A comprehensive kinetic model that estimates the required solar exposure time in large-volume containers subjected to the SODIS process has been developed. This model accounts for the spectral radiation transport from the sun to the pathogens (including the attenuation caused by the atmosphere, containers’ walls, and naturally occurring substances dissolved in water) and the mechanistic chemical reactions that happen during the solar disinfection of viruses, protozoa, and bacteria. The following conclusions highlight for each piece of the entire model:
▸SOLAR RADIATION: An algorithm has been developed to predict the real daily dose depending on the latitude and the day of the year. As a particular strength, this procedure accounts for spectral distribution, being possible to calculate the actual daily dose reaching SODIS containers for each wavelength.
▸CONTAINER: The Solar UV Calculator tool has been developed to determine the spectral irradiance within SODIS containers manufactured with alternative suitable plastic materials. PMMA and PP with 1% UV-stabiliser were identified as excellent materials for manufacturing SODIS devices (excellent optical properties and disinfection rates). Due to their mechanical properties, PPMA is recommended for static SODIS devices and PP for portable containers.
▸WATER COMPOSITION: A radiation distribution-based procedure has been developed to calculate the effective available incident radiation in large-volume containers as a function of naturally occurring substances present in water such as (bi)carbonates, soluble carbohydrates, solids and humic acids. The two latter and iron act as radiation-attenuating factors, but iron also enhances bacterial damage, probably because of its permeation into the cell and contribution to the intracellular Fenton process.
▸KINETIC MODEL: A set of three mechanistic kinetic models were developed to predict solar water disinfection of viruses, protozoa, and bacteria and successfully reproduced experimental data. The three kinetic models consider the effect of the water temperature, the photoinactivation, and the UV-T synergistic effects, but their own ways to reproduce the behaviour of each pathogen. Under dark conditions, thermal inactivation caused by the rise of water temperature was only observed for protozoa (above 40°C) and bacteria (above 30°C), but achieving similar disinfection rates at 50°C. In contrast, the synergistic effect was strongly observed for viruses (above 30°C), moderately for protozoa (above 40°C), and softly for bacteria (above 30°C). Nonetheless, under illuminated conditions, bacteria showed to be the most sensitive microorganism, followed by viruses and, finally, protozoa. Due to the relative simplicity of the kinetics for viruses and protozoa, in which the action of light was only noticeable by direct damage, the dependence on the spectral irradiance was included fairly reproducing the spectral absorption of DNA/RNA. In the case of the bacterial model, the enhancement caused by H2O2 addition was studied and modelled, helping to elucidate the internal cellular mechanisms and their kinetic parameters.
▸Validation of the developed algorithm to predict actual daily dose for other wavelengths (equivalent wavelengths for other types of damage or pathogens).
▸Study of PC ageing. Evolution of mechanical and optical properties and effect on the durability of containers and the disinfection rates.
▸Economic and life cycle assessments of new SODIS container materials.
▸Study of the impact of naturally occurring substances on solar water disinfection processes to inactivate viruses and protozoa in large-volume containers manufactured with PET or other materials.
▸Kinetic modelling of the effect of iron as enhancer of disinfection when it permeates into the cell.
▸Incorporation of the spectral dependence to the bacterial solar disinfection kinetic model.
▸Optimisation of kinetic parameters to other strains of bacteria, virus, and protozoa.
▸Validation of the global kinetic modelling approach with field experiments around the world