Chlorine has been widely used for disinfecting treated sewage. As sewage systems become widespread nationwide, there are growing numbers of cases where sewage treatment plants discharge treated sewage into headwaters and fishing areas. The use of sodium hypochlorite for disinfection purposes, however, tends to produce carcinogenic substances (organic chlorine compounds such as trihalomethanes) due to the function of free chlorine. There is thus a need to develop disinfection technology that takes the place of chlorine disinfection in order to control residual chlorine in final effluent. We have developed an ultraviolet (UV) disinfection system as a promising substitute for conventional disinfection systems; its bactericidal effect has been verified through performance tests.