Even if you cannot always see microorganisms, they are there. Microorganisms are not the most exciting topic nor are they very appealing; however it is important to understand the affects that bacteria, fungus and algae have on your cooling tower performance and maintenance strategy. Test your knowledge at the end to find out if you can correctly identify microorganism growth and how to treat or control it.

• Bacteria: Are microorganisms living within the water of your cooling tower.


• Legionella. Is a form of bacteria that grows within the water system of a cooling tower causing Legionnaires Disease.


• Fungi: A fungus is part of the plant family of organisms and is quickly reproduced by spores. Fungus grow in areas that lack sunlight. Mold & Yeast are two common fungus types in a cooling tower.


• Algae: is thick and green in color. It is a biological growth that forms mostly in areas with sunlight, such as the cooling tower deck or a crossflow tower hot water basin.

All Depot cooling towers are an open loop cooling system, which means that water and air come in direct contact with each other to complete the evaporative thermal process. Because of the open system design, microbes are being "scrubbed" from the air contributing to quicker biological growth. Additionally the source and quality of make-up water also contributes to increased amounts of microbes. The dirtier the water, the greater potential of biological growth. Maintaining water quality is one way to help control biological growth. Cooling towers within refineries, food processing plants, coal plants etc. where clean water is hard to come by should be on extra alert to microorganism growth.

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  The various microorganisms within your cooling water system cause biological fouling or biofilm. Biofilm has a significant impact in decreasing cooling performance. Biofilm acts as an insulator counteracting the cooling process. Additionally, it interrupts even water distribution and water flow by clogging areas such as nozzles, fill media and drift eliminators. Biofilm also induces corrosion causing metals to galvanize like your gearboxes, motors, distribution piping and hardware.




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  Legionella becomes a serious issue when it is entrained within the water vapor that has exited or evaporated from the cooling tower. When this evaporated air is circulating the air around which a person is breathing, it causes a form of pneumonia or lung infection. Legionella is a serious health condition and is why cooling tower design, water treatment, temperature control and low drift are important in cooling tower operations.




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  Fungus compromises the structural integrity for your cooling tower by causing wood rot and layers of biofilm.




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  Algae growth also clogs your nozzles, affecting the first stage in the thermal process: Water distribution. Uneven water distribution will cause additional performance issues and damage throughout the rest of the water cycle.

There is more than one solution in keeping your water system free from microbes. It is important to first understand that certain organisms will grow depending on their environment and the PH balance of your water. Knowing exactly what biological growth is populating in your cooling tower is important to the water treatment program you choose. Different organisms require certain nutrients to grow, thus the wrong treatment applied to your water system could contribute to other microbiological growth instead of treating it. For example one type of treatment may kill algae, but increase the bacterial count.

1) Growth within your cooling tower structure that causes wood rot is which microorganism?

Answer: Fungus

2) What microorganism needs sunlight to grow? What problems does this organism cause?

Answer: Algae. Algae causes clogged nozzles affecting water distribution.

3) What bacteria causes health issues and what are the best ways to prevent this health concern?

Answer: Legionella. Low drift rate, water temperature control and water treatment can help prevent legionella from occurring.

4) Biological fouling or biofilm causes clogged nozzles and fill. Biofilm also causes corrosion in ________, _________, ____________ and ____________.

Answer: Gearboxes, motors, distribution piping and hardware

5) True or False: Some water treatments can kill some microorganisms but implement the growth of others.

Answer: TRUE

6) What are the two groups of chemical treatments that can control biological fouling?

Answer: Oxidizing and non-oxidizing treatments

**Determining the right cooling tower water treatment program and chemicals can be complex and is unique to each system environment. Cooling Tower Depot does not provide water treatment services, thus we recommend discussing the implementation of any water treatment program with a water treatment specialist. Additionally, review the below resources for further and more detailed information about microorganisms, legionella and water treatment options.