Efficacy of Copper and Silver ions and reduced levels of free chlorine in inactivation of Legionella pneumophila

Water disinfection systems utilizing electrolytically generated copper and silver ions (200 and 20, 400 and 40, or 800 and 80 μg/liter) and low levels of free chlorine (0.1 to 0.4 mg/liter) were evaluated at room (21 to 23°C) and elevated (39 to 40°C) temperatures in filtered well water (pH 7.3) for their efficacy in inactiviting Legionella pneumophila (ATCC 33155). At room temperature, a contact time of at least 24 h was necessary for copper and silver (400 and 40 μg/liter) to achieve a 3-log₁₀ reduction in bacterial numbers. As the copper and silver concentration increased to 800 and 80 μg/liter, the inactivation rate significantly (P ≤ 0.05) increased from K = 2.87 x 10^-3 to K = 7.50 x 10^-3 (log₁₀ reduction per minute). In water systems with and without copper and silver (400 and 40 μg/liter), the inactivation rates significantly increased as the free chlorine concentration increased from 0.1 mg/liter (K = 0.397 log₁₀ reduction per min) to 0.4 mg/liter (K = 1.047 log₁₀ reduction per min). Compared to room temperature, no significant differences were observed when 0.2 mg of free chlorine per liter with and without 400 and 40 μg of copper and silver per liter was tested at 39 to 40°C. All disinfection systems, regardless of temperature or free chlorine concentration, showed increase inactivation rates when 400 and 40 μg of copper and silver per liter was added; however, this trend was significant only at 0.4 mg of free chlorine per liter.