Synergistic UV-Ozone Effects on the Treatment of Pathogens in Secondary Effluent

Ozone and ultraviolet (UV) light have been shown to be effective in treating emerging pathogens that are not affected by the current standard treatment of chlorine. The hypothesis of this study was that a combined treatment of UV light and ozone would result in greater pathogen reduction than a large dose of either one individually. In the one-way studies for ozone and UV light, the ozone treatments took at least twice as long as the UV treatments to achieve the same extent of reduction in the microbial population. Specifically, 60 min for ozone and 30 min for UV light for a 4-log reduction in Heterotrophic plate counts (HPC's); 10 min for ozone and 3 min for UV light for a 3-log reduction in total coliforms and fecal coliforms; and, 6 min for ozone and 3 min for UV light for a 2-log reduction in coliphage. The ozone treatments, however, unlike the UV treatments, resulted in no positive re-growth in bacteria after treatment. Ozone and UV light acting simultaneously exhibited not merely additive, but synergistic effects, in reducing the microbial populations, with the exception of when low levels (2 min UV and 1 min ozone) of both factors were used. Specifically, the following treatment combinations yielded the maximum microbial reductions: low level of UV (1 min) and high level of ozone (4 min) for HPC's; high level of UV (2 min) and high level of ozone (4 min) for total Coliforms and fecal coliforms; and either one of the foregoing treatments for the overall microbial population. The synergistic effects between UV light and ozone allowed for lower UV and ozone levels to achieve the same extent of microbial reduction achieved by either factor acting separately. Thus, 2 min of ozone and 2 min of UV light applied simultaneously was just as effective as 6-8 min of ozone only or 2.5 min of UV light only. This synergistic effect should lead to reduced energy consumption while maintaining the level of disinfection required. If UV light and ozone were to be applied successively, the most effective sequence of application would be ozone first followed by UV light.