UV Disinfection and Dose Distributions: Optimizing Design and Reducing Risk

Operation of a UV system was simulated at a wastewater treatment plant. With the UV system operating at a target MS2 RED of 25 mJ/cm2, the E. coli RED delivered by the UV system typically ranged from 15 to 20 mJ/cm2, notably lower than the MS2 RED. In contrast, with the system operating at a target T1UV RED of 18 mJ/cm2, the E. coli RED delivered was typically in the range of 18 to 20 mJ/cm2 with some values spiking higher. Because of these differences, the inactivation of E. coli achieved by the UV system was lower than would have been expected based on the delivered MS2 RED and somewhat greater than would be expected based on the T1UV RED. The analysis shows that sizing a wastewater UV system based on MS2 REDs can lead to inefficient and poor inactivation of indicator microbes such as E. coli, fecal coliform, and total coliform. In contrast, T1UV REDs are an excellent surrogate for indicator microbe inactivation if the required level of disinfection occurs in the disperse region of the UV dose response curve. Ideally, UV systems are sized and operated based on an RED specific to the target microbe, whether that be indicator microbes or target pathogens.