Why does the relationship between ORP and various factors, such as ORP and pH, ORP and DO, cause changes in ORP values? To understand the relationship between changes in ORP values and various factors, we need to start with the definition of ORP!
1、 The definition of ORP is the full English name for ORP, which translates to oxidation-reduction potential. It is the difference between the oxidation-reduction potential of the indicator electrode and the comparison electrode in the liquid, which can provide a comprehensive indicator of the oxidation-reduction state of the entire 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 dissolved oxygen concentration is low, and the reducing environment is dominant. 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. The traditional oxidation-reduction water treatment technology has shortcomings such as inaccurate control conditions, waste of chemicals, and unfriendly to the environment. However, with the help of ORP measuring instruments and the use of ORP electrical signals as detection and control means, the precise control level of oxidation-reduction water treatment technology can be greatly improved, thereby enhancing the treatment effect. Its detection principle is similar to pH, and many pH online detection instruments have a two channel detection method, including the ORP detection channel. In summary, ORP is an important direction for the development of automatic control technology and anaerobic precise control in sewage treatment plants, which is of great significance for saving energy, controlling the metabolic pathways of anaerobic microorganisms, and improving treatment efficiency.
2、 Due to the numerous oxidation-reduction reactions that occur in wastewater treatment and the different factors that affect ORP in each reactor, it is difficult to determine which factor is the main cause of ORP changes. For example, there are many organic substances in the activated sludge treatment system, and large changes in organic matter concentration cause small changes in ORP, but it is difficult to determine which organic matter is the main cause of ORP changes. Therefore, before studying the indicative effect of ORP changes on wastewater treatment, it is necessary to first understand the factors that affect its changes.
1. As is well known, dissolved oxygen (DO) refers to the amount of oxygen dissolved in water. In an aerobic tank, the DO at the outlet should be controlled at 2mg/l, and for pure oxygen aeration, it should be controlled at 4mg/l. The DO of the anoxic denitrification tank should be 0.5mg/l. In anaerobic tanks, molecular oxygen is basically non-existent, and nitrate nitrogen is preferably less than 0.2mg/l. DO, as an oxidant in wastewater treatment, is the most direct cause of the increase in system ORP. In pure water, there is a linear relationship between ORP and DO logarithm, and ORP increases with the increase of DO. 2. In pH wastewater treatment, pH value is an important control factor. The optimal pH for the growth of aerobic microorganisms and fermentative acid producing bacteria is 6.5-8.5, while the optimal pH for anaerobic methane producing bacteria is 6.8-7
2. To control the appropriate pH value, it is generally achieved by adjusting with alkali. The metabolic activity of microbial pollutants has a significant impact on pH value. During the acid production stage, acid producing bacteria decompose large organic molecules to produce fatty acids and carbon dioxide, which have a pH lowering effect. However, the production of ammonia during protein decomposition has a pH increasing effect; During the methane production stage, methane producing bacteria can use acetic acid to produce methane, which can increase the pH value of the system. The pH value is an important factor causing the rise and fall of ORP, and the higher the pH value, the lower the ORP; The lower the pH value, the higher the ORP. It is worth mentioning that although there is a certain correlation between pH and ORP in wastewater, the correlation between pH and ORP is not as strong as in pure water due to the influence of microbial activity, dissolved oxygen, and other factors on ORP.
3. Temperature
Temperature is a very important indicator in the process of wastewater treatment. Aerobic microorganisms are active at 15-30 ℃, while anaerobic microorganisms have optimal temperatures around 35 ℃ and 55 ℃.
In the process of anaerobic wastewater treatment, temperature changes have a significant impact on the composition and proliferation of microorganisms, methane production rate, and sedimentation performance of sludge. Therefore, to ensure the stability of anaerobic tank operation, the wastewater temperature is generally adjusted to 35 ℃ or 55 ℃ through cooling towers and steam heating before entering the anaerobic tank.
Research practice has shown that the higher the solution temperature, the lower the ORP of the solution; The impact of temperature is also the same in the process of wastewater treatment. In addition, the higher the temperature in the water treatment process, the lower the ORP, which is also related to the decrease in water molecule clusters caused by the increase in temperature.
In addition, changes in temperature can also lead to changes in acidity, gas solubility, biological activity, and the equilibrium between water pollutants, thereby affecting ORP.
4. Composition of Microorganisms
In wastewater biological treatment systems, there exists a unique ecosystem.
In a two-phase anaerobic bioreactor, effective separation of acid producing bacteria and methane producing bacteria has been achieved, facilitating system control and management. In the UASB dominated by flocculent mud, acid producing bacteria and methane producing bacteria were sequentially screened along the direction of water flow. In anaerobic granular sludge and anaerobic biofilm, the dominant bacterial species shift from acid producing bacteria to methane producing bacteria from the outside to the inside.
In anaerobic reaction systems, it is necessary to control the DO concentration and ORP very low, especially in the methane production stage, where the oxidation-reduction potential cannot exceed -330mV.
The presence of dissolved oxygen (DO) is inevitable in the inflow, but under the unique ecosystem, the ORP of the system quickly decreases to the range suitable for the growth of methane bacteria through the synergistic and symbiotic effects between aerobic microorganisms, facultative microorganisms, and anaerobic microorganisms. This phenomenon of low redox potential not only exists in anaerobic reactors, but also in flocculent sludge in aeration tanks. 5. The activity of microorganisms in anaerobic activated sludge can be represented 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 oxygen consumption and production of reducing substances, and the faster the decrease in ORP. ORP, as a comprehensive indicator reflecting the macroscopic redox properties of water bodies, has various influencing factors. In addition to the main influencing factors mentioned above, there are also factors such as pressure, organic matter, solid matter, and microbial species. These factors are not isolated, they influence and constrain each other. Therefore, the redox properties of water bodies are also the result of a combination of multiple factors.
3、 The application of ORP in wastewater treatment. In the early days, redox potential was mainly used in the treatment of industrial wastewater, especially in the treatment of wastewater generated from metal precision processing. Later, it was gradually widely used in municipal wastewater treatment plants. There are various valence ions and dissolved oxygen in the sewage system, namely multiple redox pairs. Through ORP online monitoring instruments, the oxidation-reduction potential in sewage can be detected in a very short period of time, without the need for sampling and measurement in the laboratory. This can greatly shorten the testing process and improve work efficiency. The important redox reactions in sewage treatment systems include the biodegradation of organic pollutants such as carbon, nitrogen, and phosphorus, hydrolysis and acidification of organic matter, nitrification and denitrification reactions, anaerobic phosphorus release by organisms, aerobic phosphorus uptake, etc.
1. The oxidation-reduction potential required by microorganisms varies in different stages of sewage treatment. Generally, aerobic microorganisms can grow above+100mV, with the optimal range being+300 to+400mV; Facultative anaerobic microorganisms engage in aerobic respiration above+100mV and anaerobic respiration below+100mV; The requirement for obligate anaerobic bacteria is -200 to -250 mV, with obligate anaerobic methanogens requiring -300 to -400 mV, and the optimal range being -330 mV. The normal redox environment in the aerobic activated sludge process system is between+200 and+600 mV. 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 wastewater, management personnel can artificially control the occurrence of biological reactions. By changing the environmental conditions of the process operation, such as increasing the aeration rate, increasing the dissolved oxygen concentration, and adding oxidizing substances, measures can be taken to increase the oxidation-reduction potential, reduce the aeration rate, decrease the dissolved oxygen concentration, and add carbon sources and reducing substances to reduce the oxidation-reduction potential, thereby promoting or inhibiting the progress of the reaction. Therefore, managers can use ORP as a control parameter in aerobic biological treatment, anaerobic biological treatment, and anaerobic biological treatment to achieve better treatment results.
Aerobic biological treatment:
ORP has a good correlation with COD removal and nitrification. By controlling the aerobic aeration rate through ORP, insufficient or excessive aeration time can be avoided, ensuring the water quality of the treated effluent. Anaerobic biological treatment: There is a certain correlation between ORP and nitrogen concentration in the denitrification state during anaerobic biological treatment, which can be used as a criterion to determine whether the denitrification process has ended. Relevant practice has shown that in the denitrification process, 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, as the reducing substances decrease, the ORP value will increase and tend to stabilize over a certain period of time. In summary, there is a good correlation between ORP and the biodegradation of COD and BOD, as well as ORP and nitrification reactions, for aerobic biological treatment in sewage treatment plants. For anaerobic biological treatment, there is a certain correlation between ORP and nitrate nitrogen concentration in denitrification state during the anaerobic biological treatment process, which can be used as a criterion to determine whether the denitrification process has ended. 3. Controlling the treatment efficiency of the phosphorus removal process and improving the phosphorus removal efficiency for biological phosphorus removal includes two steps: firstly, in the anaerobic environment, the release stage of phosphorus is carried out. Fermentation bacteria produce fatty acids under ORP conditions of -100 to -225 mV, which are absorbed by polyphosphate accumulating bacteria and released into the water. Secondly, in the aerobic tank, phosphorus accumulating bacteria begin to degrade the fatty acids absorbed in the upper stage and convert them from ATP to ADP to obtain energy. The storage of this energy requires the adsorption of excess phosphorus from water, and the reaction of phosphorus adsorption requires an ORP of+25 to+250mV in the aerobic tank for biological phosphorus removal storage to occur. Therefore, the staff can control the treatment efficiency of the phosphorus removal process section and improve the phosphorus removal effect through ORP. When staff do not wish for denitrification or nitrite accumulation during a nitrification process, an ORP value exceeding+50mV must be maintained. Similarly, management personnel must maintain an ORP value exceeding -50mV in the sewer system to prevent the formation and reaction of sulfides, in order to prevent the generation of odors (H2S) in the sewer system.
4. In addition to adjusting the aeration time and intensity of the process to save energy and reduce consumption, workers can also use the significant correlation between ORP and dissolved oxygen in water to adjust the aeration time and intensity of the process through ORP, achieving the goal of energy conservation and consumption reduction while meeting the biological reaction conditions. In summary, ORP has simple detection methods, low equipment prices, high measurement accuracy, and real-time display of detection data. Through ORP online detection, staff can quickly grasp the sewage purification reaction process and water pollution status information based on real-time feedback, thereby achieving refined management of the sewage treatment process and efficient management of water environment quality. However, as mentioned earlier, there are numerous redox reactions that occur in wastewater treatment, and the factors that affect ORP in each reactor are also different. Therefore, in sewage treatment, staff need to further study the correlation between dissolved oxygen, pH, temperature, salinity and ORP in water based on the actual situation of the sewage plant, and establish ORP control parameters suitable for different water bodies.
