Four years of experience with silver-copper ionization for control of Legionella in a German university hospital hot water plumbing system

Silver-copper ionization was used for controlling Legionella distribution in a German university hospital hot water plumbing system for 4 years. In the beginning, silver concentrations were not allowed to exceed 10 mu g/L because of drinking water regulation limits in Germany. Water samples were monitored for Legionella counts, temperature, and silver and copper concentrations. A significant (P<.001) 3.8-log reduction of Legionella counts, from 40,000 cfu/L to 7 cfu/L, was found during the first year with silver-copper ionization, Nevertheless, the long-term efficacy of silver concentrations <10 mu g/L was not sufficient. Legionella counts increased to 10,000 cfu/L during the third year, During the fourth year, we studied the influence of higher silver concentrations on Legionella distribution. With an average silver level of 30 mu g/L, only a 1.3-log reduction in Legionella, to 500 cfu/L, was achieved. The effect was not significant (P=.071); therefore, it must be considered that Legionella developed a tolerance to silver ions.