Why Are Bacteria Harmful To Cooling Towers

close up image of bacteria growing in a closed loop system

Why Is It Important To Lay Up My Cooling Tower?

If left to grow unchecked, the bacteria that live in your cooling tower will colonize pipes and other wetted surfaces. Over time these colonies will grow into thick biofilms that reduce heat transfer, prevent corrosion inhibition strategies, and even cause corrosion themselves. Cooling tower water commonly supports large populations of bacteria and algae. Air pulled through the tower contains particulates and those particulates harbor microbes. The cooling tower scrubs the particulates out of the air and the water offers the microbes a place to grow and multiply. The water may look clear, but microbe counts as high as one million or more cells per milliliter can thrive in cooling tower water. That’s up to a trillion bacteria in 1,000 gallons of tower water!

infographic of the levels of bacteria that can grow in closed loop systems

How Much Bacteria is Too Much?

The Cooling Tower Institute, ASHRAE and other professional organizations have identified a population level of 105 CFU/ml* or lower as efficient. Bacteria are over 98% water and do not impact evaporation or heat transfer as free-floating cells. When the population exceeds the carrying capacity of the environment, bacteria secrete a coat of extracellular polymer, better known as slime. This slime traps other particulates, harbors both living and dead bacteria, and eventually grows into a complex community knows as a biofilm. Biofilm has an insulating effect on heat transfer, can clog piping and reduce flow rates, and can promote corrosion beneath the biofilm by a variety of mechanisms. If left to grow out of control, the biofilm will support rich, complex communities that may include multicellular organisms such as rotifers and roundworms.

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What Are The Problems With Biofilm In My System?

Biofilm forms a boundary between the water and the copper and steel in your tower and heat exchangers. This boundary reduces heat transfer efficiency. In fact, biofilm creates even more heat transfer problems than calcium scale. Biofilm also prevents corrosion inhibitors or the inhibiting effects of PowerPure from reaching the base metal. Biofilm can harbor Legionella and other potentially harmful species. Microbiologically influenced corrosion, or MIC, can occur within biofilm and attack tube sheets, end bells, and other system components that are protected during normal tower operation. Biofilm also supports under-deposit corrosion that can weaken metal components and shorten equipment life.

What About Algae in My System?

Close up image of algae that can grow in cooling towers

Algae can be a problem in cooling towers where a significant amount of sunlight reaches the water. Much like bacteria though, a little algae in the sunlit parts of a cooling tower does not reduce evaporation or heat transfer. Thick mats of algae can promote under-deposit corrosion and harbor other damaging bacteria. Long strands of algae that break free can clog strainers or other fine orifices in your system. Algae dies quickly once it breaks free of its hold in the sun and flows into the darkness of a piping system. Dead algae decompose and can provide nutrients for bacterial communities, but the main impact of algae is the clogging of intake screens and other strainers in the system.

How Can I Prevent Bacteria In My Cooling Tower?

One of the goals of a good water treatment program is to manage the population of bacteria at or below the recommended level of 105 cfu/ml. Most bacteria are not inherently harmful, but bad things can happen when they grow out of control. It is important to realize that we encounter these bacteria every day when breathing the air around us, touching countertops or sinks, or working in dirty areas. There are no dangers at very low exposure levels. Keeping bacteria populations at or below the 105 CFU/ml level will prevent biofilm formation. Chemical treatment programs use biocides to control bacteria. Chardon’s PowerPure system also maintains very low levels of bacteria, usually less than 103 CFU/ml.

What Happens When My System Is Off Line?

Your equipment is protected by Chardon’s treatment program while it is online and operating, but what about during short-term shutdowns or the offseason? Bacteria grow exponentially; that is, one cell divides to form two cells, those two divide to form four, then eight, then sixteen, and so on. When water is left in the system without biocide or PowerPure to keep the bacteria under control, microbiological activity continues even though the system is no longer circulating. If left unchecked, bacteria will multiply and begin forming the biofilm.

biofilm build up in a close loop system

What Can I Do To Prevent Biofilm When My System Is Off Line?

 Following an accepted layup procedure is the best practice. We outline our layup procedure in a previous cooling tower resource. The best practice for protecting systems during seasonal or long-term layup is to drain condensers and heat exchangers as soon after shut down as possible. Microbiological fouling can proceed quickly and the cleaning and inspection will be easier when performed soon after shut down. First cycle the system down to help flush out particulates, then circulate a product such as our CT Lay-up or VaProtect, and finally drain the system completely. VaProtect will provide protection for up to six months. An airtight desiccant lay up program should be considered for longer periods of inactivity.

What If I Can’t Drain My System?

Cooling tower systems that operate intermittently during the cooler months will have biofilm problems and the problems could be severe if operation only occurs for a few hours a week. Chemical treatment products can remain active in the system even when it is not circulating, but all biocides have a certain half-life and protection rarely lasts more than a couple days. The PowerPure effect on microbes also diminishes quickly once the system goes offline. Serious consideration should be given to isolating secondary components and draining them per the outline above. If draining is not possible, program the energy management system to circulate water through offline components twice daily in coordination with the biocide program or with the PowerPure running to help prevent biofilm formation.

If you have questions about this or any other technical matter, call one of our Technical Support Team Members at 800-848-9526.

*Microbiologists use the term “colony forming unit,” or CFM, instead of bacteria because only colonies can be seen or counted, not individual cells.