NEW YORK'S ASHOKAN RESERVOIR: New York City is turning to ultraviolet (UV) technology to supplement its use of chlorine for drinking water disinfection.
© David Goehring

The Big Apple wants to use the sun's power to provide clean drinking water for its nine million residents without adding more of the potentially harmful chlorine it uses as a disinfectant. More specifically, New York City officials are building a water disinfection facility some 30 miles (50 kilometers) north of Manhattan on a 153-acre (62-hectare) property in the Westchester County towns of Mount Pleasant and Greenburgh that will use ultraviolet (UV) light to destroy water-borne pathogens such as Escherichia coli, giardia, and cryptosporidium in reservoirs that serve city dwellers.

The city currently uses chlorine to disinfect its drinking water, which is piped in from New York State's Delaware County and Catskill watersheds about 100 miles (160 kilometers) north. But the U.S. Environmental Protection Agency (EPA)—empowered by the federal Safe Drinking Water Act (SDWA) of 1974 (and amended in 1986 and 1996) to set national safety standards—has urged communities since 1996 to cut back on chlorine, which produces harmful by-products when added to water, including trihalomethanes and haloacetic acids, known cancer-causing agents. The trick has been to reduce the chemical as much as possible without increasing the amount of disease-causing microorganisms in the water.

First proposed in 2003, the city got the ball rolling on the UV disinfection project shortly after the EPA in 2006 tightened restrictions on microbial pathogens allowed in drinking water. The UV facility, set to fire up in 2012, is expected to be the world's largest (pdf). When completed, it will consist of 56 40-million-gallon (151-million-liter) UV disinfection units designed to disinfect up to 2.4 billion gallons (nine billion liters) of water (pdf) per day.

All surface and ground water entering New York City's drinking water distribution system is currently treated with fluoride (pdf) to help prevent tooth decay as well as chlorine to kill deleterious microbes as designated by the New York State Sanitary Code and the SDWA.

The city in November 2005 commissioned Trojan Technologies, Inc., a wholly owned subsidiary of Danaher Corp. in Washington, D.C., to build a UV drinking water disinfection system to help comply with the EPA's rules.

"UV is a short wavelength that alters the DNA (genetic material) of the bacteria in the water, making it unable to reproduce," says Jason Cerny, Trojan's lead mechanical designer. "If you leave a pan of water out, UV rays from the light will clean it. We've just increased the scale, added a more efficient source of UV, and more power."