Cooling Towers & Legionella

Cooling Towers and Legionella

Cooling towers are a major component in air conditioning systems used in large buildings in cities and towns or anywhere evaporative cooling systems are used. These cooling towers are an energy, cost and space-effective solution for the removal of heat created in the evaporative air cooling process. Water is used in an evaporation process to pull the heat from the system and move it outside, away from the space being cooled.

Due to the location and open nature of these cooling towers the basin that captures circulating water is exposed to dust and other debris. When left unchecked bacteria can flourish, creating the ideal breeding ground for harmful Legionella.

Specific factors contributing to the growth of legionella in cooling towers include:

• Water temperatures ranging from 20°C to 50°C. Legionella’s optimal growth range is between 35°C to 46°C.
• Stagnant, non-circulating water
• pH levels between 5.0 and 8.5
• Various forms of sediment the feed the growth of microflora conducive to the spread of legionella
• The presence of certain microorganisms, including algae and Pseudomonas, which contain essential nutrients required for legionella growth.

How to control Legionella Growth In Cooling Towers

There are four basic steps, which, if you take them, can control or prevent the growth of legionella in cooling towers. If you don’t take these steps, the likelihood of legionella growth is high.

I’ll mention the 4 steps here and then expand on each in more detail. The 4 steps to controlling legionella are: Regular maintenance of the cooling tower, system wide filtration, treating the water, and ongoing monitoring.

1. Regular Cooling Tower Maintenance

Cooling towers are constantly exposes to particles or debris in the air in their immediate environment. As a result, it’s only natural that contaminates will blow into the cooling tower. Screens can sometimes be used to reduce debris from blow-in but it’s impossible to eliminate debris completely. This means your regular maintenance routine needs to include removing blow-in materials that settle in the tower and particularly in the basin.

2. System-Wide Filtration

Even with frequent cooling tower maintenance dust and debris will continue to gather in the cooling tower basin. As a result, the water requires continuous filtration to maintain a satisfactory level of cleanliness. An effective filtration system will:

• Arrest organic composites and biological organisms
• Remove food and materials that nurture the growth of bacterial
• Eradicate dust and debris drawn into the cooling tower

3. Treatment of Water

The water in the cooling tower and the makeup water entering the cooling tower require treatment. Each cooling tower has its own set of unique features including location, size, source of water, and so, and therefore the water treatment program will vary from one cooling tower to another. But in general, a water treatment program that stops or reduces the chance of legionella will include:

• Minimizing scale formation
• Eliminating biological contamination  
• Monitoring and controlling the water pH levels
• Reducing the instance of corrosion and fouling

4. Ongoing Monitoring

An ongoing program of system wide monitoring and plant commissioning will keep the system functioning and maintain water quality levels. An optimum monitoring program will utilise technology, people and systems to achieve the following:

• maintaining water quantity and quality by monitoring temperature, pH levels and solubility of dissolvable materials
• Maintaining historical records and seasonal patterns effecting water quality
• Alerting maintenance personnel to significant chances effecting water quality

Follow the steps above and you’ll achieve two things: 1. you’ll maximise the efficiency of your cooling tower and 2. You’ll minimise the chance of legionella growth