Efficacy of copper and silver as residual disinfectants in drinking water

Abstract

Contamination events and biofilms can decrease the amount of free chlorine available in drinking water systems. The efficacy of 100 mu g/L silver and 400 mu g/L copper, individually and combined, were evaluated as secondary, longer-lasting residual disinfectants against Salmonella enterica serovar Typhimurium, Escherichia coli, Listeria monocytogenes, and Mycobacterium fortuitum at 24 degrees C and 4 degrees C. A >5.0-log(10) reduction was observed in E. coli and L. monocytogenes after three hours and S. Typhimurium following seven hours of exposure to silver. M. fortuitum was the most resistant species to silver (1.11-log(10) after seven hours). Copper did not significantly reduce S. Typhimurium and E. coli at 24 degrees C; >= 2.80-log(10) reductions were observed in the Gram-positive L. monocytogenes and M. fortuitum. Longer exposure times were required at 4 degrees C to achieve significant reductions in all species. A synergistic effect was observed when silver and copper were combined at 24 degrees C. In addition, silver was not affected by the presence of organic matter at concentrations that completely inhibited 0.2 mg/L chlorine. The results of this study suggest that combinations of silver and copper show promise as secondary residual disinfectants. They may also be used in conjunction with low chlorine levels or other disinfectants to provide additional, long-lasting residuals in distribution systems.