Providing effective and sustainable water disinfection methods, without harmful by-products, is essential to protect public health and safety. It is hypothesized that the application of liquid-thin-film (LTF) technology can enhance the bactericidal activity of both ultraviolet (UV) irradiation and ozone (O3) treatments. Therefore, this study aimed to examine the bactericidal activity and synergistic effect of the combined UV + O3 + LTF treatment against Escherichia coli in water processing. The results showed that LTF technology significantly increased the disinfection efficiency of UV and O3 in potable water production. Thus, the combined UV + LTF, O3 + LTF, and UV + O3 + LTF treatments were more effective than the individual treatments under identical conditions. Particularly, the combined double-UV (41.7 mJ cm−2) + O3 (100% O3) + LTF (65 L min−1) treatment exhibited the highest bactericidal activity, resulting in a 5.8-log decrease in the E. coli load within 30 min. Pearson's correlation analysis demonstrated a significant correlation between the synergistic effects of the combined treatments and UV dosage (r = 0.88, p < 0.0001) and aqueous O3 concentrations (r = 0.84, p < 0.0001). These findings suggest that combined UV + O3 + LTF treatment could be a viable alternative method for water disinfection.
Liquid-thin-film (LTF) enhances the bactericidal activity of UV rays and O3 disinfectant.
The combined UV radiation, ozone, and LTF had synergistic bactericidal effects.
UV + O3 + LTF combined treatment had the highest bactericidal activity against E. coli.
E. coli were susceptible to UV + O3 + LTF at a low chemical dosage and short exposure time.
Currently, ∼844 million people lack access to clean water, particularly residents of developing countries (WHO and UNICEF 2017). In Vietnam and most developing countries, large amounts of untreated domestic wastewater are discharged into natural water bodies, which is an environmental and health risk. There has been an increase in the incidence of water-borne diseases, such as cholera, typhoid fever, dysentery, hepatitis A, and intestinal infections, owing to the contamination of drinking water by pathogenic microbes, such as Vibrio cholera, Salmonella typhi, Shigella spp., Ascaris lumbricoides, Cryptosporidium parvum, Schistosoma, and enteropathogenic Escherichia coli (WHO and UNICEF 2017). Therefore, removing potentially harmful chemical, microbiological, or physical contaminants through water treatment techniques is essential to improve the quality of drinking water. Conventionally, water is treated using a multibarrier treatment process consisting of pre-oxidation, flocculation, sedimentation, filtration, and disinfection. Disinfection is the final step in drinking water processing to prevent water-borne disease transmission.
Presently, there are emerging concerns regarding disinfection methods for drinking water treatment. An ideal disinfection method should be inexpensive, simple to implement, and able to rapidly and effectively eliminate numerous microbes without generating harmful by-products. Conventional disinfection methods include chemical and physical treatment. Although chemical disinfection remains a widely investigated approach, the application of this method may have problems related to the generation of potentially harmful disinfection by-products (DBPs). For example, chlorination is a widely used method for water and wastewater disinfection; however, this method may produce potential carcinogenic agents, such as trihalomethanes (THMs), haloacetic acids (HAAs), and bromate (Boorman et al. 1999; Srivastav et al. 2020). Compared with chlorine (standard redox potential E0 = 1.36 V/NHE), ozone (O3) is a strong oxidizing agent (E0 = 2.07 V/NHE). Ozone can effectively inactivate several microbes, including bacteria, viruses, spores, cysts, and Giardia, and Cryptosporidium cysts (Cho et al. 2003; Von Gunten 2003; Pichel et al. 2019), making it a suitable alternative to chlorination. In water disinfection, the normal concentration of ozone application is in the range of 5–10 mg L−1 (Huang et al. 2003; Blatchley et al. 2012). However, ozone must be produced on-site owing to its instability and low solubility in water (14 mmol L−1 at 20 °C), and it is difficult to determine the necessary ozone oxidizing dose (Cho et al. 2003). Hence, this method is expensive and has high energy requirements for industrial applications. Additionally, although high ozone doses can improve disinfection efficiency, they can also promote the formation of DBPs, especially bromate formation in water containing bromide (Huang et al. 2003; Von Gunten 2003). Recently, physical disinfection methods, such as ultrasound, ultra-high pressure, high-voltage pulsed electric fields, plasma technology, cavitation, and UV radiation, have attracted increasing research attention for water treatment (Tsolaki & Diamadopoulos 2010; Kheyrandish et al. 2017; Pichel et al. 2019; Gorito et al. 2021). Although physical disinfection does not generate residual toxicity, these methods have substantial energy consumption and high operating costs (Tsolaki & Diamadopoulos 2010). Therefore, it is important to develop advanced and sustainable methods for water disinfection. To date, several studies have evaluated the possibility of combining different water disinfection methods to improve water quality and obtain synergistic benefits (Lubello et al. 2002; Koivunen & Heinonen-Tanski 2005; Magbanua et al. 2006; Jung et al. 2008; Blatchley et al. 2012; Dang et al. 2016; Giannakis et al. 2016; Gorito et al. 2021); one such potential approach is the combination of ultraviolet (UV) radiation and O3 treatment.
UV radiation has been extensively applied in food sterilization and water disinfection because of its broad-spectrum disinfectant ability and non-residual toxicity (Hijnen et al. 2006; Bowker et al. 2011; Kheyrandish et al. 2017). However, the implementation of UV is limited because of the presence of UV-absorbing compounds and particles in water, which inhibit UV light transmittance into bacterial cells, thereby preventing total bacterial inactivation (Tsolaki & Diamadopoulos 2010; Farrell et al. 2018). An alternative approach is the combined application of UV light and oxidants, such as O3, peracetic acid (PAA), chlorine, and hydrogen peroxide (H2O2) (Lubello et al. 2002; Koivunen & Heinonen-Tanski 2005; Giannakis et al. 2016). Lubello et al. (2002) reported that the combined PAA (2 mg L−1)/UV (120 mW s cm−2) treatment method achieved improved disinfection efficiency with 3.6-log reduction in E. coli load, whereas only 2.8-log and 3.0-log reduction, respectively, were achieved by the two individual treatments (PAA and UV). In contrast, combined H2O2/UV (120 mW s cm−2) disinfection at low H2O2 dosages did not significantly enhance UV effectiveness; only at high H2O2 dosages (>20 mg L−1), a slight increase in disinfection efficiency (∼1 log) was observed by the combined H2O2/UV treatment (Lubello et al. 2002). Koivunen & Heinonen-Tanski (2005) studied the disinfection efficiencies of PAA, H2O2, and sodium hypochlorite (NaOCl) against E. coli, Enterococcus faecalis, Salmonella enteritidis, and coliphage MS2 viruses and found that combined PAA/UV disinfection achieved significantly higher bactericidal activity than other methods. Additionally, Giannakis et al. (2016) observed an increase in the disinfection efficiency of UV irradiation with the addition of H2O2. Despite the increased disinfection effect of the combined treatments, the use of high-dose chemical agents may induce the generation of DBPs.
Recently, liquid-thin-film (LTF) technology, which involves a liquid-film-forming apparatus (LFFA), has been extensively studied in water and wastewater treatment and various other applications because of its specific properties (Imai & Zhu 2011; Vo et al. 2014; Dang et al. 2016; Imai & Dang 2017; Nguyen et al. 2018). LTF is known to enhance the contact area between gas and water, provide relatively long-term durability in water, and facilitate gas dissolution in water (Imai & Zhu 2011; Nguyen et al. 2018). A number of studies have reported that the application of LTF can improve water disinfection. For instance, Vo et al. (2014) reported that an LTF of pressurized carbon dioxide (CO2) at 0.7 MPa and room temperature inactivated more than 3.3-log bacteriophage Qβ and ∼3.0-log bacteriophage ΦX174 within 25 min. Imai & Dang (2017) employed an LTF of pressurized CO2 in the treatment of sea ballast water and succeeded in eliminating E. coli within 3 min under identical treatment conditions (0.7 MPa, 20 °C, 25 L min−1, 50% WVR, ΔP = 0.12 MPa, and 15 cycles of pressure cycling). Dang et al. (2016) reported that using an LTF of pressurized CO2 and sodium hypochlorite showed efficient performance in the inactivation of Enterococcus sp. in seawater. Additionally, Dang et al. (2020) found that the use of the combined LTF (2,400 L min−1) and UV (4.53 × 10−18 mJ cm−2) resulted in a ∼95% reduction in E. coli population within 75 min, whereas only 32% of the E. coli load was reduced by UV treatment alone at the same UV dosage. Though LTF has greatly improved the bactericidal activity of UV light; with the design of a non-submerged UV lamp system, low UV dosage (i.e., 4.53 × 10−18 mJ cm−2), and high flow rate (i.e., 2,400 L min−1), the disinfection efficiency remains low and has not fulfilled the requirement of National Technical Regulation QCVN 01-1:2018/BYT on the quality of water supplied for domestic purposes (E. coli level should not exceed 1 CFU 100 mL−1) (Dang et al. 2020). Studies are yet to examine whether using LTF combined with UV and a small amount of O3 in water treatment could improve disinfection efficiency.
It is hypothesized that the use of LTF can substantially improve the solubilization of ozone in water, thereby, enhancing its bactericidal activity. Additionally, UV radiation at 254 nm may induce the formation of free radicals (i.e., •OH; E0 = 2.8 V/NHE) from photolytic ozone decomposition, which could further enhance the bactericidal effect of the treatments (Chin & Bérubé 2005; Jung et al. 2008). Furthermore, it is expected that an increase in the interfacial contact efficiency between gas and water caused by LFFA may help increase UV ray scattering. The bactericidal effect of UV is relatively dependent on UV transmittance. Therefore, the combination of UV and LTF may promote UV activity and increase its bacterial inactivation efficiency (Dang et al. 2020). Chin & Bérubé (2005) reported that a combined O3/UV advanced oxidation process reduced the DBPs’ formation potentials (∼80% THMs and 70% HAAs). However, since limited information is available on the effect of LTF technology on the bactericidal performance of combined UV/O3 treatments, it is unclear whether combined UV/O3 treatment could be an effective primary disinfectant.
Therefore, this study aimed to evaluate the bactericidal efficacy and synergistic effects of combined UV radiation, O3, and LTF treatment against E. coli in contaminated water. The sensitivity of the bacteria to the combined UV + O3 + LTF treatment was investigated under various UV dosages and ozone supply rates. Additionally, the bactericidal effects of UV radiation and O3 were examined and compared in both combination and individual treatments, with and without LTF technology.
MATERIALS AND METHODS
The E. coli (ATCC 11303) inoculum was prepared by inoculating the bacterial stock (American Type Culture Collection, Manassas, VA, USA) in 100 mL of Luria-Bertani (LB) broth (Wako Chemical Co. Ltd, Osaka, Japan). The E. coli culture was incubated at 37 °C with shaking at 150 rpm for 20 h, and the permanent stock was preserved in 30% glycerol at –60 °C.
For each disinfection experiment, 100 μL of E. coli culture glycerol stock was transferred to 100 mL of LB broth and incubated at 37 °C under continuous shaking at 150 rpm for 20 h. Bacterial biomass was then centrifuged for harvesting, and rinsed with physiological saline solution three times (10 min at 10,000 rpm at 4 °C) using a refrigerated centrifuge (Allegra X-30R, Beckman Coulter, Inc, USA). The pellet was then re-suspended in saline solution. The E. coli culture was immediately used to prepare artificial micro-polluted water samples.
Preparation of artificial micro-pollution water sample
Disinfection experiments were performed using an artificial micro-polluted water sample. The water sample was prepared by adding the bacterial culture to tap water. Tap water was obtained from the local water supply of Hue City (HueWACO Co., Ltd). The basic water quality parameters of the tap water were as follows: total iron, 0.01 mg L−1; Mn, 0.001 mg L−1; turbidity, 0.02–0.1 NTU; pH, 7.0–7.5; hardness, 22 mg CaCO3 L−1; nitrite, 0.003 mg N-NO2 L−1; and residual chlorine, 0.5–0.6 mg L−1. For all experiments, a 0.02 M solution of sodium thiosulfate pentahydrate (Na2S2O3·5H2O; Wako, Japan) was added to the tap water to completely quench residual chlorine. The N,N-diethyl-p-phenylenediamine (DPD; HI701-0 free chlorine reagent, Hanna Instruments SRL, Romania) colorimetric method with an ion-specific meter (HI701, Hanna Instruments SRL, Romania) was employed for chlorine content determination (to confirm residual chlorine disposal). The samples were continuously aerated for 60 min under UV irradiation to completely remove the remaining thiosulfate (Ahmad et al. 2015). Thereafter, the prepared E. coli culture was added to the water sample to achieve an initial bacterial concentration of 5–6 log10 CFU mL−1. The dissolved oxygen (DO) and temperature of the samples were measured using a DO meter (Pro 2030, YSI Incorporated, USA).
The aqueous ozone concentration of the samples was immediately measured using the DPD colorimetric method (HI 93757-0 ozone reagent, Hanna Instruments SRL, Romania) with a checker disc 38054 (HI38054, Hanna Instruments SRL, Romania) in the range of 0–2.3 mg L−1 with the smallest increment of 0.1 mg L−1. The gas ozone concentration produced by the ozone generator equipment was detected using the double-beam UV absorption method (UVO3-2000S, Shenzhen O3 Tech Co., Ltd, China) with an ozone analyzer flow (RS 485, Shenzhen O3 Tech Co., Ltd, China).
E. coli colonies were evaluated using the spread-plate technique. Briefly, a series of 10-fold dilutions were prepared using sterile saline (0.85% NaCl), and 100 μL of either diluted or undiluted samples was plated on the surface of Chromocult® Coliform media plates (Merck & Co., Inc., Darmstadt, Germany). The inoculated medium was incubated for 24 h at 37 °C. Dark blue colonies suspected to be E. coli were counted on each plate containing 25–300 CFU, and data were reported as the mean number of CFU mL−1. In the case of undiluted samples with less than 30 CFU/plate, the E. coli colonies were counted using the pour technique (1 mL of the sample was poured into coliform agar, which was maintained at 45 °C). After incubating the plates for 24 h at 37 °C, the dark blue colonies were counted. Each sample was examined three times.
Apparatus and procedure for UV, UV + LTF and UV + O3 + LTF disinfection experiments
In the combined UV + LTF disinfection experiments, 105 L of water was introduced into the apparatus, and the UV lamp system and LFFA were simultaneously switched on. The water pump (150 W, AP5400, LifeTech, Guangdong Zhenhua Electrical Appliance Co., Ltd, China), which was connected to the LFFA (FB-50h, Japan), was operated at a flow rate of 65 L min−1. During a treatment period of 75 min, the UV intensities were 20.8 and 41.7 mJ cm−2, corresponding to one and two UV lamps (single-UV and double-UV), respectively. For UV treatment alone, the LFFA and pump were removed from the experimental apparatus.
In the experimental setup for ozone application, 105 L of sample water was fed into the apparatus. Ozone was then introduced into the water by the ozone generator (at a gas flow rate of 1.0 L min−1) through the outlet tube to a stone diffuser (19 mm × 30 mm, Youmo Aquapure, China) to create ordinary ozone bubbles in the water. Disinfection was conducted at various ozone supply ratios (25% O3 + 75% air, 50% O3 + 50% air, 75% O3 + 25% air, and 100% O3) for 75 min.
In the combined O3 + LTF treatments, ozone gas was fed into the fluid at a flow rate of 1.0 L min−1. The O3 generator was connected to an LFFA (FB-50h, Japan) at a water flow rate of 65 L min−1 to create fine ozone bubbles and LTF. The sensitivity of bacteria to combined O3 + LTF treatments was determined at various ozone supply ratios (25, 50, 75, and 100%), which were applied for 75 min.
In the combined UV + O3 + LTF treatments, an appropriate ozone dosage was fed into the reactor via the LFFA to produce fine O3 bubbles and LTF and to facilitate water circulation. The UV lamp system was started at the same time as O3 + LTF. The remaining experiments followed the combined O3 + LTF method described above. To investigate regrowth, the treated samples were analyzed after 3 d of storage in the dark at room temperature (25.3–25.5 °C), and the reactivation potential was determined using the plating method.
Treated water was collected from the six valves of the reactor at different timepoints (0, 5, 10, 15, 20, 25, 30, 45, 60, and 75 min) (Figure 1). The bacterial concentrations were determined as described above. The experiments were repeated thrice.
Presentation of results
Disinfection efficiency was assessed by the log10 of the reduction ratio from the colony number before and after disinfection. The synergistic bactericidal effect of the combined UV + O3 + LTF treatment was calculated using the following formula (Dang et al. 2016):
Synergy value (log units) = log reduction caused by the combined UV + O3 + LTF – (log reduction caused by UV alone + log reduction caused by O3 alone + log reduction caused by LTF alone).
Following this formula, a positive value indicates synergistic benefit because the combined treatment efficiency is greater than the summed efficiency of the individual treatments. In contrast, a negative value indicates antagonism. A zero value indicates no synergy because the combined treatment efficiency is equal to the summed efficiency of the individual treatments.
All statistical analyses were performed using the R program (version 4.0.5, http://cran.R-project.org). Pearson's correlation coefficient was performed to access the relationship between synergy values and other variables such as UV dosage and dissolved O3 concentrations, and statistical significance was set at 5% (p < 0.05).
RESULTS AND DISCUSSION
Bactericidal activity of the UV treatment and combined UV + LTF treatment under different UV dosages
The results of the present study showed that the combined UV + LTF treatment yielded significantly higher E. coli inactivation efficiency than UV irradiation alone at all UV doses (Figure 2). Approximately 2.5-log and 3.5-log decreases in the E. coli count were obtained within 75 min at equivalent UV dosages of 20.8 and 41.7 mJ cm−2, respectively. However, combined single-UV + LTF treatment caused an approximately 3.6-log decrease in the E. coli count within 75 min. Additionally, the highest decrease in the bacterial load (>5.5 log) was achieved by the double-UV + LTF combined treatment, resulting in complete inactivation of E. coli within 60 min. In contrast, E. coli was not inactivated by LTF (65 L min−1) in the absence of UV light. These results indicated that LTF significantly enhanced the UV disinfection efficacy. Particularly, the combined UV + LTF treatments yielded synergistic benefits. Pearson's correlation analysis indicated a significant positive correlation between the equivalent UV dosage and synergy value (r = 0.88, p < 0.0001). Accordingly, a 1.1-log average synergistic value was achieved within 75 min by single-UV + LTF combined treatment, and a 2.0-log average synergistic value was obtained within 60 min by double-UV + LTF combined treatment. These results indicated that a combination of high UV dosage and LTF induced a higher synergistic bactericidal effect against E. coli within a short exposure time.
The bactericidal activity of UV radiation is dependent on the physical light adsorption process. However, the adsorption of UV irradiation by water is limited by impurities, thus limiting its bactericidal activity (Tsolaki & Diamadopoulos 2010; Farrell et al. 2018). In this study, it could be speculated that the combined treatment improved contact between UV radiation and water, which facilitated bacterial cell penetration by UV radiation and improved UV bactericidal activity (Dang et al. 2020).
Bactericidal effect of ozone treatment and combined O3 + LTF treatment against E. coli in water
Furthermore, O3 treatment alone exhibited considerable disinfection efficiency against E. coli, with 100% O3 treatment inducing a 1.8-log decrease in the E. coli count, followed by 75% O3 treatment (1.4-log decrease), 50% O3 treatment (0.4-log decrease), and 25% O3 treatment (0.3-log decrease) (Figure 3(a)). In the case of the combined O3 + LTF treatment, there was a considerable increase in the disinfection efficiency against E. coli with increasing O3 supply rates. Additionally, there was a positive correlation between the O3 + LTF disinfection efficiency against E. coli and the O3 concentration of the water (r = 0.81, p < 0.0001). Particularly, O3 + LTF combined treatment exhibited higher bactericidal activity against E. coli than O3 treatment alone at all O3 supply rates, with 100% O3 + LTF treatment inducing a 3.4-log decrease in E. coli count after 75 min, followed by 75% O3 + LTF (3.2-log decrease), 50% O3 + LTF (0.5-log decrease), and 25% O3 + LTF (0.4-log decrease) (Figure 3(b)). Furthermore, the O3 + LTF combined treatment exhibited synergistic effects against E. coli at 25, 50, 75, and 100% O3 supply rates, with average synergy values of 0.1, 0.1, 1.8, and 1.5 log, respectively, within 75 min. Moreover, there was a significantly positive correlation between the dissolved O3 concentrations and synergy values (r = 0.84, p < 0.0001), indicating that the synergistic effect of the combined O3 + LTF treatment depended on the dissolved O3 level of the water.
The low disinfection efficacy of O3 treatment alone confirmed that O3 has poor solubility in water (Cho et al. 2003). In this study, LTF technology significantly increased the disinfection efficiency of O3 in water. The use of LFFA allows the conversion of all liquids into LTFs and promotes water movement in the system, which increases the gas–liquid contact area by the interior interface (gas bubbles and liquid film) (Imai & Zhu 2011; Imai & Dang 2017). It is assumed that LTF technology can considerably improve O3 solubility in water because of the larger surface area and longer residence time of the gas bubbles in water (Imai & Zhu 2011; Nguyen et al. 2018), which enhance O3 diffusion into bacterial cells. O3 is an extremely powerful disinfectant, mainly because of its ability to oxidize substances in bacterial cell membranes. When O3 penetrates the phospholipid layer, it can damage the cell wall structure (Ding et al. 2019) and destroy the membrane surface (Wu et al. 2018; Ding et al. 2019).
Bactericidal activity of combined UV + ozone + LTF treatments against E. coli in water
Furthermore, the bactericidal activity of the combined treatments against E. coli was significantly influenced by both the UV dosage and O3 supply rate (Figure 4(a) and 4(b)). Particularly, single-UV + 100% O3 + LTF combined treatment caused a 5.1-log decrease in the E. coli count, followed by single-UV + 75% O3 + LTF (4.4-log decrease), single-UV + 50% O3 + LTF (4.1-log decrease), and single-UV + 25% O3 + LTF, with the lowest efficacy (3.9-log decrease). The results showed that higher O3 ratios enhanced the bactericidal efficacy of the combined UV + O3 + LTF treatment, confirming the findings reported in the previous section. A similar relationship was observed between the O3 supply rate and disinfection efficiency of the combined double-UV + O3 + LTF treatment, although the highest inactivation efficiency and synergistic benefits were observed at a UV dosage of 41.7 mJ cm−2 (double-UV lamps, Figure 4(b)). The treatment period required for the complete inactivation of E. coli decreased with increasing O3 percentage (60 min at 25% O3, 60 min at 50% O3, 45 min at 75% O3, and 30 min at 100% O3).
Notably, UV + O3 + LTF combined treatments exhibited synergistic benefits against the pathogen at all UV and O3 dosages (Supplementary material, Table S1). Particularly, the synergy values of the single-UV + O3 + LTF combined treatment under 25, 50, 75, and 100% O3 conditions were 1.1, 1.2, 0.5, and 0.7 log within 75-min treatment, whereas those of double-UV + O3 + LTF combined treatment were 1.5 (25% O3) and 1.6 (50% O3) log after 60 min, 2.0 log (75% O3) after 45 min, and 3.0 log (100% O3) within 30 min of treatment. These data suggest that higher UV dosages and O3 supply ratios with shorter exposure times or lower UV dosages and O3 supply ratios with longer exposure times can be applied to inhibit bacteria.
Furthermore, the O3 concentrations of water samples treated using the UV + O3 + LTF combined treatment and O3 ratios of 25, 50, 75, and 100% reached 0.1, 0.2, 0.3, and 0.6 mg L−1, respectively, within 5–15 min of treatment. However, the concentrations dropped below the detection limit (<0.1 mg L−1) after 30 min of treatment. Further experimentation after the treatment periods revealed no regrowth potential for E. coli in the dark after exposure to the double-UV + O3 + LTF combined treatment.
Jung et al. (2008) reported that combined O3 (2 mg L−1) and UV radiation (14 mJ cm−2) treatment exhibited synergistic bactericidal effects against Bacillus subtilis spores. Additionally, Blatchley et al. (2012) reported that combined UV254 (12.4 mJ cm−2) and O3 (2–3 mg L−1) treatment did not exhibit any synergistic effect against E. coli in wastewater; however, combined double-UV + LTF disinfection completely inactivated more than 5.0-log E. coli and exhibited 2.0-log synergy after 60 min. In addition, combined double-UV (41.7 mJ cm−2) + O3 (100% O3, 0.6 mg L−1) + LTF (65 L min−1) treatment reduced the treatment period to 30 min, with average synergy value of 3.0 log. These results confirm the bactericidal efficacy of the combined UV + O3 + LTF disinfection method, indicating its potential future application for water treatment.
Although the disinfection mechanisms of the combined UV + O3 + LTF treatment are yet to be fully elucidated, some hypotheses could be drawn. The disinfection efficacy of dissolved O3 depends on its rapid penetration of bacterial cell membranes, where it oxidizes material, resulting in cell rupture and death (Wu et al. 2018; Ding et al. 2019). Additionally, UV radiation at 254 nm can induce the formation of hydroxyl radicals (•OH, E0 = 2.80 V/NHE) from photolytic O3 decomposition, which may further contribute to the increase in inactivation rates (Chin & Bérubé 2005; Jung et al. 2008). The findings of this study revealed that the combined disinfection methods (UV + O3 + LTF) were effective against the pathogen, which could largely be attributed to the improved gas–water interaction efficiency as well as the water circulation caused by the LFFA's action, thus improving cell penetration by UV radiation and O3 efficacy against the bacteria. The better results obtained from the combined UV + O3 + LTF treatment against E. coli could be attributed to the better diffusion of UV light into water (Dang et al. 2020), better dissolution of O3 in water, and generation of hydroxyl free radicals (Chin & Bérubé 2005; Jung et al. 2008; Takahashi et al. 2021).
Moreover, increasing the contact between UV and E. coli may induce radiation damage to nucleic acids in the bacterial cells, leading to cell replication dysfunction, mutagenesis, irreversible damage, and cell death (Xu et al. 2018). UV radiation can cause lipid peroxidation and breakdown of protein-like materials, as well as genetic damage, at high doses, thereby altering the bacterial cell membrane (Xu et al. 2018). Unlike UV disinfection, the bactericidal activity of O3 is related to its oxidation of the constituents of the cell wall (such as proteins and amino acids), damage to cellular structures, release of intracellular substances (Ding et al. 2019), and destruction of the membrane surface (Wu et al. 2018; Ding et al. 2019). Overall, it can be concluded that the concomitant effects of UV dosage and O3 concentration increased the sensitivity of the bacterial cells to the combined UV + O3 + LTF treatment.
The findings of this study illustrated that LFFA significantly improved the disinfection efficiency of UV and O3 treatments individually and as a combined treatment. The bactericidal efficacy of the treatments and their synergistic effects were in the order of combined double-UV + O3 (100% O3) + LTF treatment > combined double-UV + LTF treatment > combined O3 (100% O3) + LTF treatment > double-UV alone > O3 (100% O3) alone. Additionally, the combined UV + O3 + LTF treatment exhibited considerably higher bactericidal efficacy than either UV or O3 treatment alone. The simultaneous application of UV (41.7 mJ cm−2), O3 (100% O3), and LTF (65 L min−1) reduced the bacterial load of E. coli by approximately 5.8 log within 30 min, while the sum of the log reductions due to the individual treatments was approximately 2.8 log. The synergistic effect of the combined treatment was attributed to the simultaneous enhancement of three main factors: UV radiation transmittance, aqueous ozone solubility, and hydroxyl radical formation.
In summary, the findings of this study highlight the bactericidal activity and synergistic effect of combined UV + O3 + LTF treatment and indicate that the method possesses a potential for use in future applications for water disinfection. However, further studies are necessary to elucidate the mechanism of DBP formation during disinfection and the possibility of applying this treatment to other kinds of microorganisms.
This work was supported by the Vietnam Ministry of Education and Training [grant number B2023–DHH–20]; Hue University [grant number DHH 2021–01–182]; GSGES Seeds Research Funding Program 2020-2021, Kyoto University (Japan); and Hue Tronic Company.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.