THE INTERACTION OF SILVER IONS AND HYDROGEN-PEROXIDE IN THE INACTIVATION OF ESCHERICHIA-COLI - A PRELIMINARY EVALUATION OF A NEW LONG-ACTING RESIDUAL DRINKING-WATER DISINFECTANT

The inactivation efficiencies of silver ions, hydrogen peroxide and th eir combination was studied as part of a performance evaluation of the combined disinfectant for drinking water applications. The major adva ntages of such combined disinfectant include, low toxicity of its comp onents, long lasting residual effect and low disinfection by product f ormation. Specific strains of E. coli (E. coli-B (SR-9) and E. coli K- 12) were used in this study as target microorganisms and the separate and combined inactivation efficiencies of silver and hydrogen peroxide were evaluated at different concentrations and exposure durations. Bo th, silver and hydrogen peroxide exhibited a significant inactivation performance even at concentrations that do not pose any health risk ac cording to the EEC, WHO and the USEPA (the USEPA Maximum Contaminant L evel (MCL) of silver is 90 ppb, and currently, there is no MCL for hyd rogen peroxide but it is approved as a food additive in the USA). Comb inations of 1:1000 silver:hydrogen peroxide (w) exhibited higher inact ivation performance as compared with each of the disinfectants alone a nd in some cases a synergistic effect was observed, i.e., the combined disinfectant exhibited higher inactivation performance than the sum o f the inactivation levels of the separate disinfectants. Thus, for exa mple, one hour exposure to 30 ppb silver, 30 ppm hydrogen peroxide and their combination yielded 2.87, 0.65 and 5 logs of inactivation respe ctively. While the rate of inactivation shown by this combined disinfe ctant, now available commercially in a stabilized formulation is relat ively slow, it may well hold promise as a secondary disinfectant provi ding long lasting residuals and biofilm control required for distribut ion systems. Its disinfection action may de similar to chloramines, th e use of which has been recently outlawed in France and in Germany and which are now under careful scrutiny in other countries due to the fo rmation of undesirable by-products.