How is Legionella risk controlled in water lines?

Legionella is a bacterium that can be present in water systems and can cause serious infection through inhalation. Managing the risk begins with understanding where and why the bacterium grows.

Where does Legionella grow?

The ideal conditions for Legionella are quite specific:

• Temperature: the 20–45°C range is the most favourable for growth. That is why the warm zones of hot-water plumbing and cooling circuits are critical points.
• Stagnation: dead legs where water waits, rarely used taps and reservoirs create risk.
• Biofilm: the most important factor. Legionella shelters and multiplies within the biofilm layer on the inner pipe surface.

When these three factors come together — a warm, stagnant, biofilm-laden line — the risk reaches its highest level.

Why is biofilm so important?

Biofilm is a protective layer formed by microorganisms attaching to the pipe surface. For Legionella it is both a shelter and a food source. More importantly, biofilm physically protects the bacterium from disinfectants.

This explains the core challenge in Legionella control: killing free bacteria in the water is easy, but reaching the bacteria within the biofilm is hard. An effective control strategy must be able to penetrate the biofilm layer.

The limit of shock chlorination

A common method is periodic shock chlorination: a high dose of chlorine is delivered to the line to reduce the load. This is momentarily effective, but biofilm re-forms quickly and the risk accumulates again until the next shock. Intermittent intervention is a structurally weak solution to a problem that requires continuity.

The advantage of continuous disinfection

The uncharged, small molecular structure of hypochlorous acid enables it to penetrate the biofilm layer. When continuous low-dose hypochlorous acid is delivered to hot and cold water lines, the re-growth of biofilm is suppressed; instead of spiking intermittently, the risk is held stably low.

The practical advantages of this approach:

• Biofilm growth is continuously suppressed, preventing intermittent re-accumulation.
• Dosing is continuously monitored; control becomes measurable and documentable.
• The same generation unit feeds both the water line and — where needed — surface disinfection.

A holistic approach

Legionella control is not the job of a single device; it is the combined execution of temperature management, elimination of dead legs, regular monitoring and effective disinfection. Continuous hypochlorous acid dosing is one of the most critical and most measurable components of that whole. For healthcare facilities, hotels and large facility water systems, this approach makes the risk manageable and documentable.

What the guidance says

The World Health Organization’s dedicated guidance builds Legionella risk management not on spot sampling but on a water safety plan covering the building’s water system as a whole. It sets out a framework for assessing and managing hazardous environments including cooling towers, pools and spa baths, and foregrounds temperature control, elimination of dead legs and a continuous disinfectant residual as the core measures.1

What those three measures have in common matters: none of them is “raise the dose”. All three work by removing the conditions in which the bacterium can multiply.

Why a sample result alone is not enough

Legionella lives not in the water itself but in the biofilm on the pipe wall. That distinction has a measured counterpart.

LeChevallier and colleagues measured surface-attached biofilm bacteria as 150 to more than 3,000 times more resistant to hypochlorous acid than free-floating cells. The same study showed that for free chlorine the rate-limiting factor is transport of disinfectant into the biofilm — and that consequently, raising the chlorine level does not raise disinfection efficiency.2

In a second study the same team identified what allows bacteria to survive chlorination in distribution systems: surface attachment, shelter inside particles, and adaptation following earlier exposure.3

For an operator the conclusion is clear: a normal-looking free-chlorine residual is not proof that a line is microbiologically clean.

pH is in play here too

How much of the chlorine dosed into a line is in the active form is a separate variable. The US Environmental Protection Agency’s CT tables show that at 10 °C and 1.0 mg/L free chlorine, 3-log inactivation requires 79 mg·min/L at pH 6, 162 at pH 8 and 234 at pH 9.4

In a high-pH hot water circuit, then, two disadvantages compound: a smaller share of the chlorine is in the active form, and reaching the biofilm with that share is already limited. This is why a disinfectant generated at neutral pH makes a difference in these circuits.