Fundamentals of ORP for Cooling Towers

The Measurement of Oxidation/Reduction Potential (ORP) is by No Means New. However, it has only been within the last decade that a significant number of power plants have begun to realize the benefits of ORP measurement for cooling water systems, makeup water systems and the steam/water cycle. The ORP measurement utilizes an inert metal (typically platinum) measuring electrode that develops a millivolt potential due to the transfer of electrons within the oxidation/reduction (redox) process. The millivolt potential is established on the measuring electrode with respect to a reference electrode, typically silver/silver chloride, which is similar to that used in pH measurements. In an oxidizing environment, such as that caused by the presence of an oxidizing microbiocide, a higher ORP will exist, while a lower ORP will exist in a more reducing environment.

Microbiological Control

Historically, microorganisms and macroorganisms have been controlled by adding an oxidizing biocide such as chlorine, bromine or ozone to effectively limit microbial activity. Oxidizing biocides are toxic to the organism growth by removing electrons from it, thus irreversibly oxidizing protein groups such that normal enzyme activity is lost, which results in the death of the cell.To ensure proper biological control, it is necessary both to measure the total number of organisms present in the system and to identify the types of organisms. Typical microbiological control programs are designed to reduce 99 percent or more of the organisms in the water by determining the concentration of oxidant required to both kill any existing microorganisms and also prevent any further growth from occurring. Often, a residual of the oxidant of choice, based upon the determined concentration requirement, is maintained to ensure that any excursions of organic growth are immediately arrested.

Various shortcomings associated with the residual method, however, often result in overfeed or underfeed conditions. Oxidant overfeed will result in high chemical costs as well as potential damage to system components and metallurgy. Oxidant underfeed will result in potentially irreversible damage and loss of efficiency due to microorganism growth. Both problems should be avoided, but the only way to improve residual control is to increase periodic plate counts and perform more setpoint adjustments. This method can yield only slightly better results and will waste time, money and energy in performing the frequent checks that are required.

Research shows that a microorganism`s ability to survive in water is mainly influenced by the ORP of the water. One of the most common questions asked about ORP measurement when used for biocide control is, "Can ORP be used to measure residual oxidant (chlorine, bromine, etc.)?" This approach acknowledges that there is indeed a problem with residual control, but obviously lacks a fundamental understanding of exactly what the problem is. Whereas residual measurements simply respond to the concentration of excess oxidant that exists, ORP responds to the oxidant`s toxicity to the organisms.For each individual site and application, an ORP value must be established based upon laboratory tests for organism growth. The control setpoint will typically be the optimal ORP value that, when maintained, will consistently prevent growth of microorganisms at the minimal oxidant dosage required, which may change significantly from day to day or season to season. However, it is usually observed that control by ORP will significantly reduce the cost of chemicals for a given application while minimizing organism growth. Any costs associated with maintenance for the ORP measurement are small compared to the cost-savings associated with the improved biocide control.

Cooling Water

Both microorganisms and macroorganisms can enter a cooling water system through the incoming water or through the air itself (if a cooling tower is present). In fact, a cooling water system can provide optimum conditions for growth, because temperature and pH ranges are usually ideal, and nutrients such as sunlight, organic matter and inorganic salts are found in abundance. Organisms potentially requiring control include barnacles, clams, jellyfish, mussels, algae, fungi and bacteria. Minimizing bacteria growth is perhaps the most difficult type of microbiological control, because so many different species of bacteria can exist in cooling water systems. Slimes can form that clog heat exchangers. Some aerobic bacteria types form strong acids, which can lower the pH of the water in general, as well as locally drop pH levels to as low as 1.0. Typically, anaerobic bacteria can grow underneath the aerobic bacteria, causing site corrosion. "Iron" bacteria can produce iron deposits that cause plugging, pitting corrosion and reduced heat transfer. Certain types of bacteria can destroy nitrite corrosion inhibitors. Other bacteria types can form ammonia, which can attack copper-based construction materials.

Controlling microbiological growth in a cooling water system is commonly achieved with chlorine addition, although sodium hypochlorite, bromine, chlorine dioxide and ozone are becoming more common as their benefits are realized. ORP control can be done with a feedback loop with the sensor placed either before or after the heat exchanger, although a downstream location may be better to compensate for any drop in oxidant levels within the heat exchanger. The ORP control setpoint must be determined for each individual site, because there will be many variations among organism type, water chemistry, temperature and oxidant type. For the most part, control setpoints for oxidizing biocide addition will be within the +550 to +650 mV range.

Dechlorination (or a similar removal of another type of oxidizing biocide from the water) for environmental protection is typically performed by adding a reducing agent such as sodium bisulfite or sulfur dioxide in order to properly reduce the oxidant. Research has shown that addition of enough sulfur dioxide or sodium bisulfite to provide an ORP value of just below +200 mV will result in the reduction of the oxidant.  Not all oxidants require this procedure, because some, such as ozone, will break down quickly on their own.

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Aldo Zaffalon
Aldo Zaffalon


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