Solar water disinfection in high-volume containers: are naturally occurring substances attenuating factors of radiation?

Access to safe drinking water is still a world challenge. The SODIS process is an easy and affordable household water treatment (HWT) for low income countries. The main limitations of its standard procedure are the low treated volume (2 L Polyethylene terephthalate [PET] bottles are commonly used) and the uncertainty in the required solar exposure time, which is strongly dependent on the water composition and irradiance. This work evaluates the use of high-volume (25 L) PET containers for the SODIS process, especially focusing on the effect of water composition on the efficacy of E. coli bacteria inactivation as a reference for faecal contamination. The optical properties (absorption and scattering) of the substances dissolved in the water were found to have a dramatic impact since the radiation distribution inside a high-volume container is not homogenous and strongly influences the disinfection rate. Simulation of the radiation distribution within the container allows modelling and predicting the required solar exposure time based on the average radiation intensity and its uniformity index as a function of the water composition. Bicarbonates, soluble carbohydrates, solids and humic acids greatly agreed with the model while iron was found to have a more complicated role since its main effect as attenuator of radiation is partially counteracted by the improvement in the disinfection rate due to an internal photo-Fenton mechanism within the cell. With this particular exception, it can be concluded that most of the naturally occurring substances in water mainly affect the performance of the SODIS process in high-volume containers by their potential role as attenuators of light. The potential role of the external substances in electron transfer mechanisms is completely overshadowed by the loss of radiation in the total volume of the container.