MICROORGANISMS:

The purpose of UV disinfection is to inactivate microorganisms such as Giardia, Cryptosporidium, and viruses in drinking water without producing known disinfection byproducts (DPBs). Prior to 1998, UV disinfection was not considered a viable treatment for disinfection because of the high UV doses thought to be necessary for Cryptosporidium. Previously, the inactivation of Cryptosporidium oocysts was greatly underestimated by in vitro tests. One early bench scale study showed that a dose of 246 mJ/cm² gave a 96.3% Cryptosporidium reduction using the DAPI-PI test and a 98.8% reduction using in vitro excitation, less than a 2 log reduction. According to research by Bukhari (Bukhari et al., 1999), the inactivation of Cryptosporidium oocysts using mouse infectivity was 3.9 log inactivation for 19 mJ/cm² dose and over 4.5 log inactivation for a dose of 66 mJ/cm².

Traditional methods of disinfection use chemicals to destroy cellular structures to prohibit proper cell function. In UV disinfection, UV light damages the nucleic acid of the cell so it cannot reproduce (USEPA, 2003). Nucleotides of DNA and RNA absorb UV light in the 200 to 300 nm range, which is the wavelength that the UV lamps emit. Absorption of the UV light varies from microorganism to microorganism because of differences in DNA and RNA.

Some microorganisms have the ability to repair damage after exposure to UV light (USEPA, 2003). Photoreactivation may occur in bacteria when exposed to reactivating light between 310 and 490 nm, activating the enzyme mechanisms to repair the damaged area. Dark repair of bacterial does not require reactivating light; damaged areas are removed and regenerated from the complimentary strand of DNA in bacteria. However, the residual disinfectant in the distribution system will most likely prevent repair of UV damage in bacteria. Cryptosporidium, and viruses do not show significant signs of repair after UV exposure. Repair has occurred in Giardia when exposed to small UV doses (0.5 mJ/cm²) (Linden et al., 2002) but because typical UV doses for drinking water are approximately 40 mJ/cm², Giardia repair should not be an issue.

The log inactivation for Cryptosporidium at various UV doses is shown here.

Inactivation of Cryptosporidium by UV Light (USEPA, 2003)

This figure shows the log inactivation for Giardia at various UV doses.

Inactivation of Giardia by UV Light (USEPA, 2003)

The two tables below list the UV sensitivity of various pathogenic and non-pathogenic microorganisms in water.

UV Sensitivity of Pathogenic Microorganisms in Water¹ (USEPA, 2003)

UV Sensitivity of Non-Pathogenic Bacteria, Bacteriophage, and Spore-Forming Bacteria in Water¹ (USEPA, 2003)

UV Dose:

The UV dose is generally equal to I*T, where I is the irradiance in units of mW/cm² and T is the exposure time in seconds. This is analogous to the C*T disinfectant dose for chlorine. The UV dose to inactivate microbes varies with the microbe. Bacteria generally require lower doses than viruses, which means that in a situation where UV light is the only disinfectant, the dose will probably be determined by virus inactivation (Cotton et al., 2001). However, in most cases, virus inactivation can be achieved by chlorine or chloramines. Most microorganisms exhibit a first order relationship between UV dose and log inactivation.

According to J. G. Jesky (Jesky et al., 2001) the UV dose can be estimated using one of three methods: biodosimetry, actinometry, or mathematical modeling. In biodosimetry a known quantity of a microorganism is injected into the influent. After exposure to UV light, the log-inactivation is determined by measuring the microorganisms remaining in the effluent and comparing to the influent counts. The reduction equivalent dose (RED) is then determined using the dose-response curve which is determined on the bench-scale collimated beam apparatus. Actinometry determines the UV dose by measuring the degeneration of a chemical species by the UV light. The UV dose of UV reactors can also be determined by mathematical modeling.