Legionella: What It Is, How It Spreads, and How to Prevent It

Legionella is a bacterium that thrives in both natural and artificial aquatic environments; therefore, hot water storage systems, cooling towers, humidifiers, and various types of water systems are potential sites for its proliferation. The most common species is Legionella pneumophila.

The Legionella bacterium survives in an inactive state at temperatures below 20°C, it finds a favorable environment for growth between 20°C and 49°C, with a particularly optimal range between 32°C and 42°C, while it cannot survive above 49°C. Between 49°C and 60°C, 90% of bacteria die within two hours; this time drops to just two minutes if the temperature rises above 60°C, and instant death occurs at temperatures above 70°C.

Legionella Infections

Domestic hot water systems are therefore among the environments most at risk, especially if they contain dead-end branches where water stagnates; the formation of a biofilm inside the pipes can promote the growth and proliferation of bacteria, which is why it is important to avoid areas of stagnant water as much as possible.

Once Legionella bacteria have formed, they can be contracted through the air —that is, by inhaling aerosols (tiny suspended droplets) and vapors —and not simply by drinking the water. The disease (legionellosis), however, is not contagious and therefore cannot be transmitted from person to person.

The facilities where the risk associated with Legionella is most closely monitored are those that use large amounts of hot water and serve vulnerable populations, such as hospitals and nursing homes.

There are various methods for preventing the growth of Legionella or for eliminating it once it has formed in domestic hot water systems. These methods fall into two main categories: physical treatments and chemical treatments.

Physical treatment methods are one of the primary approaches for controlling and reducing Legionella in water systems. They are based on non-chemical methods that aim to directly address the environmental conditions in which the bacteria can proliferate. Among the most common solutions are:

• Thermal shock
• Thermal Disinfection
• Mechanical Filters
• Ultraviolet (UV) rays

thermal shock

Since, as mentioned, Legionella does not survive at temperatures above 70°C, this treatment involves circulating water at a temperature of at least 70°C throughout the entire system. During the process, it is obviously essential that no one be using domestic hot water; for this reason, this procedure must be scheduled in advance, and the necessary precautions must be taken.

Thermal Disinfection

In thermal disinfection, the temperature is maintained constantly between 55°C and 60°C in order to eliminate Legionella bacteria and prevent their growth, as well as to eliminate them should they manage to form. One of the risks of this process is the difficulty in ensuring that the temperature is continuously maintained at at least 55°C throughout the entire distribution system, and not just in the storage tank.

Mechanical Filters

One option is to use mechanical membrane filters with a filtration capacity of 0.2 μm, which are effective at trapping Legionella bacteria, and install them at water draw-off points. A drawback of this method is that the filters must be replaced at regular intervals, and they must be installed in very clean systems to avoid the risk of filter clogging. Although this system is effective, it is often used in combination with one of the other methods.

Ultraviolet (UV) rays

This method involves exposing water to UV rays in order to damage the DNA of Legionella bacteria and prevent them from reproducing. This process does not guaranteeuniform treatment throughout the entire system (it must be applied at the points of use) and is less effective than other methods; for this reason, it is rarely used on its own for domestic hot water systems. It is also a relatively expensive method that does not affect the biofilm that has formed on the pipes.

Chemical treatments involve adding additives that eliminate or prevent the growth of Legionella. These types of treatments include:

• Shock Hyperchlorination
• Continuous hyperchlorination
• Chlorine dioxide
• Copper and silver ions

Shock Hyperchlorination

Shock hyperchlorination involves adding chlorine at a concentration between 20 and 50 mg/L for a minimum of 2 hours in order to completely eliminate the bacteria. Similar to thermal shock treatment, this process requires that the system remain out of service for the entire duration of the treatment, and that appropriate precautions be taken to prevent accidents.

Continuous hyperchlorination

Continuous hyperchlorination involvesthe continuous addition of chlorine (such as sodium or calcium hypochlorite) at concentrations high enough to prevent the growth of Legionella but low enough not to harm human health. The concentration of free chlorine is usually between 1 and 3 mg/L.

Sodium hypochlorite (NaClO) is also used as a disinfectant for drinking water, but its effectiveness is greatly affected by the water’s pH: at a pH above 7.5, its effectiveness decreases, and it also becomes more corrosive to hot-water pipes.

One drawback of using chlorine is that it can corrode certain types of pipes, such as copper pipes, which are widely used in healthcare settings. In addition, chlorine can form byproducts that are harmful to human health, which is why the system must be thoroughly rinsed before use.

Chlorine dioxide

Chlorine dioxide is particularly interesting because a concentration of just 0.4 mg/L is sufficient to ensure disinfection and eliminate any biofilm that may be present; although it eliminates biofilm, this additive does not compromise the water’s potability or the integrity of the pipes.

Copper and silver ions

This treatment is less well known but nonetheless very effective. Copper and silver ions attack the bacterial membrane, causing the bacteria to die; however, this process must be constantly monitored to ensure that the ion concentration is high enough to guarantee its effectiveness. Another disadvantage of this method is that it is not suitable for galvanized pipes (zinc inhibits copper ions) and may cause corrosion.

The methods described above are the main ways to prevent Legionella; however, there are other, less commonly used methods, such as the use of ozone (which is effective but expensive, complex, and potentially risky for piping as well as human health if toxic byproducts are formed). Each method has its strengths and weaknesses, which is why it is sometimes advisable to use two methods simultaneously to eliminate any risk of bacterial growth. For more information on Legionella prevention methods, contact our Enertech Solution team.