Fighting Legionella: methods for sanitizing water distribution systems

Hot water production and distribution systems provide the ideal environment for the growth of Legionella, a bacterium responsible for lung infections and therefore potentially dangerous to humans. For this reason, it is important to periodically carry out sanitization procedures that help eliminate the bacteria and protect human health. There are several ways to sanitize these systems—here are the most common ones.

• What is Legionella and why does it thrive in water production and distribution systems?
• How to fight Legionella
• Which solution should you choose?

What is Legionella and why does it thrive in water production and distribution systems?

Legionella is a family of bacteria that thrives in water-rich environments. Among them is a particularly dangerous species for humans: Legionella pneumophila, a bacterium that can cause lung infections which, over time, may spread to other internal organs and, in the most severe cases, lead to death.

Legionella pneumophila is found worldwide, but it affects Italy with particular frequency. It is estimated that each year, several thousand people in the country come into contact with this bacterium. A few hundred of them develop Legionellosis, the infection caused by the bacterium, which has a mortality rate ranging from 5% to 15%.

The Legionella bacterium is anaerobic, meaning it lives in the absence of oxygen, and thrives in stagnant conditions, especially in the presence of scale or sediment. It can survive in temperatures ranging from 5°C to 55°C, but proliferates most actively between 24°C and 42°C. However, even in the absence of these external conditions, Legionella is capable of surviving: the bacterium has developed the ability to produce bacterial biofilms—clusters of bacteria encased in a protective gelatinous substance—that can resist both thermal and chemical treatments.

These characteristics clearly explain why water production and distribution systems—especially those for domestic hot water—are a natural habitat for this bacterium, and why it is so important to combat its proliferation in order to prevent risks to human health.

How to fight Legionella

To reduce the danger posed by Legionella, it is important to take preventive action already during the design phase of the system. This includes avoiding the creation of areas where water can stagnate (systems with recirculation are an optimal solution in this regard) and, where possible, opting for copper piping, as this metal inhibits bacterial growth.

An alternative is the design of instant domestic hot water production systems, as these do not include water storage tanks. The potential challenges in this case are the high cost of the equipment and possible space constraints, since the system needs to be installed close to the point of use.

In addition to measures taken during the design phase, it is important to pay close attention to system maintenance, including the periodic sanitization of storage tanks, reservoirs, and all components where water stagnation and sediment buildup may occur.

Cleaning can be complemented by chemical water conditioning treatments aimed at preventing the growth of algae and other bacteria. While these treatments do not target Legionella specifically, they help create a hostile environment by eliminating the microorganisms that the bacterium feeds on. Additionally, such interventions inhibit the formation of bacterial biofilms, which—as we’ve seen—are Legionella’s main defence mechanism in unfavourable temperature conditions. As a result, their use enhances the effectiveness of other disinfection methods, which may include various types:

1. Thermal Disinfection. Heat is Legionella’s main enemy, which is why the bacterium can be controlled using the so-called “thermal shock” method: at 70°C, Legionella dies instantly; at 60°C, it dies in 32 minutes; and at 66°C, in just 2 minutes. This method is certainly effective, especially because it does not require the use of chemicals or specialized equipment. However, its main drawback lies in the need to bring the entire system—including all distribution terminals—to the target temperature. Otherwise, bacterial colonies in cooler areas may survive and multiply. To eliminate Legionella through heat, it is therefore essential to ensure that water reaches at least 60°C at all outlets and that this temperature is maintained for over 30 minutes. Moreover, this method is not effective against biofilm, meaning Legionella may still survive despite the heat treatment.
2. Chlorine: it is a powerful oxidizing agent, and when added to water at high concentrations (> 3 mg/l), it puts significant stress on Legionella. The main advantages of this method lie in the fact that the chemical agent can easily reach all areas of the system and is effective even against biofilm. The downside is that adding chlorine leads to the formation of by-products, and there are maximum concentration limits to ensure the water remains potable (recommended limit: 0.2 mg/l). Additionally, certain types of piping may suffer from corrosion due to excessive chlorine levels. Therefore, if disinfection is carried out using this method, it is crucial to thoroughly flush the system before restoring water distribution.
3. Chlorine Dioxide: a valid alternative to chlorine is chlorine dioxide, a molecule that—even in small amounts (0.4 mg/l)—can achieve effective disinfection and biofilm removal, without affecting the piping, compromising water potability, or generating by-products. The only drawback is that chlorine dioxide must be produced on-site using equipment specifically suited to the system.
4. UV Rays: it is possible to eliminate Legionella by irradiating the water flow with ultraviolet (UV) lamps. However, this method is only effective when used near the points of use and has no impact on biofilm or stagnant water areas. Additionally, the cost of the UV lamps required is high, and the volume of water that can be treated this way is limited. For all these reasons, UV rays are typically used in combination with other methods (usually thermal shock) to disinfect the peripheral points of the system.
5. Copper-Silver Ionization since copper inhibits the growth of Legionella, copper ions can be generated through electrolysis and combined with silver ions to produce an antibacterial effect. Copper is particularly effective against biofilm and can prevent the formation of new Legionella colonies for several weeks. However, this method has some drawbacks: first, ion concentrations must be carefully monitored to ensure effective disinfection while staying within safe limits for drinking water. Additionally, zinc deactivates silver ions, meaning this solution cannot be used in systems with galvanized (zinc-coated) piping.
6. Hydrogen Peroxide and Silver: another chemical method for eliminating Legionella involves the use of a stable solution of hydrogen peroxide and silver. As we’ve seen, Legionella thrives in oxygen-poor environments, and since hydrogen peroxide does not produce harmful by-products, its use to combat the bacterium is both efficient and low-risk. However, this method is relatively new and still not widely adopted, so there is limited data available to fully assess its effectiveness.
7. Ozone: ozone is a powerful oxidizing agent and significantly less harmful to water potability compared to chlorine. Since Legionella does not thrive in oxygen-rich environments, ozone has proven to be effective in fighting the bacterium. Moreover, ozone not only targets Legionella but also eliminates many other bacteria and microorganisms that may be present in the water, allowing for broad-spectrum disinfection with a single agent.
8. Nanotubes: a final, innovative solution for combating Legionella involves the use of nanotubes applied directly at water outlets or along distribution pipes. These nanotubes act as a mechanical filter (membrane filter) for Legionella bacteria. The tubes have a diameter of approximately 0.2 μm (or 0.0002 mm), meaning they must be replaced regularly and can only be used in combination with other methods—and only in systems that are already free of impurities, otherwise they risk immediate clogging.

Which solution should you choose?

As we have seen, there are many methods to combat Legionella, and choosing the most suitable one for a given system depends on multiple factors. Performing anti-Legionella cycles using high-temperature water remains an effective solution, but thermal shock is not the only available option—it can be especially beneficial when combined with other methods.

If considering thermal disinfection, it is important to ensure that the system can reach and maintain a temperature above 60°C. Not all heat pump systems are capable of achieving this performance, and in some cases, it becomes necessary to add an electric heating element—an option that significantly increases energy consumption.
Robur heat pumps are designed to reach the temperature required for effective thermal anti-Legionella treatment, thus helping to simplify the sanitization procedures of water systems.

Sources: Guidelines for the Prevention and Control of Legionellosis – Permanent Conference for Relations between the State, the Regions, and the Autonomous Provinces of Trento and Bolzano.