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Validating UV Reactors for Low Dose Wastewater Applications

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Ultraviolet light (UV) disinfection is a process that is being applied increasingly for treating wastewater, reuse water and drinking water. As this technology has moved rapidly into the mainstream for disinfection applications, bioassay validation of UV reactor performance has been embraced as a method of proving UV reactor performance. Bioassay validation is the ultimate proof that a UV reactor achieving its disinfection goals. MS2 and Bacillus subtilis are organisms that are chosen for testing, typically in the new applications of reuse and drinking water. They are good choices because they are resistant to UV, and can measure high doses, consistent with the high doses required for UV resistant target organisms. However, in typical secondary wastewaters, relatively low doses are required to meet targets. Although bioassay validation measures disinfection directly, results can be misleading if the organisms used to measure a response in testing are very different from the organisms being regulated. To understand why this is, it is necessary to understand that a UV reactor is a non-ideal reactor and produces a dose distribution rather than any single dose during operation. This paper will discuss ideal and non-ideal operation, resulting dose distributions, and the effects of these dose distributions on bioassay testing.

Through computational fluid dynamic and light intensity modeling of wastewater reactors, we showed that MS2 is an inappropriate choice as a validation organism for the typical low doses required to meet regulatory targets for indigenous organisms in secondary wastewater. Validation with organisms having similar UV inactivation kinetics to those being regulated, namely indigenous organisms, will avoid the pitfalls of MS2 validations. Indigenous organism bioassays in a variety of secondary activated sludge wastewaters should yield universal performance curves. Care is required when sizing with higher doses near the transition region of the collimated beam dose response curve between free and particle-associated organisms. Testing with indigenous organisms together with MS2 would give the required information to size for low and intermediate doses.

This paper investigates the impact of differences in microbial response upon validation results. Differences in UV resistance between MS2 and indigenous organisms are large enough to lead impacts in validation, while differences within the range of responses of indigenous organisms do not have an impact on validation.
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Document Type: Research Article

Publication date: 2005-01-01

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