Implementing tier 2 analysis on biodosimetric test results
With both goals to develop a sizing program and an operation matrix the biodosimetric results of an in-line cross-flow reactor equipped with 6 medium pressure lamps were analyzed per the US EPA Ultraviolet Disinfection Guidance Manual (UVDGM) Tier 2 approach. Using a Tier 1 analysis was quickly ruled out for this reactor, as the surrogate microorganism used during biodosimetry was B. subtilis instead of the standard MS-2 phage. The expanded uncertainty term (E) was by far the most challenging parameter to determine as part of the Tier 2 analysis. The current draft version of the UVDGM does not include a detailed guidance on how to compute uncertainties from correlations that involve a large number of experimental data with multiple parameters such as Reduced Equivalent Fluence (REF), flow, water transmittance, lamp UV output and sensor reading.
The biodosimetric data from the 6-lamp reactor was properly correlated using standard and multiple linear regression analyses. An emphasis was put on the importance of choosing a regression equation that is statistically representative but also relevant to the UV disinfection principles. As a result, Tier 2 design fluences of 268 to 292 J/m2 were found for 3.0-log inactivation of Cryptosporidium, which is much lower than the equivalent Tier 1 fluence of 420 J/m2, hereby confirming the conservativeness of the assumptions underlying the Tier 1 analysis. The Tier 2 analysis detailed in this paper has been implemented in an Excel sizing spreadsheet that is now being used to bid and size UV systems for the disinfection of drinking water.
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