Synthetic and natural waters disinfection using natural solar radiation in a pilot plant with CPCs

Solar disinfection of synthetic and natural waters from the Douro River, northern Portugal was studied in a pilot plant with compound parabolic collectors. Inactivation of Enterococcus faecalis was slower than Escherichia coli possibly due to the cell wall composition of the Gram-positive and Gram-negative bacteria, respectively. The high content of peptidoglycan, teichoic acids, polysaccharides, and peptidoglycolipids, in E. faecalis cell wall, when compared with E. coli, acts as a protective coating. Higher inactivation rate constants were obtained for higher initial bacteria concentrations; however a greater dose of UV energy was required. The flow rate effect in disinfection of synthetic waters was negligible. However, for natural waters with low bacteria contamination, the effect of the mechanical stress on the inactivation increased with the flow rate. Competition for the reactive oxidant radicals was observed in binary systems, containing similar concentrations of E. coli and E. faecalis. No bacterial regrowth was observed for E. faecalis in synthetic waters. Oppositely, regrowth occurred for natural waters. This behaviour can be due to the natural water chemical composition, with the presence of various organic and inorganic species.