Validation of a solar-thermal water disinfection model for Escherichia coli inactivation in pilot scale solar reactors and real conditions

In the present work, the synergistic SODIS-thermal model, describing the E. coli inactivation by solar exposure (SODIS) considering the synergistic effect of solar UV photons and solar heating of water under controlled conditions of irradiance and temperature, is validated under real field conditions. The main objective of this work is to demonstrate its capability to predict the solar bacterial inactivation in several solar reactor designs, different scales, and under real field conditions, i.e. variable solar irradiation, water turbidity and temperature. The model was proven to be able to predict satisfactorily the E. coli inactivation under different climate conditions in plastic 2-L PET (polyethylene terephthalate) bottles, the most widely used for SODIS application, in isotonic and natural well water. This model predicts also, with a high acceptance level (NRMSLE < 20%), the E. coli inactivation in turbid water, experimentally studied with an artificial turbidity agent (kaolin) and natural red soils to simulate the turbidity between 5 and 300 NTU. The simulation results for turbid water were performed using the Radiative Transfer Equation for the incident irradiance. In addition, the model was applied for different reactor designs (volumes ranged 2.5-22.5 L) and materials (polycarbonate, borosilicate and methacrylate) concluding that transmittance affects significantly to the incident radiation and hence to the bacterial inactivation. The predicted water disinfection of the synergistic SODIS-thermal model has important implications in photo-reactor design as a potential tool for comparing the efficiency of new prototypes and for automatized control systems for SODIS reactors. A 'safetime' and 'safe UV-A dose' were defined as the minimal time or UV-A dose necessary to achieve a certain bacterial reduction.