During the operation of wastewater treatment systems, most businesses often focus on monitoring indicators such as pH, DO, COD, Ammonia, or Total Nitrogen to evaluate treatment efficiency. However, there is a parameter that, although not included in the discharge standards, very quickly reflects the operating status of the biological system: ORP (Oxidation Reduction Potential). When monitored properly, ORP can help operators early identify changes in the treatment environment, evaluate the efficiency of nitrification, denitrification, and other biological processes, thereby proactively adjusting the system before indicators like Ammonia or Total Nitrogen begin to fluctuate.
ORP (Oxidation Reduction Potential) is a parameter that represents the tendency for oxidation or reduction reactions to occur in an aqueous environment, usually measured in mV (millivolts). Simply put, ORP reflects the "oxidizing" or "reducing" level of the environment, thereby indicating whether the current conditions are suitable for microbial groups to perform the treatment process.
Unlike COD, Ammonia, or TSS, which are indicators reflecting wastewater quality, ORP does not assess the level of pollution but reflects the oxidation-reduction state of the treatment environment. Therefore, ORP is not an indicator in discharge standards but an operational parameter, helping operators detect changes in the biological system early on.
In biological treatment systems, each stage requires a different environmental condition for microorganisms to function efficiently. Instead of waiting until the effluent COD, Ammonia, or Total Nitrogen increases to detect an issue, monitoring ORP can help operators early identify changes in the treatment environment right from inside the tank.
When combined with parameters such as DO, pH, and temperature, ORP provides a more comprehensive view of the microbial system's operating status, supporting the evaluation of nitrification, denitrification, and many other biological treatment processes. This is also why many current wastewater treatment systems have begun to integrate ORP sensors to support automated operation.
The ORP value is not fixed but changes according to the conditions of each biological treatment stage. This change reflects the oxidation-reduction environment in the treatment tank and helps operators evaluate whether the biological process is occurring correctly.
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ORP Diagram
ORP is not an independent operational parameter but reflects the oxidation-reduction state of the treatment environment. The ORP value is influenced by many factors such as DO, pH, temperature, and the operating conditions of the microbial system. Therefore, when ORP fluctuates abnormally, businesses should not evaluate it in isolation but need to combine it with other operational parameters to accurately determine the cause and provide appropriate solutions.
In anaerobic tanks, ORP is usually negative, reflecting an environment with no dissolved oxygen, which is suitable for the activity of anaerobic microorganisms. This is the condition for the decomposition of organic compounds and the characteristic biological reactions of the anaerobic treatment stage.
When moving to the anoxic tank, the ORP will increase compared to the anaerobic tank but remain lower than in the aerobic tank. This is a favorable condition for the denitrification process to occur efficiently. If ORP fluctuates abnormally, businesses should combine checking DO, the carbon source, and the recirculation mode to accurately assess the cause.
In the aerobic tank, ORP is usually maintained at a positive level, reflecting a favorable oxidizing environment for the activity of aerobic microorganisms. This is the condition for the oxidation of organic matter and nitrification. In reality, if ORP continuously decreases while the wastewater flow and load remain almost unchanged, businesses should not immediately increase the air supply. First, it is necessary to simultaneously check the DO, influent organic load, and the condition of activated sludge to determine the correct cause.
Note: The ORP value of each stage is for reference only and may vary depending on the treatment technology, wastewater characteristics, and operating conditions of each system.
| Stage | ORP Trend | Meaning |
| Anaerobic tank | Negative | Suitable condition for anaerobic microorganisms to decompose organic matter. |
| Anoxic tank | Slightly negative | Favorable condition for the denitrification process. |
| Aerobic tank | Positive | Suitable condition for aerobic microorganisms and the nitrification process. |
The ORP value is for reference only and may vary according to the treatment technology, wastewater characteristics, and operating conditions of each system.
In actual operation, the ORP value does not depend on a single factor but is simultaneously affected by environmental conditions and the operating status of the microbial system. Therefore, when ORP changes abnormally, businesses should not immediately adjust the system based solely on this parameter but need to consider the overall related operating conditions.
DO and ORP both reflect the oxidizing conditions of the environment, but they are not the same parameter. DO indicates the amount of dissolved oxygen in the water, while ORP reflects the tendency for oxidation-reduction reactions to occur.
Usually, when DO decreases, the ORP value also tends to decrease because the environment becomes less oxidizing. However, there are still cases where DO is at a suitable level but ORP fluctuates due to the influence of the organic load, microbial activity, or other biological reactions. Therefore, businesses should monitor both DO and ORP simultaneously to have a more complete view of the system's status.
Temperature directly affects the biological reaction rate and metabolic activity of microorganisms. When the temperature changes significantly, the ORP value can also fluctuate, even if other operating conditions remain largely unchanged.
For some industries with high-temperature wastewater or seasonal fluctuations, monitoring temperature and ORP simultaneously will help operators more accurately assess the system's condition, avoiding inappropriate adjustments based solely on one parameter.
Besides DO and temperature, ORP is also influenced by pH, influent organic load, and the operating status of the microbial system. When the pollution load suddenly increases or the microbial system becomes unstable, the oxidation-reduction reactions in the treatment tank also change, leading to fluctuations in ORP.
Therefore, ORP only truly shows its value when evaluated alongside other operating parameters. Considering multiple factors simultaneously will help identify the correct cause instead of adjusting the system based on individual parameters.
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ORP in the wastewater treatment system
Although ORP is a useful parameter during operation, using it incorrectly can still lead to inaccurate assessments. Here are some common mistakes:
ORP constantly changes according to operating conditions and wastewater load. Therefore, a single ORP value at one point in time is not enough to assess the system's condition. Operators should monitor the fluctuation trend of ORP over several days or operating cycles to recognize abnormal signs.
A fairly common mistake is to only look at ORP while ignoring other parameters such as DO, pH, temperature, or wastewater analysis results. In reality, ORP only reflects a part of the treatment environment's state. Combining multiple parameters will help evaluate the system's operating efficiency more accurately and limit inappropriate adjustment decisions.
ORP sensors operate in a wastewater environment, making them prone to fouling from biological sludge or chemicals. If not cleaned and calibrated periodically, the measured values can deviate, leading operators to misjudge the actual condition of the system.
Checking, cleaning, and calibrating sensors according to the manufacturer's recommendations is essential to ensure that ORP data always accurately reflects operating conditions.
In many cases, ORP is the parameter that helps operators detect microbial imbalances early before indicators like Ammonia or Total Nitrogen begin to exceed the limit.
ORP is not an indicator for assessing the quality of effluent wastewater, but it is a highly useful parameter in the operation of a wastewater treatment system. When monitored in combination with DO, pH, temperature, and wastewater analysis results, ORP can help businesses early identify changes in the treatment environment, evaluate the operational efficiency of the microbial system, and proactively adjust the system before issues arise.
With experience in the design, construction, operation, and renovation of industrial wastewater treatment systems, Dai Nam is always ready to accompany businesses in evaluating operating parameters, optimizing treatment efficiency, and developing solutions suitable for specific practical conditions.
