Les Flynn | CAPE HydroTek

| IMAGE 1: New technologies will allow operators to control Legionella growth in their cooling towers without disrupting existing infrastructure. (Photo credit: Gu Bra, pexels.com)

Cooling towers are a critical asset in a wide array of industrial processes in chemical and pharmaceutical manufacturing, power generation, food and beverage processing, refining and a host of other sectors. These diverse industries all face similar challenges related to scaling, source water volatility and biofilm formation. While most industrial processes have systems in place for addressing each of these challenges, those processes must also be designed to handle a bacterial outbreak, like Legionella, in a cooling tower system.

Unfortunately, the conditions found in nearly all cooling towers are often ideal for Legionella formation, and an outbreak can become a serious health concern for facility team members, visitors and other building occupants. Community members can also be at risk due to the propensity of Legionella to disperse well beyond a facility in aerosolized cooling tower emissions.

To prevent an outbreak, facilities have traditionally relied on biocides to directly combat the growth of Legionella in cooling tower water. However, biocides can introduce additional challenges, and industries need new technologies and methods for preventing Legionella from forming in the first place.

How Legionella Forms in Cooling Towers

Legionella is a broad genus of bacteria found in fresh water like lakes and streams. In nature, water currents, changing temperatures and a balanced ecosystem generally keep the bacteria in check and prevent it from becoming a public health hazard. In cooling towers, however, the water is usually warm and stagnant or slow moving. Most cooling towers exist to reject heat using evaporative cooling: As warm water is exposed to moving air, a portion evaporates, drawing heat from the remaining water before it recirculates.

This process leaves the water warm and increasingly mineral-concentrated, conditions that favor both Legionella growth and biofilm formation. Biofilm (a buildup of scale, organic debris and microorganisms on wetted surfaces) both shields Legionella from disinfection and feeds it, supplying organic matter and microbial byproducts while hosting free-living amoeba that Legionella replicates within before rupturing them to release new bacteria. This combination of nutrients, shelter and protection makes cooling towers a near-ideal environment for the bacteria to thrive.

Additionally, poor circulation or taking a cooling tower offline for service can also lead to Legionella formation, even in systems that have historically not experienced Legionella-related challenges.

The health risks associated with a Legionella outbreak are well known. The most common risk is Legionnaires’ disease, an illness similar to pneumonia that comes from inhaling water droplets contaminated with the bacteria. Because of the health risks, many states and regions have strict regulations in place that require periodic testing for Legionella formation in industrial process water.

The Problems With Biocides

Most biocides work by attacking bacteria directly and either destroying its cellular structure or limiting its ability to reproduce. Historically, facility operators have prevented Legionella formation through biocide treatment cycles and regularly taking a cooling tower offline for cleaning and biofilm removal. While this strategy can prove to be effective, it is costly and time-consuming and can bottleneck a facility’s overall performance.

Additionally, some biocides can interfere with manufacturing processes or accumulate in downstream systems, adding to a facility’s maintenance burden and reducing equipment lifespan and effectiveness. Many facilities address this by adding a treatment step to remove biocides from the water before discharge or reuse, but that introduces yet another system that comes with its own expense and maintenance, solely to remove a chemical the facility added in the first place. Biocide use also has a technical limitation separate from these logistical costs: Biofilm formation can create pockets that shelter bacteria from biocides, reducing overall treatment effectiveness regardless of dosage or frequency.

There are costs associated with relying on biocides to treat cooling tower water. Any potential supply chain volatility exposes a facility to rising chemical costs, which can disrupt that facility’s ability to operate and remain within regulatory compliance. Add this risk to the additional maintenance and operational costs associated with removing biocides from process water, and chemical treatment quickly becomes a significant operational cost.

Until recently, facility operators have not had many effective alternatives to biocides for combatting Legionella. However, recent advancements in water treatment technologies have found new approaches for treating process water and destabilizing the environment that harbors Legionella formation.

| IMAGE 2: Cooling towers are an essential part of many manufacturing processes but can also be the perfect environment for Legionella to thrive. (Image courtesy of CAPE HydroTek)

Non-Biocide Solutions to Legionella

As water treatment technology advances, new solutions are coming to market to fight Legionella formation while avoiding the drawbacks associated with biocides. Most of these solutions are easy to implement and require minimal maintenance, space and setup time. Also, the immediate installation costs associated with these technologies usually yields long-term savings through reduced chemical costs, lower maintenance needs and a reduction in scaling and biofilm formation.

Three such technologies include ultraviolet (UV) treatment, ozone generation and catalytic water treatment (CWT). Each solution has strengths and weaknesses, and facility operators will need to examine each to determine the best fit for their facility.

UV treatment often involves treating process water in a sidestream loop and disinfects water by attacking Legionella directly. UV lights are well-known and long proven; however, while UV is effective at combatting Legionella, it does not address the biofilm or scaling that will continue to shelter Legionella from treatment.

Ozone generation can control the growth of Legionella and prevent the formation of biofilms. As an oxidizer, it attacks the cell membrane and destroys Legionella, while also penetrating and breaking down biofilm layers when properly dosed. The system uses on-site oxygen and leaves minimal residue in the treatment train. For operators seeking a more aggressive solution for controlling Legionella, ozone generation can be a promising option. However, it may not be a fit if a facility does not have an on-site oxygen supply or has very low tolerances for the minimal residues that come from ozone generation.

Finally, CWT is a unique technology in that it does not attack Legionella directly. Instead, it facilitates interfacial electron transfer and improves transport and hydration structure, which can make existing redox reactions (like chlorine disinfection) more effective, disrupting the primary drivers behind biofilm formation. This removes Legionella’s main food source in the cooling tower and makes it harder for the bacteria to establish and grow.

CWT systems can be deployed upstream of the cooling tower, within the cooling circuit or on a side stream, so no modifications to the tower itself are required, making the technology well-suited to facilities facing persistent or recurring biofilm challenges. Unlike biocides, however, CWT does not kill Legionella outright, so it should be paired with other treatment methods if an active outbreak needs to be addressed. Ultimately, CWT’s real value lies in changing the water’s underlying chemistry so that Legionella never gets a foothold in the first place. This shifts treatment from reactive bacterial control to proactive prevention, with downstream benefits including reduced biocide dependence, lower chemical handling and disposal costs and less strain on tower components from scale and corrosion.

While biocides are effective at controlling the growth of Legionella in cooling towers, long-term Legionella control should not include trading one contaminant for another. With advancements in water treatment technology, new reduced-chemical options are changing how the industry thinks about biocides. Chemical treatments remain fast-acting and reliable, making them a reasonable backup should Legionella circumvent or overwhelm a low-chemical approach, but that reliability comes at a cost, both financially and in the downstream burden biocides place on equipment and treatment.

Ultimately, facility operators should weigh cost, water chemistry impact and long-term effectiveness when choosing a treatment strategy, recognizing that the most resilient programs often combine proactive, chemical-free prevention with biocides held in reserve rather than relying on either approach alone.

| IMAGE 3: Biofilm formation is a critical part of Leigonella growth. Keeping infrastructure clean is critical in preventing outbreaks. (Image courtesy of CAPE HydroTek)

Les Flynn is president of CAPE HydroTek, a catalytic water conditioning and treatment technology solutions provider. He may be reached at lflynn@cape-inc.com. For more information, visit capehydrotek.com.

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