Disinfection is achieved using low pressure mercury arc lamps, which produce ultraviolet light that have germicidal properties. The radiation kills or deactivates pathogens, which includes bacteria (e-coli), viruses and cysts such as Cryptosporidium and Giardia. Radiation leaves no residual product that continues to disinfect beyond the treatment period. Most water is treated just prior to consumption; therefore there is no need for the usually small amount of disinfecting chlorine which is intentionally added to the water at the end of the treatment process to reduce the risk of re-contamination in the distribution system.