Quantifying uncertainty in pathogen removal, membrane integrity monitoring and health consequences in the treatment of drinking water
Minnery, John Gerard William
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We used Monte Carlo and Probability Bounds Analysis, to examine uncertainty in removal of oocysts of Cryptosporidium and viruses in membranes used to treat drinking water. In chapter 2, we examined variability and uncertainty in the determination of parameters used in pressure-based direct integrity tests of hollow fiber micro- and ultra-filtration membranes. We found that variability and model uncertainty related to the contact angle may result in defect size resolution that is greater than 3 micrometers. This implies that the test is not always compliant with Environmental Protection Agency requirements established to detect defects large enough to allow the passage of oocysts of Cryptosporidium. Chapter 3 analyzes the decline in pathogen removal from commercial hollow fiber membranes given that fibers will break and that breakage may not be detected and therefore may accumulate. The analysis used reported fiber breakage rates and simple models to determine the flow rate of unfiltered water through broken fibers given hydraulic configurations and operating conditions. We determined that the initial decline in [oo]cyst removal from one broken fiber per membrane is large. Thereafter the decline is gradual. We found the effect of hydraulic differences between manufacturers were small, as was the effect of differences in initial removal in a new membrane. The decline in virus removal on the lower boundary is gradual. We conclude that a qualitative examination of the potential rate of decline in pathogen removal given the resolution of both direct and indirect integrity monitoring should inform policy on awarding pathogen removal. In Chapter 4, we applied Quantitative Microbial Risk Analysis (QMRA) to examine the probability of infection by Cryptosporidium given that fibers may break on a daily basis. This later examination uses an uncertainty analysis of the levels of oocysts in source water bin classifications. The probability of fiber breakage increases the probability of infection, by up to a factor of 10 in the most contaminated waters, whereas the increased risk in less contaminated waters is negligible. This is the first application of Probability Bounds Analysis in a QMRA to determine the health risk of undetected fiber breakage. These results may inform policy on the frequency and resolution of integrity monitoring.
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