Solar disinfection of drinking water (SODIS): an investigation of the effect of UV-A dose on inactivation efficiency

The effect of solar UV-A irradiance and solar UV-A dose on the inactivation of Escherichia coli K-12 using solar disinfection (SODIS) was studied. E. coli K-12 was seeded in natural well-water contained in borosilicate glass tubes and exposed to sunlight at different irradiances and doses of solar UV radiation. In addition, E. coli K-12 was also inoculated into poly(ethylene) terephthalate (PET) bottles and in a continuous flow system (10 L min⁻¹) to determine the effect of an interrupted and uninterrupted solar dose on inactivation. Results showed that inactivation from approximately 10⁶ CFU mL⁻¹ to below the detection level (4 CFU/mL) for E. coli K-12, is a function of the total uninterrupted dose delivered to the bacteria and that the minimum dose should be >108 kJ m⁻² for the conditions described (spectral range of 0.295–0.385 μm). For complete inactivation to below the limit of detection, this dose needs to be received regardless of the incident solar UV intensity and needs to be delivered in a continuous and uninterrupted manner. This is illustrated by a continuous flow system in which bacteria were not fully inactivated (residual viable concentration 10² CFU/mL) even after 5 h of exposure to strong sunlight and a cumulative dose of >108 kJ m⁻². This has serious implications for attempts to scale-up solar disinfection through the use of re-circulatory continuous flow reactors.