Preliminary studies into the disinfection of potable water using solar radiation

Abstract

The efficacy and kinetics of water disinfection using solar radiation was studied, with respect to potential household or individual use in regions where treated drinking water is unavailable. Small volumes of sterilized water (1 L or less) were deliberately contaminated with various concentrations of Escherichia coli, and then exposed to solar radiation for several hours with periodic bacterial enumeration using plate count methods. An agar which was selective for fecal coliforms (mFC) was found to be inappropriate in this work as it often gave false negative counts, compared to the nonselective total plate count agar. The die-off of bacteria could be modelled, as a function of cumulative solar dose, in an exponential decay model. Decay rate constants of 0.071 to 0.486 m2∙MJ−1 were estimated. Volumes of 250 mL or less (in a 1 L bottle), and waters with lower bacterial concentrations or higher ionic strength, exhibited higher rates of die-off. Bacterial counts could be reduced from several thousands to less than 1 per millilitre (lower detection limit) during one day of solar irradiation. The source of the water and bottle material had a significant effect on the die-off rate. A Weibull distribution-based model has a conceptually better theoretical basis, and also fits the experimental data adequately at low and high doses. At intermediate solar doses, the Weibull model fit tended to underestimate the bacterial inactivation whereas the exponential decay model overestimated it. While solar disinfection appears useful for bacterial inactivation, it may be ineffective in situations where the major contaminants are the more UV resistant viruses or parasites, or where significant competition from physical or chemical water characteristics exists. It is not likely to be a suitable alternative to conventional chemical disinfection, but may be applicable in situations where no other disinfection is available. Key words: solar disinfection, potable water, coliform bacteria, inactivation kinetics, modelling.