In a previous article, we’ve shared how controlling bacteria and pathogens in a cooling tower is critical to maintaining human health, heat transfer efficiency, and equipment lifespan. We shared “three-legged stool” approach for maintaining biological control in cooling towers: mechanical design, operational practices, and chemical treatment. In this article, we’re going to focus-in on the chemical leg of the stool – what chemicals are commonly used, their purposes and tradeoffs, and what operators need to know about employing them properly. EAI is a proponent of the philosophy that any of the typical cooling biocides can be effective if its employment is matched properly with operator training, testing protocols, and monitoring technology.

A biocide is a general term for a physical substance that can destroy living organisms such as pathogens, like bacteria, viruses and fungi. Biocides are chemicals that can be oxidizing or non-oxidizing and are typically added to water to control pathogenic growth.

Why are Biocides Added to Cooling-Tower Water?

Open loop cooling towers are particularly susceptible to waterborne pathogens such as Aspergillus, Mycobacteria, E. coli, and Legionella pneumophila and require assessment before treatment with biocides. The distinction of biocide as a physical substance exists because there are other treatments such as ultraviolet light or heat that can also act in a similar manner but are sources of energy rather than physical substance. This distinction is important because often it is an effective practice to mix the use of biocides with sterilizing techniques like UV or heat radiation.

The Most Common Biocide

Chlorine is the most commonly used biocide for water treatment. It has been grandfathered in due to its use since 1897 when the town of Maidstone England became the first town to treat its entire water supply with chlorine.^[1] Chlorine has been used over a century due to its reliability and effectiveness against a wide spectrum of pathogenic organisms. But, chlorine is not a catch-all, some pathogens such as Sphingomonas, Acinetobacter and Bacillus have shown resistance to chlorine and other disinfectants. Cryptosporidium (aka “Crypto”) is an extremely chlorine-tolerant parasite due to it’s thick-walled oocyst which allows it to survive for long periods of time outside the body.^[2] Oxidizing biocides, such as chlorine, initiate the electrochemical process of oxidation in which the oxidizing agent such as chlorine pulls in electrons. This pull of electrons removes them from the pathogens such as bacteria and causes them to die or, at least, prevents them from continuing their growth cycle. Then pathogens such as Crypto have an extra thick oocyst, it blocks this process from occurring.

Here is a summary of the most commonly employed oxidizing biocides in cooling towers (generally members of the halogen periodic element family):

• Chlorine: In cooling towers, chlorine is typically fed as liquid sodium hypochlorite (aka bleach) at 12.5% strength.. This is typically the cheapest option in terms of direct costs, but it does come with some downsides. Bleach is typically much more corrosive than alternative oxidizing biocides and can degrade easily with exposure to sunlight and over time. It is important to store bleach properly and observe expiration dates. In some larger facilities, it makes financial sense to have bleach delivered in “mini-bulk” method, where a truck comes to refill a large base tank in lieu of having individual drum or tote packages delivered at high frequency.
• Bromine (solid or liquid). Bromine is more expensive than chlorine, but does not have nearly the same corrosive potential as chlorine. Bromine in solid form has the advantage of reducing freight and delivery complications that come with the liquid form, and can have extended life / stability while in storage. Solid bromine can be in either tablets or granular. While bromine has always been more expensive than equivalent doses of bleach, the supply chain disruptions of 2020-2023 saw massive price increases in bromine costs. 
• “Stabilized” Bromine (aka BroMax): A mixture of chlorine and bromine. This hybrid product blends the cost, corrosion, pH, and stability implications of both pure type of oxidizing biocides.

One alternative to note is ozone generated from UV lamps. Ozone and its constituents in solution can be a highly effective oxidizing agent. These generators can link several lamps together to generate enough ozone for most any size cooling tower. These generators can bing dramatic reduction in costs for traditional chemicals (the electricity operating expense is a tiny fraction of the total cost of buying and employing traditional liquid or solid oxidant. They can also be helpful for onsite operators who now have less bulk chemicals to haul and manage. The downside is that ozone can be harder to control with the traditional oxidant measurement tools, is little dosing adjustment, and it has a much shorter half life and lower residual than the traditional halogen-based biocides. In the case of a power interruption or other problem that interrupts the generator, the lack of residual introduces greater risk of losing biological control. In some cases, it may still be beneficial to feed a traditional non-oxidizing biocide to supplement ozone.

For  oxidizing biocides, it is important to note that any increase in cycles of concentration will result in a higher mean residence time for the recirculating water (all else being equal). This means that the relative amount of biocide dosed may need to increase to maintain adequate biological control, as halogens like chlorine do degrade over time.

Control Mechanisms for Oxidizing Biocides

In towers with modern automated controllers, biocides can be directly controlled using “online” probes and sensors. In this case, “online” means that they are taken directly without disturbing the production process, but even better they are actually connected to the internet. 

The most common sensors are “oxidation reduction potential” or ORP probes. When ORP drops below a certain threshold, additional biocide will be dosed. While this is effective, it is not the highest control method available. This is because ORP is a function of several factors besides biocide concentration, such as temperature and pH.

As a more expensive, but higher quality alternative, chlorine analyzers can directly and automatically measure halogen residuals in the circulating water.

Regardless of which method is used, operators much be mindful to ensure that probes are calibrated within the manufacturer’s recommended frequency and that results are validated with additional manual sampling methods periodically.

Non-oxidizing Biocides

Non-oxidizing biocides (NOBs) are compounds that act against bacteria through several mechanisms: preventing duplication, stopping cellular processes and metabolic reactions or destroying the cell wall. The biocidal action is carried out through specific mechanisms that make NOBs characterized by a major tendency to induce resistance phenomenon. Non-oxidizing biocides do not act through redox reaction and, in general, they are less reactive than oxidizing biocides. This characteristic makes them less corrosive and more stable allowing longer storage. Non-oxidizing biocides typically include ammonium and phosphonium compounds, organic molecules with bromine and isothiazolinones.^[3] Due to variety of methods of cell damage, non-oxidizing biocides may be used in certain applications where oxidizing biocides may fail. They are also less sensitive to pH and temperature change.

In a cooling tower, the amount of non-oxidizing biocide being fed is a dependent on the system volume and the blowdown rate, which will result in biocide loss from the system that needs to be replaced.There are complex formulas based on the “kill time” and density of each biocide that will be used in conjunction with system volume and density. 

Other things to note

It is incredibly important for facilities to ensure that labels on your biocide tanks and packaging have application and dosing guidance, and that you are following in that. In the unfortunate event of a Legionella investigation (especially one that leads to a human infection) or any other type of investigation or litigation stemming from a waterborne pathogen, this can be a huge liability blindspot for the operating team.

Biocides, such as chlorine, are traditionally used in water treatment as a primary and secondary source of sterilization. The primary source of sterilization will kill on contact and the secondary source will act as a residual in the water and attack pathogenic growth in the future. But, as discussed above, there are limits to the effectiveness of physical biocides.

Conclusion

Remember that chemical treatment alone is not sufficient in establishing long-term biological control in cooling systems. Reach out to EAI Water to talk further about your Water Management Program.