Synergistic effects of sequential treatment by UV irradiation and chemical disinfectant for drinking water disinfection

Abstract The sequential use of disinfection strategies for drinking water treatments has been shown to provide levels of inactivation greater than those corresponding to the single-step disinfection effects (Corona-Vasquez et al., 2002; EPA, 1999). Therefore, a disinfectants combined system could be applied both for the inactivation of microorganisms, resistant to traditional treatments, and also for limiting the dosage of single disinfectants, reducing cost and risk of by-products formations. In this work, the inactivation of the bacteria E. coli was initially investigated by sequential action of UV irradiation (bench-scale collimated beam apparatus equipped with a low pressure UV lamp) and either chlorine dioxide or free chlorine. The results showed that a pre-irradiation of 70 J.m UV dose (equal to approximately 1.2 log of inactivation) at 20°C and pH 7.4, increased the following germicidal effect of both chlorine and chlorine dioxide (between 0.04 and 1 mg min L). A different order of the disinfectants was also studied and a similar performance was observed. In all cases, an enhanced signal was observed and an inactivation of approximately 1 log greater than that seen in the inactivation curve for free chlorine and chlorine dioxide alone was achieved. An in-situ application of the sequential disinfection system consisted in a combined use of UV irradiation and chlorination to eliminate a saprophyte flora interfering during the faecal contamination indicator analyses. Likewise, the outlet of a reservoir for drinking water has been firstly disinfected by chlorine dioxide and then irradiated by UV. In both cases, an increase of the microbiological quality of the water was achieved, compared to the quality observed with a single step disinfection. This observation led to more bench scale investigations concerning the potential synergistic effects of UV associated with chemical oxidants for water disinfection. In particular, preliminary studies were addresses to synergistic action for virus MS-2 bacteriophages inactivation.