Using ultraviolet (UV) light for drinking water disinfection dates back to 1916 in the U.S. Over the years, UV costs have declined as researchers develop and use new UV methods to disinfect water and wastewater. Currently, several states have developed regulations that allow systems to disinfect their drinking water supplies with UV light. Running a UV light system requires a basic level of operator skill and relatively clean source water. On the down side, however, UV offers no residual disinfection within the distribution system.
What is UV disinfection? UV light, which continues to be a reliable means of disinfection, involves exposing contaminated water to radiation from UV light. The treatment works because UV light penetrates an organism’s cell walls and disrupts the cell’s genetic material, making reproduction impossible. A special lamp generates the radiation that creates UV light by striking an electric arc through low-pressure mercury vapor. This lamp emits a broad spectrum of radiation with intense peaks at UV wavelengths of 253.7 nanometers (nm) and a lesser peak at 184.9 nm. Research has shown that the optimum UV wavelength range to destroy bacteria is between 250 nm and 270 nm. At shorter wavelengths (e.g.185 nm), UV light is powerful enough to produce ozone, hydroxyl, and other free radicals that destroy bacteria.
International set the minimum UV light requirement at 38 mWs/cm2 for class A point of use (POU) and point of entry (POE) devices that treat visually clear water. The U.S. Environmental Protection Agency (EPA) lists UV disinfection as an approved technology for small public water systems. In addition, EPA is considering the following variations of conventional UV treatment as “emerging” technologies: pulsed UV, medium-pressure UV, and UV oxidation (i.e., used in combination with
Closed Vessel Ultraviolet Reactor
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Reprinted with permission from the Atlantic Ultraviolet Corporation.
NATIONAL DRINKING WATER CLEARINGHOUSE
peroxide or ozone).
Advantages Generally, UV is simple to install and requires little supervision, maintenance, or space. Improved safety, minimum service time, low operation and maintenance costs, and the absence of a chemical smell or taste in finished water are primary factors for selecting UV technology rather than traditional disinfection technologies. UV treatment breaks down or removes some organic contaminants. UV achieves 1-log reduction of Giardia lamblia at an intensity of 80-120 mWs/cm2, and 4-log reduction of viruses at an intensity of 90-140 mWs/cm2. Only recently has the scientific community begun to accept UV as a highly effective tool for Cryptosporidium control. UV light disinfection does not form any significant disinfection byproducts, nor does it cause any significant increase in assimilable organic carbon (AOC). Research has confirmed that UV effectiveness is relatively insensitive to temperature and pH differences. In addition, researchers found that UV application does not convert nitrates to nitrites, or bromide to bromines or bromates. Recent pilot studies show that UV-treated drinking water inhibits bacterial growth and replication in the distribution system; however, conditions within distribution...