wastewater: no residual. However, growing concern over chlorinated disinfection by-products associated with more stringent regulations (EPA, 1996), coupled with the ineffectiveness of chlorine against hardy protozoan pathogens, has led to a search for disinfection alternatives since the 1970s. In 1998 and soon thereafter it was discovered that UV was in fact very effective against many chlorine-resistant pathogens, including Cryptosporidium (Clancy et al., 1998) and Giardia (Linden et al., 2002), which propelled UV into the forefront of disinfection alternatives and allowed the U.S. Environment Protection Agency (EPA) to establish more stringent regulations for the control of Cryptosporidium and Giardia in drinking water. Now UV is a fast-growing disinfection process being incorporated all over the world and is encouraged as an effective water disinfection process in the most recent EPA drinking water regulations (EPA, 2006).
UV is a physical process, harnessing the power and energy of photons to effect destruction of pathogenic microorganisms and break apart chemical bonds of pollutants. In order for UV to be effective, the photons both have to be absorbed by the target pathogen or chemical (first law of photochemistry) and pack enough energy to cause a lasting photochemical effect.
In disinfection applications the photon target is the DNA of a microorganism. The absorbance spectrum of the target is one determinant of the wavelengths that will be most effective. UV irradiation covers the electromagnetic spectrum from 100 nm to 400 nm (100-200 nm is the vacuum UV range), where the most important wavelengths for disinfection are between 240 nm and 280 nm, the peak wavelengths of DNA absorbance (Figure 1). In chemical destruction applications, the important factors are the absorbance features of the target pollutant and the efficiency of the photonic transformation of chemical bonds (known as the quantum yield). Two examples of target pollutants are also presented in Figure 1.
Engineered UV systems were first developed at the turn of the 20th century with the mercury arc lamp. Current conventional UV lamps include mercury vapor lamps of the low-pressure (LP) and medium-pressure (MP) variety, indicating the internal mercury vapor pressure, which dictates the emission spectrum illustrated in Figure 2. LP lamps are low in power, high in UV efficiency, and generate near monochromatic emission at 253.7 nm, near the peak DNA absorbance. MP lamps are higher in power, lower in UV efficiency, and generate a broadband polychromatic emission with characteristic peaks between 200 nm and 400 nm. These differences translate into engineering decisions and opportunities for each lamp, specific to the application considered. On the frontiers of UV lamp technology are nonmercury-based sources including high-energy, pulsed UV-based lamps, often with xenon gas, and UV-based light-emitting diodes (LEDs) currently under development. An example of the types of spectra emitted from a pulsed UV source is presented in Figure 2. These new UV sources may soon radically