Comparison of inactivation characteristics between Gram-positive and Gram-negative bacteria in water by synergistic UV and chlorine disinfection

Chlorination-induced spread of antibiotic resistance genes in drinking water systems

Chlorine, the most widely utilized disinfectant for drinking water globally, has recently been implicated in facilitating the spread of antibiotic resistance genes (ARGs), raising concerns about its underestimated environmental and ecological risks. However, given the current fragmented research focus and results, a comprehensive understanding of the potential mechanisms and influencing factors behind chlorination-promoted ARGs transmission in drinking water systems is crucial. This work is the first to systematically review the variations in abundance, transmission mechanisms, influencing factors, and mitigation strategies related to ARGs during the chlorination process. The results indicated that chlorination could induce genetic mutations and promote horizontal gene transfer through multiple pathways, including increased reactive oxygen species, enhanced membrane permeability, stimulation of the SOS response, and activation of efflux pumps. In addition, this work delves into significant discoveries regarding the factors affecting ARG transmission in drinking water, such as chlorine concentration, reaction time, disinfection byproducts, pipe materials, biofilms, and the water matrix. A series of effective strategies from water source to point-of-use were proposed aimed at mitigating ARGs transmission risks in the drinking water system. Finally, we address existing challenges and outline future research directions to overcome these bottlenecks. Overall, this review aims to advance our understanding of the role of chlorination in the dissemination of ARGs and to inspire innovative research ideas for optimizing disinfection techniques, minimizing the risks of antibiotic resistance transmission, and enhancing the safety of drinking water.

Identification of correlation relationships and establishment of regression models among multiple microbial indicators in reclaimed waters

Monitoring and controlling microbial water quality is crucial for ensuring water reuse safety. In particular, existing water reuse guidelines and regulations normally prescribed coliform bacteria as microbial indicators. However, the use of non-unified coliform groups may bring difficulties to compare and optimize the conformity efforts on microbial surveillance. This study has identified the correlation relationships in each pair of four microbial indicators in reclaimed waters, namely the heterotrophic plate counts (HPCs), total coliforms (TC), fecal coliforms (FC) and E. coli (r = 0.861-0.987). Ultimately, the built regression model for internal conversion is expressed as: log10HPC (MPN/mL) = 0.737 × log10TC (MPN/L) = 0.830 × log10FC (MPN/L) = 0.872 × log10E. coli (MPN/L) with further verification and validation. The developed model can be used to help water reuse regulators and practitioners improve the efficiency in universal microbial risk detection and management. Besides, the resistant microbes in HPCs (e.g. disinfection resistant bacteria and pathogens) after reclaimed water treatment and disinfection also call for future attention.

Spatio-temporal variation of microbial indicators of river water and treatment efficiencies of drinking water treatment plants along the upper Mahaweli river segment of Sri Lanka

The microbial pollution status of river surface water is important to ensure a river-based quality drinking water supply for the public. The present study aimed to investigate bacterial contamination status in the upper Mahaweli River, the main drinking water supplier to the hill country of Sri Lanka. Both the raw surface water and treated water, taken at 14 drinking water treatment plants (DWTPs) along the river segment of 60 km between Kotmale and Victoria reservoirs, were tested for total bacterial counts (TBC), total coliform counts (TCC) and faecal coliform counts (FCC). Irrespective of the flow, the selected river segment showed significant contamination variations of TBCs, TCCs and FCCs between dry and wet seasons due to point source discharges and drains of septic waste. The level of pollution was enhanced in the wet season due to the diffusion of pollutants with the runoff. Locations of DWTPs could be clustered into 3 categories according to microbial contamination as very low (Kotagala, Thalawakelle-Galkanda, Kandy South, Pundaluoya, Ulapane, Greater Kandy), moderate (Nawalapitiya, Paradeka, Elpitiya, Nillambe, Balagolla, Haragama, University Plant, Polgolla) and high (Thalawakelle-Nanuoya). Although the bacterial reduction efficiencies of many DWTPs (10 out of 14) were higher than 77.9%, treated water samples from 10 DWTPs exceeded the maximum TBC recommended by the US Environmental Protection Agency (2.70 log10 CFU/mL) for drinking water, during wet season due to high bacterial load in the surface water. Isolates from treated water were Gram-positives with Staphylococci (74.3%) and Micrococci (25.7%). Total coliforms and faecal coliforms were absent in treated water satisfying the SLS 614:2013 guidelines for drinking water. The disinfection process of DWTPs should be improved to ensure the safety of drinking water, particularly during the wet season. Incorporation of more microbiological parameters into routine testing practices is important in understanding bacterial load and diversity to improve treatment strategies.

Experimental study on the effect of light source arrangements on the disinfection performance of upper-room 222 nm Far-UVC

The air disinfection efficacy of upper-room 222 nm Far-UVC was experimentally investigated in a real-size chamber under well-mixed air conditions. Two bacteria (Escherichia coli, Staphylococcus epidermidis) and two bacteriophages (MS2, and P22) were selected for the test. The study considered different lamp source arrangements, including single and double sources, stationary and rotating operating modes, and an overlapping mode with a 45° irradiation angle. A numerical view-factor model was developed to analyze the irradiance distributions. Four irradiation angles, 30°, 45°, 60°, and 90°, were chosen. The results show that the lamps operating with an irradiation angle of 45° provide the highest chamber-averaged irradiance. This suggests an optimal irradiance level for a given room dimension, as inferred from the view factor model. Experimental results indicated that the overlapping mode with a 45° irradiation angle consistently outperformed both the stationary mode and rotating mode in disinfection. This can be attributed to the higher chamber-averaged irradiance, which is also supported by the numerical model predictions. The increment ratios ranged from 14.9 % to 42.9 % compared to the stationary mode. The susceptibility constants of Escherichia coli, Staphylococcus epidermidis, MS2, and P22 were measured as 0.572 m2/J, 0.099 m2/J, 0.060 m2/J, and 0.081 m2/J respectively.

Potential pathogenic microorganisms in rural wastewater treatment process: Succession characteristics, concentration variation, source exploration, and risk assessment

Pathogenic microorganisms can cause infection, sepsis, and other diseases in humans. Although municipal wastewater plants are important sources and sinks for potential pathogenic microorganisms, data on rural wastewater treatment processes are limited. The proximity of rural wastewater facilities to human settlements and the trend toward wastewater resourcing could pose risks to humans. Here, a typical village in southern China was selected to analyze potential pathogenic microorganisms in wastewater, sewage sludge, and aerosols during the collection, treatment, and discharge of domestic wastewater. The succession characteristics and concentration variations of potential pathogenic microorganisms throughout the wastewater treatment process were identified using high-throughput sequencing and culture methods. Bacteria-associated health risks in facility aerosols were estimated based on average daily dose rates from inhalation and dermal exposure. Lower amounts of pathogenic bacteria and pathogenic fungi were detected in the effluent of the 1-ton treatment scale and the 10-ton treatment scale facilities, compared to those in the influent. Pathogen effluent concentrations were significantly lower than influent concentrations after treatment in rural wastewater facilities. 16 and 29 potential pathogenic bacteria and fungi were detected in aerosols from wastewater treatment facilities, respectively. Furthermore, the potential pathogen concentrations were higher than those in the background air. Aerobic units are the main source of pathogen emissions from aerosols. There were 42 potential pathogenic bacteria and 34 potential pathogenic fungi in the sewage sludge. Biochemical units were the main source of potential pathogens in sewage sludge, and more potential airborne pathogens originated from wastewater. In rural wastewater resourcing processes with greater pollutant exposure, the effluent of rural wastewater treatment facilities (WWTFs), downstream rivers, and facility aerosols, could be important potential sources of microbial risk. Inhalation is the main pathway of human exposure to airborne bacteria. Therefore, more attention should be focused on microbiological risk in rural wastewater treatment processes.

Enhancing Escherichia coli Inactivation: Synergistic Mechanism of Ultraviolet Light and High-Voltage Electric Field

This study investigated the bactericidal effects of ultraviolet (UV) radiation, a high-voltage electric field (HVEF), and their combination on Escherichia coli. The results indicated that UV and combined disinfection were more effective with longer exposure, leading to significant reductions in microbial activity. Specifically, the single UV disinfection alone reduced activity by 3.3 log after 5 min, while combined disinfection achieved a 4.2 log reduction. In contrast, short-term HVEF treatment did not exhibit significant bactericidal effects, only achieving a reduction of 0.17 log in 5 min. Furthermore, prolonged exposure to both UV disinfection and an HVEF was found to damage cell membranes, ultimately causing cell death, while shorter durations did not. Despite rapid cell count decreases, flow cytometry did not detect apoptotic or necrotic cells, likely due to rapid cell rupture. This study suggests that combining UV radiation and an HVEF could be a promising approach for inhibiting bacterial reproduction, with HVEF enhancing UV effects. These findings provide insights for using combined HVEF and UV disinfection in food safety and preservation.

Disinfection of wastewater by a complete equipment based on a novel ultraviolet light source of microwave discharge electrodeless lamp: Characteristics of bacteria inactivation, reactivation and full-scale studies

Ultraviolet (UV) light is widely used for wastewater disinfection. Traditional electrode-excited UV lamps, such as low-pressure mercy lamps (LPUV), encounter drawbacks like electrode aging and rapid light attenuation. A novel UV source of microwave discharge electrodeless lamp (MDEL) has aroused attention, yet its disinfection performance is unclear and still far from practical application. Here, we successfully developed a complete piece of equipment based on MDELs and achieved the application for disinfection in wastewater treatment plants (WWTPs). The light emitted by an MDEL (MWUV) shared a spectrum similar to that of LPUV, with the main emission wavelength at 254 nm. The inactivation rate of Gram-negative E. coli by MWUV reached 4.5 log at an intensity of 1.6 mW/cm2 and a dose of 20 mJ/cm2. For Gram-positive B. subtilis, an MWUV dose of 50 mJ/cm2 and a light intensity of 1.2 mW/cm2 reached an inactivation rate of 3.4 log. A higher MWUV intensity led to a better disinfection effect and a lower photoreactivation rate of E. coli. When inactivated by MWUV with an intensity of 1.2 mW/cm2 and a dose of 16 mJ/cm2, the maximum photoreactivation rate and reactivation rate constant Kmax of E. coli were 0.63 % and 0.11 % h-1 respectively. Compared with the photoreactivation, the dark repair of E. coli was insignificant. The full-scale application of the MDEL equipment was conducted in two WWTPs (10,000 m3/d and 15,000 m3/d). Generally 2-3 log inactivation rates of fecal coliforms in secondary effluent were achieved within 5-6 s contact time, and the disinfected effluent met the emission standard (1000 CFU/L). This study successfully applied MDEL for disinfection in WWTPs for the first time and demonstrated that MDEL has broad application prospects.

An evaluation of conventional and nature-based technologies for controlling antibiotic-resistant bacteria and antibiotic-resistant genes in wastewater treatment plants

Antibiotic resistance is a globally recognized health concern which leads to longer hospital stays, increased morbidity, increased mortality, and higher medical costs. Understanding how antibiotic resistance persists and exchanges in environmental systems like soil, water, and wastewater are critically important for understanding the emergence of pathogens with new resistance profiles and the subsequent exposure of people who indirectly/directly come in contact with these pathogens. There are concerns about the widespread application of prophylactic antibiotics in the clinical and agriculture sectors, as well as chemicals/detergents used in food and manufacturing industries, especially the quaternary ammonium compounds which have been found responsible for the generation of resistant genes in water and soil. The rates of horizontal gene transfer increase where there is a lack of proper water/wastewater infrastructure, high antibiotic manufacturing industries, or endpoint users - such as hospitals and intensive agriculture. Conventional wastewater treatment technologies are often inefficient in the reduction of ARB/ARGs and provide the perfect combination of conditions for the development of antibiotic resistance. The wastewater discharged from municipal facilities may therefore be enriched with bacterial communities/pathogens and provide a suitable environment (due to the presence of nutrients and other pollutants) to enhance the transfer of antibiotic resistance. However, facilities with tertiary treatment (either traditional/emerging technologies) provide higher rates of reduction. This review provides a synthesis of the current understanding of wastewater treatment and antibiotic resistance, examining the drivers that may accelerate their possible transmission to a different environment, and highlighting the need for tertiary technologies used in treatment plants for the reduction of resistant bacteria/genes.

Combined applications of UV and chlorine on antibiotic resistance control: A critical review

Environmental health problems caused by antibiotic-resistant bacteria (ARB) and antibiotic-resistant genes (ARGs) have become a global concern. ARB and ARGs have been continuously detected in various water environments, which pose a new challenge for water quality safety assurance. Disinfection is a key water treatment process to eliminate pathogenic microorganisms in water, and combined chlorine and UV processes (the UV/Cl2 process, the UV-Cl2 process, and the Cl2-UV process) are considered potential disinfection methods to control antibiotic resistance. This review documented the efficacy and mechanism of combined UV and chlorine processes for the control of antibiotic resistance, as well as the effects of chlorine dose, solution pH, UV wavelength, and water matrix on the effectiveness of the processes. There are knowledge gaps in research on the combined chlorine and UV processes for antibiotic resistance control, in particular the UV-Cl2 process and the Cl2-UV process. In addition, changes in the structure of microbial communities and the distribution of ARGs, which are closely related to the spread of antibiotic resistance in the water, induced by combined processes were also addressed. Whether these changes could lead to the re-transmission of antibiotic resistance and harm human health may need to be further evaluated.

Effects of water quality on bacterial inactivation by ferrate(VI)

Ferrate (Fe(VI)) is an emerging green oxidant which has great potential and prospect in water disinfection. However, the effects of water quality on Fe(VI) disinfection remain unclear. This study systematically investigated the effects of pH, organic matters and inorganic ions on Fe(VI) inactivation of Escherichia coli (E. coli). Results showed that pH was the dominant influencing factor and the inactivation efficiency as well as inactivation rate constant was negatively correlated with pH (6.8-8.4). HFeO4- was found to be the critical Fe(VI) species contributing to the inactivation. As for organic matters (0-5 mg C/L), protein and humic acid significantly accelerated the decay of Fe(VI) and had negative effects on the inactivation efficiency, while polysaccharide slightly inhibited the inactivation due to the low reactivity with Fe(VI). As for inorganic ions, bicarbonate (0-2 mM) could stabilize Fe(VI) and decreased the inactivation rate constant, while ammonium (0-1 mM) had little effect on the inactivation of E. coli. In addition, the comprehensive effects of water quality on Fe(VI) disinfection in actual reclaimed water were also evaluated. The inactivation of E. coli in secondary effluent and denitrifying effluent was found to be inhibited compared to that in phosphate buffer. Overall, this study is believed to provide valuable information on Fe(VI) disinfection for water and wastewater treatment practices.