Compound parabolic collector solar disinfection system for the treatment of harvested rainwater

Abstract

A cost-effective solar disinfection (SODIS) system fitted with a compound parabolic collector (CPC) (SODIS-CPC) was designed for the treatment of rainwater. One system (SODIS-CPC-1) was installed next to a 2000 L rainwater harvesting tank without pre-treatment (Tank 1) and a second system (SODIS-CPC-2) was installed next to a 2000 L tank connected to a pre-filtration first flush (FF) diverter [Tank 2 (FF)]. To analyse the efficiency of the SODIS-CPC systems, roof-harvested rainwater was exposed to direct sunlight in a borosilicate glass reactor for eight hours. Hereafter various physicochemical (e.g. pH, turbidity), chemical (e.g. anions, cations) and microbial (indicator counts using culture-based analysis) parameters were investigated in the untreated and SODIS treated rainwater. The anion and cation concentrations analysed in all the samples were within drinking water guidelines as stipulated by various national and international water associations. The Escherichia coli (E. coli) counts were reduced to below the detection limit in all the samples collected from SODIS-CPC-1 and SODIS-CPC-2 (FF), while the HPC were reduced to within the drinking water guideline of <1.0 × 10⁴ CFU per 100 mL. During the first four sampling sessions, the total coliform counts were reduced to below the detection limit (<1 CFU per 100 mL) after SODIS treatment; however, during the last three sampling sessions, counts with a mean ranging from 1 to 75 CFU per 100 mL were still recorded in the SODIS treated rainwater. For both systems, ethidium monoazide bromide quantitative polymerase chain reaction (EMA-qPCR) analysis revealed a mean reduction of 99.5% in viable Legionella copy numbers when a maximum mean ambient UV-A radiation of 29.5 W m⁻² was observed. Similarly, EMA-qPCR analysis revealed a mean reduction of 99.8% in viable Pseudomonas copy numbers when the rainwater temperatures were ≥52 °C in the SODIS treated samples (after eight hours) and a maximum mean ambient UV-A radiation of 20.5 W m⁻² was recorded on the sampling day. Thus, while the SODIS-CPC systems improved the quality of harvested rainwater, based on the presence of viable Legionella and Pseudomonas spp. after SODIS treatment, it is recommended that the treated rainwater is utilised for domestic purposes.