Combination of low pressure UV with chlorine (NaOCI) for disinfection of drinking water

Chlorine has been used as a disinfectant for more than a century because of its cost effectiveness, ease of use, oxidizing efficiency and capability to guarantee residual disinfectant in water distribution systems. However, the inactivation efficacy of chlorine has been questioned since outbreaks of micro organisms like Cryptosporidium parvum oocysts and Giardia cysts, very resistant to single step chlorine inactivation, were recorded; furthermore the reaction of chlorine with a wide variety of natural organic matter (NOM) to form disinfection by products (DBP) like trihalomethanes (THM) and haloacetic acids (HAA) was a trigger to develop and study alternative disinfection systems. Hence, because of the resistance of some micro organisms to chlorination and DBP formation, UV light disinfection started to gain ground as a promising DBP-free disinfection method. The major drawback of the technology is that it does not guarantee residual disinfection in the distribution network. Therefore, the combination of UV and free chlorine can be practically implemented to solve both issues at the same time. The main objectives of this study were to investigate the inactivation efficacy of free chlorine on Bacillus Subtilis spore, as a surrogate for Cryptosporidium parvum, and MS2 phage, as a surrogate for viruses; to assess the inactivation efficacy of UV on MS2 phage; and to investigate the synergism of the sequential application of UV and free chlorine. The results of this study show that MS2 phages are found to be very sensitive to chlorination with 100% inactivation at a CT value of less than 0.44 mgmin/L while Bacillus subtilis spores are found to be very resistant to chlorination and showed 1-log inactivation at a CT value of 1000 mgmin/L. The result showed that the pH of experimental sample has great impact on inactivation efficacy of chlorine on Bacillus subtilisspore. The pH has a big effect on hypochlorite/hypochlorous acid ratio which resulted decreases on available free chlorine for inactivation. MS2 phages are found to be resistant to UV irradiation compared to \chlorination. At a UV dose of 23 mJ/cm21.55-log inactivation of MS2 phage was obtained. Synergetic effect of sequential application of UV as a primary disinfectant followed by free chlorine as a secondary residual disinfectant could not be observed because the removal of MS2 phages with free chlorine were very effective, 100% inactivation was obtained with a dose of 0.5 mg Cl2/L and a contact time of 1 minute for this reason secondary disinfection enhancement by primary UV irradiation could not be observed. Total trihalomethanes (TTHMs) formation has also been observed during inactivation of MS2 phage by single step chlorination as well as sequential application of UV and chlorine. At a dose of 0.3 mg Cl2/L and contact time of 1 hour with chlorine alone disinfection 0.08 µg/L of TTHMs were formed while forsequential application of UV (46 mJ/cm2) and chlorine (0.3 mg Cl2/L) TTHMs formed were 0.07 µg/L showing that primary cause for THMs was chlorination disinfection process. The results from this study suggest that the efficiency of chlorine to inactivate MS2 was very high and inactivation of Bacillus subtilis spores with free chlorine is highly dependent on ratio of hypochlorite/hypochlorous acid. MS2 phages are found to be resistant to UV irradiation and primary UV irradiation does not cause TTHMs formation