Continuous-flow solar UVB disinfection reactor for drinking water

Development and modeling of a novel type of photoreactors with exterior ultraviolet (UV) reflector for water treatment applications

Ultraviolet (UV) water disinfection method has emerged as an alternative to chemical methods of disinfection. In typical UV photoreactors for water treatment, water flows in the space between the lamp's sleeve and outer shell. The contact of water and sleeve causes fouling, which reduces the effectiveness of UV. To clean the photoreactor, the quartz sleeve must be replaced; this may lead to quartz or lamp breakage and mercury leakage into water during cleaning. In this study, a novel type of multi-lamp UV photoreactors is proposed, in which the UV lamps are placed out of the water channel and their UV irradiation is redirected into the channel using an outer cylindrical reflector. This allows for the installment of a self-cleaning mechanism for the water channel. A well-validated three-dimensional CFD model is utilized to model the performance of this photoreactor for microbial inactivation. The impacts of several geometrical and optical parameters are investigated on the inactivation of microorganisms. The results revealed that the difference in log reduction values (LRV) between fully specular and fully diffuse reflector ranges from 10 to 47% as the lamp-to-channel distance increases. For the volumetric flow rate of 25 GPM, the LRV of a photoreactor with fully diffuse reflector can be 46% higher than a fully specular one. In addition, the performance of the proposed photoreactor is compared against a classic L-shaped annular photoreactor. The results show that the new design can provide equal or better microbial performance compared to the classic photoreactor, but it removes many of their common issues such as quartz fouling, lamp overheating at low flow rates, and sleeve breakage during lamp replacement.

A Low Cost and Eco-Sustainable Device to Determine the End of the Disinfection Process in SODIS

The lack of safe drinking water is one of the main health problems in many regions of the world. In order to face it, Solar water disinfection (SODIS) proposes the use of transparent plastic containers, which are filled with contaminated water, and exposed to direct sunlight until enough UV radiation is received to inactivate the pathogens. However, a reliable method for determining the end of the disinfection process is needed. Although several approaches have been proposed in the literature for this purpose, they do not strictly accomplish two critical constraints that are essential in this type of project, namely, low cost and sustainability. In this paper, we propose an electronic device to determine when the lethal UV dose has been reached in SODIS containers, which accomplishes both constraints mentioned above: on the one hand, its manufacturing cost is around EUR 12, which is much lower than the price of other electronic solutions; on the other hand, the device is sufficiently autonomous to work for months with small low-cost disposable batteries, thereby avoiding the use of rechargeable batteries, which are considered hazardous waste at the end of their useful life. In our approach, we first analyze different low cost UV sensors in order to select the most accurate one by comparing their response with a reference pattern provided by a radiometer. Then, an electronic device is designed using this sensor, which measures the accumulated UV radiation and compares this value with the lethal UV dose to determine the end of the disinfection process. Finally, the device has been manufactured and tested in real conditions to analyze its accuracy, obtaining satisfactory results.

Comparative Life Cycle Assessment of Water Disinfection Processes Applicable in Low-Income Settings

Access to safe, sufficient water for health and sanitation is a human right, and the reliable disinfection of water plays a critical role in addressing this need. The environmental impact and sustainability of water disinfection methods will also play a role in overall public health. This study presents an investigation of the environmental life cycle impacts of four ultraviolet disinfection systems utilizing ambient solar radiation directly and indirectly for water disinfection in comparison to chlorination and water delivery for application in low-income settings. Product inspection and existing literature were used to define a life cycle functional unit of 1 m³ of water for each system, which allowed quantification of material use, infrastructure requirements, and life cycle of the original components of each system and those needed to keep them operational for the studied lifespans (1, 5, 10, and 20 years) and scales (30, 100, 500, and 1000 L per day). For all studied cases, chlorine had the lowest impact in all impact categories, but end-user acceptance of chlorine in some settings is low, driving interest in low-impact alternatives. Disinfection based on low-pressure mercury lamps had the next lowest normalized impact in most categories and may represent a viable alternative, particularly for long-term (10+ years), high production (500+ liters per day) scenarios.

Solar disinfection (SODIS) technologies as alternative for large-scale public drinking water supply: Advances and challenges

Gastrointestinal waterborne diseases, continue to stand out among the most lethal diseases in developing countries, because of consuming contaminated water taken from unsafe sources. Advances made in recent decades in methods of solar water disinfection (SODIS) have shown that SODIS is an effective and inexpensive method of providing drinking water, capable of substantially reducing the prevalence and mortality of waterborne diseases. The increased impact of SODIS in communities lacking drinking water services depends on a successful upgrade from conventional SODIS (based on PET bottle reactors) in high flow continuous flow systems for solar water disinfection (CFSSWD). This review aimed to identify the main limitations of conventional SODIS that hinder its application as a large-scale drinking water supply strategy, and to propose ways to overcome these limitations (without making it economically inaccessible) based on the current frontier of advances technological. It was found that the successful development of the CFSSWD depends on overcoming the current limitations of conventional SODIS and the development of systems whose configurations allow combining the properties of solar pasteurization (SOPAS) and SODIS. Different improvements need to be made to the main components of the CFSSWD, such as increasing the performance of solar radiation collectors, photo and thermal reactors and heat exchangers. The integration of disinfection technologies based on photocatalytic and photothermal nanomaterials also needs to be achieved. The performance evaluation of the CFSSWD should be made considering resistant microorganisms, such as the environmental resistance structures of bacteria or protozoa (spores or (oo)cysts) as targets of disinfection approaches.

Insights into Solar Disinfection Enhancements for Drinking Water Treatment Applications

Poor access to drinking water, sanitation, and hygiene has always been a major concern and a main challenge facing humanity even in the current century. A third of the global population lacks access to microbiologically safe drinking water, especially in rural and poor areas that lack proper treatment facilities. Solar water disinfection (SODIS) is widely proven by the World Health Organization as an accepted method for inactivating waterborne pathogens. A significant number of studies have recently been conducted regarding its effectiveness and how to overcome its limitations, by using water pretreatment steps either by physical, chemical, and biological factors or the integration of photocatalysis in SODIS processes. This review covers the role of solar disinfection in water treatment applications, going through different water treatment approaches including physical, chemical, and biological, and discusses the inactivation mechanisms of water pathogens including bacteria, viruses, and even protozoa and fungi. The review also addresses the latest advances in different pre-treatment modifications to enhance the treatment performance of the SODIS process in addition to the main limitations and challenges.

Using Algal Virus Paramecium bursaria Chlorella Virus as a Human Adenovirus Surrogate for Validation of UV Treatment Systems

Adenovirus is among the most UV-resistant waterborne human pathogens. There is a need to identify nonpathogenic surrogates for adenovirus for the water treatment industry. In this study, the feasibility of using the algal virus Paramecium bursaria chlorella virus (PBCV-1) as an adenovirus surrogate for validation of UV reactors was evaluated. The UV dose-response behavior of PBCV-1 to monochromatic UV radiation at 254 nm and action spectrum for wavelengths ranging from 214 to 289 nm were measured. A culture-based infectivity assay was used to evaluate viral inactivation, and a quantitative PCR assay was used to quantify DNA damage. A UV₂₅₄ dose of 150 mJ/cm² resulted in roughly 5-log₁₀ units of reduction of PBCV-1, which is similar to that of adenovirus. Furthermore, the inactivation action spectrum of PBCV-1 was similar to that of adenovirus between 214 and 289 nm. A simplified and inexpensive prepurification method was also developed to prepare PBCV-1 viral suspensions with similar inactivation behavior to purified PBCV-1. Overall, PBCV-1 appears to represent an appropriate adenovirus surrogate for UV system performance evaluation and illustrates the potential of using algal viruses as nonpathogenic, easy to culture, and readily available surrogates for human pathogens.

Spectrum of virucidal activity from ultraviolet to infrared radiation

The COVID-19 pandemic has sparked a demand for safe and highly effective decontamination techniques for both personal protective equipment (PPE) and hospital and operating rooms. The gradual lifting of lockdown restrictions warrants the expansion of these measures into the outpatient arena. Ultraviolet C (UVC) radiation has well-known germicidal properties and is among the most frequently reported decontamination techniques used today. However, there is evidence that wavelengths beyond the traditional 254 nm UVC - namely far UVC (222 nm), ultraviolet B, ultraviolet A, visible light, and infrared radiation - have germicidal properties as well. This review will cover current literature regarding the germicidal effects of wavelengths ranging from UVC through the infrared waveband with an emphasis on their activity against viruses, and their potential applicability in the healthcare setting for general decontamination during an infectious outbreak.

MS2 coliphage and E. coli UVB inactivation rates in optically clear water: dose, dose rate and temperature dependence

Natural ultraviolet irradiance disinfection is known to play a significant role in both natural wastewater treatment systems and drinking water disinfection processes, while the influence of ultraviolet B (UVB) delivering method on sunlight disinfection outcome is still unclear. This study aims to determine the effects of environmentally relevant temperatures, UVB doses (J m^-2) and dose rates (W m^-2) on the inactivation and log reduction values (LRVs) of the F-RNA coliphage MS2 and Escherichia coli in optically clear water. E. coli and MS2 were separately incubated and irradiated at five different doses of UVB light that delivered using six UVB dose rates. The results of the study demonstrate that the UVB dose delivering method (combination of dose rate and exposure time) influences inactivation and LRVs of E. coli and MS2 at all UVB doses investigated (up to seven-fold difference). Two phases were identified within the UVB dose rate, UVB inactivation or LRV curves for both organisms; a UVB dose rate limited inactivation phase and a dose rate saturation inactivation phase. The results contribute to a better understanding of UVB disinfection in the environment and natural wastewater treatment systems, potentially improving the design and operation of high rate algal ponds.

Ultraviolet A and B wavelength-dependent inactivation of viruses and bacteria in the water

UVA and UVB can be applied to solar disinfection of water. In this study, the inactivation and photoreactivation of viruses and bacteria in the UVA-B range were analyzed. MS2 and T4 bacteriophages, and Escherichia coli were used as surrogates to quantify dose-response behaviors. Inactivation in UVC was used to validate the methodology and to expand the inactivation action spectra. The results showed log-linear inactivation for MS2 and T4 in the 254-320 nm wavelength range. T4 inactivation was consistently faster than MS2 (except at 320 nm), and for both phages, inactivation decreased with increasing wavelength. The dose-response of bacteria exhibited a lag at low doses, possibly because the photons must strike a discrete number of critical targets before growth stops. A tail was present at high doses for some wavelengths, perhaps due to clumping or the presence of subgroups with higher resistance. The inactivation action spectra for bacteria exhibited a reduction in inactivation as wavelength increased. No bacterial inactivation was observed beyond 320 nm at doses applied. After inactivation at 297 nm (UVA), bacteria regained viability through photoreactivation, and repair increased with increase in photoreactivating light exposure time. This implies additional doses above inactivation thresholds are required to cause irreversible damage. These results are useful for designing solar disinfection systems.

Sunlight-mediated inactivation of health-relevant microorganisms in water: a review of mechanisms and modeling approaches

Health-relevant microorganisms present in natural surface waters and engineered treatment systems that are exposed to sunlight can be inactivated by a complex set of interacting mechanisms. The net impact of sunlight depends on the solar spectral irradiance, the susceptibility of the specific microorganism to each mechanism, and the water quality; inactivation rates can vary by orders of magnitude depending on the organism and environmental conditions. Natural organic matter (NOM) has a large influence, as it can attenuate radiation and thus decrease inactivation by endogenous mechanisms. Simultaneously NOM sensitizes the formation of reactive intermediates that can damage microorganisms via exogenous mechanisms. To accurately predict inactivation and design engineered systems that enhance solar inactivation, it is necessary to model these processes, although some details are not yet sufficiently well understood. In this critical review, we summarize the photo-physics, -chemistry, and -biology that underpin sunlight-mediated inactivation, as well as the targets of damage and cellular responses to sunlight exposure. Viruses that are not susceptible to exogenous inactivation are only inactivated if UVB wavelengths (280-320 nm) are present, such as in very clear, open waters or in containers that are transparent to UVB. Bacteria are susceptible to slightly longer wavelengths. Some viruses and bacteria (especially Gram-positive) are susceptible to exogenous inactivation, which can be initiated by visible as well as UV wavelengths. We review approaches to model sunlight-mediated inactivation and illustrate how the environmental conditions can dramatically shift the inactivation rate of organisms. The implications of this mechanistic understanding of solar inactivation are discussed for a range of applications, including recreational water quality, natural treatment systems, solar disinfection of drinking water (SODIS), and enhanced inactivation via the use of sensitizers and photocatalysts. Finally, priorities for future research are identified that will further our understanding of the key role that sunlight disinfection plays in natural systems and the potential to enhance this process in engineered systems.

Ray Tracing for Fluence Rate Simulations in Ultraviolet Photoreactors

The performance of photochemical reactors is governed by the spatial distribution of radiant energy within the irradiated region of the reactor. Ray tracing has been widely used for simulation of lighting systems. The central hypothesis of this work was that ray tracing can provide accurate simulations of fluence rate fields within ultraviolet (UV) photoreactors by accounting for the physical and optical phenomena that will govern fluence rate fields in UV photoreactors. Ray tracing works by simulating the behavior of a large population of rays emanating from a radiation source to describe the spatial distribution of radiant energy (i.e., fluence rate) within a system. In this study, fluence rate calculations were performed using commercial ray tracing software for three basic UV reactors, each with a single low-pressure Hg lamp. Fluence rate calculations in the ray tracing program were based on the formal definition of fluence rate, calculated as the incident radiant power from all directions on a small spherical receptor, divided by the cross-sectional area of that sphere. The results of this study demonstrate that ray tracing can provide predictions of fluence rate in UV radiative systems that are close to experimental measurements and the predictions of other numerical methods.

Tetraselmis as a challenge organism for validation of ballast water UV systems

Transport and release of waterborne organisms as a result of ballasting and de-ballasting operations is widely acknowledged to represent an important mechanism for invasions by non-indigenous species. Regulatory requirements have been implemented globally to require treatment of ballast water before its release to the environment as a means of minimizing risks of invasion. UV-based processes represent an option for ballast water treatment; however, their use will require development of appropriate methods for reactor validation. To address this need, Tetraselmis was examined as challenge organism using a most probable number (MPN) assay for quantification of the concentration of viable (reproductively active) cells in suspension. A low pressure collimated-beam reactor was used to investigate UV₂₅₄ dose-response behavior of Tetraselmis. Based on the experimental conditions applied, Tetraselmis indicated 4.5-5 log₁₀ units of inactivation for UV₂₅₄ doses of approximately 120 mJ/cm², with no apparent change of resistance resulting from repeated exposure. A medium pressure UV collimated-beam reactor equipped with a series of narrow bandpass optical filters was used to investigate the action spectrum of Tetraselmis for wavelengths ranging from 228 nm-297 nm. Radiation with wavelengths in the range 254-280 nm was observed to be most efficient for inactivation of Tetraselmis. Additionally, DNA was extracted from Tetraselmis to allow measurement of its absorption spectrum. These results indicated strong absorbance from 254 nm to 280 nm, thereby suggesting that damage to DNA plays an important role in the inactivation of Tetraselmis sp. However, deviations of the action spectrum shape from the shape of the DNA absorption spectrum suggest that UV-induced damage to biomolecules other than DNA may contribute to Tetraselmis inactivation at some wavelengths in the UVC range.

Constructed wetlands and solar-driven disinfection technologies for sustainable wastewater treatment and reclamation in rural India: SWINGS project

SWINGS was a cooperation project between the European Union and India, aiming at implementing state of the art low-cost technologies for the treatment and reuse of domestic wastewater in rural areas of India. The largest wastewater treatment plant consists of a high-rate anaerobic system, followed by vertical and horizontal subsurface flow constructed wetlands with a treatment area of around 1,900 m² and a final step consisting of solar-driven anodic oxidation (AO) and ultraviolet (UV) disinfection units allowing direct reuse of the treated water. The implementation and operation of two pilot plants in north (Aligarh Muslim University, AMU) and central India (Indira Gandhi National Tribal University, IGNTU) are shown in this study. The overall performance of AMU pilot plant during the first 7 months of operation showed organic matter removal efficiencies of 87% total suspended solids, 95% 5-day biological oxygen demand (BOD₅) and 90% chemical oxygen demand, while Kjeldahl nitrogen removal reached 89%. The UV disinfection unit produces water for irrigation and toilet flushing with pathogenic indicator bacteria well below WHO guidelines. On the other hand, the AO disinfection unit implemented at IGNTU and operated for almost a year has been shown to produce an effluent of sufficient quality to be reused by the local population for agriculture and irrigation.

Intracellular mechanisms of solar water disinfection

Solar water disinfection (SODIS) is a zero-cost intervention measure to disinfect drinking water in areas of poor access to improved water sources, used by more than 6 million people in the world. The bactericidal action of solar radiation in water has been widely proven, nevertheless the causes for this remain still unclear. Scientific literature points out that generation of reactive oxygen species (ROS) inside microorganisms promoted by solar light absorption is the main reason. For the first time, this work reports on the experimental measurement of accumulated intracellular ROS in E. coli during solar irradiation. For this experimental achievement, a modified protocol based on the fluorescent probe dichlorodihydrofluorescein diacetate (DCFH-DA), widely used for oxidative stress in eukaryotic cells, has been tested and validated for E. coli. Our results demonstrate that ROS and their accumulated oxidative damages at intracellular level are key in solar water disinfection.

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

Improvements in point-of-use (POU) drinking water disinfection technologies for remote and regional communities are urgently needed. Conceptually, UV-C light-emitting diodes (LEDs) overcome many drawbacks of low-pressure mercury tube based UV devices, and UV-A or visible light LEDs also show potential. To realistically evaluate the promise of LED disinfection, our study assessed the performance of a model 1.3 L reactor, similar in size to solar disinfection bottles. In all, 12 different commercial or semi-commercial LED arrays (270-740 nm) were compared for their ability to inactivate Escherichia coli K12 ATCC W3110 and Enterococcus faecalis ATCC 19433 over 6h. Five log10 and greater reductions were consistently achieved using the 270, 365, 385 and 405 nm arrays. The output of the 310 nm array was insufficient for useful disinfection while 430 and 455 nm performance was marginal (≈ 4.2 and 2.3-log10s E. coli and E. faecalis over the 6h). No significant disinfection was observed with the 525, 590, 623, 660 and 740 nm arrays. Delays in log-phase inactivation of E. coli were observed, particularly with UV-A wavelengths. The radiation doses required for >3-log10 reduction of E. coli and E. faecalis differed by 10 fold at 270 nm but only 1.5-2.5 fold at 365-455 nm. Action spectra, consistent with the literature, were observed with both indicators. The design process revealed cost and technical constraints pertaining to LED electrical efficiency, availability and lifetime. We concluded that POU LED disinfection using existing LED technology is already technically possible. UV-C LEDs offer speed and energy demand advantages, while UV-A/violet units are safer. Both approaches still require further costing and engineering development. Our study provides data needed for such work.