Biocides used in industrial water systems are numerous and available from several different manufacturers. Efficacy data for these biocides is typically available, however direct comparisons are difficult to make for many reasons, most notably the different methods, organisms and test conditions used by the different manufacturers. For this reason, a comprehensive review including typical organism physiological information, general biocide mechanisms and some limited toxicity data of commonly used industrial water treatment biocides was conducted. Additionally, twelve commonly used industrial water treatment biocides were selected and evaluated against a variety of organisms using standardized methods and conditions. Overall, for the organisms and protocols utilized, the products that were based on quaternary chemistries were the most effective. Blends of biocides that included quaternary chemistries also outperformed their single biocide counterparts in many cases. The efficacy results along with the physical and chemical properties of the biocides were combined and are summarized in biocide selection decision trees. Together, this data provides a useful reference document for water treatment professionals.


One of the major problem sources within cooling towers is the presence and growth of biofouling microorganisms. Microbial-based fouling can cause damage to the structure and function of these systems directly through impeding the flow of water and indirectly by providing sites for corrosion to occur. In addition, cooling towers can harbor pathogenic organisms detrimental to human health, specifically Legionella pneumophila. The major organisms that impact cooling towers are typically algae and bacteria, while fungi (yeast and mold) impact cooling towers to a much less extent.

Types of Organisms

The category “algae” is very expansive and encompasses a number of structurally different types. They are most commonly classified based upon their pigmentation; they include green algae, brown and red algae and blue-green algae (cyanobacteria). The phylogeny of these organisms is often debated as well as their evolutionary endosymbiotic events. Structurally, within the outer membrane, the cyanobacteria are distinct in that peptidoglycan is present within the cell wall which lends strength and added protection to the cell.2 Other algae have cell walls composed of cellulose and a variety of glycoproteins. Several different polysaccharides help to differentiate algae as well: manosyl from microfibrils in green and red algae, alginic acid in brown algae and agarose, carrageenan and porphyran in red algae for example. While they do exhibit multiple variations in structure, the common link is their ability to metabolize carbon dioxide photoautotrophically.3 The cell walls of bacteria are made of peptidoglycan, or peptide-cross linked polysaccharide chains. Gram positive bacteria have a thick cell wall with multiple layers of peptidoglycan and teichoic acid. Gram positive bacteria are common pathogenic bacteria in humans, although not the typical pathogens found in cooling towers. Gram positive bacteria also contain the spore-forming bacteria most notably Bacillus and Clostridium. Gram negative bacteria, on the other hand, have a thinner cell wall also consisting of peptidoglycan, as well as a second lipid bilayer membrane outside of the cell wall which contains lipopolysaccharides and lipoproteins on its outer surface.

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