Free available chlorine (FAC) remains widely utilized as a disinfectant in water treatment due to its low cost, ease of use, and generally high efficacy. However, certain microorganisms (e.g., bacterial endospores, Cryptosporidium spp. oocysts, Giardia spp. cysts, Mycobacteria) exhibit considerable chlorine-resistance. This project has demonstrated that the effectiveness of chlorine-based disinfection processes can be significantly improved by employing sunlight-driven chlorine photolysis to generate the potent oxidants hydroxyl radical (^•OH) and ozone (O₃) in situ (Figure 1). This approach was found to accelerate the inactivation of various chlorine-resistant microorganisms, including Bacillus subtilis endospores, C. parvum oocysts, M. avium, and Coxsackievirus B5 (CVB5). For example, the cumulative FAC exposure (CT_FAC, in mg/L*min) required to achieve 99% (2-log₁₀) inactivation of B. subtilis endospores could be lowered by more than two-thirds by exposing an initial 8 mg/L concentration of FAC to continuous solar irradiation at pH 8 and 10 °C (Figure 1). No spore inactivation was observed during exposure to equivalent levels of solar irradiation in the absence of FAC. Solar chlorine photolysis was found to yield roughly 90% (1-log₁₀) inactivation of C. parvum oocysts under similar conditions, though this could be increased to as much as 99.9% (3-log₁₀) by doubling irradiation time from 60 to 120 minutes (with a corresponding doubling of CT_FAC). Only ~1/4 of the observed oocyst inactivation was attributable to direct inactivation by sunlight, and no inactivation of oocysts was observed during exposure to FAC alone. The enhancement effects observed during solar chlorine photolysis were found to be attributable to varying degrees to a combination of (a) direct inactivation by O₃ and/or FAC, and (b) sensitization of microbes to FAC and/or O₃ by co-exposure to O₃ and hydroxyl radical (^•OH), respectively.

This work has led to the generation and compilation of an extensive dataset of rate constants and treatment requirements for achieving inactivation of B. subtilis spores, C. parvum oocysts, M. avium, and CVB5 by solar chlorine photolysis under a wide variety of conditions (including variable solar irradiance, temperature, pH, chlorine concentrations, oxygen concentrations, and dissolved organic matter (DOM) concentrations). On the basis of these data, a coupled photochemical/disinfection model was developed to enable prediction of O₃ and ^•OH formation and consequent microbial inactivation during FAC photolysis, then validated using experimental measurements of oxidant concentrations and B. subtilis spore, C. parvum oocyst, and M. avium inactivation rates during treatment under various solution conditions.

Additional work by the project team showed that – compared to dark chlorination – solar photolysis of FAC in reagent waters or natural water samples generally led to higher yields of organic and inorganic disinfection byproducts – including trihalomethanes (THMs), haloacetic acids (HAAs), chlorate (ClO₃^–) and bromate (BrO₃^–), with DBP yields increasing with irradiation time. This appears to be due to modifications of DOM by O₃, ROS, and/or RHS (predisposing precursor materials to higher DBP formation in reactions with FAC), as well as direct formation of DBPs through reactions of precursors with O₃, ROS, and/or RHS (Figure 2). However, experiments undertaken over a wide range of water chemistries illustrated that under appropriate conditions (e.g., moderate dissolved organic carbon levels, low bromide levels, low carbonate levels), DBP yields could be maintained at or below regulatory limits. Formation of chlorite (ClO₂^–) was only observed under select conditions in the absence of oxygen, while perchlorate (ClO₄^–) was not observed under any conditions.

Research findings from the project have been communicated to technical and non-technical audiences through peer-reviewed publications in leading journals, oral and poster presentations at international conferences, and outreach to the public through regular K-8 open house events hosted by the UW College of Engineering (see Figure 3 for open house photos).

Overall, this work could have important implications for disinfection practice in low-resource settings. For example, the combination of chlorination with the widely accessible practice of solar disinfection (SODIS – typically applied in UVA-transparent plastic bottles) could provide a simple, practical means of improving inactivation of chlorine-resistant waterborne pathogens such as Cryptosporidium oocysts and Giardia cysts in point-of-use treatment (Figure 4). The project findings could also have significant implications for preventing disease transmission in chlorinated outdoor swimming pools, as a large proportion of gastroenteritis outbreaks arising from fecal contamination of pools are attributable to Cryptosporidium oocysts.

In addition to contributing to advancement of scientific knowledge and technology development, the project supported the professional development of an early career investigator (the PI), three graduate students (two UW PhD students and one visiting PhD student), and ten undergraduate students (six from the UW, and four from outside universities). The project also provided a total of twenty-five Seattle-area high school teachers (Figure 5) with the opportunity to participate in hands-on experimental work through a summer workshop program focusing on inquiry-based curriculum development in collaboration with the project team (for an overview, see: http://faculty.washington.edu/doddm/Personal/WaterWorks.html).

Last Modified: 11/29/2017
Modified by: Michael Dodd