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."

During the disinfection process, water will pass through the 56 UV units, each of which contains 144 low-pressure, high-output ultraviolet lamps that produce 40 millijoules per square centimeter of light that together can treat an average of 1.3 billion gallons (4.9 billion liters) of water daily (pdf). The lamps are similar to fluorescent bulbs, but without the phosphor coating designed to protect people from prolonged exposure to UV rays.

Based on the city's specifications (pdf), the facility will use no more than 6.3 megawatts of power when the water is at the maximum flow of 2.4 billion gallons per day. (On normal days, when only 1.3 billion gallons. or liters, are flowing, energy usage should not exceed 4.45 megawatts.)

New York City is not the only city planning to use UV rays to purify its water. San Francisco's Tesla Portal drinking water facility earlier this month awarded a $5-million contract to Pittsburgh's Calgon Carbon Corp. to begin installing 12 of its Sentinel Chevron 48 UV reactors to treat up to 320 million gallons (11.5 million liters) of water daily.

There's a strong need for expanded use of technology that can clean water without the use of chemicals, says Thomas Harmon, an associate engineering professor at the University of California, Merced, and co-founder of the Center for Embedded Network Sensing (CENS), a research group that specializes in using wireless sensors to, among other things, measure and monitor water pollution. "Our water sources have become more and more vulnerable because our population has grown and sprawled out of the city," he says. "We no longer have the pristine reservoirs out on the edges of town, making it difficult to keep our source water clean."

New York's Catskill and Delaware watersheds do not require significant filtration. (They are able to shed most of their naturally occurring minerals as the water flows through the thousands of miles of pipes, aqueducts and tunnels on its way to the city's taps.) But other water sources require more than UV radiation and chlorine to make them potable. Sionix, an Anaheim, Calif.–based corporation, which manufactures a treatment system that uses microscopic air bubbles to clean water, is testing whether its technology can also reduce levels of metal, namely iron and manganese (both toxic to the body in large amounts) in Santiago Creek near Villa Park Dam in Orange County, Calif.

The creek's water has yet to be tapped for drinking water, which means it would be a new source for the surrounding communities, says Sionix CEO Jim Houtz, who adds that his company also does UV disinfection.

The Sionix Elixir system uses pressurized air that produces microscopic bubbles that adhere to suspended particles in water. The bubbles force the particles to the surface, where the system skims them out into a separate hopper. The system removes particles and parasitic organisms as small as one micron in diameter, Houtz says. (A micron is one millionth of a meter, or about four one hundred-thousandths of an inch.)

Sionix by the end of this month plans to ship a $2-million Elixir treatment system to Little Rock, Ark.–based Innovated Water Equipment, Inc., which works with oil drilling companies. Innovated Water plans to use Elixir to treat the brine water discharge from oil wells—either for reuse or to be returned safely to the environment.