By Ludensky, M., Sweeny, P., Lammering, D., A. Zakarian, and E. Meyer
Biofouling in cooling towers is undesirable because it can reduce heat transfer efficiency, restrict water flow, and accelerate corrosion rates. Of even greater concern is the fact that pathogen growth in cooling towers can lead to disease transmission. Given the favorable growth environment of a cooling tower, these microorganisms can reproduce, proliferate and form complex biofilm communities. Legionella bacteria, which cause Legionnaires’ disease, are one of the greatest concerns from a public health standpoint because infections are often lethal and cooling towers are the most frequently reported non-potable water source of Legionnaires’ disease outbreaks (Llewellyn 2017).
Planktonic (free-floating) Legionella are relatively easy to kill, with CT values reported as low as 4 ppm-min (Kuchta 1983). In a diverse biofilm community, Legionella can be much harder to treat. Legionella can act as a parasite and multiply inside amoebae and ciliated protozoa. Living inside other organisms provides another layer of protection from biocides, beyond that already provided by the biofilm (EPA 2016). This means that in order to successfully control Legionella in cooling water systems the selected biocide program must be effective against a variety of both planktonic and sessile populations.