As World Water Week celebrates its 21st anniversary in Stockholm this week, New York City is two months away from opening the world's largest ultraviolet (UV) drinking-water disinfection plant. When the lights go on, the facility's 56 massive UV units will neutralize waterborne pathogens in all the drinking water coming from the city's major sources—the Delaware County and Catskill watersheds. The facility will process up to nine billion liters daily, adding a second layer of sanitation to the chlorine treatment that has been applied for years.



The Delaware–Catskill watersheds, located 160 kilometers north of the metropolis, have historically not required filtration or multiple methods of disinfection. More stringent U.S. Environmental Protection Agency (EPA) regulations in recent years and increased development around these bodies of water over the past decade, however, have prompted the city to add more protection against potentially disease-causing microorganisms.



The new, $1.6-billion Catskill–Delaware Ultraviolet Disinfection Facility—built some 50 kilometers north of Manhattan on 62 hectares in the towns of Mount Pleasant and Greenburgh in Westchester County, N.Y.—is scheduled to go live by October 29. As water flows through each of its 151-million-liter disinfection units, the UV light will alter the DNA of cryptosporidium, giardia and other waterborne pathogens, rendering them unable to replicate. Blooms of these microorganisms can cause nausea, cramps, diarrhea and even more serious maladies.



Water-flow pipes connect to either end of the 7,200-kilogram rectangular stainless steel UV disinfection reactors, each of which is about 5.8 meters long, 1.7 meters wide and 2.3 meters tall. Water moves through each reactor at about 1.5 meters per second, passing within centimeters of that unit's 210 UV lamps. The Catskill–Delaware facility as a whole is expected to use up to 6.3 megawatts of power when the water is at the maximum flow. On normal days, when about five billion liters are flowing, energy usage should not exceed 4.5 megawatts, according to Trojan Technologies, Inc., the wholly owned subsidiary of Danaher Corp., which built the facility.



Trojan's operation will dwarf the San Francisco Public Utilities Commission's Tesla Treatment Facility, which opened in July 2011. That $114-million project is California's largest UV water disinfection facility, treating up to 1.2 billion liters of water per day for the Bay Area from the Hetch Hetchy Reservoir in Yosemite National Park.



In 2006 the EPA began requiring unfiltered surface water treatment systems, including the one delivering New York City's drinking water, to either filter its drinking water or install some other barrier for microorganisms besides chlorine treatment. The city's alternative to UV would have been to build a much more expensive filtration facility (pdf) that passed drinking water through a series of porous materials—typically layers of sand, gravel and charcoal—helping to remove tiny chemicals, hazardous materials and toxins.



The EPA's requirements make sense, given that neither chlorine nor UV treatments can by themselves mitigate all threats to drinking water. Whereas cryptosporidium is highly resistant to chlorine, UV has proved effective at controlling the parasite. On the other hand, adenovirus is notoriously resistant to UV disinfection but can be killed using chlorine.



Although adenovirus is not typically found in surface water repositories, New York's watersheds are not entirely free of risk because of the growing population and increased development in those areas, says Mark Sobsey, University of North Carolina at Chapel Hill professor of environmental sciences and engineering and director of the school's Environmental Microbiology Laboratory. "We have to assume that there is some risk of adenoviruses getting into these water sources from human fecal contamination, such as septic tank effluents that may discharge into some waters and eventually enter the reservoirs," he adds.



The good news—between the new UV facility and the requirement that all municipal and community drinking water supplies use a chemical disinfectant, any risk that adenovirus would contaminate the city's water supply is negligible, Sobsey says.