Global modeling of lake-water indirect photochemistry based on the equivalent monochromatic wavelength approximation: The case of the triplet states of chromophoric dissolved organic matter

Chromophoric dissolved organic matter (CDOM) plays key role as photosensitizer in sunlit surface-water environments, and it is deeply involved in the photodegradation of contaminants. It has recently been shown that sunlight absorption by CDOM can be conveniently approximated based on its monochromatic absorption at 560 nm. Here we show that such an approximation allows for the assessment of CDOM photoreactions on a wide global scale and, particularly, in the latitude belt between 60°S and 60°N. Global lake databases are currently incomplete as far as water chemistry is concerned, but estimates of the content of organic matter are available. With such data it is possible to assess global steady-state concentrations of CDOM triplet states (³CDOM*), which are predicted to reach particularly high values at Nordic latitudes during summer, due to a combination of high sunlight irradiance and elevated content of organic matter. For the first time to our knowledge, we are able to model an indirect photochemistry process in inland waters around the globe. Implications are discussed for the phototransformation of a contaminant that is mainly degraded by reaction with ³CDOM* (clofibric acid, lipid regulator metabolite), and for the formation of known products on a wide geographic scale.

Solar water disinfection in large-volume containers: from the laboratory to the field. A case study in Tigray, Ethiopia

The lack of safe drinking water affects communities in low-to-medium-income countries most. This barrier can be overcome by using sustainable point-of-use water treatments. Solar energy has been used to disinfect water for decades, and several efforts have been made to optimise the standard procedure of solar water disinfection (SODIS process). However, the Health Impact Assessment of implementing advanced technologies in the field is also a critical step in evaluating the success of the optimisation. This work reports a sustainable scaling-up of SODIS from standard 2 L bottles to 25 L transparent jerrycans (TJC) and a 12-month field implementation in four sites of Tigray in Ethiopia, where 80.5% of the population lives without reliable access to safe drinking water and whose initial baseline average rate of diarrhoeal disease in children under 5 years was 13.5%. The UVA dose required for 3-log reduction of E. coli was always lower than the minimum UVA daily dose received in Tigray (9411 ± 55 Wh/m²). Results confirmed a similar decrease in cases of diarrhoea in children in the implementation (25 L PET TJC) and control (2 L PET bottles) groups, supporting the feasibility of increasing the volume of the SODIS water containers to produce safer drinking water with a sustainable and user-friendly process.

A model to predict the kinetics of direct (endogenous) virus inactivation by sunlight at different latitudes and seasons, based on the equivalent monochromatic wavelength approach

Sunlight plays an important role in the inactivation of pathogenic microorganisms such as bacteria and viruses in water. Here we present a model that is able to predict the kinetics of direct virus inactivation (i.e. inactivation triggered by sunlight absorption by the virion, without the role played by photochemically produced reactive intermediates generated by water-dissolved photosensitizers) on a global scale (from 60 °S to 60 °N latitude) and for the different months of the year. The model is based on the equivalent monochromatic wavelength (EMW) approach that was introduced recently, and which largely simplifies complex polychromatic calculations by approximating them with a monochromatic equation at the proper wavelength, the EMW. The EMW equation was initially established for mid-July conditions at a mid-latitude, and was then extended to different seasons and to the latitude belt where the day-night cycle is always observed throughout the year. By so doing, the first-order rate constant of direct virus photoinactivation can be predicted on a global scale, with the use of a relatively simple equation plus tables of pre-calculated input data, as a function of latitude, month, and key water parameters. The model was here applied to the virus organism phiX174, a somatic phage that is often used as proxy for pathogenic viruses undergoing fast direct inactivation, and for which a wide array of published inactivation data is available. Model predictions are validated by comparison with field data of inactivation of somatic phages by sunlight.

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.

Kinetic modeling of the synergistic thermal and spectral actions on the inactivation of Cryptosporidium parvum in water by sunlight

Water contamination with the enteroprotozoan parasite Cryptosporidium is a current challenge worldwide. Solar water disinfection (SODIS) has been proved as a potential alternative for its inactivation, especially at household level in low-income environments. This work presents the first comprehensive kinetic model for the inactivation of Cryptosporidium parvum oocysts by sunlight that, based on the mechanism of the process, is able to describe not only the individual thermal and spectral actions but also their synergy. Model predictions are capable of estimating the required solar exposure to achieve the desired level of disinfection under variable solar spectral irradiance and environmental temperature conditions for different locations worldwide. The thermal contribution can be successfully described by a modified Arrhenius equation while photoinactivation is based on a series-event mechanistic model. The wavelength-dependent spectral effect is modeled by means of the estimation of the C. parvum extinction coefficients and the determination of the quantum yield of the inactivation process. Model predictions show a 3.7% error with respect to experimental results carried out under a wide range of temperature (30 to 45 °C) and UV irradiance (0 to 50 W·m^-2). Furthermore, the model was validated in three scenarios in which the spectral distribution radiation was modified using different plastic materials common in SODIS devices, ensuring accurate forecasting of inactivation rates for real conditions.

Kinetic modeling of the synergistic thermal and spectral actions on the inactivation of viruses in water by sunlight

Sunlight can be an effective tool for inactivating pathogens in water disinfection processes. In clear water, photoinactivation of viruses is driven by the absorption of UVB radiation and it is more efficient at shorter wavelengths. Moreover, the temperature can significantly improve the efficiency of the process. To date, no kinetic model has been reported that describes the simultaneous thermal and spectral effects that occur during the solar inactivation of viruses. This work presents a novel comprehensive kinetic model for the solar inactivation of MS2 coliphage as a function of the water temperature, irradiance, and spectral distribution of the incident radiation. The model is based on a combination of the modified Arrhenius equation, a wavelength-dependent first-order inactivation model with the quantum yield, and thermal parameters estimated from laboratory data. Model predictions have a 9% error with respect to experiments in the temperature range from 30 to 50 °C and UV irradiance range from 15 to 50 W/m². Moreover, the model was validated in three scenarios using different plastic materials that modify the spectral range of the radiation reaching the water, confirming an accurate prediction of inactivation rates for real solar disinfection systems worldwide using containers made of any material.

Material selection and prediction of solar irradiance in plastic devices for application of solar water disinfection (SODIS) to inactivate viruses, bacteria and protozoa

Solar water disinfection (SODIS) is a simple, inexpensive and sustainable Household Water Treatment (HWT) that is appropriate for low-income countries or emergency situations. Usually, SODIS involves solar exposure of water contained in transparent polyethylene terephthalate (PET) bottles for a minimum of 6 h. Sunlight, especially UVB radiation, has been demonstrated to photoinactivate bacteria, viruses and protozoa. In this work, an in-depth study of the optical and mechanical properties, weathering and production prices of polymeric materials has been carried out to identify potential candidate materials for manufacturing SODIS devices. Three materials were ruled out (polystyrene (PS), polyvinyl chloride (PVC) and polyethylene (PE)) and four materials were initially selected for study: polymethylmethacrylate (PMMA), polypropylene (PP), polycarbonate (PC) and polyethylene terephthalate (PET). These plastics transmit sufficient solar radiation to kill waterborne pathogens with production costs compensated by their durability under solar exposure. A predictive model has been developed to quantitatively estimate the radiation available for SODIS inside the device as a function of the material and thickness. This tool has two applications: to evaluate design parameters such as thickness, and to estimate experimental requirements such as solar exposure time. In this work, this model evaluated scenarios involving different plastic materials, device thicknesses, and pathogens (Escherichia coli bacterium, MS2 virus and Cryptosporidium parvum protozoon). The developed Solar UV Calculator model is freely available and can be also applied to other customized materials and conditions.