Synergistic Bacterial Inactivation by Silver Ions and Free Chlorine in Natural Waters

Using high amounts of chlorine to disinfect contaminated natural waters for drinking purposes can produce an unpleasant taste and odor and contribute to the formation of toxic byproducts. These challenges can be addressed through the combined use of lower amounts of chlorine and silver. Several studies in well water or solutions inoculated with bacteria or viruses have demonstrated that this combination produces a synergistic effect in the inactivation of pathogens. This study investigated the synergistic inactivation of bacteria in natural waters (from a pond with 4.82 NTU and upstream with 11.9 NTU in Virginia) using low doses of silver (added as silver nitrate) and free chlorine (from Aquatabs). There was a significant synergistic effect at 3-h contact time, and the ${\log }_{10}$ reductions of E. coli and total coliform bacteria (TCB) were $\ge 1.44$ and $\ge 2.73$, respectively, with the lower-turbidity water, and 0.87 and 1.29, respectively, with the higher-turbidity water. Chlorine effectiveness was significantly reduced by higher turbidity, whereas silver effectiveness was not. Thus, for waters with higher turbidity, silver alone or a combination of low doses of silver and chlorine may produce a higher bacteria inactivation than chlorine alone. In addition, bacteria inactivation by the MadiDrop+ (MD, a commercial silver-ceramic tablet that releases silver ions for point-of-use water disinfection), with low doses of free chlorine in water from a stream in South Africa, was tested. The MD alone at 8-h contact time obtained $\sim 1\text{-}{\log }_{10}$ reduction for E. coli and $\sim 2\text{-}{\log }_{10}$ reduction for TCB. However, some of the MD-free chlorine combinations achieved a similar bacteria reduction with a substantial reduction of contact time (between 6 and 7 h less). Overall, these results show that the silver–chlorine synergistic effect demonstrated in previous studies with solutions inoculated with pathogens is also present in the more realistic scenario of natural waters that contain more complex matrices.