Synergistic effect of combined UV-LED and chlorine treatment on Bacillus subtilis spore inactivation
Graphical abstract
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.
Section snippets
Spore preparation and enumeration
Spore suspensions of B. subtilis (ATCC 6633) were prepared as described by Rochelle et al. (2010). Briefly, B. subtilis cells were first inoculated into a nutrient broth to allow the growth of vegetative cells. After incubation for 24 h at 37 °C, 2 mL of the solution was transferred to a 200-mL modified sporulation medium supplemented with 0.1 mM of MnSO4. The spores were incubated for five days at 37 °C and 125 rpm, and then harvested by centrifugation at 4000 ×g for 15 min at 4 °C.
Spore inactivation in the UV265/Cl2 and UV280/Cl2 processes
Spore inactivation by exposure to Cl2 only, UV only, and UV265/Cl2 and UV280/Cl2 treatment was compared (Fig. 1). Treatment with Cl2 alone at the same exposure time as irradiation caused a log reduction below 0.1 log, which is negligible, however, the UV-LEDs could efficiently inactivate spores. The inactivation rate constants for UV265 and UV280 were 0.024 and 0.017 cm2/mJ, respectively. This suggests that 265 nm inactivated B. subtilis spores more efficiently than 280 nm, which is consistent
Conclusion
The results from this study demonstrated the beneficial effects of combined UV-LED and chlorine treatment processes on the inactivation of B. subtilis spores. The inactivation rate constant of the UV265/Cl2 and UV280/Cl2 treatment processes at an UV fluence of 125 mJ/cm2 were two times higher than those obtained during UV265 and UV280 treatment alone. The reactive radicals formed by UV/Cl2 treatment played a significant role in enhancing spore inactivation. The spore inactivation by UV/Cl2-UV
Acknowledgement
This study was supported by Key Program of the National Natural Science Foundation of China (No. 51738005), National Key R&D Program of China (No. 2016YFE0118800), and the Collaborative Innovation Center for Regional Environmental Quality, China.
References (52)
- et al.
Inactivation of adenovirus using low-dose UV/H2O2 advanced oxidation
Water Res.
(2012) - et al.
Effect of coupled UV-A and UV-C LEDs on both microbiological and chemical pollution of urban wastewaters
Sci. Total Environ.
(2012) - et al.
Investigating synergism during sequential inactivation of Bacillus subtilis spores with several disinfectants
Water Res.
(2006) - et al.
Investigating synergism during sequential inactivation of MS-2 phage and Bacillus subtilis spores with UV/H2O2 followed by free chlorine
Water Res.
(2011) - et al.
Sequential inactivation of Cryptosporidium parvum oocysts with ozone and free chlorine
Water Res.
(2000) - et al.
Comparison of low-and medium-pressure ultraviolet lamps: photoreactivation of Escherichia coli and total coliforms in secondary effluents of municipal wastewater treatment plants
Water Res.
(2009) - et al.
Inactivation credit of UV radiation for viruses, bacteria and protozoan (oo)cysts in water: a review
Water Res.
(2006) - et al.
Synergistic effect of sequential or combined use of ozone and UV radiation for the disinfection of Bacillus subtilis spores
Water Res.
(2008) - et al.
UV inactivation and resistance of rotavirus evaluated by integrated cell culture and real-time RT-PCR assay
Water Res.
(2009) - et al.
The removal of estrogenic activity with UV/chlorine technology and identification of novel estrogenic disinfection by-products
J. Hazard. Mater.
(2016)
Comparison of UV-LED and low pressure UV for water disinfection: photoreactivation and dark repair of Escherichia coli
Water Res.
Point-of-use water disinfection using ultraviolet and visible light-emitting diodes
Sci. Total Environ.
Inactivation of E. coli, B. subtilis spores, and MS2, T4, and T7 phage using UV/H2O2 advanced oxidation
J. Hazard. Mater.
The solar UV environment and bacterial spore UV resistance: considerations for Earth-to-Mars transport by natural processes and human spaceflight
Mutat. Res. Fundam. Mol. Mech. Mutagen.
Application of UV light emitting diodes to batch and flow-through water disinfection systems
Desalination
Inactivation of Cryptosporidium parvum oocysts with ozone
Water Res.
Studies on photokilling of bacteria on TiO2 thin film
J. Photochem. Photobiol. A
Synergistic effect of the sequential use of UV irradiation and chlorine to disinfect reclaimed water
Water Res.
Formation of disinfection by-products in the ultraviolet/chlorine advanced oxidation process
Sci. Total Environ.
Light-emitting diodes as an emerging UV source for UV/chlorine oxidation: carbamazepine degradation and toxicity changes
Chem. Eng. J.
Degradation of natural organic matter by UV/chlorine oxidation: molecular decomposition, formation of oxidation byproducts and cytotoxicity
Water Res.
Chlorine photolysis and subsequent OH radical production during UV treatment of chlorinated water
Water Res.
Application of GaN-based ultraviolet-C light emitting diodes–UV LEDs–for water disinfection
Water Res.
PPCP degradation by UV/chlorine treatment and its impact on DBP formation potential in real waters
Water Res.
UV/chlorine process for ammonia removal and disinfection by-product reduction: comparison with chlorination
Water Res.
Inactivation of antibiotic resistance genes in municipal wastewater effluent by chlorination and sequential UV/chlorination disinfection
Sci. Total Environ.
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