Legionellosis–America’s “most important waterborne disease.”
by Dr. Joseph Cotruvo
Editor’s Note: The piece below is excerpted from a Water Technology column by Dr. Joseph Cotruvo. It appeared the WT‘s “Professor POU/POE” series, which is the trade journal’s version of the Gazette‘s own ongoing series by technical wizard Pure Water Annie. —Hardly Waite.
The most important waterborne diseease risk in the United States is legionellosis, and it can be fatal. Distribution and plumbing system deficiencies are the most significant sources of waterborne disease in the U.S. and probably in all developed countries. In the past, source water contamination and inadequate water treatment or treatment breakdowns were the major sources of traditional waterborne diseases.
Since the passage in 1974 and implementation of the Safe Drinking Water Act after about 1978, however, new EPA regulations were implemented. Public water systems have undertaken major improvements in installations and operations of treatment technologies and water quality monitoring, and the traditional waterborne diseases have been reduced significantly. However, the types of illnesses have changed to water distribution-related causes that can’t be entirely eliminated at the central municipal water treatment plant. This calls for a radically different approach to protect public health by regulators and the public.
Legionellosis has been a reportable disease only since 2001. The disease is not caused by ingestion of the water, but rather by inhalation of aerosols such as during showering or from inhaling blow down from cooling system heat exchangers, or probably even humidifiers. Those at particular risk are the elderly and especially people with impaired immune systems such as those who are hospitalized and in extended care facilities, but they are everywhere in the community. It is well known that legionella are detectable in a high percentage of plumbing systems, including in homes, hotels and other buildings, and a substantial number of people in the general population are susceptible because of their ages or health or immune status. Several hospital-related outbreaks of legionella related diseases are reported annually around the world, but undoubtedly most are not identified or attributed to the water system. CDC has estimated that between 8,000 and 18,000 people are hospitalized with Legionnaires’-related disease each year in the U.S. It is not known how many of those are attributable to exposure from the plumbing or cooling systems.
Treatment technologies for managing Legionella and other regrowth microorganisms
Treatment is a real challenge and a cookie cutter approach is not likely to be widely successful. The problem of controlling microorganisms colonizing plumbing and distribution systems is not trivial because many of them are associated with biofilms or protozoa such as amoebas. Thus, even though they may be susceptible to disinfection in suspension, the disinfectants may not have ready access to them in biofilms so that they can be killed. Disinfectants that have been employed have had limited success. They include chlorine, chloramine, chlorine dioxide, ozone, UV light, copper/silver ionization and shock thermal and steam treatments. Each of them has its benefits and weaknesses, and often a combination of treatments must be applied on a regular basis supported by monitoring to indicate the conditions of the system and the time to re-treat. Chlorine is a powerful disinfectant, but even when applied in a temporary hyperchlorination mode (e.g., 50 ppm for several hours) total eradication may not be achieved. Chloramines, which are far less potent than free chlorine, have demonstrated considerable success in reducing legionella counts in some water plumbing systems. This could be due to the lower chemical reactivity of chloramine and greater hydrophobicity that allows greater penetration into biofilms. Chlorine dioxide is a potent disinfectant that also has had mixed success. On-site generation and survival of a residual in far plumbing reaches and in hot water systems can be a problem. Ozone and UV might have some efficacy in recirculating systems, but they will be primarily effective against organisms in the water column. Copper/silver in combination and individually have shown successes when they are properly managed and maintained. Shock thermal treatment for several hours at temperatures above 70oC have shown temporary success, but a complete strategy would require a combination of initial biofilm cleanout with a disinfectant system that will retard regeneration of the biofilm.
The concept of final barrier protection has value in situations where sufficient risk exists. For example, instant hot water delivery systems leave a smaller volume of water to stagnate and provide a growth environment. Temperature control valves at faucets and showerheads allow maintaining hot water lines at temperatures above the ~55oC upper legionella growth temperature, and then blending with cold water at the point-of-use to prevent scalding risk.