Synergistic effect of combined UV-LED and chlorine treatment on Bacillus subtilis spore inactivation

Highlights

• The combined UV-LEDs and chlorine systems on B. subtilis spore inactivation were evaluated.

• Significant enhanced spore inactivation were obtained in the combined systems.

• Role of hydroxyl radical formed from chlorine photolysis was investigated.

• UV-LEDs and chlorine disinfection were both accelerated after combined system treatment.

Abstract

An ultraviolet (UV)-based advanced oxidation process (AOP) for disinfecting water is introduced in this paper. This study aimed to evaluate the potential of UV light-emitting diodes (UV-LEDs)/chlorine AOP (UV/Cl₂) for Bacillus subtilis spore inactivation. Chlorine was combined with 265 and 280 nm LEDs (UV₂₆₅/Cl₂, UV₂₈₀/Cl₂) and investigated. The addition of 4.0 mg/L of free chlorine at pH 7.0 in the presence of 125 mJ/cm² of UV irradiation resulted in an additional 1.8-log reduction in UV₂₆₅/Cl₂ and 1.5-log reduction in UV₂₈₀/Cl₂. There was no observed enhancement in spore inactivation with the addition of a radical scavenger, t-BuOH, which indicated the role of ^•OH in the synergistic effect. To quantitatively evaluate the synergism, the primary treatment with UV/Cl₂ was followed by further UV or Cl₂ treatment. After UV/Cl₂ pretreatment at different pH levels, the 265 and 280 nm LEDs treatment enhanced an approximate 0.4–0.5-log reduction compared to UV only, and Cl₂ treatment enhanced an approximate 0.7–1.1-log reduction compared to Cl₂ only. In addition, at pH 7.0, in UV₂₆₅/Cl₂-Cl₂ and UV₂₈₀/Cl₂-Cl₂, the inactivation rate constant k increased by approximately 2 and 1.5 times, respectively. The CT for the lag phases (CT_lag) reduced to approximately 67 and 58%, respectively. Similar results were obtained at pH 7.5 and 8.0, and in the secondary effluent. The synergistic effect on spore inactivation suggested that the pathogen inactivation efficiency of sequential UV and chlorine disinfection processes, which are commonly applied, can be significantly enhanced by adding chlorine prior to UV treatment.

Introduction

Owing to its high efficiency against Cryptosporidium and Giardia and the non-formation of disinfection by-products (DBPs), ultraviolet (UV) treatment has developed rapidly and is a primary disinfection process in the United States and Europe (Brownell et al., 2008; Hijnen et al., 2006). Low-pressure (LP) and medium-pressure (MP) mercury lamps are commonly used as UV light sources. However, these have some shortcomings, for example, the limited design of uniform light distribution reactors that cause low levels of pathogen inactivation, have a shorter lifetime, and contain mercury, which is a hazardous environmental contaminant (Aoyagi et al., 2011). Therefore, over the last decade, UV-light emitting diodes (UV-LEDs), which are an emerging semiconductor technology, have been developed for a new source of UV disinfection. Many studies have demonstrated the effectiveness of pathogen inactivation in water by UV-LEDs emitting light of various wavelengths (Chevremont et al., 2012; Li et al., 2017; Lui et al., 2016; Oguma et al., 2013). However, UV disinfection is less effective at controlling some viruses, such as rotavirus, hepatitis A, or adenovirus (Battigelli et al., 1993; Eischeid et al., 2009; Li et al., 2009). Adenovirus, which is known as the most resistant microorganism, requires an UV dose of 186 mJ/cm² using low-pressure UV (LP UV) to achieve 4-log inactivation (EPA, 2006).

Chlorine is highly effective against viruses, especially at a lower pH. Less than 0.24 mg of Cl₂ min/L CT value could result in a 4-log reduction in adenovirus infectivity (Thurston-Enriquez et al., 2003). Thus, sequential and combined application of UV and chlorine disinfection processes have gained attention for providing a double protective barrier. Previous studies investigated the sequential disinfection with UV and chlorine in wastewater and reported a synergistic effect on the inactivation of heterotrophic bacteria, total coliforms, and the removal of antibiotics resistance genes (Wang et al., 2012; Y. Zhang et al. 2015). However, for Bacillus subtilis spore inactivation, Cho et al. (2006) observed no synergism.

UV-based advanced oxidation processes (AOP), such as UV/H₂O₂, UV/Cl₂, and UV/peroxydisulfate (UV/PDS), used as the disinfection process, have been studied in inactivation pathogens due to these high oxidation capabilities (Cho et al., 2011; Rattanakul and Oguma, 2016; Sun et al., 2016). By using UV/H₂O₂ AOP, the inactivation of E. coli, bacteriophages MS2, T4, and T7, and adenovirus were evaluated and significantly enhanced compared to UV alone due to formation of hydroxyl radicals (^•OH) (Bounty et al., 2012; Mamane et al., 2007). Sun et al. (2016) examined the UV/PDS AOP for water disinfection, and reported that the inactivation of E. coli and MS2 were enhanced, but of B. subtilis spore was slightly enhanced, and ^•OH showed the highest disinfection efficacy.

Recently, UV/Cl₂ AOP becomes attractive, because radical production from photolysis of hypochlorous acid/hypochlorite ions (HOCl/OCl⁻) is more efficient than from H₂O₂ due to higher molar absorption coefficient and quantum yields (Chuang et al., 2017; Watts and Linden, 2007). The UV/Cl₂ AOP produces both OH and chlorine radicals Cl. However, OH was found to dominate contaminant degradation, and OH concentrations were more than five times of Cl for the UV/Cl₂ AOP (Chuang et al., 2017). Zyara et al. (2016) reported that the UV/Cl₂ treatment was more effective in inactivation seventeen different coliphages than chlorine alone. Rattanakul and Oguma (2016) investigated the inactivation mechanisms of MS2 in the UV/Cl₂ treatment and found that the viral genome damage due to ^•OH oxidation led to synergistic effect on MS2 inactivation. In the UV/Cl₂ AOP, The UV wavelength influences chlorine absorption and pH shifts the fraction of chlorine species and then affects the production of radicals (Watts and Linden, 2007). Wang et al. (2017a) reported that 280 nm LEDs UV/Cl₂ is an efficient AOP for the degradation of carbamazepine. However, studies on the simultaneous application of UV-LEDs and chlorine for water disinfection are limited.

The objectives of this study were to (1) evaluate the enhanced inactivation efficiency of combined UV-LEDs and chlorine treatment processes compared to individual disinfection processes, and (2) examine the specific role of ^•OH in synergic effect. Considering that the emittance wavelength of 265 nm LEDs is close to the DNA absorption peak of 260 nm (Bolton, 2000), and the emittance wavelength of 280 nm LEDs can reduce the photoreactivation of E.coli (Li et al., 2017), 265 nm and 280 nm LEDs were used as UV sources. B. subtilis spores were selected as the model microorganism as they, are the common standard surrogate organisms for UV reactor validation in Europe (Bohrerova et al., 2006), and are highly resistant to chlorine (Cho et al., 2006). The implications of the combined UV-LED and chlorine treatment process for multiple-barrier disinfection approaches were also discussed.