Solar Photocatalytic Disinfection of Water with Immobilised Titanium Dioxide in Re-circulating Flow CPC Reactors

Abstract

It is estimated that 780 million people lack access to improved water supplies and many more are forced to rely on sources that are microbiologically unsafe. While piped-in water supplies are the ultimate goal for the provision of water in developing countries, low cost point-of-use disinfection treatments could help to significantly reduce the incidence of water borne disease. Solar disinfection of water (SODIS) is a simple method of treating water where the UV and thermal energy of the sun act to inactivate pathogenic microorganisms in water; however, the recommended protocol is 6 h under direct sunlight, the efficiency depends on environmental factors, and some pathogens are more resistant to solar disinfection. The use of compound parabolic collectors (CPC) and immobilised titanium dioxide for photocatalysis were investigated as enhancement technologies for solar disinfection. The reactors consisted of borosilicate glass tubes (1.5 m in length) as either single tubes of diameter 50 mm or two concentric tubes (inner tube of diameter 32 mm) with and without CPC. For solar photocatalytic disinfection (SPC-DIS), the inside wall of the 50 mm tube was dip coated with TiO2 (TiO2, Evonik P25) and/or the outside wall of the 32 mm tube. SPC-DIS and SODIS were tested under flow conditions using different reactor configurations under real sun conditions. E. coli was used as the model microorganism for the disinfection studies in 0.9% saline solution and the total volume in each experiment was 7 L. It was found that the use of CPCs improved the SODIS and SPC-DIS efficiency. The disinfection kinetics were observed to follow a log-linear with shoulder and/or tailing model. The kinetic parameters were determined using the UVA dose as the independent variable and the configurations were compared for E. coli inactivation efficiency based on the log of the residual concentration (Log Nres) and the first order rate constant (k). Three reactor configurations showed a residual bacterial count below the detection limit and they were compared based upon the first order rate constant. The concentric tube arrangement (a tube within a tube) with CPC was the most effective configuration. The following order was found for k where coated refers to TiO2 coating and the equals sign indicates no significant difference; uncoated external – coated internal ≥ double coated tube ≥ uncoated double tube. It is known that E. coli is inactivated by SODIS and it may be a ‘soft’ target for comparing the effectiveness of SODIS vs SPC-DIS. Nevertheless, photocatalysis presents advantages in terms of the non-recovery of inactivated organisms and the inactivation of SODIS resistance organisms