Solar Energy, Vol.158, 303-310, 2017
Estimating lethal dose of solar radiation for enterococcus inactivation through radiation reaching the water layer. Application to Solar Water Disinfection (SODIS)
SODIS is one of the least frequently used technologies in development cooperation interventions for creating safe household drinking water. The primary weaknesses are the need for a supply chain of SODIS devices, usually transparent plastic bottles, and the uncertainty regarding the point at which solar disinfection is achieved. The parameter 'Lethal UV dose' (W h m(-2) or kJ m(-2)) is the most commonly used to determine a suitable disinfection level. In general, it is calculated according to the radiation dose delivered on the reactor surface. However, it should be determined taking into account the peculiarities of the water used, the SODIS device, and the characteristic radiation of the location of use in order to obtain a reference value for a protective treatment under different conditions. This study proposes an equation to estimate the lethal UV dose, according to the radiation that really achieves water. The equation has been employed in order to discuss the solar disinfection of wild strains of enterococcus that is evidenced in wastewater by using several SODIS devices. Three plastic bags and a typical PET bottle were used. Bags share a functional design to facilitate the SODIS application. A radiation dose in a water layer of UVA-B (280-400 nm) of 40 W h m(-2) has been determined to achieve a disinfection of two log units for enterococci in both polymer bags, however, UVA radiation acts of only (315-400 nm) 110 W h m(-2) are necessary in the water layer for the same level of disinfection. Reference values of a lethal UV dose in the water layer obtained in the laboratory were corroborated in field experiments that were performed in Tangier (Morocco). Field experiments outperform disinfection results obtained in laboratory experiments due to the higher irradiance conditions that were encountered. This work concludes that time of solar exposure required for a safe application of the SODIS method can be estimated with the utilization of simple and versatile measurements for several conditions of irradiance, water UV transmittance, and the type of SODIS device.