| Popis: |
Ultraviolet (UV) irradiation is being explored as an alternative primary disinfectant for water and wastewater but provides no residual disinfectant. Monochloramine, a chlorine and ammonia derivative, is a much weaker disinfectant than free chlorine and provides a residual with less formation of the toxic by-products produced by free chlorine. In this report, the effects of the sequence of low doses of UV irradiation followed by monochloramine exposure are investigated for disinfection of the bacterium Salmonella typhimurium. It was hypothesized that synergy occurs, whereby the combination of these water disinfectants applied in sequence achieves greater inactivation than either disinfectant applied alone. UV irradiation at doses of 5 and 7 mJ/cm^2 reduced S.typhimurium by 2.2 and 3.4 log[10], respectively. A dose of 1.0 mg/L monochloramine at pH 7.0 and temperature of 5°C, reduced S.typhimurium by about 0.5 log[10] after 20 minutes. There was no evidence of a synergistic effect on S.typhimurium reduction by sequential application of UV irradiation and monochloramine under the conditions tested. Upon monochloramine exposure, the UV-irradiation S.typhimurium increased in infectivity titer. This phenomenon confounded the effort to determine if there was a synergistic effect on S.typhimurium by sequential UV-monochloramine disinfection. The infectivity titer increase of UV-irradiated S.typhimurium during monochloramine may have been due to DNA repair, because this phenomenon was not observed for a bacterial strain that was deficient in DNA repair. Sequential application of UV followed by monochloramine disinfection of the repair deficient Salmonella strain did result in an inactivation greater than the additive effects of the disinfectants alone, suggesting that synergism may have occurred. |
