AUTHORS: Landeen LK, Yahya MT, and Gerba CP
PUBLICATION REF: Applied & Environmental Microbiology 55: 3045-3050,1989

PURPOSE OF STUDY:
Relatively high levels of chlorination are known to be effective in inactivating Legionella pneumophila in drinking water. However, high levels of free chlorine may degrade rapidly, especially at high temperatures, producing only temporary suppression rather than continuous disinfection. Also, high chlorine levels may lead to the development of resistant strains and be corrosive to plumbing fixtures. Therefore, the authors investigated the effectiveness of electrolytically generated copper and silver ions together with low concentrations as an alternative disinfection treatment.

MATERIALS AND METHODS:
Pellets of Legionella pneumophila were suspended in samples of filtered well water and bacterial inactivation rates were determined using chlorination alone at low concentrations of 0.1, 0.2, 0.32, & 0.4 mg/liter. Then electrolytically generated copper & silver ions at copper to silver ratio of 200 and 20, 400 and 40, and 800 and 80 ug/liter were tested both separately and in combination with chlorination to evaluate any incremental bactericidal effect. The majority of experiments were conducted at room temperature with some tests performed at elevated temperatures (39 to 40). Inactivation rates for each experimental regimen were calculated by linear regression analysis. Buffering was avoided because phosphate buffer had been found to interfere with the disinfection efficacy of copper in previous test using Escherichia Coli.

RESULTS:
Bacterial inactivation with chlorination alone was progressively greater as concentrations were increased from 0.1 to 0.4 mg/liter. Although the rates were relatively slower, a similar inactivating effect was demonstrated in tests of copper and silver ions alone: the ratio of 800 (copper) to 80 (silver) ug/liter was significantly faster than ratios of either 200 and 20 or 499 and 40 ug/liter.
When the chlorination and copper/silver disinfection methods were combined, inactivation rates and amounts of bacterial reduction were enhanced. The 400 to 40 copper and silver regimen augments the measure rate of bacterial inactivation at all tested chlorine levels; the difference was statistically significant at a chlorine concentration of 0.4 mg/liter. Although reported by other investigators, significantly increased inactivation rates at high temperatures were not observed in this study.

CONCLUSIONS:
Electronically generated copper & silver ions increased the level of inactivation of Legionella pneumophilia achieved in filtered well water above the levels observed with free chlorine alone. This effect was consistent at all copper & silver ratios tested and at various low concentrations of chlorine. The improvement was statistically significant when 400 ug/liter of silver were added to a chlorine concentration of 0.4 mg/liter. Thus copper/silver ion treatment was shown to provide effective disinfection against Legionella pneumophilia while maintaining free chlorine levels sufficiently low to avoid the known disadvantages of relatively high concentrations