Key Finding: Legionella spp. were detected in water-main biofilms within two municipal drinking water distribution systems, demonstrating that distribution network biofilms can serve as a reservoir for this potentially lethal pathogen. Maintaining adequate disinfectant residuals, controlling water temperature in distribution lines, and implementing regular flushing and monitoring programs are essential strategies to minimize Legionella risk in piped water systems.
Waak, Michael B.; LaPara, Timothy M.; Halle, Cynthia; Hozalski, Raymond M.
ENVIRONMENTAL SCIENCE & TECHNOLOGY, 52 (14):7630-7639; 10.1021/acs.est.8b01170 JUL 17 2018
Abstract: The maintenance of a chlorine or chloramine residual to suppress waterborne pathogens in drinking water distribution systems is common practice in the United States but less common in Europe. In this study, we investigated the occurrence of Bacteria and Legionella spp. in water-main biofilms and tap water from a chloraminated distribution system in the United States and a system in Norway with no residual using real-time quantitative polymerase chain reaction (qPCR). Despite generally higher temperatures and assimilable organic carbon levels in the chloraminated system, total Bacteria and Legionella spp. were significantly lower in watermain biofilms and tap water of that system (p < 0.05). Legionella spp. were not detected in the biofilms of the chloraminated system (0 of 35 samples) but were frequently detected in biofilms from the no-residual system (10 of 23 samples; maximum concentration = 7.8 x 10(4) gene copies cm(-2)). This investigation suggests water-main biofilms may serve as a source of Legionella for tap water and premise plumbing systems, and residual chloramine may aid in reducing their abundance.
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Source: Water Feed
Legionella in Distribution System Biofilms: Risks, Detection, and Control
Legionella pneumophila and related Legionella species are gram-negative bacteria responsible for Legionnaires’ disease, a severe form of pneumonia, and Pontiac fever, a milder flu-like illness. The bacteria thrive in warm, stagnant water environments and are particularly well-adapted to colonizing the complex microbial biofilms that develop on the interior surfaces of water mains, storage tanks, and plumbing materials. This study provides direct evidence that distribution system biofilms in treated municipal water networks can harbor Legionella, even downstream of conventional disinfection treatment processes.
The detection of Legionella in water-main biofilms is significant because biofilm communities can protect embedded bacteria from residual disinfectants such as chlorine and chloramine. The extracellular polymeric substances (EPS) matrix produced by biofilm organisms acts as a physical barrier that reduces disinfectant penetration and allows protected Legionella cells to persist and multiply. Free-living amoebae—which are also common in distribution biofilms—can serve as intracellular hosts that shelter Legionella from both disinfection and immune responses, further complicating control.
Temperature management is a critical control lever. Legionella proliferates most rapidly between 25°C and 42°C and is effectively inactivated above 60°C. Distribution systems with stagnant dead-ends, low-flow sections, or aged pipes that lose thermal stability are at elevated risk for Legionella colonization. Maintaining hot water above 60°C at the point of generation and cold water below 20°C at the tap are the cornerstone recommendations of water safety plan (WSP) frameworks endorsed by the World Health Organization.
Proactive distribution system management strategies to control Legionella include systematic flushing programs to eliminate stagnant sections, routine microbiological monitoring using culture or qPCR methods, disinfectant residual verification at system extremities, and pipe rehabilitation or replacement programs targeting aged infrastructure with heavy biofilm accumulation. For building water systems, point-of-entry UV disinfection systems provide an additional barrier against Legionella and other opportunistic pathogens that may be introduced from the distribution network. AMPAC’s commercial UV purification systems deliver validated doses sufficient for 4-log inactivation of Legionella and other waterborne pathogens without introducing disinfection byproducts.
Frequently Asked Questions
What is Legionella and why is it dangerous in drinking water?
Legionella is a genus of bacteria that causes Legionnaires' disease, a potentially fatal pneumonia, and Pontiac fever. It spreads primarily through inhalation of contaminated aerosols from water systems, not direct consumption. Case fatality rates for Legionnaires' disease range from 5–30%, with higher mortality in immunocompromised patients and healthcare settings.
How does Legionella survive in drinking water distribution systems?
Legionella survives and multiplies by colonizing biofilms on pipe surfaces and by living as an intracellular parasite within free-living amoebae found in distribution system water. The biofilm EPS matrix protects it from residual disinfectants. Warm water temperatures (25–42°C) and stagnant flow conditions further favor its proliferation.
Can conventional water treatment eliminate Legionella from distribution systems?
Conventional treatment—including coagulation, filtration, and chlorination—effectively reduces Legionella in treated finished water. However, the bacteria can re-colonize distribution system biofilms downstream of treatment, particularly in areas with low disinfectant residuals, elevated water temperature, or stagnant conditions. Distribution system management is therefore as important as treatment plant performance.
What disinfectant residual levels are needed to control Legionella?
Regulatory guidance typically requires a minimum free chlorine residual of 0.2 mg/L or monochloramine residual of 0.5 mg/L throughout the distribution system. However, these minimums may be insufficient to penetrate biofilms. Periodic superchlorination (shock chlorination) combined with flushing is used to reduce established biofilm Legionella populations in problem areas.
How is Legionella detected in water samples?
Standard methods include culture on buffered charcoal yeast extract (BCYE) agar (ISO 11731), which can take 10–14 days, and quantitative polymerase chain reaction (qPCR), which provides faster results. qPCR detects both viable and non-viable cells and is useful for rapid screening, while culture remains the reference method for regulatory compliance.
What is a Water Safety Plan (WSP) and how does it address Legionella?
A Water Safety Plan is a risk management framework endorsed by the WHO that systematically identifies and controls hazards throughout the water supply chain from catchment to consumer. WSPs for building and distribution systems specifically address Legionella risk through temperature management, stagnation prevention, regular flushing, disinfection residual monitoring, and scheduled microbiological testing.
Do UV purification systems protect against Legionella in building water systems?
Yes. UV disinfection at validated doses (minimum 40 mJ/cm² for 4-log reduction) effectively inactivates Legionella by damaging its DNA. Point-of-entry UV systems installed where municipal water enters a building provide continuous protection without adding chemical byproducts. UV is particularly valuable for facilities serving vulnerable populations such as hospitals, aged care facilities, and hotels.
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