tunnel No. 2 serves Brooklyn and Queens and has been in service since 1936.

Proposed in 1954, Tunnel No. 3 was designed to be built in stages, the city says. Preliminary work began in 1970 but the project faced numerous hurdles including funding, and there have been 24 deaths associated with its construction.

The $2.4-billion first leg was activated in 1998. It is a 13-mile stretch that runs from the Hillview Reservoir in Yonkers, south through the Bronx and into Manhattan, then east at Central Park and on into Astoria, Queens.

The next leg of the tunnel is under construction in Queens and Brooklyn. Set for completion in 2018 but activation around 2021, the third section has two distinct legs from Red Hook, Brooklyn, to Maspeth, Queens. It will also connect with the Richmond Water tunnel, which serves Staten Island.

Completion of the Manhattan leg provides the borough with 350 million gallons of water that originates from the Catskill/Delaware watersheds. The water is disinfected at the Dept of Environmental Protection's ultraviolet (UV) filtration facility in Mount Pleasant and Greenburgh, N.Y., which was completed in January. The 160,000-sq-ft facility, which can treat 2 billion gallons a day and uses gravity instead of pumps to convey water through the system, is the largest UV water treatment facility nationwide.