Point-of-use water disinfection using ultraviolet and visible light-emitting diodes

Unveiling Mechanically Driven Catalytic Processes: Beyond Piezocatalysis to Synergetic Effects

Mechanically driven catalysis (MDC) has emerged as an effective strategy for environmental remediation, renewable energy conversion, and cancer therapy; this functions by converting mechanical forces to drive catalytic reactions. This review examines four primary mechanisms, namely, piezocatalysis, flexocatalysis, tribocatalysis, and sonocatalysis, each involving specific catalytic pathways for harnessing mechanical energy at the nanoscale. However, significant challenges arise in decoupling the effects related to each individual mechanism in order to better understand and manipulate their synergies. In this review, the fundamental principles underpinning MDC are systematically interpreted. Beyond mechanistic insights, recent advancements in performance enhancement strategies for these catalysts are highlighted. Potential applications using these mechanistic approaches in environmental remediation (pollutant and antibiotic degradation and microbial disinfection), renewable energy conversion (hydrogen production and greenhouse gas conversion), and biomedical treatments (particularly cancer therapy) are discussed. Finally, the mechanistic synergies and limiting factors are explored, addressing challenges related to the overlooked combined effects of ultrasound as the activation source, complexities in mechanical force interactions at the nanoscale, and the need for targeted application strategies. Additionally, the industrial potential of these catalytic processes with consideration to scalability and practical deployment is evaluated. While challenges remain, this review provides a roadmap for advancing mechanically driven catalyst design and implementation toward real-world applications, offering potential into its future trajectory and transformative impact across numerous fields.

Mini-cores of activated carbon block simulate full-sized performance for removing organics and arsenate from drinking water✰

Activated carbon block (ACB) filters are widely used in point-of-use (POU) drinking technology to remove tastes, odors, and organic compounds from drinking water, and when modified can even remove inorganic pollutants (e.g., arsenate, lead, copper). To introduce ACB technologies to the POU market, thorough assessment and testing are required to meet the National Sanitary Foundation 53 certification. Testing to gain this certification can be costly and time-consuming and is usually only done on the final product before commercial distribution. We developed and explored how a cylindrical "plug" cored from an ACB can be used in a mini-core apparatus with low water volumes, to mimic full-sized ACB performance. These mini-cores allow the same outside-in radial flow conditions as the full-sized ACB. After addressing potential hydraulic channeling problems, tests with chloroform or arsenate confirmed the ability of the mini-core ACB "plugs" to mimic the performance of full-sized cartridge unit. The benefit of the mini-core ACB "plug" lab-scale approach lays the foundation for testing methodologies that can evaluate a range of pollutants, water chemistries, or material modifications using a small fraction of water compared to full-sized ACB filter. Overall, the development of a mini-core ACB testing apparatus is a key advancement towards sustainable water purification, impacting environmental health, resource conservation, and global access to safe water.

Microbiome shifts elicited by ornamental lighting of granite facades identified by MinION sequencing

Night-time outdoor illumination in combination with natural sunlight can influence the visible phototrophic colonizers (mainly algae) growing on stone facades; however, the effects on the microbiome (invisible to the naked eye) are not clear. The presence of stone-dwelling microbes, such as bacteria, diatoms, fungi, viruses and archaea, drives further biological colonization, which may exacerbate the biodeterioration of substrates. Considering the microbiome is therefore important for conservation of the built heritage. The impact of the following types of lighting on the relative abundance and diversity of the microbiome on granite ashlars was evaluated in a year-long outdoor pilot study: no lighting; lighting with a metal halide lamp (a traditional lighting system currently used to illuminate monuments); and lighting with a novel LED lamp (an environmentally sound prototype lamp with a biostatic effect, halting biological colonization by phototrophs, currently under trial). Culturable fractions of microbiome and whole-genome sequencing by metabarcoding with Oxford Nanopore Sequencing (MinION) was conducted for bacteria and fungi in order to complement both community characterization strategies. In addition, the possible biodeteriorative profiles of the isolated strains, relative to calcium carbonate precipitation/solubilisation and iron oxidation/reduction, were investigated by plate assays. Alpha and beta diversity indexes were also determined, along with the abundance of biocide and antibiotic resistance genes. Culture-dependent microbiological analysis failed to properly show changes in community composition, for which metagenomic approaches like MinION are better suited. Thus, MinION analysis identified shifts in the granite microbiome elicited by ornamental lighting. The novel LED lamp with the biostatic effect on phototrophs caused an increase in the diversity of bacteria and fungi. In this case, the microbiome was more similar to that in the unlit samples. In the samples illuminated by the metal halide lamp, dominance of bacteria was favoured and the presence of fungi was negligible.

Investigation of baffle configurations on the water disinfection efficiency using ultraviolet C light-emitting diodes

In this study, the effect of baffle configuration on the water disinfection efficiency of a planar photoreactor equipped with ultraviolet C light-emitting diodes (UV-C LEDs) was investigated. The results indicated that the configuration of the baffles influenced the hydrodynamics inside the flow channel and thus affected the microbial trajectory, and exposure time. Accordingly, a modified serpentine configuration was developed to enhance the UV light exposure of microbes in water and improve the reactor performance for microbial inactivation. According to the simulation results, the quarter-circle baffles used in the modified serpentine configuration increased the microbial path length along the flow channel. However, because the cross-sectional area of the flow channel decreased, this configuration increased the water velocity. A modified serpentine configuration with a baffle radius of 5 mm achieved the longest microbial exposure time and highest inactivation value for Escherichia coli. At a water flow rate of 160 mL/min, this configuration achieved a UV fluence of 15.2 mJ/cm2 and an inactivation value of 3.8 log, which were approximately 22% and 0.4 log higher than those obtained with the traditional serpentine configuration, respectively. In addition, the maximum water flow rate at which the UV reactor achieved an inactivation value of 4.0 log was 154 mL/min at a baffle radius of 5 mm. This flow rate was 11.5% higher than that obtained with the traditional serpentine configuration. These close agreements between the experimental and simulation results confirmed the strong capability of the proposed modified serpentine configuration to improve reactor performance.

Assessing the Relative Sustainability of Point-of-Use Water Disinfection Technologies for Off-Grid Communities

Point-of-use (POU) water disinfection technologies can be adopted to provide access to safe drinking water by treating water at the household level; however, navigating various POU disinfection technologies can be difficult. While numerous conventional POU devices exist, emerging technologies using novel materials or advanced processes have been under development and claim to be of lower cost with higher treatment capacity. However, it is unclear if these claims are substantiated and how novel technologies compare to conventional ones in terms of cost and environmental impacts when providing the same service (i.e., achieving a necessary level of disinfection for safe drinking water). This research assessed the sustainability of four different POU technologies (chlorination using sodium hypochlorite, a silver-nanoparticle-enabled ceramic water filter, ultraviolet mercury lamps, and ultraviolet light-emitting diodes). Leveraging open-source Python packages (QSDsan and EXPOsan), the cost and environmental impacts of these POU technologies were assessed using techno-economic analysis and life cycle assessment as per capita cost (USD·cap-1·yr-1) and global warming potential (kg CO2 eq·cap-1·yr-1). Impacts of water quality parameters (e.g., turbidity, hardness) were quantified for both surface water and groundwater, and uncertainty and sensitivity analyses were used to identify which assumptions influence outcomes. All technologies were further evaluated across ranges of adoption times, and contextual analysis was performed to evaluate the implications of technology deployment across the world. Results of this study can potentially provide valuable insights for decision-makers, nonprofit organizations, and future researchers in developing sustainable approaches for ensuring access to safe drinking water through POU technologies.

Performance of high-efficiency UV-C LEDs in water disinfection: Experimental, life cycle assessment, and economic analysis of different operational scenarios

The widespread use of low or medium pressure mercury lamps in UV-C water disinfection should consider recent advances in UV-C LED lamps that offer a more sustainable approach and avoid its main drawbacks. The type of water and the mode of operation are critical when deciding on the treatment technology to be used. Therefore, this study investigates the potential application of UV-C LED disinfection technology in terms of kinetics, environmental assessment, and economic analysis for two scenarios: the continuous disinfection of a wastewater treatment plant (WWTP), and disinfection of harvested rainwater (RWH) in a residential household that operates intermittently. Experiments are conducted using both the new UV-C LED system and the conventional mercury lamp to disinfect real wastewater. Removal of total coliforms and Escherichia coli bacteria, with concentrations of approximately 105 and 104 CFU per 100 mL has been followed to assess the performance of both types of UV-C lamps. The experimental study provides kinetic parameters that have been further used in the environmental assessment conducted from a life cycle perspective. Additionally, considering the significant role of electricity consumption, a preliminary economic analysis has been conducted. The results indicate that first-order kinetic constants of pathogens removal with UV-C LEDs achieve 1.4 times higher values than Hg lamp. Regarding the environmental and economic assessment, for disinfection systems operating continuously, LEDs result in environmental impacts 5 times higher than Hg lamp in most categories, indicating that Hg lamps offer a viable option both from economic and environmental point of view. However, for installations with intermittent operation, LEDs emerge as the most competitive alternative, due to their ability to be turned on and off without affecting their lifespan. This study shows that UV-C LED lamps hold promise to replace conventional mercury lamps in a near future.

Studying the viability and growth kinetics of vancomycin-resistant Enterococcus faecalis V583 following femtosecond laser irradiation (420-465 nm)

Enterococcus faecalis is among the most resistant bacteria found in infected root canals. The demand for cutting-edge disinfection methods has rekindled research on photoinactivation with visible light. This study investigated the bactericidal activity of femtosecond laser irradiation against vancomycin-resistant Enterococcus faecalis V583 (VRE). The effect of parameters such as wavelength and energy density on the viability and growth kinetics of VRE was studied to design an optimized laser-based antimicrobial photoinactivation approach without any prior addition of exogenous photosensitizers. The most effective wavelengths were 430 nm and 435 nm at a fluence of 1000 J/cm2, causing a nearly 2-log reduction (98.6% and 98.3% inhibition, respectively) in viable bacterial counts. The colony-forming units and growth rate of the laser-treated cultures were progressively decreased as energy density or light dose increased at 445 nm but reached a limit at 1250 J/cm2. At a higher fluence of 2000 J/cm2, the efficacy was reduced due to a photobleaching phenomenon. Our results highlight the importance of optimizing laser exposure parameters, such as wavelength and fluence, in bacterial photoinactivation experiments. To our knowledge, this is the first study to report an optimized wavelength for the inactivation of VRE using visible femtosecond laser light.

Reinforcing Feedbacks for Sustainable Implementation of Rural Drinking-Water Treatment Technology

Progress toward universal access to safe drinking water depends on rural water service delivery models that incorporate water safety management. Water supplies of all types have high rates of fecal contamination unless water safety risks are actively managed through water source protection, treatment, distribution, and storage. Recognizing the role of treatment within this broader risk-based framework, this study focuses on the implementation of passive chlorination and ultraviolet (UV) disinfection technologies in rural settings. These technologies can reduce the health risk from microbiological contaminants in drinking water; however, technology-focused treatment interventions have had limited sustainability in rural settings. This study examines the requirements for sustainable implementation of rural water treatment through qualitative content analysis of 26 key informant interviews, representing passive chlorination and UV disinfection projects in rural areas in South America, Africa, and Asia. The analysis is aligned with the RE-AIM framework and delivers insight into 18 principal enablers and barriers to rural water treatment sustainability. Analysis of the interrelationships among these factors identifies leverage points and encourages fit-for-purpose intervention design reinforced by collaboration between facilitating actors through hybrid service delivery models. Further work should prioritize health impact evidence, water quality reporting guidance, and technological capabilities that optimize trade-offs in fit-for-purpose treatment design.

Photoinactivation of the bacteriophage PhiX174 by UVA radiation and visible light in SM buffer and DMEM-F12

OBJECTIVE

It has been observed that viruses can be inactivated by UVA radiation and visible light. The aim of this study is to investigate whether a medium that contains a photosensitizer might have an influence on viral reduction under irradiation by UVA, violet or blue light. Test virus is the bacteriophage PhiX174 in the photosensitizer-free SM buffer and DMEM-F12, which contains the known photosensitizer riboflavin.

RESULTS

The determined PhiX174 D90 doses in SM buffer and DMEM were 36.8 J/cm² and 13.6 J/cm² at 366 nm, 153.6 J/cm² and 129.1 J/cm² at 408 nm and 4988 J/cm² and 2477.1 J/cm² at 455 nm, respectively. It can be concluded that the medium has a large influence on the results. This might be caused by the photosensitizer riboflavin in DMEM-F12. As riboflavin is a key component in many cell culture media, irradiation experiments with viruses in cell culture media should be avoided if the investigation of intrinsical photoinactivation properties of viruses is aimed for.

Tailored hybrid microbial water disinfection system using sequentially assembled microbial fuel cells and an ultraviolet C light-emitting diode

An integrated ultraviolet C light-emitting diode (UV-C LED) water disinfection system activated by microbial fuel cells (MFCs) was developed, and optimized via electric circuit and device voltage profiling. The intensity of the renewable energy operated, self-powered UV-C LED for E. coli inactivation was calculated by bio-dosimetry to be 2.4  × 10-2 μW cm-2 using fluence-based rate constant (k) of ∼1.03 (±0.11) cm2/mJ to obtain the reduction equivalent fluence kinetics value. Finally, the first-order rate constant for E. coli inactivation during the tailored hybrid disinfection system was found to be 0.53 (±0.1) cm2/mJ by multiplying intensity with 1.09 (±0.1) × 10-5 s-1 derived from the linear regression of E. coli inactivation as a function of time. Furthermore, selected model microbial consisting of two bacteria (Salmonella sp. and Listeria sp.) and three viruses (MS2 bacteriophage, influenza A virus, and murine norovirus-1) were treated with UV-C LED irradiation under controlled experimental conditions to validate the disinfection efficiency of the system. Consequently, the required to achieve significant removal (i.e., >3-log; 99.9%) UV fluence and dose time were calculated to be 4-7 cm2/mJ and 54-76 h and 33-53 cm2/mJ and 400-622 h for model bacterial and viral, respectively. This study expands the applicability of microbial electrochemical system (MES) for microbial disinfection and could be utilized in future MFCs implementation studies for predicting and measuring the kinetics of microbial elimination using a tailored hybrid water treatment system.

An Approach to Improve Energy Efficiency during Antimicrobial Blue Light Inactivation: Application of Pulse-Width Modulation Dimming to Balance Irradiance and Irradiation Time

Antimicrobial blue light (aBL) is an effective non-destructive inactivation technique and has received increasing attention. Despite its significance, the existing research has not thoroughly delved into the impacts of irradiance and irradiation time on enhancing energy efficiency during aBL inactivation and the explanation of the enhancement effect of pulse exposure. In this paper, a series of Escherichia coli inactivation experiments with different duty cycles, pulse frequencies, and irradiation times were conducted, and the relative concentrations of reactive oxygen species (ROS) were measured under corresponding conditions. A two-dimensional (2-D) Hom model was proposed to evaluate the effect of irradiance and irradiation time. The results show that, compared to continuous exposure, pulsed aBL (duty cycle = 25%) can save ~37% of the energy to achieve the same inactivation effect and generate a 1.95 times higher ROS concentration. The 2-D Hom model obtains the optimal combination of average irradiance and time according to the desired reduction and shows that the irradiation time has a higher weight than the irradiance (1.677 and 1.083, respectively). Therefore, using pulse exposure with a lower average irradiance for a longer period of time can achieve a better inactivation effect when consuming equivalent energy. The proposed pulse-width modulation dimming approach helps promote the application of the aBL technique.

Simultaneous Irradiation with UV-A, -B, and -C Lights Promotes Effective Decontamination of Planktonic and Sessile Bacteria: A Pilot Study

Surfaces in highly anthropized environments are frequently contaminated by both harmless and pathogenic bacteria. Accidental contact between these contaminated surfaces and people could contribute to uncontrolled or even dangerous microbial diffusion. Among all possible solutions useful to achieve effective disinfection, ultraviolet irradiations (UV) emerge as one of the most "Green" technologies since they can inactivate microorganisms via the formation of DNA/RNA dimers, avoiding the environmental pollution associated with the use of chemical sanitizers. To date, mainly UV-C irradiation has been used for decontamination purposes, but in this study, we investigated the cytotoxic potential on contaminated surfaces of combined UV radiations spanning the UV-A, UV-B, and UV-C spectrums, obtained with an innovative UV lamp never conceived so far by analyzing its effect on a large panel of collection and environmental strains, further examining any possible adverse effects on eukaryotic cells. We found that this novel device shows a significant efficacy on different planktonic and sessile bacteria, and, in addition, it is compatible with eukaryotic skin cells for short exposure times. The collected data strongly suggest this new lamp as a useful device for fast and routine decontamination of different environments to ensure appropriate sterilization procedures.

Techno-economic evaluation of UV light technologies in water remediation

Disinfection commonly follows conventional treatments in wastewater treatment and remediation plants aiming at reducing the presence of pathogens. However, the presence of the so called "micropollutants" has emerged as a serious concern, therefore developing tertiary treatments that are not only able to remove pathogens but also to degrade micropollutants is worth investigating. Nowadays, UV-C photo-degradation processes are widely used for disinfection due to their simplicity and easy operation; additionally, they have shown potential for the removal of contaminants of emerging concern. Conventional mercury lamps are being replaced by light-emitting diodes (LEDs) that avoid the use of toxic mercury and can be switched on and off with no effect on the lamp lifetime. This work aims to comparatively evaluate the performance of several photo-degradation technologies for the removal of two targeted micropollutants, the pharmaceutical dexamethasone (DXMT) and the herbicide S-metolachlor (MTLC), using UV irradiation doses typical of disinfection processes. To this end, the technical performance of UV-A/UV-C photolysis, UV-A/UV-C photocatalysis, UV-C/H₂O₂ and UV-C/NaOCl has been compared. The influence of operating conditions such as the initial concentration of the pollutants (3 mg L^-1 - 30 mg L^-1, concentrations found in membrane or adsorption remediation steps), pH (3-10), and water matrix (WWTP secondary effluent, and ultrapure water) on the degradation efficiency has been studied. The economic evaluation in terms of electricity and chemicals consumption and the carbon footprint has been evaluated. UV-C photolysis and UV-C photocatalysis appear as the most suitable technologies for the degradation of DXMT and MTLC, respectively, in terms of kinetics (1.53·10^-1 min^-1 for DXMT and 1.96·10^-2 min^-1 for MTLC), economic evaluation (1 € m^-3 for DXMT and 32 € m^-3 for MTLC) and environmental indicators (0.5 g-CO₂ for DXMT and 223.1 g-CO₂ for MTLC).

Quercetin mediated antimicrobial photodynamic treatment using blue light on Escherichia coli O157:H7 and Listeria monocytogenes

Interest in using an antimicrobial photodynamic treatment (aPDT) for the microbial decontamination of food has been growing. In this study, quercetin, a substance found ubiquitously in plants, was used as a novel exogenous photosensitizer with 405 nm blue light (BL) for the aPDT on foodborne pathogens, and the inactivation mechanism was elucidated. The inactivation of Escherichia coli O157:H7 and Listeria monocytogenes in PBS solution by the quercetin and BL combination treatment reached a log reduction of 6.2 and more than 7.55 at 80 J/cm² (68 min 21 s), respectively. When EDTA was added to investigate the reason for different resistance between two bacteria, the effect of aPDT was enhanced against E. coli O157:H7 but not L. monocytogenes. This result indicated that the lipopolysaccharide of Gram-negative bacteria operated as a protective barrier. It was experimentally demonstrated that quercetin generated the superoxide anion and hydrogen peroxide as the reactive oxygen species that oxidize and inactivate cell components. The damage to the bacterial cell membrane by aPDT was evaluated by propidium iodide, where the membrane integrity significantly (P < 0.05) decreased from 40 J/cm² compared to control. In addition, DNA integrity of bacteria was significantly (P < 0.05) more decreased after aPDT than BL treatment. The inactivation results could be applied in liquid food industries for decontamination of foodborne pathogens, and the mechanisms data was potentially utilized for further studies about aPDT using quercetin.

Deactivating environmental strains of Escherichia coli, Enterococcus faecalis and Clostridium perfringens from a real wastewater effluent using UV-LEDs

Environmental bacteria strains are known to be more resistant but studies on UV-LEDs are scarce, especially for Clostridium perfringens and Enterococcus faecalis. UV-LEDs of different wavelengths (268 nm, 279 nm and 307 nm) have been used for treating real wastewater from the effluent of the municipal plant in Linares (Spain), with real organic matter content, for E. coli, Enterococcus faecalis and Clostridium perfringens disinfection. Experimental results demonstrate that 268 nm was the most effective wavelength for inactivation of the three different bacteria strains: E. coli showed an inactivation rate of 0.561 at 268 nm vs. 0.245 at 279 nm and 0.0029 for 307 nm; E. faecalis inactivation rate was 0.313 at 268 nm, 0.231 at 279 nm and 0.0023 at 307 nm; and C. perfringens inactivation rate was 0.084 at 268 nm, 0.033 at 279 nm and 6.9e-4 at 307 nm. In general, 307 nm wavelength showed a significantly lower inactivation rate so it would not be recommended for practical applications. C. Perfringens required higher UV doses and longer times to achieve complete inactivation.

Inactivation of antibiotic resistant bacteria from stormwater runoff using UVA/LED and its potential risks

Recently, increasing attention has been paid to antibiotic resistance in stormwater runoff. However, there is no available literature about the control of antibiotic resistant bacteria (ARB) through 365 nm ultraviolet light-emitting diode (UVA/LED). In this study, batch experiments were conducted to investigate ARB inactivation kinetics, effects of light intensity and water matrix (including suspended solid (SS) concentration, initial pH and bacteria concentration), and potential transmission risks after UVA/LED irradiation. Results showed that ARB inactivation efficiencies reached 6.31 log reduction at 8 mW/cm² (86 J/cm²) of UVA/LED for 180 min. ARB inactivation efficiencies increased with the increase of light intensity, and showed a linear relationship. ARB inactivation decreased with increasing SS levels, and the largest inactivation efficiencies was 3.56 log reduction at 50 mg/L of SS. Initial pH had slight effect on ARB inactivation through UVA/LED irradiation. A low initial bacteria concentration (10⁵ CFU/mL) was not necessarily associated with good ARB inactivation (3.59 log reduction). After UVA/LED irradiation, ARB was hardly detected during 12 hr of dark repair, and the transfer frequency of kanamycin resistance gene was increased to 5.43 × 10^-4. These suggested that the application of UVA/LED to inactivate ARB in stormwater runoff was feasible and desirable in this study.

Current understanding on antibacterial mechanisms and research progress of tea polyphenols as a supplementary disinfectant for drinking water

Disinfection by-products (DBPs) generated during the disinfection of drinking water have become an urgent problem. So, tea polyphenol, a natural green disinfectant, has attracted widespread attention in recent years. This review summarizes the antibacterial mechanism of tea polyphenols and the recent findings on tea polyphenols as disinfectants for drinking water. These studies show that tea polyphenol is an antibacterial agent that works through different mechanisms and can be used as a supplementary disinfectant because of its higher lasting effect and economical cost. The dosage of tea polyphenols as a disinfectant of ultrafiltration effluent is the lowest among all the tea polyphenols disinfection methods, which can ensure the microbial safety of drinking water. This application of tea polyphenols is deemed a practical solution to solving the issue of disinfecting drinking water and reducing DBPs. However, it is necessary to further explore the influence of factors such as pipeline materials on the disinfection process and efficacy to expand the application scope of tea polyphenols. The large-scale application of tea polyphenols still needs to be fine-tuned but with new developments in tea polyphenol purification technology and the long-term need for drinking water that is safe for human consumption, tea polyphenols have good prospects for further development.

Action Spectra of Bacteria and Purification of Pollutant Water at Faucets Using a Water Waveguide Method

Ultraviolet (UV) radiation treatment is an effective method for purifying pollutant water contaminated with bacteria and/or chemicals. As an emerging technology, purification by deep ultraviolet light-emitting diodes (DUV-LEDs) is promising. Few studies have used the point-source characteristics of LEDs and have instead replaced mercury vapor lamps with LEDs. Here, we show our recent progress in the instantaneous purification of contaminated water by combining the point-source characteristics of DUV-LEDs with a water waveguide (WW). Before the demonstration, we determined the efficacy of disinfection as a function of irradiation wavelength (action spectra) by constructing a wavelength tunable DUV light source. We found that, as a function of irradiation wavelength, there is a strong correlation between the dose-based inactivation rate constants and deoxyribonucleic acid (DNA) absorbance. Based on this correlation, the emission wavelength of 265 nm was determined as the most effective wavelength for disinfecting water contaminated with bacteria. Instantaneous 2-log disinfection levels of water contaminated with Escherichia coli O1 or Pseudomonas aeruginosa were demonstrated by using the DUV-LED WW method. We also discuss how far-UVC radiation shorter than 230 nm, which has recently been attracting attention and is known as a safe and effective disinfection wavelength for the human body, cannot give a higher-dose-based inactivation rate constant compared to that of 265 nm irradiation due to the larger absorption coefficient of water with a wavelength shorter than 230 nm.

Blue Light Inhibits E. coli, but Decisive Parameters Remain Hidden in the Dark: Systematic Review and Meta-Analysis

Blue light (400-500 nm) alleviates overexposure risks associated to UV light and has therefore gained increased interest in multiple applications. This meta-analysis deals with decontamination of E. coli through the use of blue light based from nine recent publications identified via a systematic literature search. In these studies, various pathogenic and non-pathogenic E. coli strains grown in nutritional broths were exposed to wavelengths ranging from 395 to 460 nm. Five meta-analyses were performed using Cochrane's software for meta-analyses (Review Manager): one including all studies to estimate the effect of E. coli reduction and four subgroup-analyses considering reported intensities, wavelengths, exposure dose as well as serovars/pathovars. Random effects models were used. All included studies used colony-forming units to estimate the impact of E. coli reduction. None of the included studies involved an organic matrix (e.g., skin, food related surface). Exposure to blue light had a significant and large reducing effect on viable counts of E. coli. However, substantial heterogeneity across studies was observed. Among subgroups, reported intensity and wavelength showed the clearest impact on E. coli reduction. With respect to the reported exposure dose, the picture across the spectrum was scattered, but effect sizes tend to increase with increasing exposure dose. Substantial heterogeneity was also present with respect to all serovar/pathovar subgroups among the included studies. The present body of reports does not display a strong basis for recommendation of relevant intensities, wavelengths and exposure doses for superficial blue light decontamination in medical or food safety contexts. A serious shortcoming in most studies is the absence of a clear documentation of inoculum preparation and of study parameters. We suggest improvement for study protocols for future investigations.

Dual-wavelength light radiation for synergistic water disinfection

Development of the narrow-band mercury-free light sources, such as light emitting diodes (LEDs) and excilamps, has stimulated research on inactivation of pathogenic microorganisms by dual-wavelength light radiation. To date, dual-wavelength light radiation has emerged as an advanced tool for enhancing microbial inactivation in water in view of potential synergistic effect. This is the first review that aims at elucidating its mechanisms under dual-wavelength light exposure and surveying a body of related literature in terms of yes-or-no synergy. We have proposed three key inactivation mechanisms, which function in the estimated spectrum ranges I (190-254 nm), II (250-320 nm) and III (300-405 nm) and provide a synergistic effect when combined. These mechanisms involve proteins damage and DNA repair suppression (I), direct and indirect DNA damage (II) and generation of reactive oxygen species (ROS) by endogenous photosensitizers (III), such as porphyrins and flavins. A synergy under dual-wavelength light irradiation simultaneously or sequentially occurs if coupling two wavelengths of different ranges (I + II, I + III, II + III) in order to trigger different inactivation mechanisms. Recent advances of dual-wavelength light strategy in photodynamic therapy could be applied for water disinfection. They bring opportunities for applying the sources of near-UV and visible radiation and making the disinfection processes more energy- and cost-effective. From this standpoint, the synergistically efficient dual-wavelength combinations II + III and the combinations within the extended to 700 nm range III (near-UV + VIS) appear to be promising for developing novel advanced oxidation processes for disinfection of real turbid waters.

Recent Update on UV Disinfection to Fulfill the Disinfection Credit Value for Enteric Viruses in Water

Ultraviolet (UV) radiation alone or in combination with other oxidation processes is increasingly being considered for water disinfection because of stringent regulatory requirements for pathogen inactivation. To fulfill this requirement, an appropriate UV dose or fluence (mJ/cm²) is applied to combat enteric viruses in surface or treated water. There is a need for a cumulative review on the effectiveness of current and emerging UV technologies against various types of human enteric viruses. We extracted the kinetics data from 52 selected experimental studies on enteric virus inactivation using low pressure (LP-UV), medium pressure (MP-UV), UV-LED, and advanced oxidation processes (AOPs) and applied a simple linear regression analysis to calculate the range of UV fluence (mJ/cm²) needed for 4-log₁₀ inactivation. The inactivation of adenoviruses with LP-UV, MP-UV, and UV/H₂O₂ (10 mg/L) required the highest fluence, which ranged from 159 to 337, 45, and 115 mJ/cm², respectively. By contrast, when using LP-UV, the inactivation of other enteric viruses, such as the Caliciviridae and Picornaviridae family and rotavirus, required fluence that ranged from 19 to 69, 18 to 43, and 38 mJ/cm², respectively. ssRNA viruses exhibit higher sensitivity to UV radiation than dsRNA and DNA viruses. In general, as an upgrade to LP-UV, MP-UV is a more promising strategy for eliminating enteric viruses compared to AOP involving LP-UV with added H₂O₂ or TiO₂. The UV-LED technology showed potential because a lower UV fluence (at 260 and/or 280 nm wavelength) was required for 4-log₁₀ inactivation compared to that of LP-UV for most strains examined in this critical review. However, more studies evaluating the inactivation of enteric viruses by means of UV-LEDs and UV-AOP are needed to ascertain these observations.

Ultra-high irradiance (UHI) blue light: highlighting the potential of a novel LED-based device for short antifungal treatments of food contact surfaces

Microbial food spoilage is an important cause of health and economic issues and can occur via resilient contamination of food surfaces. Novel technologies, such as the use of visible light, have seen the light of day to overcome the drawbacks associated with surface disinfection treatments. However, most studies report that photo-inactivation of microorganisms with visible light requires long time treatments. In the present study, a novel light electroluminescent diode (LED)-based device was designed to generate irradiation at an ultra-high power density (901.1 mW/cm²). The efficacy of this technology was investigated with the inactivation of the yeast S. cerevisiae. Short-time treatments (below 10 min) at 405 nm induced a ~4.5 log reduction rate of the cultivable yeast population. The rate of inactivation was positively correlated to the overall energy received by the sample and, at a similar energy, to the power density dispatched by the lamp. A successful disinfection of several food contact surfaces (stainless steel, glass, polypropylene, polyethylene) was achieved as S. cerevisiae was completely inactivated within 5 min of treatments. The disinfection of stainless steel was particularly effective with a complete inactivation of the yeast after 2 min of treatment. This ultra-high irradiance technology could represent a novel cost- and time-effective candidate for microbial inactivation of food surfaces. These treatments could see applications beyond the food industry, in segments such as healthcare or public transport. KEY POINTS : • A novel LED-based device was designed to emit ultra-high irradiance blue light • Short time treatments induced high rate of inhibition of S. cerevisiae • Multiple food contact surfaces were entirely disinfected with 5-min treatments.

Effects of UV Radiation on the Chlorophyte Micromonas polaris Host-Virus Interactions and MpoV-45T Virus Infectivity

Polar seas are under threat of enhanced UV-radiation as well as increasing shipping activities. Considering the ecological importance of marine viruses, it is timely to study the impact of UV-AB on Arctic phytoplankton host-virus interactions and also test the efficacy of ballast water (BW) UV-C treatment on virus infectivity. This study examined the effects of: (i) ecologically relevant doses of UV-AB radiation on Micromonas polaris RCC2258 and its virus MpoV-45T, and (ii) UV-C radiation (doses 25-800 mJ cm-2) on MpoV-45T and other temperate algal viruses. Total UV-AB exposure was 6, 12, 28 and 48 h (during the light periods, over 72 h total). Strongest reduction in algal growth and photosynthetic efficiency occurred for 28 and 48 h UV-AB treatments, and consequently the virus production rates and burst sizes were reduced by more than half (compared with PAR-only controls). For the shorter UV-AB exposed cultures, negative effects by UV (especially Fv/Fm) were overcome without impacting virus proliferation. To obtain the BW desired log-4 reduction in virus infectivity, a UV-C dose of at least 400 mJ cm-2 was needed for MpoV-45T and the temperate algal viruses. This is higher than the commonly used dose of 300 mJ cm-2 in BW treatment.

Review of microbial touchscreen contamination for the determination of reasonable ultraviolet disinfection doses

Background: Touchscreens are usually microbially contaminated and can therefore act as fomites inside and outside healthcare environments. Due to the increasing use of such touchscreens and the growing awareness of infection risks, approaches that allow safe and automatic disinfection are desired. Ultraviolet (UV) irradiation, with its known antimicrobial efficacy, could achieve this goal, but should be executed with limited touchscreen degradation, disinfection duration, and energy consumption. It should also pose as little harm as possible to humans even in case of failure. Materials and methods: A literature search was performed first to identify the microorganisms most commonly found on touchscreens. Then, the 90% reduction doses (D90 doses) for the different relevant microorganisms and UV spectral ranges were determined from the literature, and irradiation doses are suggested that should reduce most of these important microorganisms by 5 log-levels. Results: The most frequent microorganisms are staphylococci, bacilli, micrococci, enterococci, pseudomonads and E. coli with small differences between hospital and community environments, if antibiotic resistance properties are ignored. The determined irradiation doses for a 5 log-reduction of the most frequent microorganisms are about 40 mJ/cm2, 80 J/cm2, 500 J/cm2 and 50 mJ/cm2 for the UV spectral ranges UVC, UVB, UVA and far-UVC, respectively. These doses are also sufficient to inactivate all nosocomial ESKAPE pathogens on touchscreens by at least 99.999%. Conclusion: Disinfection is achievable in all UV spectral ranges, with UVC being the most effective, enabling automatic disinfection within a minute or less. The much higher doses required in the UVB and UVA spectral range result in much longer disinfection durations, with the advantage of a reduced risk to humans. For all kinds of UV irradiation, the doses should be limited to reasonable values to avoid irradiating an already more or less sterile surface and to prevent degradation of touchscreen devices.

Disinfection Performance of a Drinking Water Bottle System With a UV Subtype C LED Cap Against Waterborne Pathogens and Heterotrophic Contaminants

The purgaty One systems (cap+bottle) are portable stainless-steel water bottles with UV subtype C (UVC) disinfection capability. This study examines the bottle design, verifies disinfection performance against Escherichia coli, Pseudomonas aeruginosa, Vibrio cholerae, and heterotrophic contaminants, and addresses the public health relevance of heterotrophic bacteria. Bottles were inoculated with deliberately contaminated potable water and disinfection efficacy examined using colony forming unit (CFU) assay for each bacterial strain. The heterotrophic plate count (HPC) method was used to determine the disinfection performance against environmental contaminants at day 0 and after 3days of water in stationary condition without prior UVC exposure. All UVC irradiation experiments were performed under stationary conditions to confirm that the preset application cycle of 55s offers the desired disinfection performance under-tested conditions. To determine effectiveness of purgaty One systems (cap+bottle) in disinfection, inactivation efficacy or log reduction value (LRV) was determined using bacteria concentration between UVC ON condition and controls (UVC OFF). The study utilized the 16S ribosomal RNA (rRNA) gene for characterization of isolates by identifying HPC bacteria to confirm if they belong to groups that are of public health concern. Purgaty One systems fitted with Klaran UVC LEDs achieved 99.99% inactivation (LRV4) efficacy against E. coli and 99.9% inactivation (LRV3) against P. aeruginosa, V. cholerae, and heterotrophic contaminants. Based on the 16S rRNA gene analyses, the study determined that the identified HPC isolates from UVC irradiated water are of rare public health concern. The bottles satisfactorily inactivated the target pathogenic bacteria and HPC contaminants even after 3days of water in stationary condition.

UVC inactivation of MS2-phage in drinking water - Modelling and field testing

UVC disinfection has been recognised by the WHO as an effective disinfection treatment to provide decentralized potable water. Under real conditions there are still unknowns that limit this application including the influence of suspended solids and natural organic matter. This work aims to investigate the influence of two key parameters, suspended solids and natural organic matter, on the efficiency of UVC disinfection of surface water to achieve the drinking water quality requirements established by the WHO for point of use (POU) technologies. Kaolinite (turbidity agent) and humic acids (HA, model of organic matter) were used in a factorial design of experiments (Turbidity from 0 to 5 NTU, and HA from 0 to 3.5 mg/L) to investigate their effect on UVC inactivation of MS2 phage in surface water. A collimated beam (12 W) and a commercial UVC disinfection flow system (16 W) designed to provide drinking water at households were used. The UVC flow system both in the laboratory and in the field was able to achieve the reduction requirements established by WHO (LRV >3.5 for all tested conditions), confirming the good performance of the studied UVC disinfection system. The results found in the lab were used to establish a numerical model that predicts the disinfection rate constant as a function of water turbidity and transmittance at 254 nm (confidence level>95%). The model permitted to elucidate the critical effect of low concentrations of HA in reducing the inactivation rate by 40% for 3.5 mg/L-HA compared with 0, the non-significant detrimental effect of turbidity lower than 5 NTU, and the lack of synergistic effects between both parameters at these levels. The UVC flow system was also tested in the field, in Tzabalho, Chiapas (Mexico), and Antioquia (Colombia), with spiked MS2 into natural surface water. This investigation opens a potential application to monitor the performance of UVC systems with surface water by monitoring transmittance at 254 nm as a tool to control UVC domestic systems to deliver safe drinking water in a household without the need of expensive and laborious biological monitoring tools.

Solar Water Disinfection to Produce Safe Drinking Water: A Review of Parameters, Enhancements, and Modelling Approaches to Make SODIS Faster and Safer

Solar water disinfection (SODIS) is one the cheapest and most suitable treatments to produce safe drinking water at the household level in resource-poor settings. This review introduces the main parameters that influence the SODIS process and how new enhancements and modelling approaches can overcome some of the current drawbacks that limit its widespread adoption. Increasing the container volume can decrease the recontamination risk caused by handling several 2 L bottles. Using container materials other than polyethylene terephthalate (PET) significantly increases the efficiency of inactivation of viruses and protozoa. In addition, an overestimation of the solar exposure time is usually recommended since the process success is often influenced by many factors beyond the control of the SODIS-user. The development of accurate kinetic models is crucial for ensuring the production of safe drinking water. This work attempts to review the relevant knowledge about the impact of the SODIS variables and the techniques used to develop kinetic models described in the literature. In addition to the type and concentration of pathogens in the untreated water, an ideal kinetic model should consider all critical factors affecting the efficiency of the process, such as intensity, spectral distribution of the solar radiation, container-wall transmission spectra, ageing of the SODIS reactor material, and chemical composition of the water, since the substances in the water can play a critical role as radiation attenuators and/or sensitisers triggering the inactivation process.

LED irradiated photo-Fenton for the removal of estrogenic activity and endocrine disruptors from wastewater treatment plant effluent

The goal of this work was to evaluate the performance of the LED irradiated photo-Fenton process on the removal of (i) estrogenic activity and (ii) seven endocrine disruptors (EDs) (4-octylphenol, 4-nonylphenol, bisphenol A, estrone, 17β-estradiol, 17α-ethinylestradiol, and estriol) from real wastewater treatment plant effluent (WWTPE). EDs are a group of contaminants of emerging concern present in WWTPE and which may be recognized by hormone receptors, thus harming animal and human health. The yeast estrogenic screen test (YES) was used to quantify estrogenic activity promoted by EDs in WWTPE samples before and after photo-Fenton treatment. Tests were performed following a factorial design with different iron (20, 40, and 60 mg L^-1) and hydrogen peroxide (100, 200, and 300 mg L^-1) concentrations in a laboratory scale LED photoreactor (λ = 455 nm, 1.5 L, 1.6 × 10^-6 Einstein s^-1). EDs were analyzed by gas chromatography coupled to a mass spectrometer. Control experiments consisted of Fenton process, iron only, LED irradiation only, and H₂O₂ only. Optimum experimental conditions for LED photo-Fenton resulted in 62% removal of estrogenic activity and 59% mineralization. In addition, treated WWTPE was not toxic to Aliivibrio fischeri and more than 80% of EDs were removed during LED irradiated photo-Fenton. Although Fenton process showed similar efficiency to that obtained by LED photo-Fenton, a higher volume of sludge was generated in the dark. Finally, results obtained in this study confirm the applicability of LED irradiated photo-Fenton process for improving the quality of WWTPE as an alternative to solar photo-Fenton in case solar radiation is not available, thus reducing hazards associated to WWTPE reuse or discharge.

Investigating the Use of Ultraviolet Light Emitting Diodes (UV-LEDs) for the Inactivation of Bacteria in Powdered Food Ingredients

The addition of contaminated powdered spices and seasonings to finished products which do not undergo further processing represents a significant concern for food manufacturers. To reduce the incidence of bacterial contamination, seasoning ingredients should be subjected to a decontamination process. Ultraviolet light emitting diodes (UV-LEDs) have been suggested as an alternative to UV lamps for reducing the microbial load of foods, due to their increasing efficiency, robustness and decreasing cost. In this study, we investigated the efficacy of UV-LED devices for the inactivation of four bacteria (Listeria monocytogenes, Escherichia coli, Bacillus subtilis and Salmonella Typhimurium) on a plastic surface and in four powdered seasoning ingredients (onion powder, garlic powder, cheese and onion powder and chilli powder). Surface inactivation experiments with UV mercury lamps, UVC-LEDs and UVA-LEDs emitting at wavelengths of 254 nm, 270 nm and 365 nm, respectively, revealed that treatment with UVC-LEDs were comparable to, or better than those observed using the mercury lamp. Bacterial reductions in the seasoning powders with UVC-LEDs were less than in the surface inactivation experiments, but significant reductions of 0.75-3 log10 colony forming units (CFU) were obtained following longer (40 s) UVC-LED exposure times. Inactivation kinetics were generally nonlinear, and a comparison of the predictive models highlighted that microbial inactivation was dependent on the combination of powder and microorganism. This study is the first to report on the efficacy of UV-LEDs for the inactivation of several different bacterial species in a variety of powdered ingredients, highlighting the potential of the technology as an alternative to the traditional UV lamps used in the food industry.

Inactivation Effect of Violet and Blue Light on ESKAPE Pathogens and Closely Related Non-pathogenic Bacterial Species - A Promising Tool Against Antibiotic-Sensitive and Antibiotic-Resistant Microorganisms

Due to the globally observed increase in antibiotic resistance of bacterial pathogens and the simultaneous decline in new antibiotic developments, the need for alternative inactivation approaches is growing. This is especially true for the treatment of infections with the problematic ESKAPE pathogens, which include Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species, and often exhibit multiple antibiotic resistances. Irradiation with visible light from the violet and blue spectral range is an inactivation approach that does not require any additional supplements. Multiple bacterial and fungal species were demonstrated to be sensitive to this disinfection technique. In the present study, pathogenic ESKAPE organisms and non-pathogenic relatives are irradiated with visible blue and violet light with wavelengths of 450 and 405 nm, respectively. The irradiation experiments are performed at 37°C to test a potential application for medical treatment. For all investigated microorganisms and both wavelengths, a decrease in colony forming units is observed with increasing irradiation dose, although there are differences between the examined bacterial species. A pronounced difference can be observed between Acinetobacter, which prove to be particularly light sensitive, and enterococci, which need higher irradiation doses for inactivation. Differences between pathogenic and non-pathogenic bacteria of one genus are comparatively small, with the tendency of non-pathogenic representatives being less susceptible. Visible light irradiation is therefore a promising approach to inactivate ESKAPE pathogens with future fields of application in prevention and therapy.

Chlorination effects on DNA based characterization of water microbiomes and implications for the interpretation of data from disinfected systems

Quantitative PCR (qPCR) and next generation sequencing (NGS) are nucleic acid based microbiology techniques that provide new insights into drinking water quality, but considerable uncertainty remains around their correct interpretation. We noticed the presence of bacterial DNA from various putative pathogens, including from faecal indicator bacteria (FIB), in disinfected water, when culturable FIB were absent. To understand these observations better we studied the effect of chlorination on conventional and DNA based microbial water quality assessments. Surface water chlorination reduced plate counts for various FIB by up to >6 log units, intact cell counts by flow cytometry by 3.3 log units, and 16S rRNA gene copies by qPCR by 1.5 and 1.6 log units for total bacteria and total coliforms, respectively. Nanopore sequencing of 16S rRNA amplicons with the portable MinION device revealed the DNA from several families containing putative pathogens appeared to be more resistant than that of other bacteria to degradation by chlorine disinfection. For instance, 16S rRNA genes assigned to the Enterobacteriaceae family, members of which are mostly the target of coliform tests, increased in relative abundance from 0.001 ± 0.0002% to 0.0036 ± 0.003% after chlorine treatment. Hence, metagenomic drinking water data needs to be interpreted with caution. Plate counts and flow cytometry in combination with DNA based analysis provide more robust insight than NGS or qPCR alone.

Potential biofilm control strategies for extended spaceflight missions

Biofilms, surface-adherent microbial communities, are associated with microbial fouling and corrosion in terrestrial water-distribution systems. Biofilms are also present in human spaceflight, particularly in the Water Recovery System (WRS) on the International Space Station (ISS). The WRS is comprised of the Urine Processor Assembly (UPA) and the Water Processor Assembly (WPA) which together recycles wastewater from human urine and recovered humidity from the ISS atmosphere. These wastewaters and various process streams are continually inoculated with microorganisms primarily arising from the space crew microbiome. Biofilm-related fouling has been encountered and addressed in spacecraft in low Earth orbit, including ISS and the Russian Mir Space Station. However, planned future missions beyond low Earth orbit to the Moon and Mars present additional challenges, as resupplying spare parts or support materials would be impractical and the mission timeline would be in the order of years in the case of a mission to Mars. In addition, future missions are expected to include a period of dormancy in which the WRS would be unused for an extended duration. The concepts developed in this review arose from a workshop including NASA personnel and representatives with biofilm expertise from a wide range of industrial and academic backgrounds. Here, we address current strategies that are employed on Earth for biofilm control, including antifouling coatings and biocides and mechanisms for mitigating biofilm growth and damage. These ideas are presented in the context of their applicability to spaceflight and identify proposed new topics of biofilm control that need to be addressed in order to facilitate future extended, crewed, spaceflight missions.

Photoinactivation results of Enterococcus moraviensis with blue and violet light suggest the involvement of an unconsidered photosensitizer

Microorganisms can be photoinactivated with 405 and 450 nm irradiation, due to endogenous photosensitizers, which absorb light of these wavelengths and generate reactive oxygen species that destroy the cells from within. The photosensitizers assumed to be responsible are porphyrins in the spectral region around 405 nm and flavins at about 450 nm. The aim of this study was to investigate this hypothesis on enterococci, considering that they do not contain porphyrins. In photoinactivation experiments with Enterococcus moraviensis, 405 nm and 450 nm irradiation both led to a reduction of the bacterial concentration by several orders of magnitude with 405 nm irradiation being much more efficient. The measurement and analysis of the fluorescence spectra revealed no signs of porphyrins whereas flavins seemed to be rapidly converted to lumichrome by 405 nm radiation. Therefore, probably none of the usual suspects, porphyrins and flavins, was responsible for the photoinactivation of Enterococcus moraviensis during 405 nm irradiation. Fluorescence experiments revealed the spectra of lumichrome and NADH, which are both known photosensitizers. Presumably, one of them or both were actually involved here. As NADH and flavins (and therefore their photodegradation product lumichrome) are abundant in all microorganisms, they are probably also involved in 405 nm photoinactivation processes of other species.

Design and implementation of water purification system based on deep ultraviolet light emitting diodes and a multi-pass geometry reactor

A novel reactor was designed and implemented for water purification using deep ultraviolet light emitting diodes (LEDs). The focus was on minimizing the number of LEDs required for effective germicidal action. Simulation studies were carried out on the flow of water as well as the irradiance of UV. Variation was made in the beam divergence of the UV sources and reflectivity of optical coatings used for photon recycling. Based on optimized reactor designs, water purification was carried out both in the static and flow-through configuration. Water from various sources was spiked with a known bacterial strain, exposure studies were carried out and germicidal effect was determined. Our results indicate that under optimal design, a 3 mL volume of water shows a three order inactivation using a single UV-LED in a static reactor in 180 s. For a flow-through geometry, only three LEDs were used in the reactor implementation, and a multi-pass procedure was used to purify 150 mL of water from an Escherichia coli CFU count of 4.3 × 10⁴/mL to 12/mL. While slow, this process requires less than 2 W, and can be powered from rechargeable sources. Faster processes can be implanted using multiple such reactor units in parallel, and can be optimized to the requirement and power levels.

Applications of Light-Emitting Diodes (LEDs) in Food Processing and Water Treatment

Light-emitting diode (LED) technology is an emerging nonthermal food processing technique that utilizes light energy with wavelengths ranging from 200 to 780 nm. Inactivation of bacteria, viruses, and fungi in water by LED treatment has been studied extensively. LED technology has also shown antimicrobial efficacy in food systems. This review provides an overview of recent studies of LED decontamination of water and food. LEDs produce an antibacterial effect by photodynamic inactivation due to photosensitization of light absorbing compounds in the presence of oxygen and DNA damage; however, such inactivation is dependent on the wavelength of light energy used. Commercial applications of LED treatment include air ventilation systems in office spaces, curing, medical applications, water treatment, and algaculture. As low penetration depth and high-intensity usage can challenge optimal LED treatment, optimization studies are required to select the right light wavelength for the application and to standardize measurements of light energy dosage.

Visible Light as an Antimicrobial Strategy for Inactivation of Pseudomonas fluorescens and Staphylococcus epidermidis Biofilms

The increase of antimicrobial resistance is challenging the scientific community to find solutions to eradicate bacteria, specifically biofilms. Light-Emitting Diodes (LED) represent an alternative way to tackle this problem in the presence of endogenous or exogenous photosensitizers. This work adds to a growing body of research on photodynamic inactivation using visible light against biofilms. Violet (400 nm), blue (420 nm), green (570 nm), yellow (584 nm) and red (698 nm) LEDs were used against Pseudomonas fluorescens and Staphylococcus epidermidis. Biofilms, grown on a polystyrene surface, were irradiated for 4 h. Different irradiance levels were investigated (2.5%, 25%, 50% and 100% of the maximum irradiance). Surviving cells were quantified and the inactivation kinetic parameters were estimated. Violet light could successfully inactivate P. fluorescens and S. epidermidis (up to 6.80 and 3.69 log10 reduction, respectively), while blue light was effective only against P. fluorescens (100% of maximum irradiance). Green, yellow and red irradiation neither increased nor reduced the biofilm cell density. This is the first research to test five different wavelengths (each with three intensities) in the visible spectrum against Gram-positive and Gram-negative biofilms. It provides a detailed study of the potential of visible light against biofilms of a different Gram-nature.

Multiwell plates for obtaining a rapid microbial dose-response curve in UV-LED systems

UV light-emitting diodes (UV-LEDs) have emerged as a new technology for water disinfection. Multiwell plates are a common tool in biological research, but they have never been used for UVC/UVB-inactivation experiments of microorganisms. In this study, a novel, rapid and simple UVC/UVB-inactivation assay was developed for a UV-LED system using a multiwell plate setup (96- and 24-well plates). The relative incident irradiance distribution across the exposed area was examined by spectroradiometry and nitrate-nitrite uniformity assay. The two methods showed a good correlation and high distribution factors (>0.89 and >0.94 for 96- and 24-well plates, respectively). In addition, the potential of the new system for determining disinfection efficacy of E. coli and MS2 coliphage by UV-LEDs emitting at central wavelengths of 265 nm and 285 nm was demonstrated. The inactivation rate constants were comparable to those obtained using UV-LED systems with the conventional dish (or beaker) setup, but the multiwell plate method allowed for many more repetitions. The proposed system is an alternative for UV-inactivation dose-response assay, especially when screening assays are desired, since it has the advantage of being fast, comprehensive (with a large number of simultaneous replicates) and easily adapted to various applications as UV-LED based photocatalysis experiments, UV effect on biofilm formation and UV-based AOP degradation experiments.

Analysis of microbial contamination of household water purifiers

Household water purifiers are increasingly used to treat drinking water at the household level, but their influence on the microbiological safety of drinking water has rarely been assessed. In this study, representative purifiers, based on different filtering processes, were analyzed for their impact on effluent water quality. The results showed that purifiers reduced chemical qualities such as turbidity and free chlorine. However, a high level of bacteria (10²-10⁶ CFU/g) was detected at each stage of filtration using a traditional culture-dependent method, whereas quantitative PCR with propidium monoazide (PMA) treatment showed 10⁶-10⁸ copies/L of total viable bacteria in effluent water, indicating elevated microbial contaminants after purifiers. In addition, high-throughput sequencing revealed a diverse microbial community in effluents and membranes. Proteobacteria (22.06-97.42%) was the dominant phylum found in all samples, except for purifier B, in which Melainabacteria was most abundant (65.79%). For waterborne pathogens, Escherichia coli (10⁰-10⁶ copies/g) and Pseudomonas aeruginosa (10⁰-10⁵ copies/g) were frequently detected by qPCR. Sequencing also demonstrated the presence of E. coli (0-6.26%), Mycobacterium mucogenicum (0.01-3.46%), and P. aeruginosa (0-0.16%) in purifiers. These finding suggest that water from commonly used household purifiers still impose microbial risks to human health.

Efficacy of UVC-LED in water disinfection on Bacillus species with consideration of antibiotic resistance issue

Ultraviolet light emitting diode (UV-LED) has attracted extensive attention as a new technology to replace traditional mercury lamp for water disinfection. This study reported for the first time the application of UVC-LEDs in range of 200-280 nm for the treatment of two Gram-positive tetracycline resistant bacteria (TRB) from Bacillus species and their tetracycline resistant gene (TRG). The results showed that UVC-LEDs can inactivate TRB up to 5.7-log and inhibit TRG expression, especially at 268 nm. The required fluence was approximate to that of the referential non-resistant bacteria using the same UVC-LED, but far less than that of TRB using mercury lamp. After UVC-LED irradiation, photoreactivation was the dominant mechanism to repair TRB, just like non-resistant bacteria. But contrary to non-resistant bacteria, the regrowth ratio of TRB was remarkably high at 24 h since the end of the irradition, nevertheless the number of the regrown bacteria in the irradiated water was still less than that in the non-irradiated water. Whereas TRB restored resistance after repair even applying 268 nm at a fluence up to 46.08 mJ/cm² (maximum in this study). This study highlights the merits of UVC-LED to effectively inactivate TRB in a prompt, energy-efficient and resistance-reducing way, while future study on TRB regrowth and resistance resilience is needed.

Application of Ultraviolet Light-Emitting Diodes (UV-LED) to Full-Scale Drinking-Water Disinfection

Ultraviolet light-emitting diodes (UV-LEDs) have recently emerged as a viable technology for water disinfection. However, the performance of the technology in full-scale drinking-water treatment systems remains poorly characterised. Furthermore, current UV disinfection standards and protocols have been developed specifically for conventional mercury UV systems and so do not necessarily provide an accurate indication of UV-LED disinfection performance. Hence, this study aimed to test the hypothesis that a full-scale UV-LED reactor can match the Cryptosporidium inactivation efficiency of conventional mercury UV reactors. Male-specific bacteriophage (MS2) was used as the Cryptosporidium spp. surrogate microorganism. The time-based inactivation efficiency of the full-scale reactor was firstly compared to that of a bench-scale (batch-type) UV-LED reactor. This was then related to mercury UV reactors by comparing the fluence-based efficiency of the bench-scale reactor to the USEPA 90% prediction interval range of expected MS2 inactivation using mercury UV lamps. The results showed that the full-scale UV-LED reactor was at least as effective as conventional mercury UV reactors at the water-quality and drive-current conditions considered. Nevertheless, comparisons between the bench- and full-scale UV-LED reactors indicated that improvements in the hydraulic flow profile and power output of the full-scale reactor could help to further improve the efficiency of UV-LED reactors for municipal drinking water disinfection. This represents the world’s first full-scale UV-LED reactor that can be applied at municipal water treatment works for disinfection of pathogenic microorganisms from drinking water.

Rationally designed tubular coaxial-electrode copper ionization cells (CECICs) harnessing non-uniform electric field for efficient water disinfection

Though well known for its anti-microbial property, copper is usually not considered for drinking water disinfection because of its health risk to human bodies under efficient biocidal concentration. Herein, we have rationally designed and constructed a tubular coaxial-electrode copper ionization cell (CECIC) that enables superior disinfection performance (~6-log removal of E. coli) with a very low effluent copper concentration (~200 μg/L). A non-uniform electric field with enhanced strength near the center electrode is generated in the chamber attributed to the coaxial center-outer electrode configuration. Exposure to the strong electric field subsequently increases the permeability of cell membrane, the excessive uptake of Cu ions into microbes, and thus the reinforced bacteria inactivation. The in-situ ionization results in a Cu ion concentration gradient with higher concentrations in the regions closer to the center. In addition, being driven by the electrophoresis and dielectrophoresis forces, the bacterial cells are transported to the vicinity of the center electrode, where both the electric field strength and Cu ion concentration are higher. These mechanisms in the CECIC synergistically result in the high inactivation efficiency with low Cu concentration in the effluent. The low-cost, high-efficiency, and disinfection-byproduct-free CECIC has shown significant potential in point-of-use applications.