Assessing Point of Use Water Treatment Technologies under Real-Use Conditions: The Field Challenge Test Technique

Date

2023-09-28

Authors

Zimmer, Camille

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Abstract

Point of use water treatment (POUWT) technologies can be the final and sometimes only barrier against waterborne illness in contexts where there is insufficient access to a safely managed on-premises water supply. Microbiological effectiveness of POUWT devices is currently evaluated under controlled laboratory conditions using water spiked with virus, bacteria, and/or protozoa or their surrogates to measure log10 reduction values or LRVs, in a process called challenge testing. However, laboratory-based POUWT challenge tests do not adequately assess microbe reduction under real-use conditions, thus omitting variations relative to factors such as user behaviours and water quality. The overall aim of this work was to develop a method with which POUWT technologies can be evaluated under real-use conditions, which we refer to as the field challenge test technique. To this end, we validated the use of probiotic Escherichia coli (E. coli Nissle, EcN) and S. cerevisiae (baker’s yeast) as field-appropriate, food-safe surrogates for pathogenic bacteria and protozoans, respectively. We implemented the innovative field challenge test technique using validated EcN and S. cerevisiae surrogates. In summer 2021, 144 one-on-one surveys were conducted of backcountry campers in the Juan de Fuca provincial park in British Columbia, Canada. The field challenge test consisted of spiking a 1 L sample of water with EcN and S. cerevisiae and requesting participants to treat the spiked water as they normally would, using their own POUWT device. Post-treatment water samples were enumerated in comparison to the original spike to calculate LRVs. Using field challenge testing, we were able to ascertain the performance of POUWT methods under real-use conditions. Our field-based LRVs were generally lower than claimed by POUWT device manufacturers for the bacterial microbe class, but for the protozoan microbe class, LRVs were similar to those claimed by manufacturers. Using the framework of quantitative microbial risk assessment (QMRA), we quantified and compared health risk estimates when using laboratory-gathered vs field-gathered LRVs of POUWT devices. Health risks attributable to the bacterial pathogen class were higher based on field-gathered LRVs (i.e., obtained by field challenge testing) in comparison to corresponding manufacturer-claimed LRVs.For the protozoan pathogen class, calculated health risks were similar due to homogeneity between field-obtained and manufacturer-claimed LRVs. The field challenge technique and corresponding QMRA analysis have numerous implications, including validation of POUWT sanitary inspection criteria, quantifying health impacts of contextual factors, or to inform technology selection.

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Keywords

Point of use water treatment (POUWT), quantitative microbial risk assessment (QMRA), microbiological challenge testing, drinking water, Saccharomyces cerevisiae, Escherichia coli

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