Influence of photoreactivating light intensity and incubation temperature on photoreactivation of Escherichia coli following LP and MP UV disinfection.

AIMS: To investigate the effects of fluorescent light intensity, sunlight intensity and temperature on photoreactivation of Escherichia coli after low-pressure (LP) and medium-pressure (MP) ultraviolet (UV) disinfection. METHODS AND RESULTS: Two E. coli strains were irradiated with LP and MP UV lamps, and exposed to various fluorescent light (0-23 kLux) and sunlight intensities (1-80 kLux), and temperatures (4-50 degrees C). Escherichia coli concentrations were enumerated at hourly intervals to determine photoreactivation rates and final photoreactivation levels. Higher photoreactivation rates and levels were observed with increasing fluorescent light intensities, while high sunlight intensity (>12 kLux) caused a one-log decrease in E. coli concentrations. When exposed to near-optimum growth temperatures (23-37 degrees C), photoreactivation levels were higher than those with too high (50 degrees C) or too low (4 degrees C) temperatures. Overall, photoreactivation following MP UV disinfection was lower than that following LP UV disinfection. CONCLUSIONS: Photoreactivation of bacteria following UV disinfection can be a problem in tropical countries where sunlight is abundant and temperatures are high, unless high sunlight intensity is present or if MP UV disinfection is employed. SIGNIFICANCE AND IMPACT OF THE STUDY: With the increased use of UV disinfection, it is imperative that photoreactivation be taken into account in the design of reactors based on site-specific conditions of temperature and light intensity exposure.

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

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.