Appendix B: The use of biocides for Legionella control
All biocides used in Public Works and Government Services Canada (PWGSC) facilities shall be registered with Health Canada's Pest Management Regulatory Agency and shall have a pest control registration number on the primary display panel of the product label. A registered biocide must be used according to the label instructions and the precautionary information on the label, to ensure that it does not pose unacceptable risk to humans.
Biocides are compounds selected for their ability to kill microbes, but with low toxicity to humans, plants, and, animals. For Legionella bacterial control, they are generally employed in the water treatment system of cooling towers and in the water supply system. Biocide use is a key to controlling risks in Legionella susceptible systems.
The ideal biocide shall be effective against a wide range of bacteria, algae, protozoa, and fungi, and should have a long activity time. It shall have no toxicity to humans and be environmentally acceptable. It shall be quick-acting and effective at low concentrations over a wide pH range. It must be compatible with other chemicals used, and should not cause deterioration of materials with which it comes into contact. It shall be capable of penetrating foam, sludge, slime, and scale within the system without foaming.
The effectiveness should not be reduced by contaminants within the water system or by substances present in the make-up water. Its concentration shall be easily measurable using simple test procedures. There are no biocides that meet all of these requirements. In practice, it is often necessary to use more than one type of biocide with other additives to achieve desired results.
Each facility will have a local specialist provider for water treatment. While approaches may vary by provider, for each building system, the goal must be to control microbial growth, including Legionella. Both American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and Cooling Technology Institute (CTI) have detailed information related to water treatment, approaches, biocide applications, and controls. These best practices should be consulted and followed.
In general, there are two types of biocide: oxidizing and non-oxidizing.
Oxidizing biocides include halogens such as chlorine, and bromine, and, chlorine dioxide. Chlorine and bromine react very rapidly with microbiological species and chemicals in the water. This reactivity is both their strength and their weakness. Rapid reaction means a quick and effective kill, but it also means that the biocide reacts very quickly with other chemicals in the system, such as scale inhibitors and corrosion inhibitors. Hence, they leave very little residuals for continuous protection against bacteria.
Chlorine- and bromine-based formulations are only effective at concentrations exceeding 0.5–1 ppm; these levels can, however, cause rapid corrosion of piping and other materials. Also, the effectiveness of these biocides varies with pH, and careful control of pH in the range of 7–10 is required. Chlorine-based formulations do not penetrate biofilms very easily; hence, some form of dispersant may also be required.
Chlorine is generally used in the form of sodium hypochlorite, a chemical that liberates free chlorine in the presence of water.
Chlorine dioxide has better properties than chlorine or bromine for Legionella control. It is fast-acting and more effective at higher temperatures than chlorine, and less corrosive. It reduces the formation of chlorinated by-products and is very effective at concentrations as low as 0.1 ppm. It does not lose effectiveness over a wide pH range of 4–10. Also, it better penetrates biofilms that can harbour Legionella bacteria.
Chlorine dioxide is approved in many countries for potable water treatment. It can be fed to domestic cold water systems as well as to heating water systems. Feeding chlorine dioxide to the domestic cold water system delivers better water protection as it treats the system further upstream and allows more contact time for killing the bacteria. Even at maximum feed rates, chlorine dioxide leaves no noticeable taste or odour.
Bromine is moderately effective against Legionella, and it requires a higher concentration of 0.5–1.5 ppm. It is more effective than chlorine as a biocide at higher pH. Its effectiveness is less dependent on pH as compared to chlorine, and it is not as corrosive. Stabilized bromine chloride at concentrations of 4–10 ppm may also be used as a biocide.
There are numerous other oxidizing biocides available, and their use must be carefully selected and controlled for the application.
Non-oxidizing type biocides include organic compounds such as BNPD (2-bromo-2-nitropropane-1, 3-diol), glutaraldehyde, dithiocarbamates, isothiazolin, DBNPA (di-bromo-nitrilo-propionamide), and some quaternary ammonium compounds.
These chemicals are typically slow to act and are added to water systems such as cooling towers in large, weekly doses, then allowed to decrease until the next addition. This type of treatment is based on overdosing the system during addition, and then leaving a residual that destroys the bacteria over a period of time.
Non-oxidizing biocides have several drawbacks. They are often toxic to humans and animal species. They are dangerous to store and handle, and disposal of cooling water containing these chemicals may be difficult and costly, due to environmental regulations. These biocides are used in larger quantities and may be more expensive than oxidizing biocides.
Alternating the use of biocides
Alternating the use of different types of biocides during water treatment minimizes the growth of resistant strains of bacteria. Alternating the dose and frequency of application is also helpful.
A good biocide program includes the use of alternate types at regular intervals and at the proper dosage. This strategy minimizes the development of resistant strains of bacteria.
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