Evidence Shows Need to Address Pathogenic Bacteria in U.S. Drinking Water Systems

Water Glass Filled from the Tap with a Close Up of Legionella Bacteria

By: Daryn Cline, ASHRAE Member & Director of Environmental Technologies at EVAPCO, Inc. and Sarah Ferrari, Former Product Development Manager at EVAPCO, Inc.

Legionnaires’ disease is on the rise. Unfortunately, efforts to prevent Legionnaires’ disease to date have focused on building water systems rather than the municipal water supply. Building water systems are too far downstream to correct the problem, and, as a result, these efforts have been ineffective in curbing the incidence of legionellosis.

It is unrealistic to place on building owners and operators the burden and risk for providing pathogen-free water throughout buildings when the water delivered to their buildings contains Legionella bacteria. Legionella is commonly found in source waters (Figure 1a) and in the soil, and thus, if untreated at the municipal water treatment plant and in the distribution systems, the Legionella bacteria will necessarily find its way into buildings. More needs to be done by the municipal water suppliers to minimize Legionella bacteria in the drinking water supply before it can enter building water systems. Legionnaires’ disease is the number one waterborne disease associated with potable water in the United States, and it is lethal; approximately 10% of those who contract the disease will lose their lives.

Minimizing water pathogen risks is a shared responsibility between municipal water suppliers and building owners. At present, however, regulatory efforts seek to put responsibility for all preventive measures on building owners and operators. Consider the following:

  • Water management plans based on standards like ANSI/ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems, or the Centers for Disease Control and Prevention’s (CDC) toolkit, “Developing a Water Management Program to Reduce Legionella Growth and Spread in Buildings: A Practical Guide to Implementing Industry Standards,” typically require testing for Legionella in building water systems.
  • Current U.S. Environmental Protection Agency (EPA) regulations do not require municipal water suppliers to test directly for Legionella bacteria in their water systems.

Water management plans are recommended and encouraged for building water systems, especially in high-risk buildings like hospitals and senior citizen housing. But water suppliers must also assume responsibility for providing pathogen-free water in the first place. Typically, after Legionnaires’ disease outbreaks, water suppliers state that the water “meets all EPA regulations.” This may be true, but that does not mean that the water is Legionella-free. As discussed below, EPA regulations are inadequate when it comes to requiring municipal water utilities to take actions to eliminate bacteria in the water supply.

A Paradigm Shift is Due

Because Legionella bacteria exist in nature and are common in source waters, measures can and must be used to minimize Legionella levels in the municipal water supply. This, in turn, would minimize the presence of such bacteria in our homes and buildings. There is, however, a fundamental flaw in the EPA rules used to control or regulate Legionella bacteria. The EPA does set a maximum contaminant level goal (MCLG) of zero for Legionella. The problem is that it is simply a goal.Therefore, it is not enforceable, so water utilities/municipal providers are not required to monitor for Legionella, nor are they required to take action to minimize its presence.

This column explores three recent studies that investigated the presence of Legionella in downstream water systems, including sediments in municipal drinking water storage tanks (MDWST) with 1 to 5 million gallon (3.8 to 19 million L) capacity, point-of-use cold water taps, and cooling towers. These studies show that the EPA’s policies for controlling Legionella are not working.

Figure 1 uses these studies to illustrate the flaw in the EPA National Primary Drinking Water Regulations (NPDWR) and how this is putting additional stress on public and private systems downstream of treatment plants.

These studies sampled sites across the nation and evaluated for Legionella using quantitative polymerase chain reaction (qPCR). The qPCR technique screens and quantifies selected microorganisms like Legionella and has the distinct advantage of being able to detect even low levels of the bacteria. It works by amplifying the specific sections of DNA being investigated and measures them in real time, giving researchers information on the type and concentration of DNA.

The samples tested positive for Legionella pneumophila and Legionella pneumophila serogroup 1 (the most virulent strain) in all studies. The results demonstrate that the current methods to control the spread of Legionella are inadequate to minimize the risk of infection in municipal water distribution systems, allowing contaminated and potentially lethal water to enter facilities 24/7/365.

The Three Key Studies

These three broad, independent studies were carried out by the EPA and the CDC to measure pathogen levels across the country in diverse drinking water systems: water storage tanks, tap water, and cooling towers.

Looking at all three studies combined, approximately one-third of the drinking water samples tested positive for Legionella pneumophila at all three points along the drinking water distribution route after leaving water treatment plants. Notably, 20% to 28% of samples in each of the three studies were infected with Legionella pneumophila serogroup 1, the most lethal strain responsible for 90% of Legionnaires’ cases.

The remarkable similarity of the results provides evidence that Legionella is present when it leaves the municipal source and can and does enter building systems where people are exposed.

Water Storage Tanks Study (1)

In 2015, the EPA published the results of a study carried out to determine the level of potential pathogens including Legionella pneumophila bacteria present in municipal water storage tanks (Figure 1b). Eighty-seven sediment samples were taken from municipal drinking water storage tanks (MDWSTs) in 18 locations in 10 states that spanned five regions: Northeast, East Coast, Midwest, South and West Coast, providing a diverse sample set.

These municipalities sourced their water from a mixture of 61% groundwater and 39% surface water. At all locations, the water in the tanks contained chlorine concentrations that met EPA regulations.

The water was completely drained from each tank before sediment collection was carried out using a sterile plastic spatula and bottle. A quantitative polymerase chain reaction (qPCR) analysis was performed on each sample. 

Statistical analyses were done to establish correlations between Legionella, Acanthamoeba and temperature as well as other independent variables like total organic carbon (TOC), total organic matter (TOM), particle size and pH. These variables were recorded at the time of sampling for use in the analysis.

The results of the study indicated that Legionella bacteria were present in 66.7% of the samples; Legionella pneumophila was present in 33% of samples; and Legionella pneumophila serogroup 1 was present in 28% of the samples. In contrast, Escherichia coli (commonly known as E. coli) and Giardia were undetected. This shows that while efforts to control E. coli and Giardia work, the efforts to control Legionella do not. Another important finding: there was a significant correlation between Legionella and Acanthamoeba. Legionella bacteria will amplify and increase its virulence inside amoeba. This further supports a relationship between the two, so the detection of Acanthamoeba may be used as a potential indicator of Legionella contamination.

The results of this nationwide study, the first of its kind, demonstrate that Legionella that is present in the municipal water system will continue to survive in drinking water storage tanks in addition to surviving in large buildings’ plumbing systems. This reinforces the need for anti-Legionella measures at the water treatment plant and distribution system—before the water gets to water storage tanks where it will then be passed on to buildings and homes.

Infographic Pertaining to Legionella Distribution in Drinking Water Systems

Cold Tap Water Study (2)

The next key study on Legionella levels is the cold water tap study undertaken by the EPA aimed at determining whether Legionella was present in cold water used by the general population. This is important because, out of the 6,868 cases of Legionnaires’ disease reported in 2009– 2010, only 2.6% were outbreak events, while the remaining 97.4% were sporadic (4,5).

As with the storage tank study, samples were taken from diverse regions across the United States. Between January 2009 and December 2010, the EPA took a total of 269 samples from 68 residential and commercial locations in 25 states, one territory and one federal district.

Researchers sampled cold water from 29 kitchen sinks, 21 bathroom sinks, 17 drinking water fountains and one refrigerator water dispenser. Of the 68 taps sampled, 66 were from municipal supplies. Samples were taken after taps were allowed to run for 15 seconds to ensure that the supply was from the potable water supply and no hot water was included. No site had secondary water treatment (although the refrigerator had a filter).

A qPCR analysis was carried out and tests were performed to determine levels of Legionella pneumophila and Legionella pneumophila serogroup 1 (again, the most disease-causing variant). Twenty-nine percent of samples tested positive for Legionella pneumophila and 20% were positive for serogroup 1 (Figure 1c).

Cooling Tower Study (3)

The last key study took place in 2016, when the CDC examined samples taken from 196 cooling towers located in most of the climate zones of the United States as defined by the National Oceanic and Atmospheric Administration (NOAA). Using real time qPCR testing, Legionella bacteria was detected in cooling towers located in each of the eight regions tested.

Legionella-positive samples were cultured and further tested for specific DNA. Of 196 samples tested, 32% were positive for Legionella pneumophila and 20% were positive for Legionella pneumophila serogroup 1. The test results are almost identical to the tap water study, which is not surprising since cooling towers use tap water as their fresh, potable makeup water source (Figure 1d).

Results and Significance

The similarities between results from the three studies indicate that when Legionella bacteria are not addressed at the water treatment plant or in the distribution system, they will be present in the drinking water distribution supply with the potential to contaminate all water outlet sources after entering the building water supply. (Table 1).

The results of this study should be very concerning to homeowners, commercial building owners, and facility managers of hospitals and hotels. The conclusion drawn from this study is that the supply of fresh potable water entering their buildings is not consistently free of pathogens. This is not surprising considering the weak antimicrobial efforts used at the water treatment plant and in the drinking water distribution system.

Controlling Legionella

Legionella bacteria inhabit all types and varieties of water sources, including lakes, streams and groundwater (6) (Figure 1a). Other types of bacteria like E. coli are also present in these same sources, as are viruses like norovirus and rotavirus, as well as parasites like Giardia Lamblia (7). The public expects that water treatment plants adequately combine filtration and chemical treatment to kill these and any other waterborne pathogens. This trust, however, may be misplaced.

The Safe Drinking Water Act (SDWA) is the federal law that governs water treatment to ensure it is safe for human consumption. The National Primary Drinking Water Regulations (NPDWR) are the standard for treating drinking water and are legally enforced by the EPA. Yet, nationally, there’s a concerning divergence between theory and real accomplishment.

The EPA’s NPDWR provides that 99.9% of the Giardia parasite must be removed or inactivated from the municipal supply. There is no stated limit, however, for concentrations of Legionella. The expectation is that the treatments to remove microbes like Giardia will also result in the removal or inactivation of Legionella, but the studies suggests otherwise (8). As the three studies demonstrate, the current municipal water treatment measures are ineffective in neutralizing and inhibiting Legionella, and instead allow various bacteria and other pathogens to enter the distribution system and to pose health risks to the public.

Also, the presence of Legionella without the presence of other pathogens like Giardia demonstrates that although treatment is effective for one pathogen, it does not eliminate Legionella, as assumed in the EPA’s National Primary Drinking Water Regulations.


The key takeaway is that it is imperative that the National Primary Drinking Water Regulations be revised to require testing for Legionella bacteria and also require remedial actions as needed. Federal regulations must be modified to ensure that Legionella bacteria is dealt with at the water treatment plant and through the water distribution system, rather than making it the sole responsibility of the downstream users to remove and limit the growth of Legionella bacteria. These measures would then complement targeted downstream efforts to curb Legionella growth, such as ensuring that disinfectants are at proper levels when the water is entering the buildings.

There are new statewide efforts that represent good examples of how this could work in practice. For example, Illinois recently modified their EPA regulations to include raising residual chlorine levels to ensure a continuing antimicrobial effort as the water makes its way through the system. The same regulations require water turnover in storage tanks to address stagnation, and the elimination of dead ends in water distribution systems. Louisiana and Pennsylvania have passed similar measures, and New Jersey is considering joining this group. These efforts must be supplemented with effective measures at the municipal water treatment facilities to remove Legionella further upstream, closer to the source.

All the studies mentioned in this column support the fact that Legionella bacteria are widespread, and if not controlled will result in greater numbers of Legionella-related cases and outbreaks. Therefore, a focus on efforts to better control pathogens in the public water supply upstream by strengthening EPA requirements is the rational approach to addressing the increased threat of waterborne pathogenic bacteria-related disease cases in the future.

  1. Lu, J, Struewing I, Yelton S, Ashbolt N. 2015. “Molecular survey of occurrence and quantity of Legionella spp., Mycobacterium spp., Pseudomonas aeruginosa and amoeba hosts in municipal drinking water storage tank sediments.” Journal of Applied Microbiology 119(1):278-88.
  2. Donohue, M., et al. 2014. “Widespread molecular detection of Legionella pneumophila serogroup 1 in cold water taps across the United States.” Environmental Science and Technology 48(6):3145 - 3152.
  3. Llewellyn, A., et al. 2017. “Distribution of Legionella and bacterial community composition among regionally diverse US cooling towers.” PLOS.
  4. Hilborn, E.D., et al. 2013. “Surveillance for waterborne disease outbreaks associated with drinking water and other nonrecreational water—United States, 2009−2010.” Morbidity and Mortality Weekly Report 62(35):714 — 720.
  5. Hlavsa, M.C., et al. 2014. “Recreational water-associated disease outbreaks—United States, 2009−2010.” Morbidity and Mortality Weekly Report 63(1):6−10.
  6. U.S. EPA. 2000. “Legionella: Drinking Water Fact Sheet.” Office of Water. U.S. Environmental Protection Agency.
  7. U.S. EPA. 2000. “Giardia: Drinking Water Fact Sheet.” Office of Water. U.S. Environmental Protection Agency.
  8. U.S. EPA. 2009. “National Primary Drinking Water Standards,” EPA 816-F-09-004. U.S. Environmental Protection Agency.

Please note, this article was originally published in the April 2020 edition of the ASHRAE Journal. You can download a PDF version of the article as it appeared here.