05/08/2023 | Industrial Wastewater | 8 MINUTE READ

Say Goodbye to Bacterial Build-up: ORP Control in Cooling Towers

cooling towers

When a cooling tower is functioning correctly, it will remove heat from a facility by effectively spraying water down the tower in order to exchange heat with the building interiors. The efficiency of this process mainly depends on the water quality and the lack of bacteria in the tower. One measurement that allows you to determine how contaminated the cooling tower water has become is oxidation reduction potential (ORP).

While ORP isn’t a new measurement, it’s only been widely adopted in cooling towers for the past decade or so. The ORP measurements offers numerous benefits for makeup water systems and cooling water systems. It uses an inert metal electrode that creates a millivolt potential when electrons are transferred.

If there’s too much bacteria present in a cooling tower, an ORP sensor will provide you with low readings. In a clean environment, the ORP readings will be higher. In this guide, you’ll learn how ORP control can help you reduce bacteria buildup in a cooling tower.

cooling tower system

Importance of Water Chemistry in Cooling Tower Systems

Many businesses and industrial facilities require the use of cooling towers to ensure that the building is able to perform manufacturing, electric power generation, and air conditioning services. Despite their many advantages, there are also a few challenges that facility managers must navigate when maintaining the cooling tower. To ensure that the tower’s integrity and efficiency doesn’t waver, the water that’s situated in the tower needs to be chemically controlled.

If the water’s pH levels are too low, the cooling tower system could be contaminated or damaged. Corrosion may be accelerated as well. Over time, the cooling tower would fall into disrepair. It’s possible that any systems that are connected to the tower would also be impacted. Knowing why contamination occurs in a cooling tower is necessary to maintain it and protect it from further damage. The right water chemistry will prevent corrosion and keep bacterial growth at bay.

Introduction to ORP for Monitoring Water Chemistry

ORP is a key parameter for monitoring water chemistry in cooling tower systems. When used correctly, it allows you to test water as well as the quality of any treatment solutions you end up using. This measurement can help you mitigate microbial fouling in certain cooling tower systems, the primary of which include ones with an open loop design. The open loop design makes it significantly easier for microbial organisms to grow in the cooling tower. The water that circulates through the system will gain microbes from the surrounding air when it’s sent through the tower. Evaporation leads to bacteria in the tower’s make-up water becoming substantially more concentrated.

If this issue isn’t corrected, bacteria, algae, and fungi will rapidly increase. You may notice a biofilm along the condenser tubes, cooling tower surfaces, and pipe walls, which leads to less heat transfer capabilities and flow. To account for these contaminants, equipment will need to run longer. More water will be used as well, which ensures that the operational costs in your facility increase until these issues are dealt with.

When a facility wants to get rid of the microbial growth in a cooling tower, they typically add a type of oxidizing biocide to the water, which can be anything from chlorine and bromine to ozone. When the biocide interacts with the water, an electrochemical process occurs that causes the biocide to take an electron from the microbial organism, which destroys the cell walls and kills the organism.

cooling towers

Using ORP Control to Prevent Bacterial Growth

You can use ORP measurements to control how much biocides are used in cooling towers when preventing the growth of harmful bacteria. When you place an ORP sensor in the water, the measurement you receive will tell you if you’ve added enough biocides or if additional treatment is necessary. ORP controllers use the feedback they receive from ORP sensors to adjust the biocide dosage in real-time.

When properly implemented, this form of treatment should remove around 99% or more of organisms that are present in the water. The ORP controllers and sensors will work together to determine the exact concentration of oxidant that’s needed to eliminate all existing microorganisms and prevent additional growth from taking place. In most cases, the oxidant will leave residue that ensures organic growth can’t occur for the foreseeable future.

Some facilities will attempt to use biocide residue to limit growth without having ORP sensors and controllers in place. There are, however, a few issues that must be taken into account when using the residual technique. For one, this process could lead to the water being underfed or overfed. Oxidant overfeed means that you’ll spend a considerable sum of money on chemicals. There’s also a higher chance that system components will be damaged by these chemicals.

An oxidant underfeed scenario may result in efficiency losses because of microorganism growth as well as the potential for irreversible damage to the cooling tower. If you don’t place enough biocide in the water but believe that the water has been effectively treated, you might not notice that the water has been contaminated until it’s too late.

You can avoid both of these issues by making regular adjustments to the amount of biocides that are placed in the water. When you use ORP measurement for biocide control, keep in mind that ORP readings respond to the toxicity an oxidant is having on the organisms in the water. The optimal ORP value you should try to maintain is one that prevents the growth of harmful microorganisms while using the minimal amount of oxidant dosage that’s needed. This value can change on a daily or seasonal basis. Controlling biocides with ORP measurements should reduce the amount of chemicals you need to use.

Benefits of ORP Control in Cooling Towers

There are numerous advantages associated with using ORP control in a cooling tower. ORP control can:

  • Improve safety
  • Reduce your facility’s maintenance costs
  • Enhance efficiency

ORP control has already been successfully implemented in real-world cooling tower systems.

cooling towers

Importance of Regular Maintenance and Calibration

While ORP sensors are effective at displaying the activity of reducers and oxidizers in water, they must be properly maintained and regularly calibrated to ensure you receive precise results. Consider cleaning and calibrating the sensor between every measurement. You can choose to either make or purchase a calibration solution. Make sure that the calibration solution aligns with the specific measurement accuracy you’re looking for.

With the calibration solution in hand, rinse your ORP sensor with water before submerging it directly in the solution. You should then stir the sensor for 30-45 seconds, which is enough time for the readings to stabilize. After you receive a stable reading, make sure that your sensor matches the proper reading associated with the calibration solution. Keep in mind that proper maintenance can help you prevent such issues as inaccurate readings and sensor drift.

Balancing ORP and pH

When you’re measuring the quality of water, pH measurements are just as important in cooling tower systems. Both of these measurements should be balanced properly to ensure that the water in your cooling tower is clean and contaminant-free. ORP usually decreases when pH levels increase, which means that a low ORP reading indicates that the water is mostly free of contaminants. However, it’s best to maintain a neutral pH, which ensures that the ORP is still high enough to keep the water sanitized.

In the event that the cooling tower water is highly contaminated, you should encounter low pH levels and high ORP readings. When pH levels are low, the water is acidic. A high pH reading means that the water is alkaline and is likely comprised of chlorine. If water is acidic, there’s a good chance that it’s filled with contaminants.

Since ORP and pH readings affect one another, you should monitor and adjust them accordingly. Reacting quickly should keep your cooling tower water clean and prevent damage to the entire system. As mentioned previously, make sure that you clean these sensors often to maintain accurate readings.

dirty water


Managing the water chemistry in your cooling system is essential if you want to combat microbiological fouling and maintain system efficiency. With the right ORP control in place, you can keep the water contaminant-free. The effectiveness of a biocide directly correlates with the readings from your ORP sensor.

The ability for an organism to survive depends on the water’s current ORP. If you have a high ORP reading, this means that there’s an oxidizing agent in the water and that you’ve likely used enough biocide. When the ORP readings are low, it’s possible that the water doesn’t contain enough biocide, which increases the likelihood that fouling will occur. You can ensure safe and efficient operation in your cooling tower by regularly maintaining your ORP sensor and monitoring the results.

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Posted by Dominic O'Donnell on May 8, 2023

Sensorex is a global leader in the design and manufacture of quality sensors for water quality and process applications. The company offers more than 2000 sensor packages for pH, ORP, conductivity, dissolved oxygen, free chlorine, chlorine dioxide, UV transmittance and other specialty measurements, as well as a full line of sensor accessories and transmitters. Its expert technical support engineers solve analytical sensor challenges with custom designs and off the shelf products.

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