Investigating the Synergistic Effect of Free Chlorine and Silver Ions in Natural Waters for Antibacterial Activity, and Developing a Material for Low-Dose Chlorine Release in Water for Pathogen Inactivation

Point-of-use (POU) water treatment technologies that contain free chlorine (e.g., chlorine tablets, bleach) release high quantities of chlorine (up to 4,000 μg/L) to disinfect water. These high concentrations can create an unpleasant taste and odor in the treated water and may lead to discontinuation of the disinfection treatment. Other POU technologies that use silver provide effective inactivation of bacterial pathogens and do not change the taste and odor of the water, but they do not perform as well as chlorine against viruses. Multiple studies in well water or solutions inoculated with bacteria or viruses have evaluated the use of combinations of metals (i.e., silver and copper) and chlorine in doses that meet the World Health Organization guidelines for drinking water and have demonstrated the presence of a synergistic pathogen inactivation effect with these combinations. This dissertation describes the evaluation of the silver-chlorine synergistic effect in natural waters and the development and assessment of a material that releases low doses of chlorine in water for pathogens’ inactivation.
We first 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 (10 μg/L) and free chlorine (100 μg/L). There was a significant synergistic effect at 3-hr contact time and chlorine effectiveness was significantly reduced by higher turbidity, whereas silver effectiveness was not. In addition, bacteria inactivation by the MadiDrop+, MD (a commercial silver-ceramic tablet that releases silver ions for POU water disinfection), and low doses of free chlorine (50-200 μg/L) in water from a stream in South Africa was tested. The MD alone at 8-hr contact time obtained ~1-log10 reduction for E. coli and ~2-log10 reduction for total coliform bacteria. However, some of the MD-free chlorine combinations achieved a similar bacteria reduction with a substantial reduction of contact time (between 6- and 7-hr 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 with natural waters which contain more complex matrixes.
We also developed gels that release low doses of chlorine in water. We tested the gels against E. coli bacteria and MS2 bacteriophage virus in deionized water that contains salts to simulate groundwater. In addition, we evaluated the gels together with the MD and a copper releasing material or copper screen. Results show that after 8-hr of treatment the gels are effective for bacteria inactivation, and in combination with the MD, the inactivation was close to 2-log10 reduction (E. coli reduction for gels-MD combination, gels alone, and MD alone: 1.86-, 1.10-, and 0.69-log10 reduction, respectively). But when the gels were combined with the copper screen there was essentially no increase in the reduction of bacteria compared to when the gels were used alone. On the other hand, after 8- or 24-hr of treatment, the gels were not effective for MS2 inactivation. However, contrary to E. coli inactivation, combining the gels with the copper screen did increase the reduction of MS2 compared to the screen alone (8-hr treatment: 0.4-log10 reduction for the gels-screen combination, and 0.2-log10 reduction for the copper screen; 24-hr treatment: 3.3-log10 reduction for the gels-screen combination, and 2.1-log10 reduction for the copper screen), and when the gels were combined with the MD there was no increase in virus reduction compared to when the MD was used alone. Moreover, the greatest reduction of MS2 (0.9- and 4.4-log10 reduction for 8- and 24-hr treatment, respectively) occurred when combining all three materials: copper screen, MD, and gels.
These results from laboratory settings are encouraging and contribute towards future development of the gels to become an alternative to current commercial chlorine based POU technologies and improvement for silver based POU technologies. Future work should focus on long term usage of the gels, including stability and rechargeability evaluations, and field studies considering natural water sources, social acceptability and affordability. The ultimate goal would be to produce a certified metal-chlorine-releasing POU technology for drinking water disinfection.