Potential of UV-B and UV-C irradiation in disinfecting microorganisms and removing N-nitrosodimethylamine and 1,4-dioxane for potable water reuse: A review

Highlights

• This review assessed the water-treatment potential of UV wavelengths beyond 254 nm.

• Disinfection efficiencies for viruses, protozoa, and bacteria were compared.

• Some unconventional UV lamps showed higher removals of NDMA and 1,4-dioxane.

• UV doses for CEC inactivation were higher than those for pathogen removal.

• AOP using 185 + 254 nm LP-Hg or 222 nm KrCl lamp could be the best alternatives.

Abstract

The ultraviolet (UV)-based advanced oxidation process (AOP) is a powerful technology for removing pathogenic microorganisms and contaminants of emerging concern (CECs) from water. AOP in potable water reuse has been predominantly based on traditional low-pressure mercury (LP-Hg) lamps at 254 nm wavelength, supplemented by hydrogen peroxide addition. In this review, we assessed the potential of unconventional UV wavelengths (UV–B, 280–315 nm and UV-C, 100–280 nm) compared to conventional one (254 nm) in achieving the attenuation of pathogens and CECs. At the same UV doses, conventional 254 nm LP-Hg lamps and other sources such as, 222 nm KrCl lamps and 265 nm UV-LEDs, showed similar disinfection capability for viruses, protozoa, and bacteria, and the effect of hydrogen peroxide (H₂O₂) addition on disinfection remained unclear. The attenuation levels of key CECs in potable water reuse (N-nitrosodimethylamine and 1,4-dioxane) by 185 + 254 nm LP-Hg or 222 nm KrCl lamps were generally greater than those by conventional 254 nm LP-Hg and other UV lamps. CEC degradation was generally enhanced by H₂O₂ addition. Overall, our review suggests that 222 nm KrCl or 185 + 254 nm LP-Hg lamps with the addition of H₂O₂ would be the best alternative to conventional 254 nm LP-Hg lamps for achieving target removal levels of both pathogens and CECs in potable water reuse.

Introduction

Municipal wastewater can contain various hazardous chemicals and pathogens, making the removal or attenuation of these constituents essential for public health. Potable water reuse, an effective strategy for augmenting fresh water supplies in locations affected by droughts or rapid urbanization, can be accomplished by highly purifying municipal wastewater (EPA, 2017). AWWT for potable water reuse typically involves a series of unit processes, such as MF or UF, RO, and AOP, with the synergetic use of UV and strong oxidants such as hydrogen peroxide (H₂O₂) (Pecson et al., 2015; Farré and Gernjak, 2021). Indirect potable water reuse involves transporting the highly purified wastewater to environmental buffers, such as groundwater aquifers and surface water reservoirs, in which dilution and additional attenuation of pollutants can occur before the water is used for potable purposes. In contrast, direct potable reuse involves transporting the purified wastewater to a drinking water source or supply without passing through an environmental buffer (WHO, 2017). Any process failures during AWWT for either indirect or direct potable water reuse can cause an adverse impact on the purified water's quality and consequently public health.

UV-based AOP acts as a final protective barrier against both microbial and chemical contaminants (Bernados, 2020), capable of readily achieving a 6-log reduction (i.e., 99.9999 % removal) in viruses, bacteria, and protozoa (Tchobanoglous et al., 2015). In addition, this plays an important role in attenuating CECs that are partially removed by RO process (e.g., NDMA and 1,4-dioxane) (Farré et al., 2010; Doederer et al., 2014; Fujioka et al., 2017) such that final water quality complies with guideline values, that is, notification levels of 10 ng/L and 1 μg/L for NDMA and 1,4-dioxane, respectively (DDW/CAEPA, 2015a, b). For example, UV-based AOP is required to achieve a 0.5-log reduction of 1,4-dioxane from RO permeate (CDPH, 2013). Although NDMA can be decomposed by direct photolysis with mid UV-C (220–260 nm), hydroxyl radicals (•OH) formed through AOP are required for the destruction of 1,4-dioxane. To date, most AOPs in potable water reuse have been achieved by H₂O₂ addition (3–5 mg/L) followed by UV photolysis using an LP-Hg lamp that emits 254 nm wavelength light (Collins and Bolton, 2016; Khan et al., 2017).

In recent years, many attempts have been made to improve the disinfection or chemical degradation efficiency and reduce the chemical usage of UV-based AOP in order to reduce the overall treatment cost. UV lamps with a wavelength other than the conventional 254 nm have the potential to enhance the disinfection efficiency or formation of •OH without chemical additions. For example, a krypton chloride (KrCl) lamp with a light emission wavelength of 222 nm has a higher disinfection efficiency than conventional LP-Hg and MP-Hg lamps (Kang et al., 2018). With regard to CECs, V-UV with wavelengths shorter than 200 nm can attenuate disinfection byproduct (e.g., haloacetonitriles) concentrations more effectively than 254 nm LP-Hg lamps (Kiattisaksiri et al., 2016). Nevertheless, these have been rarely utilized for water treatment and their applicability for the removal of various pathogens and CECs has not been established.

In this review paper, we evaluate the potential of UV lamps with unconventional UV wavelengths (UV–B, 280–315 nm and UV-C, 100–280 nm) to achieve adequate reductions in pathogens and CECs for reuse of potable water reuse, and also discuss ongoing challenges in utilizing such alternative UV lamps.