Ultraviolet (UV) disinfection uses electromagnetic radiation to kill microorganisms such as protozoans, bacteria, and viruses damaging their genetic material (DNA and RNA). While UV radiation applications for treatment are a century old, date to, improved cost-effectiveness and concerns over conventional disinfection methods have increased the popularity of UV disinfection in recent decades. In the U.S., UV radiation is primarily used to treat wastewater1,2.
Disinfection involves two activities. Primary disinfection kills microorganisms in the water, while secondary disinfection maintains residual disinfectant in treated water to prevent subsequent growth prior to consumption1. UV reactors are primary disinfection mechanisms. The optimal “germicidal” portion of the electromagnetic radiation spectrum that is used to disinfect water ranges from 250-300 nm (UV-B and UV-C)1,2. UV reactors consist of mercury arc lamps, a reactor, and ballasts. Standard UV lamps range from 0.75 to 1.5 meters in length and 1.5-2.0 cm in diameter2. Contact and non-contact reactors exist, which describe whether incoming effluent directly contacts the lamps or is separated from them by transparent tube2.
UV reactor designs must consider many factors for performance3. First, the size, intensity, and wavelengths of lamps affect treatment, with more effective, “medium-pressure” lamps emitting radiation throughout frequencies in the optimal range1. Second, dissolved and suspended particles in water can reduce kill rates by preventing UV radiation from reaching microorganisms4. Particulate matter can protect microorganisms by encapsulating them or simply scattering the UV radiation1,5. Third, contact time of effluent to UV treatment influences the likely probability of killing microorganisms. In UV reactors, contact time is significantly reduced compared to chlorine6. Fourth, effective reactor designs have uniform flow with sufficient mixing to promote consistent and equal exposure of effluent. Finally, routine maintenance such as cleaning and changing bulbs helps to maintain expected performance2.
The viability of UV disinfection is changing as efficient lamps, more reliable reactors, and new techniques increase performance. Pulsed UV, UV using medium-pressure lamps, and UV oxidation are all newer, innovative techniques6. In addition, many users appreciate the lack of chlorinated smell in drinking water. UV radiation is most viable, though, for wastewater treatment, as it does not prevent pathogen growth during water distribution. Further, some organisms can use natural mechanisms to repair destroyed nucleic acids2. Overall, UV radiation will continue to play an important role in municipal treatment systems in coming decades.
The cool cover photo via http://enviroline.homestead.com/uvdisinfection.html.
1. Crittenden, J., Trussell, R. R., Hand, D. W., Howe, K. J. & Tchobanoglous, G. Water treatment principles and design. (J. Wiley: Hoboken, N.J., 2005).
2. EPA Wastewater Technology Fact Sheet: Ultraviolet Disinfection. (U.S. Environmental Protection Agency: Washington, D.C., 1999).
3. Loge, F. J., Darby, J. L. & Tchobanoglous, G. UV Disinfection of Wastewater: Probabilistic Approach to Design. Journal of Environmental Engineering 122, 1078–1084 (1996).
4. Loge, F. J., Emerick, R. W., Nelson, D., Thompson, D. & Darby, J. L. Factors Influencing Ultraviolet Disinfection Performance Part I: Light Penetration to Wastewater Particles. Water Environment Research 71, 377–381 (1999).
5. Emerick, R. W., Loge, F. J., Thompson, D. & Darby, J. L. Factors Influencing Ultraviolet Disinfection Performance Part II: Association of Coliform Bacteria with Wastewater Particles. Water Environment Research 71, 1178–1187 (1999).
6. National Drinking Water Clearing House Ultraviolet Disinfection. (2000).