Photoreactivation of Escherichia coli following medium-pressure ultraviolet disinfection and its control using chloramination

Abstract

Ultraviolet (UV) disinfection is becoming increasingly popular as an alternative disinfection technology to chlorination in recent years. In this study, we investigated the photoreactivation of Escherichia coli following medium-pressure (MP) UV disinfection of synthetic water by a bench-scale collimated beam apparatus. The UV doses ranged from 1.6 -19.7 mWs/cm2 and photoreactivation was investigated for 6 hours under fluorescent light. In addition, chloramination was applied after UV disinfection to investigate its ability to control photoreactivation. It was found that photoreactivation occurred for all UV doses tested and the increase in bacteria numbers ranged from 0.04 to 1.35 log10. However, the degree of photoreactivation decreased with increased UV doses. Chloramination experiments revealed that the addition of 0.5 mg/l of monochloramine resulted in suppression of photoreactivation for 1 hour only. An increased monochloramine dose of 1 mg/l was found to prevent photoreactivation for the entire duration of the experiment. The results of this study have shown that photoreactivation occurs even after MP UV disinfection, although it is of a lesser extent at higher UV doses. This study has also established that secondary chloramination can effectively suppress and eliminate photoreactivation with a chloramine dose of 1 mg/l.

Indicators for photoreactivation and dark repair studies following ultraviolet disinfection

“…For most of the strains in the study, the photoreactivation curves follow a similar trend, where much of the repair takes place within the first two hours of repair, followed by leveling off of the curves after that. This trend is consistent with those reported previously [16,19,28]. In this study, the maximum level of photoreactivation achieved was about 85% of the total number of the inactivated bacteria.…”

“…This reduces the efficiency of UV disinfection and has an adverse impact on the microbiological quality of the treated water. As such, photoreactivation and dark repair of microorganisms following UV disinfection have been studied quite extensively in the last few decades [1,3,5,17,19,21,22,28,29], especially for the two most commonly used UV lamps-the low-pressure (LP) and medium-pressure (MP) mercury UV lamps. LP UV lamps are traditionally used in UV disinfection and emit monochromatic UV radiation at 254 nm, which is close to the optimum germicidal wavelength of 260 nm [4].…”

Influence of photoreactivating light intensity and incubation temperature on photoreactivation ofEscherichia colifollowing LP and MP UV disinfection

“…Thus, the higher levels of recovery could have been due to the higher light intensity (23 kLux) used in this study. There has been limited studies on photoreactivation of E. coli ATCC 15597 following UV disinfection (Quek et al. 2005) but the results here indicate that the maximum level of photoreactivation achieved is similar to that for E. coli ATCC 11229.…”

Inactivation and Photorepair of Enteric Pathogenic Microorganisms with Ultraviolet Irradiation

“…However, because UV radiation from the sun is present in the environment, natural defense mechanisms have evolved in bacteria and other microorganisms that allow UVinactivated microorganisms to reverse UV-induced damage through such repair pathways as photoreactivation and dark repair (Koivunen and Heinonen-Tanski, 2005;Quek and Hu, 2008). Since these defense mechanisms are obstacles to safe disinfection levels and the application of UV disinfection, scientists have studied photoreactivation and dark repair following UV treatment extensively in the past few decades (Quek and Hu, 2008;Quek et al, 2006). Just as different strains of Escherichia coli have different repair abilities, different types of bacteria exhibit different sensitivities to UV applications and have different repair abilities.…”