Last month, the City announced the completion of a $1.5 billion Westchester facility that will treat New York City’s water supply with ultraviolet light. The City says that it is the largest such facility in the world.

According to the City’s Department of Environmental Protection, the facility “will provide an added layer of protection against pathogens and other harmful microorganisms for the drinking water consumed by…residents of New York City and portions of Westchester County.” The 270,000 square-foot facility is designed to treat more than 2 billion gallons of water each day.

The new facility will provide treatment for Cryptosporidium and Giardia, “naturally occurring microorganisms that can be found in surface waters and can cause gastrointestinal ailments in humans,” according to a DEP press release. Both organisms are naturally occurring but are found at only very low levels in New York City’s water supply, the DEP added.

Nonetheless, the agency said that new federal regulations for treating drinking water were created due to the fact that Cryptosporidium is resistant to disinfection with chlorine.

The DEP described the technology at the new facility as “revolutionary”. As drinking water passes through the facility, it is exposed to ultraviolet light that “destroys the genetic code of the microorganisms, rendering them unable to reproduce or cause infection.” The DEP noted that researchers -with funding from the city of New York- discovered that exposing water to low levels of ultraviolet light was effective at rendering Cryptosporidium and Giardia harmless to humans in the late 1990’s.

New York City has one of the largest, predominantly unfiltered municipal water supply systems on the planet. The system, stretching across 125 miles, relies on mountain water flowing into upstate reservoirs, which is delivered to the city through a network of tunnels and aqueducts.

The system delivers over one billion gallons of drinking water to nine million people in New York City and Westchester County every day.

In an April interview with the Gotham Gazette, water supply expert David Soll said that the City was building the ultraviolet facility “so they don’t have to use as much chlorine to get rid of bacteria.”

New York actually has three water supply systems: the Croton, Catskill and Delaware systems. The Catskill and Delaware systems are “very interlinked” and supply ninety-percent of New York City’s drinking water. That water is not filtered because “the natural landscape acts as a filter,” said Soll.

Soll added, “because Catskill and Delaware water is coming from a place that is not very densely populated, with relatively little industry, the city could take watershed protection measures to ensure that it could still meet the health standards.”

Soll explained that, in addition to the ultraviolet facility, the City of New York is also building a massive water filtration plant in the Bronx as ordered by the federal government. “There was no way New York could comply with safety standards for water coming from the Croton region. There’s too much development. That’s why they are required [by the federal government in 1993] to build the plant,” he said.

The Bronx plant—which would filter about 10% of the city’s water supply and is located in Van Cortlandt Park—is very close to completion said a DEP spokesman.

To learn more about the ultraviolet facility, read the City’s October 9th press release.