The efficacy of THPS (TetrakisHydroxymethyl Phosphonium Sulphate) in the control of L. Pneumophila and its host organisms is well established. Enhanced performance can however be achieved by formulating the active in combination with novel biopenetrant(s).The new generation formulated of THPS products are highly effective at controlling Legionella in water systems while being low foaming and having a very favorable environmental fate profile.
Comprehensive management of industrial water systems generally includes a biocidal treatment program for the minimization of bio-fouling and its associated risks. Of particular concern from a public health stand point is the control of Legionella pneumophila, the bacterium responsible for Pontiac Fever and the pneumonia known as Legionnaires' disease. Infection resulting from the inhalation of mists contaminated with this organism can be fatal in elderly or immuno-suppressed patients.
Control of Legionella and other bio-fouling organisms in cooling tower environments is complicated by the fact that many of the organisms of interest are present as biofilms which are inherently more resistant to chemical biocides than are planktonic forms. Laboratory studies(1,2) have shown L. pneumophila biofilms to be as much as 100 times more resistant to biocides than free swimming bacteria, requiring much larger doses or longer exposures for effective inhibition.
Despite its fastidious growth characteristics in pure culture, L. pneumophila has proven to be a more robust organism when growing in its natural environment as part of a complex microbial community. Tison(3) described the enhanced temperature and pH tolerance of L. pneumophila from a thermal effluent where it was growing in association with a cyanobacterial mat. The relationship between Legionella and protozoa is however of particular significance (4,5). Legionella not only survive ingestion by various types of protozoa, but actually replicate within the digestive vacuoles. Such intracellular reproduction is likely the predominate mode of replication for Legionella in the environment. Contained within their protozoan hosts, the bacteria are protected against a variety of environmental stresses including exposure to biocides. Furthermore, intracellular growth appears to transform L. pneumophila into the infectious agent responsible for Legionnaires' disease. Berk(6) has suggested that inhalation of infected protozoan vesicles may be the actual mode of transmission for Legionellosis.
A wide variety of chemicals are available for the control of biofouling in non potable water systems and most treatment strategies employ multiple biocides which are alternated to prevent the development of resistant populations. Chemical biocides commonly used are either oxidizing (such as chlorine or bromine) or non-oxidizing. Each has advantages and limitations. Oxidizing biocides are inexpensive but corrosive at higher use concentrations and can lead to the production of toxic byproducts such as halomethanes. Amongst the nonoxidizing class, THPS (TetrakisHydroxymethyl Phosphonium Sulphate) (Figure 1) has a well established history of use in cooling tower applications and has proven extremely efficacious against Legionella pneumophila in both laboratory and field studies (7). THPS has also been shown effective in the control of GHB (general heterotrophic bacteria), SRB (sulfate reducing bacteria), algae (both green and blue-green) and protozoa known to host Legionella, via multiple modes of action.
