CITY HALL -- A new $250 million water tunnel from Brooklyn to Staten Island will be activated by Mayor Bill de Blasio on Saturday, four years after Hurricane Sandy caused extensive damage and delays to the project.

The new siphon is considered a critical backup that can deliver as much as 150 million gallons of drinking water to the borough in the aftermath of a disaster.

"Our city is better prepared to tackle 21st century threats like Sandy today than ever before," de Blasio said in a statement. "This water tunnel is one measure that will help Staten Island spring back to action in the event of a disaster that would disrupt the water supply."

De Blasio will tour the tunnel's Tompkinsville side on Saturday morning. The new siphon competed pressurization and water quality testing this fall and will serve as an alternate feed for Staten Island's drinking water.

Staten Islanders now use about 50 million gallons of water a day carried from upstate New York through New York City Water Tunnel No. 3 and the Richmond Tunnel. That will still be the main water supply for Staten Island.

The new tunnel replaces two old siphons built just under a century ago that were the borough's primary water connection until the Richmond Tunnel was completed in 1970. Those siphons were the backup connections until being removed by the Port Authority's harbor deepening project.

SANDY DAMAGE DELAYS

Work on the project started in August 2011 and a tunnel boring machine had gone about 1,600 feet toward Brooklyn from the North Shore between July 2012 and Oct. 28 of that year, when operations were suspended before Sandy struck.

The unfinished tunnel and the Staten Island shaft of the project were both flooded with sea water. The tunnel boring machine was also severely damaged.

The city said this happened because the shaft was open. During the 18-month shutdown caused by the storm surge, the city put resiliency measures in place to prevent future damage.

Plans were redrawn. Infrastructure, including the chlorination station, was moved out of the area where there is at least a 1 percent chance of flooding every year, known as the 100-year floodplain.

The tunnel boring machine was able to resume work again on April 14, 2014, after repairs and once water was removed from the tunnels and shafts.

Before Sandy, the machine had put in place 389 of 2,349 concrete rings to line the tunnel wall. Excavation finally completed in February 2015.

NEW WATER MAINS

The project also includes 6,545 feet of new water mains on Staten Island along Van Duzer Street, Victory Boulevard, Front Street and Murray Hulbert Avenue. Construction of the chlorination station is still incomplete.

"New York City has one of the most sophisticated -- and cleanest -- water systems in the world, and it's a testament to the Department of Environmental Protection's great work," Rep. Dan Donovan (R-Staten Island/Brooklyn) said in a statement. "Thanks to this project, Staten Island will continue to have a safe and resilient water supply."

De Blasio's decision to highlight the tunnel's completion on Saturday is in contrast with how he's marked Sandy's landfall in years past.

On the third anniversary of the storm last year, de Blasio went to New Dorp Beach and promised that the Build it Back housing recovery program would complete work on single-family homes by the end of 2016.

The city won't meet that deadline.

Increasing construction costs have also left a $500 million hole in Build it Back's budget that taxpayers must shoulder, despite the program serving less than a third of roughly 20,000 homeowners who originally applied for help.