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.
Chlorine is often the biocide of choice in cooling towers due to its high efficiency, low persistence (resulting in low environmental toxicity), broad efficacy spectrum, and ability to oxidize organic material. Chlorine can be added with a number of different products, including chlorine gas, sodium hypochlorite, chlorinated hydantoins and chlorinated isocyanurates (Puckorius, 1998a; Puckorius, 1998b; Nalepa, 2000; Ludensky, 2004). For all of these chlorinated products, when they are added to water, the active biocide that is released is hypochlorous acid (HOCl).
Because HOCl is so reactive, it can be depleted quickly by reaction with contaminants in the water. Public utilities often use monochloramine for drinking water disinfection because it is easier to maintain a residual through the distribution system (EPA 1999). Monochloramine has also been shown to have greater efficacy against biofilm than free chlorine (LeChevalllier 1988). A disadvantage of chloramines is their volatility, where not only is the sanitizer lost more quickly, but corrosion of structures in contact with the gases may occur (Holzwarth 1984).