What Role Does ORP Play In Wastewater Treatment? What Exactly Is ORP
What Role Does ORP Play In Wastewater Treatment? What Exactly Is ORP
01
What is ORP?
The full English name of ORP is oxidation-reduction potential, which translates to oxidation-reduction potential.
It is the difference between the redox potential of the indicator electrode and the redox potential of the comparison electrode in the liquid, and can give a comprehensive index for the redox state of the entire system.
*If the ORP value is low, it indicates that the waste water treatment system contains high levels of reducing substances or organic pollutants, low dissolved oxygen concentration, and dominant reduction environment.
* 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 oxidizing environment is dominant.
Traditional redox water treatment technology has disadvantages such as insufficient control conditions, waste of chemicals, and unfriendly environment. However, with the help of ORP measuring instruments and the use of ORP electrical signals as detection and control methods, the precise control of redox water treatment technology can be greatly improved Level, thereby improving the processing effect. The detection principle is similar to that of pH. Many online pH detection instruments have a two-channel detection method, among which there is an ORP detection channel.
All in all, ORP is an important direction for the development of automatic control technology and anaerobic precision control in sewage treatment plants. It is of great significance for saving energy, controlling the metabolic pathways of anaerobic microorganisms and improving the treatment effect.
02
Difficulties and influencing factors of ORP
Since there are many oxidation-reduction reactions in wastewater treatment, and the factors that affect ORO are different in each reactor, it is difficult to determine which of the factors that cause the change in ORP is mainly caused. For example, there are many organic substances in the activated sludge treatment system. Large changes in the concentration of organic substances cause smaller changes in ORP, but it is difficult to determine which organic matter is the main cause of the change in ORP. Therefore, before studying the indicator effect of ORP changes on wastewater treatment, we should first understand the factors that affect the changes.
1. Dissolved oxygen (DO)
As we all know, DO represents the content of oxygen dissolved in water. In an aerobic tank, the DO from the water outlet should be controlled at 2mg/l, if it is pure oxygen aeration, it should be at 4mg/l. The DO of anoxic denitrification tank should be 0.5mg/l. In the anaerobic tank, molecular oxygen is basically absent, and nitrate nitrogen is preferably less than 0.2mg/l. As an oxidant in wastewater treatment, DO is the most direct cause of the increase in ORP of the system. In pure water, the logarithm of ORP and DO is linear, and ORP increases with increasing DO.
2. pH
In wastewater treatment, pH is an important control factor. The optimal pH for growth of aerobic microorganisms and acid-producing bacteria is 6.5-8.5, and the optimal pH for anaerobic methanogens is 6.8-7.2. In order to control the appropriate pH value, it is generally controlled by adding alkali to adjust. The metabolic activity of microbial pollutants has a great influence on the pH value. In the acid production stage, acid-producing bacteria decompose macromolecular organic matter to produce fatty acids and carbon dioxide to lower the pH, but the production of ammonia in the process of decomposing proteins can increase the pH value. Function: In the methanogenesis stage, methanogens can use acetic acid to produce methanase to increase the pH value of the system. The pH value is an important factor that causes the ORP to rise and fall. The higher the pH value, the lower the ORP; the lower the pH value, the higher the ORP. It is worth mentioning that although pH and ORP have a certain correlation in sewage, the correlation between pH and ORP is not as strong as in pure water because ORP is also affected by microbial activities, dissolved oxygen and other factors.
3. Temperature
In the wastewater treatment process, temperature is a very important indicator. Aerobic microorganisms are active at 15-30℃, and the optimum temperature for anaerobic microorganisms is around 35℃ and 55℃. In the process of anaerobic wastewater treatment, temperature changes have important effects on the composition and proliferation of microorganisms, the rate of methane production, and the sedimentation performance of sludge. Therefore, in order to ensure the stable operation of the anaerobic tank, the wastewater enters the anaerobic tank. Previously, the temperature of wastewater was adjusted to 35°C or 55°C by cooling tower cooling and steam heating. Research and practice have shown that the higher the solution temperature, the lower the ORP of the solution; the same is true for the influence of temperature in the wastewater treatment process. In addition, the higher the temperature of the water treatment process, the lower the ORP, which is also related to the smaller water molecular clusters caused by the increase in temperature. In addition, changes in temperature can also lead to changes in pH, gas solubility, biological activity, and changes in the balance of water pollutants, which in turn affects ORP.
4. The composition of microorganisms
In the wastewater biological treatment system, there is a unique ecosystem. In the two-phase anaerobic bioreactor, the effective separation of acid producing bacteria and methanogenic bacteria is realized, which is convenient for system control and management. In the UASB where flocculent mud is dominant, acid-producing bacteria and methanogenic bacteria were screened in sequence along the direction of water flow. In anaerobic granular mud and anaerobic biofilm, from the outside to the inside, the dominant bacterial species change from acid-producing bacteria to methanogenic bacteria . In the anaerobic reaction system, the DO concentration and ORP must be controlled very low , especially in the methane production stage, the redox potential cannot be higher than -330mV. The presence of DO is inevitable in the water, but under the action of this unique ecosystem, through the synergy and symbiosis between aerobic microorganisms, facultative microorganisms, and anaerobic microorganisms, the ORP of the system is quickly reduced. To the range where the methane bacteria are suitable for growth. This phenomenon of low redox potential not only exists in anaerobic reactors, but also occurs in the flocculent mud in the aeration tank.
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 activity of aerobic activated sludge can also be expressed by the maximum specific COD removal rate. The higher the activity of microorganisms, the faster the rate of consumption of oxygen and the rate of production of reducing substances, and the faster the reduction of ORP. ORP is a comprehensive index reflecting the macroscopic redox properties of water bodies. There are many types of influencing factors. In addition to the above-mentioned main influencing factors, there are also factors such as pressure, organic matter, solid matter, and microorganism types. These factors are not isolated, they influence and restrict each other. Therefore, the redox property of the water body is also the result of the combined effect of many factors.
03
Application of ORP in sewage treatment
Earlier, the oxidation-reduction potential was mainly used in the treatment of industrial wastewater, especially in the treatment of some wastewater produced in metal finishing. Later, it has gradually been widely used in municipal sewage treatment plants. There are multiple variable valence ions and dissolved oxygen in the sewage system, that is, multiple redox couples. Through the ORP online monitoring instrument, the oxidation-reduction potential in the sewage can be detected in a very short time, without the need for sampling and measurement in the laboratory, which can greatly shorten the test process in time and improve work efficiency. Important redox reactions in sewage treatment systems include biodegradation of organic pollutants containing carbon, nitrogen, and phosphorus, hydrolysis and acidification of organic matter, nitrification and denitrification, biological anaerobic release of phosphorus, aerobic phosphorus absorption, etc. .1. At each stage of sewage treatment, the oxidation-reduction potential required by microorganisms is different. General aerobic microorganisms can grow above +100mV, the most suitable is +300～+400mV; facultative anaerobic microorganisms are aerobic when above +100mV Respiration, perform anaerobic respiration when the temperature is below +100mV; obligate anaerobic bacteria require -200～-250mV, of which obligate anaerobic methanogens require -300～-400mV, and the most suitable is -330mV. The normal redox environment in the aerobic activated sludge system is between +200～+600mV. The suitable ORP value range for common reaction processes in sewage biochemical treatment is shown in the following table 2. As a control strategy in aerobic biological treatment, anoxic biological treatment and anaerobic biological treatment,
by monitoring and managing the ORP of sewage, managers can artificially control the occurrence of biological reactions. By changing the environmental conditions of process operation, such as:
· Increase the amount of aeration to increase the concentration
of dissolved oxygen · Add oxidizing substances and other measures to increase the redox potential
· Reduce the amount of aeration to reduce the concentration
of dissolved oxygen · Add carbon source and reducibility The substance lowers the oxidation-reduction potential, so as to promote or prevent the reaction from proceeding.
Therefore, managers can use ORP as a control parameter in aerobic biological treatment, anoxic biological treatment and anaerobic biological treatment to achieve better treatment effects.
Aerobic biological treatment:ORP has a good correlation with COD removal and nitrification. By controlling the amount of aerobic aeration through ORP, insufficient or excessive aeration time can be avoided, and the water quality of the treated effluent can be ensured. Anoxic biological treatment:There is a certain correlation between ORP and the nitrogen concentration in the denitrification state in the process of anoxic biological treatment, which can be used as a criterion for judging whether the denitrification process is over. Relevant practice shows that in the process of denitrification and denitrification, when the derivative of ORP with respect to time is less than -5, the reaction is more thorough. The effluent contains nitrate nitrogen, which can prevent the production of various toxic and harmful substances, such as hydrogen sulfide.Anaerobic biological treatment :During the anaerobic reaction process, when reducing substances are produced, the ORP value will decrease; on the contrary, when reducing substances are reduced, the ORP value will increase and become stable for a certain period of time. All in all, for the aerobic biological treatment of sewage treatment plants, ORP and COD, BOD biodegradation, ORP and nitrification reaction have a good correlation. For anoxic biological treatment, there is a certain correlation between ORP and the concentration of nitrate nitrogen in the denitrification state during the anoxic biological treatment process, which can be used as a criterion for judging whether the denitrification process is over.3. Control the treatment effect of the phosphorus removal process section and improve the phosphorus removal effect. Biological phosphorus removal and phosphorus removal include two steps:
one is the release stage of phosphorus in an anaerobic environment. Fatty acids are produced under conditions, which are absorbed by phosphorus-accumulating bacteria and release phosphorus into the water body at the same time.
The second is that the phosphorus accumulating bacteria in the aerobic pool begin to degrade the fatty acids absorbed in the previous stage while obtaining energy from ATP into ADP. This energy storage requires the adsorption of excess phosphorus from the water. The reaction of adsorbing phosphorus requires the ORP in the aerobic pool to be + The storage of biological phosphorus removal can only occur between 25～+250mV.
Therefore, the staff can control the treatment effect of the phosphorus removal process section through ORP and improve the phosphorus removal effect.
When the staff do not want to denitrify or accumulate nitrite during a nitrification process, they must maintain an ORP value of more than +50mV. In the same way, the management personnel prevent the generation of malodor (H2S) in the sewer system, and the management personnel must maintain an ORP value of more than -50mV in the pipeline to prevent the formation and reaction of sulfide.4. Adjust the aeration time and aeration intensity of the process to save energy and reduce consumption. In addition, the staff can also use the significant correlation between ORP and dissolved oxygen in the water to adjust the aeration time and aeration intensity of the process through ORP. While satisfying the biological reaction conditions, it achieves the purpose of energy saving and consumption reduction. In summary, ORP's detection methods are simple, the equipment price is low, the measurement accuracy is high, and the detection data is displayed in real time. Through ORP online detection, the staff can quickly grasp the sewage purification reaction process and water pollution status information based on the real-time feedback information, so as to realize the refined management of the sewage treatment link and the efficient management of the water environment quality. However, as mentioned above, in wastewater treatment, many oxidation-reduction reactions occur, and the factors that affect ORP are different in each reactor. Therefore, in sewage treatment, workers need to further study the relationship between dissolved oxygen, pH, temperature, salinity and other factors in the water and ORP according to the actual situation of the sewage plant, and establish ORP control parameters suitable for different water bodies.