Effect of UV system modifications on disinfection performance

Numerical simulations have been performed to gain a better understanding of ultraviolet (UV) disinfection process performance. Similar simulations in previous studies revealed critical paths through which particles moved and experienced low UV doses. In vertical UV systems, these paths generally are found near the channel walls with characteristics of high velocity and low turbulence intensity. Moreover, these paths generally coincide with low UV intensity zones and appear to represent the primary limitation of process performance. Reactor modifications have been designed to eliminate or modify these trajectories, thereby improving process performance. In a pilot-scale open-channel system with a vertical lamp orientation, two geometric modifications with "wave" and "baffle" shapes have been developed and examined. The results of these pilot tests confirmed the improvement of process performance when compared with an unmodified UV system. As in the case of the unmodified UV systems, a numerical model that combines kinetic information from a well-mixed batch reactor with a dose-distribution function was used to predict process performance of the UV system with the wave-shaped modification. A dose-distribution function that incorporates the effects of spatial nonuniformities in both hydrodynamics (through a random-walk model) and UV intensity (through a point-source-summation model) was developed. The dose-response function for microorganisms was obtained from completely mixed batch reactor experiments with a collimated beam. Predictions of disinfection efficacy confirmed the ability of the modified systems to improve microbial inactivation.