microbiology > Legionella > in Domestic Plumbing, Hot-Water Systems

Domestic plumbing may serve as makeup to cooling towers, however, these systems most assuredly serve our buildings, commercial and otherwise, and our hospitals and other care facilities. These systems were first implicated in a nosocomial (acquired during a hospital stay) case of Legionnaires’ Disease in 1980. Since then, they have been associated with numerous outbreaks of legionellosis. The United Kingdom reported 19 of 20 hospital Legionnaires’ outbreaks, from 1980 to 1992, to be from their plumbing systems. Cases of Legionnaires’ Disease have also been attributed to plumbing systems in nursing homes, workplaces, and private residences. Hot-water systems in hospitals, hotels and other large buildings are perfect breeding habitats for legionella, as well as other bacteria that form biofilms. Legionella can flourish in a hot-water tank, especially in the bottom warm zones that can develop with accumulated scale and sediment. The complexities of hot-water piping presents an even greater problem than tanks, because biofilm and scale that form in their valves and fittings and on the pipe walls not only feed bacteria but also protect them from hot water and chemical disinfectants. Dead-legs (unused piping) create additional problems because bacteria grow well in stagnant water. Because the domestic plumbing system represents a major "reservoir" for Legionella and associated LD cases, especially to commercial buildings, hospitals and other care facilities, there is extensive emphasis on the risk assessment, control and prevention, and treatment to these systems. While the "water treater" may more traditionally deal with cooling towers, evaporative condensers and other heat-transfer associated water systems, he (or she) should also know about Legionella in other systems. Matthew R. Freije's HC Special Reports #302 and #303, respectively entitled, How to Make Plumbing Systems Less Conducive to Legionella and Other Bacteria, and Disinfecting Plumbing Systems of Legionella: Solving Problems Without Overspending (1998) deal extensively with plumbing systems and Legionella. Within his text, Mr. Freije presents five common methods of Legionella disinfection for plumbing systems and gives comprehensive coverage as to their advantages, disadvantages and costs to apply. These methods are listed (in general) below and Mr. Freije's reports should be reviewed for more complete coverage and information. Heat-and-flush (heat shock): Water in tanks is superheated and then all outlets are flushed for several minutes. The flush time required will depend on the temperature of the water when it reaches the outlets. Chlorination: For temporary disinfection, chlorine is added to water tanks at free available levels much higher than normal for potable water and flushed throughout the system. For continuous disinfection, flow-adjusted injectors are installed to release chlorine at a drinkable concentration (1 to 2 mg/L free chlorine) throughout the domestic water system. Ultraviolet radiation: An ultraviolet sterilizer can be installed on a water line to kill legionellae as water flows through the unit. Ultraviolet units are not effective if a system is already contaminated. Ozonation: Ozone is dissolved into the water system to achieve a dose of about 1 to 2 mg/L, ideally via a generator that produces ozone in proportion to the water flow rather than a generator that produces ozone at a constant rate regardless of demand. Copper-silver ionisation: A flow-through ionisation chamber containing copper and silver electrodes is installed on a hot-water line. As electrical current is applied to the electrodes, positively charged copper and silver ions are released into the hot-water system. The positive ions bond with negatively charged sites on bacteria, causing the organisms to die. In addition, ASHRAE (the American Society of Heating, Refrigeration and Air-Conditioning Engineers) has recently produced a comprehensive guideline (ASHRAE Guideline 12-2000) entitled Minimizing the Risk of Legionellosis Associated with Building Water Systems. This guideline is much more extensive in scope than its title may suggest. It provides broad coverage and ‘minimizing the risk of Legionellosis’ information for many specific water-disseminating systems including: potable and emergency water systems; heated spas; architectural fountains and waterfall systems; cooling towers including closed-circuit cooling towers and evaporative condensers; direct evaporative air coolers, misters (atomizers), air washers, and humidifiers; indirect evaporative air coolers; and metalworking systems.

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