bacteriaLegionella is easily killed by chlorine and chlorine dioxide, however during the incubation and growth phase, Legionella  is protected in the biofilm that grows on all surfaces in contact with the water. The biofilm is not affect by the concentration of free chlorine in the water.

The Legionella bacteria is engulfed by an amoeba protozoan,  however it is not  consumed, indeed the amoeba is used as an incubator by the Legionella bacteria and eventually many thousands of  Legionella will burst out from the amoeba. If the biofilm slides off  of the inside of pipes, scum channel, balance tank or the filter media as a small particle,  then the bacteria will be protected for many minutes or even hours from oxidation by chlorine. If the water is then atomized by splashing, water canon or flumes, then the bacteria may cause an infection.

Typically Legionella will cause a nasty flu such as Pontiac flu which may go un-noticed, however there are some strains of Legionella that have a mortality rate as high as 15 %.

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