How To Measure ORP In Water（oxidation reduction potential）

What is redox potential (ORP)?

ORP is the abbreviation of oxidation-reduction potential, which refers to the difference between the redox potential of the indicating electrode and the redox potential of the comparison electrode in the liquid. It can reflect a comprehensive index of the redox of the whole system.

If the ORP value is low, it indicates that the content of reducing substances or organic pollutants in the wastewater treatment system is high, the concentration of dissolved oxygen is low, and the reduction is stronger.

If the ORP value is high, it indicates that the concentration of organic pollutants in the wastewater is low, the concentration of dissolved oxygen or oxidizing substances is high, and the oxidation environment is stronger.

There are some shortcomings of the traditional redox water treatment technology, such as inaccurate control conditions, waste of chemicals, and not being environmentally friendly. However, with the help of an ORP meter, we can significantly improve the accurate control level of redox water treatment technology.

Affecting factors of ORP

Because there are many redox reactions in wastewater treatment, and the factors affecting ORP in each reactor are also different, it is difficult to evaluate which of the main factors caused the change in ORP.

For example, many organic substances exist in the activated sludge treatment system. A significant change in the concentration of organic substances causes a slight change in ORP. However, it is difficult to evaluate which kind of organic substances mainly cause the change in ORP.

Therefore, we should first understand what factors are affecting ORP change.

1) PH

In wastewater treatment, pH value is an important control factor. The optimum growth pH is 6.5 ~ 8.5 for aerobic microorganisms and fermentation acidogenic bacteria, and the optimum pH is 6.8 ~ 7.2 for anaerobic methanogens. In order to control the appropriate pH value, it is commonly controlled by adding alkali.

The pH value is greatly affected by the metabolic activity of microbial pollutants. In the acid-producing stage, acid-producing bacteria decompose macromolecular organic substances to produce fatty acids and carbon dioxide, which can reduce the pH value, but it can increase the pH value when the production of ammonia in the process of protein decompositionthe production of ammonia in the process of protein decomposition. In the methanogenesis stage, methanogens use acetic acid to produce methane, which can increase the pH value of the system.

PH value is an important factor causing the rise and fall of ORP. The higher the pH value, the lower the ORP; The lower the pH value, the higher the ORP.

Although there is a certain correlation between pH and ORP in sewage, the correlation between pH and ORP is not as high as that in pure water because ORP is also affected by microbial activities, dissolved oxygen and other factors.

2) Dissolved oxygen (DO)

As we all know, DO represents the content of oxygen dissolved in water. In the aerobic tank, we should control DO at 2mg / L in the outlet, and in the case of pure oxygen aeration, it should be 4mg / L and be 0.5mg/l in the anoxic denitrification tank. In the anaerobic tank, nitrate-nitrogen should be less than 0.2mg/l because molecular oxygen basically does not exist.

As an oxidant in wastewater treatment, DO is the most direct reason for the rise of ORP in the system. In pure water, ORP is a linear relationship with the logarithm of DO, and ORP increases with the increase of DO.

Related： How To Measure Dissolved Oxygen In Water

3) Composition of microorganisms

In the wastewater biological treatment system, there is a unique ecosystem.

We can realize the practical separation of acidogenic bacteria and methanogens in the two-phase anaerobic bioreactor. It is convenient for the control and management of the system. In the UASB dominated by flocculent mud, acidogenic bacteria and methanogens were screened out along the direction of water flow. In anaerobic granular sludge and anaerobic biofilm, the dominant bacteria change from acidogenic bacteria to methanogens from the outside to the inside.

In the anaerobic reaction system, we should control DO concentration and ORP very low, especially in the methanogenic stage, and the redox potential should be less than – 330mv.

Since DO is brought from the influent under the action of this unique ecosystem, the ORP of the system will soon fall to the suitable growth range of methanogens through the synergy and symbiosis among aerobic microorganisms, and facultative microorganisms and anaerobic microorganisms. This low redox potential is not only in anaerobic reactors but also in flocculent sludge in aeration tanks.

4) Temperature

In the process of wastewater treatment, the temperature is an essential index. Aerobic microorganisms are active at 15 ~ 30 ℃, and the optimal temperature of anaerobic microorganisms is around 35 ℃ and 55 ℃.

In the process of anaerobic wastewater treatment, the temperature change is closely related to the composition and proliferation of microorganisms, methane production rate and sludge sedimentation performance. Therefore, to ensure the anaerobic tank’s stable operation, we should adjust the wastewater temperature to 35 ℃ or 55 ℃ by cooling tower and steam heating before entering the anaerobic tank.

The research shows that the higher the solution temperature, the lower the ORP of the solution; In the wastewater treatment process, the influence of temperature is the same. In addition, the higher the temperature of the water treatment process, the lower the ORP, which is also related to the decrease of water molecular clusters caused by the increase in temperature.

In addition, the temperature change can also cause a change in pH, gas solubility, biological activity and the phase balance of water pollutants, affecting ORP.

5) Microbial activity

The activity of anaerobic activated sludge can be expressed by the maximum specific methane production rate and the maximum specific COD removal rate. The maximum specific COD removal rate can also express the activity of aerobic-activated sludge.

The higher the activity of microorganisms, the faster the rate of oxygen consumption and the production of reducing substances, and the faster the decrease of ORP.

As a comprehensive index reflecting the macro redox property of the water body, there are many influencing ORP factors. In addition to the above primary factors, ORP is also affected by pressure, organic matter, solid substance, microbial species and other factors.

These factors will affect each other. Therefore, the redox property of water is also the result of the comprehensive action of many factors.

Determination of redox potential

In the treatment of black and odorous water, the redox potential is determined by potentiometry, also called the direct determination method of the platinum electrode. We can insert the platinum and reference electrodes into the sampled water for determination. The American Wedmore ORP electrode adopts a platinum electrode, and the reference electrode is a silver chloride electrode.

The ORP electrode can absorb or release electrons in its surface-sensitive layer. Generally, it is in millivolts (MV). When the redox potential is positive, it indicates that the water quality has oxidizability. The greater the value is, the stronger the oxidizability is. When the potential is negative, it indicates that the water quality has reducibility. The greater the value is, the stronger the reducibility is.

Characteristics of redox potentiometry

The advantage of the traditional measurement method of redox potential is very simple, but the disadvantage is that it takes a long time to reach the potential equilibrium value. Due to the formation of oxide film or adsorption of other substances on the surface of the platinum electrode, the time to establish equilibrium potential will be slow.

Depending on the condition of the medium solution, the time will be in a few hours if the medium is clean. Suppose the medium is dirty, sometimes 1 or 2 days. We can set a time, such as ten minutes, twenty minutes and half an hour. When filling in the data, we need to identify the data time.

Application of ORP in sewage treatment

Redox potential was widely used in industrial wastewater treatment, especially for wastewater from some metal finishing and municipal sewage treatment plants.

There are many kinds of ions and dissolved oxygen in the sewage system. In other words, multiple redox pairs. We can measure the redox potential in the sewage through the ORP online monitoring instrument in a concise time without sampling and measurement in the laboratory. It can significantly shorten the test process and improve work efficiency.

In the sewage treatment system, important redox reactions include biodegradation of organic pollutants including carbon, nitrogen and phosphorus, hydrolysis and acidification of organic substances, nitrification and denitrification, and biological anaerobic phosphorus release, aerobic phosphorus absorption, etc.

1. The redox potential required by microorganisms differs in each sewage treatment stage.

Generally, aerobic microorganisms can grow above + 100mV, and the optimum is + 300 ~ + 400mV; Facultative anaerobic microorganisms do aerobic respiration above + 100mV and anaerobic respiration below + 100mV; Specific anaerobic bacteria at – 200 ~ – 250Mv, among which specific anaerobic methanogens is – 300 ~ – 400mV, and the most suitable is – 330mv.

The normal redox condition in the aerobic activated sludge system is between + 200 ~ + 600mV.

2. Strategy in aerobic biological treatment, anoxic biological treatment and anaerobic biological treatment.

We can artificially control the occurrence of biological reactions by monitoring and managing the ORP of sewage.

Managing operation:

• Increase aeration and dissolved oxygen concentration
• Adding oxidizing substances and other measures to improve redox potential
• Reduce aeration and dissolved oxygen concentration
• Adding carbon sources and reducing substances reduce the redox potential to promote or prevent the reaction.

Therefore, we can achieve better treatment by ORP control in aerobic biological treatment, anoxic biological treatment and anaerobic biological treatment.

Aerobic biological treatment:

There is a good correlation between ORP and COD removal and nitrification. Avoid insufficient or excessive aeration time and ensure treated effluent quality by controlling the amount of aerobic aeration through ORP.

Anaerobic biological treatment:

In the process of anaerobic reaction, the ORP value will decrease when reducing substances are produced; On the contrary, the ORP value will increase and tend to be stable in a certain period of time when reducing substances decrease.

There is a good correlation between OPP and the biodegradation of COD and BOD and nitrification for the aerobic biological treatment of sewage treatment plants.

For anoxic biological treatment, there is a certain correlation between ORP and nitrate-nitrogen concentration in the denitrification state in the process of anoxic biological treatment, which can be used as a standard to evaluate whether the denitrification is over.

Anoxic biological treatment:

There is a certain correlation between ORP and nitrogen concentration in the denitrification state in the process of anoxic biological treatment, which can be used as a standard to evaluate whether the denitrification is over. Research shows that the reaction is more complete when the derivative of ORP to time is less than – 5 in the denitrification process. The effluent contains nitrate nitrogen, which can prevent the production of various toxic and harmful substances, such as hydrogen sulfide.

3. Save energy and reduce consumption

In addition, we can also use the correlation between ORP and dissolved oxygen in the water to adjust the aeration time and intensity of the process through ORP to achieve energy-saving and consumption reduction in biological reaction conditions.

4. Improve the phosphorus removal

There are two steps for biological phosphorus removal:

First, in the release stage of phosphorus in an anaerobic environment, the fermentation bacteria produce fatty acids under the condition of ORP of – 100 ~ – 225 MV. The fatty acids are absorbed by phosphorus accumulating bacteria and release phosphorus into the water body at the same time.

Second, the phosphorus accumulating bacteria in the aerobic tank begin to degrade the fatty acids absorbed in the previous stage and convert ATP into ADP to obtain energy. This energy storage needs to adsorb excess phosphorus from water. The reaction of adsorbing phosphorus requires the ORP in the aerobic tank to be between + 25 ~ + 250Mv to store biological phosphorus removal.

Therefore, we can control the treatment of the phosphorus removal process through ORP to improve phosphorus removal.

When we do not want denitrification or nitrite accumulation in a nitrification process, it must keep the ORP value of more than + 50mV. Similarly, if we want to prevent the generation of malodor (H2S) in the sewer system, it must keep the ORP value more than – 50mV in the pipeline to prevent the formation and reaction of sulfide.

Summary

ORP meter is a simple detection method with a low price, high accuracy and real-time detection display.

We can quickly get the sewage purification reaction process and water pollution information according to the real-time monitoring information through ORP online detection to realize the management of sewage treatment and water quality.

However, there are many redox reactions and factors affecting ORP in wastewater treatment.

Therefore, we also need to further get the relationship between dissolved oxygen, pH, temperature, salinity and ORP according to the actual situation of the sewage plant and establish ORP control parameters suitable for different water.