Legionella in Water Systems: Causes, Risk Factors and Prevention

Water systems in commercial and industrial facilities can become a source of Legionella exposure when conditions allow the bacteria to grow and spread. Legionella is a waterborne bacterium that can cause serious illness in humans. The bacteria can multiply in building plumbing, hot water systems, cooling towers, and other water-based equipment, particularly when water stagnates, temperatures favor growth, or disinfection is inadequate. People can become infected by inhaling contaminated water droplets or aerosols released by these systems.

Managing Legionella in water supply systems requires appropriate water treatment, effective disinfection, and ongoing monitoring to control bacterial growth and reduce exposure risks. This guide explains what causes Legionella in water systems, the main risk factors, how exposure occurs, available detection methods, and water treatment for Legionella in commercial and industrial facilities.

What Is Legionella and How Is It Found in Water Systems?

Legionella is a genus of bacteria that occurs naturally in freshwater environments. Legionella pneumophila is the species most commonly associated with Legionnaires' disease in humans.

Although Legionella may be present at low concentrations in natural and treated water sources, conditions within man-made water systems can allow it to multiply. Exposure to Legionella can result in two primary illnesses:

• Legionnaires' disease: A serious form of pneumonia that can require hospitalization and may be fatal, particularly among high-risk individuals.
• Pontiac fever: A milder illness that generally produces flu-like symptoms and does not develop into pneumonia.

Older adults, smokers, people with chronic lung disease, and individuals with weakened immune systems have a higher risk of developing severe illness. Legionella can be found in various commercial and industrial water systems, including:

• Cooling towers and evaporative condensers
• Hot and cold water storage tanks
• Showers and faucets
• Decorative fountains and water features
• Hot tubs and whirlpool spas
• Humidifiers and misting systems

What Surges the Growth of Legionella in Water Systems?

Legionella can enter a facility through municipal water, wells, or other source water. Low concentrations are not necessarily a concern. The primary risk occurs when conditions within the water system allow Legionella to survive and multiply. Key growth conditions include:

• Water Temperature: Legionella can multiply in water approximately between 20°C and 45°C (68°F and 113°F), with growth generally favored around 25°C to 42°C (77°F to 108°F).
• Stagnation and Low Flow: Idle outlets, dead legs, oversized tanks, and low-flow branches increase water age and can reduce disinfectant residual.
• Reduced Disinfectant Residual: Loss of disinfectant protection can allow microorganisms to establish and multiply.
• Sediment, Scale, and Corrosion: Deposits can accumulate organic matter and provide surfaces that support microbial attachment and biofilm development.
• Biofilm: Biofilm can provide nutrients and protection from disinfectants, making established contamination more difficult to control.
• Poor Circulation and System Design: Long pipe runs, inadequate recirculation, and poorly designed systems can create localized areas with stagnant water, unsuitable temperatures, or reduced disinfectant residuals.
• Free-Living Amoebae: Legionella can survive and multiply within certain amoebae, which can provide additional protection from environmental conditions and treatment.

These factors often occur together. Effective water treatment for Legionella should therefore address temperature, flow, disinfectant residual, deposits, and system cleanliness rather than relying on a single control measure.

Legionella Risk Factors in Commercial and Industrial Water Systems

Commercial and industrial water systems can contain multiple areas where the conditions described above develop. Key risk factors include:

• Potable Water Distribution: Storage tanks, recirculation loops, low-flow branches, and distant outlets can develop elevated water age or reduced disinfectant residual.
• Dead Legs and Unused Piping: Abandoned or unnecessary piping can retain stagnant water and should be removed where practical.
• Low-Use Fixtures: Infrequently used faucets, showers, hose connections, and other outlets should be managed according to the facility's water management program, which may include scheduled flushing.
• Seasonal Facilities: Extended shutdowns can increase stagnation and water age. Systems should be assessed and appropriately flushed or disinfected before normal operation resumes.
• Storage Tanks: Oversized tanks and poor circulation can increase water age and create temperature variations within stored water.
• Hot Water Systems: Poorly balanced recirculation, pump problems, and inadequate temperature control can create areas favorable to Legionella growth.
• Aging Piping: Corrosion, sediment, and established deposits can support biofilm formation and interfere with treatment.
• Cooling Towers: Poor cleaning, inadequate biocide treatment, scale, corrosion, and sediment can increase Legionella growth. As cooling towers can generate aerosols, proper maintenance and water treatment are particularly important.
• Facility Type: Hospitals, long-term care facilities, hotels, universities, manufacturing plants, data centres, and large HVAC facilities may require risk-based controls that account for system complexity, aerosol-generating equipment, and occupant vulnerability.

Legionella exposure generally occurs through inhalation of contaminated aerosols. Common sources include showers, cooling tower drift, spray systems, decorative water features, and whirlpool spas. A Legionella risk assessment should therefore evaluate water temperature, circulation, water age, disinfectant residuals, system design, piping condition, storage tank management, and cooling tower maintenance.

How to Detect and Monitor Legionella in Water Systems?

Legionella cannot be reliably identified by sight, smell, or taste. Laboratory testing is required to determine whether the bacteria are present. Here are some testing methods.

• Culture Testing: Culture testing is a commonly used reference method for detecting viable Legionella organisms. Water samples are collected from selected system locations and analyzed under controlled laboratory conditions. Results typically take several days, but the method can identify organisms capable of growing under the test conditions.
• PCR Testing: Polymerase Chain Reaction (PCR) testing detects genetic material associated with Legionella and can produce results more quickly than culture testing. However, a positive PCR result does not necessarily indicate that viable bacteria are present.

The appropriate testing method depends on the facility's monitoring objectives, water management program, and applicable requirements.

How to Treat Legionella in Water Systems?

Effective legionella treatment in water should address the conditions that allow Legionella to persist and multiply. The appropriate method depends on system design, water chemistry, contamination levels, materials, operating conditions, and regulatory requirements.

• Thermal Treatment: Thermal disinfection uses controlled water temperatures and exposure times to reduce Legionella. Although hot-water storage above 60°C (140°F) can help control growth, thermal shock treatment followed by flushing is not recommended as a routine remediation method for potable water systems. Any temperature-based intervention must follow applicable guidance and include scald prevention measures.
• Chemical Disinfection: Chemical treatment may include shock chlorination, continuous chlorine treatment, chlorine dioxide, monochloramine, or copper-silver ionization. Selection should consider water chemistry, contact time, disinfectant demand, system materials, regulatory limits, and potential corrosion or equipment effects. Treatment should be selected and managed as part of the facility's water management program.
• Physical Treatment and Filtration: UV disinfection can inactivate susceptible microorganisms as water passes through the treatment chamber, but it does not provide a residual disinfectant to control growth downstream. Point-of-use filtration can provide additional protection at individual outlets, particularly in healthcare facilities. A water filter for Legionella control should be selected based on bacterial retention performance, flow rate, pressure drop, installation conditions, replacement requirements, and manufacturer specifications.
• Cleaning and Cooling Tower Treatment: Scale, sediment, and biofilm can shelter bacteria and reduce disinfection effectiveness. Storage tanks, cooling tower basins, heat transfer equipment, and other components should be inspected and cleaned according to applicable maintenance procedures. Cooling towers require ongoing filtration where appropriate, cleaning, scale and corrosion control, microbiological monitoring, and suitable biocide treatment. Monitoring and any post-treatment testing should follow the facility's water management program, applicable guidance, and public health requirements. 

Preventing Legionella Growth in Water Systems

Prevention is an important part of controlling Legionella in commercial and industrial water systems. A documented water management program helps identify hazards, monitor operating conditions, implement corrective actions, and maintain system records.

• Map the Water System: System maps should identify storage tanks, water heaters, recirculation loops, cooling towers, heat exchangers, dead legs, low-flow areas, and high-risk outlets. Mapping helps identify areas where temperature, circulation, and disinfectant conditions may favor Legionella growth.
• Control Water Temperature: Hot water should be maintained at temperatures that limit Legionella growth while incorporating scald prevention measures. Where applicable, hot water storage should be maintained at or above 60°C (140°F), while cold water should remain below 20°C (68°F) where feasible. Temperature targets should follow applicable standards, local requirements, and facility procedures.
• Reduce Stagnation: Unused piping and unnecessary dead legs should be removed where practical. Low-use fixtures should be flushed according to the facility's water management program. Reducing water stagnation helps maintain appropriate water temperatures and disinfectant residuals.
• Maintain Water Quality: Disinfectant residuals should be monitored where chemical treatment is used. Scale, corrosion, and sediment should also be controlled because they can support biofilm formation and reduce treatment effectiveness.
• Cooling Tower Monitoring: Cooling towers should be inspected, cleaned, and treated according to established procedures and schedules. Biocide programs should include monitoring of water chemistry, treatment effectiveness, scale, corrosion, and system cleanliness.
• Maintain Documentation and Provide Training: Records should cover temperature measurements, disinfectant residuals, microbiological testing where applicable, flushing, cleaning, cooling tower maintenance, and corrective actions. Personnel responsible for water systems, HVAC equipment, boilers, and chemical treatment should receive training on monitoring, sampling, chemical handling, maintenance, and corrective procedures.

Support Legionella Control with the Right Water Treatment Solutions

Manage Legionella risks with appropriate treatment, monitoring, and maintenance. Cannon Water Technology offers oxidizing biocides, biomass dispersants, chemical feeders, metering pumps, UV disinfection systems, chlorine and ORP monitors, and cooling tower test kits. Explore our water treatment products or contact our team for guidance on selecting solutions for your cooling tower, boiler, or building water system.

Frequently Asked Questions

1. What causes Legionella to grow in water systems?

Legionella can multiply when water temperatures favor growth, water stagnates, disinfectant residuals decline, and biofilms develop. Poor circulation, sediment, and inadequate water treatment can also contribute.

2. How do you treat Legionella in water systems?

Treatment may involve thermal control, chemical disinfection, system cleaning, and targeted filtration. The appropriate method depends on the water system, contamination risks, materials, and applicable guidance.

3. How can Legionella be detected in water?

Laboratory testing can detect Legionella using culture or polymerase chain reaction (PCR) methods. Culture testing identifies viable organisms capable of growing under test conditions, while PCR detects bacterial genetic material.

4. Can water filters remove Legionella?

Point-of-use microbial filters can help prevent Legionella from passing through individual taps or showers when they meet appropriate bacterial-retention specifications. They do not eliminate contamination throughout the plumbing system.

5. How can facilities prevent Legionella growth in water systems?

Facilities can reduce risk through a documented water management program that addresses temperature control, water circulation, stagnation, disinfectant residuals, system cleaning, and cooling tower maintenance. Regular monitoring helps identify conditions requiring corrective action.