Effectiveness of in-Situ Generated Monochloramine for the Control of Legionella in a Real Industrial Cooling Tower

The aim of the study was to determine the effectiveness of in-situ-formed monochloramine forthe control of Legionella growth in a real cooling tower system. The cooling tower utilizes an industrialwater (makeup) made of by blending industrial raw water and decarbonated raw water. A generatordevice injecting in-situ-formed monochloramine into the cooling system was installed. The detection ofLegionella, heterotrophic plate count (HPC - 22°C) and continuous monitoring of oxidation reductionpotential (ORP) were carried out to check the efficacy of monochloramine. Water samples were monthlycollected for 6 months from the makeup, two collecting basins and the blow-down. Monochloramine wasinjected at 4 mg/L for 60 days to achieve a mean residual concentration of 0.145 mg/L in the blow-down.Over time, at the same pumping rates, a greater monochloramine residual was achieved so the dosagewas decreased at 2.5 mg/L. L. pneumophila (SG 7-14) decreased from an initial load >10.000 cfu/L toundetectable levels and remained stable till the end of the experimental period while HPC decreasedslowly (mean count <10.000 cfu/ml) after the first 60 days.