Photolysis of free chlorine to hydroxyl radical by sunlight and ultraviolet irradiation for enhanced disinfection of chlorine-resistant waterborne pathogens

University Of Washington, Seattle WA

Investigators

Abstract

1236303 Dodd Free available chlorine (FAC) remains the most widely used disinfectant in drinking water practice worldwide. It is cheap, easily utilized as a disinfectant, portable, and in general highly effective for achieving the inactivation of a wide variety of waterborne microbial pathogens. However, it is known to be relatively ineffective as a primary disinfectant of such important pathogenic agents as Cryptosporidium parvum, Mycobacterium avium, and Giardia lamblia, This has led to widespread adoption of more effective, but often substantially more capital-, equipment-, and energy-intensive alternative disinfectants such as UV light and ozone. Recent findings suggest that inactivation of chlorine-recalcitrant microbial pathogens may actually be achievable at considerably lower expense by utilizing sunlight or monochromatic UV light to photolyze FAC to such highly-reactive oxidant species as hydroxyl radical, atomic oxygen, and ozone during conventional chlorination. In such an approach, FAC and photochemically-generated oxidants may act in tandem to yield substantially greater inactivation of various waterborne pathogens than would be achievable using chlorine alone. This investigation will utilize a combination of chemical and microbiological tools to quantify inactivation of chlorine-resistant viral, bacterial, and protozoan pathogens during conventional chlorination processes enhanced by FAC photolysis. The primary objective of this work will be to evaluate the use of sunlight for photochemical enhancement of chlorination processes. However, the investigation will also focus on potential applications of monochromatic and polychromatic UV light sources, on account of their growing frequency of application in drinking water treatment. The project team will develop and optimize experimental and analytical procedures for quantifying pathogen inactivation during photochemically-enhanced chlorination by first utilizing two common surrogates for waterborne pathogens B. subtilis spores and MS2 bacteriophage, followed by the chlorine-resistant human pathogens M. avium, Coxsackievirus B5 (CVB5), and C. parvum. These procedures will subsequently be utilized to examine the influence of such critical parameters as pH, water temperature, alkalinity, and matrix oxidant demand on inactivation efficiency for each pathogen under simulated sunlight, natural sunlight, and various artificial UV light sources in buffered laboratory reagent water systems, as well as in real water matrixes acquired from municipal water utilities in the Puget Sound region. Particular emphasis will be placed on development of kinetic models for pathogen inactivation that take into account measured water quality parameters and spectral irradiance data. Finally, formation potentials of organic and inorganic DBPs likely to be generated during application of photochemically-enhanced chlorination (e.g., trihalomethanes, haloacetic acids, ClO3-, ClO4-, and BrO3-) will be quantified under a variety of scenarios relevant to full-scale application. In addition to establishing a theoretical framework for modeling chlorine-resistant pathogen inactivation during photochemically-enhanced chlorination, this project will provide an extensive dataset for M. avium, CVB5, and C. parvum inactivation under a wide variety of conditions applicable to full-scale water treatment (including variable irradiation wavelength, intensity, temperature, pH, and alkalinity). This research could support the photochemical augmentation of conventional drinking water chlorination processes at full-scale, with minimal equipment and process retrofit. Application of such an approach could have substantial benefits over existing alternatives to chlorine-based disinfection processes. First, chlorination is used in the vast majority of water treatment facilities. Second, utilization of solar radiation in particular as a light source could net significant cost and energy savings in comparison to processes utilizing ozone or artificial UV light. In addition, if a UV process is already in place at a facility, that process could quite easily be adapted for photochemically-enhanced chlorination simply by dosing FAC upstream of the UV reactor(s). Furthermore, sunlight-enhanced chlorination could prove exceptionally useful for ensuring disinfection of chlorine-resistant pathogens during point-of-use applications in developed and developing societies, on account of its expected low costs and ease of implementation.