Application of ORP in sewage treatment

What does ORP stand for in sewage treatment?
ORP stands for redox potential in sewage treatment. ORP is used to reflect the macro redox properties of all substances in aqueous solution. The higher the redox potential, the stronger the oxidizing property, and the lower the redox potential, the stronger the reducing property. For a water body, there are often multiple redox potentials, forming a complex redox system. And its redox potential is the comprehensive result of the redox reaction between multiple oxidizing substances and reducing substances.
Although ORP cannot be used as an indicator of the concentration of a certain oxidizing substance and reducing substance, it helps to understand the electrochemical characteristics of the water body and analyze the properties of the water body. It is a comprehensive indicator.
Application of ORP in sewage treatment There are multiple variable ions and dissolved oxygen in the sewage system, that is, multiple redox potentials. Through the ORP detection instrument, the redox potential in the sewage can be detected in a very short time, which can greatly shorten the detection process and time and improve work efficiency.
The redox potential required by microorganisms is different at each stage of sewage treatment. Generally, aerobic microorganisms can grow above +100mV, and the optimum is +300～+400mV; facultative anaerobic microorganisms perform aerobic respiration above +100mV and anaerobic respiration below +100mV; obligate anaerobic bacteria require -200～-250mV, among which obligate anaerobic methanogens require -300～-400mV, and the optimum is -330mV.
The normal redox environment in the aerobic activated sludge system is between +200～+600mV.
As a control strategy in aerobic biological treatment, anoxic biological treatment and anaerobic biological treatment, by monitoring and managing the ORP of sewage, the staff can artificially control the occurrence of biological reactions. By changing the environmental conditions of the process operation, such as:
●Increasing the aeration volume to increase the dissolved oxygen concentration
●Adding oxidizing substances and other measures to increase the redox potential
●Reducing the aeration volume to reduce the dissolved oxygen concentration
●Adding carbon sources and reducing substances to reduce the redox potential, thereby promoting or preventing the reaction.
Therefore, managers 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 aerobic aeration volume through ORP, insufficient or excessive aeration time can be avoided to ensure the water quality of the treated water.
Anoxic biological treatment: ORP and the nitrogen concentration in the denitrification state have a certain correlation in the anoxic biological treatment process, which can be used as a criterion for judging whether the denitrification process has ended. Relevant practice shows that in the process of denitrification, when the derivative of ORP 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, when reducing substances are produced, the ORP value will decrease; conversely, when reducing substances decrease, the ORP value will increase and tend to be stable in a certain period of time.
In short, for aerobic biological treatment in sewage treatment plants, ORP has a good correlation with the biodegradation of COD and BOD, and ORP has a good correlation with nitrification reaction.
For anoxic biological treatment, there is a certain correlation between ORP and the nitrate nitrogen concentration in the denitrification state during anoxic biological treatment, which can be used as a criterion for judging whether the denitrification process has ended. 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:
First, in the phosphorus release stage under anaerobic conditions, fermentation bacteria produce fatty acids under the condition of ORP at -100 to -225mV. Fatty acids are absorbed by polyphosphate bacteria and phosphorus is released into the water body at the same time.
Second, in the aerobic pool, polyphosphate bacteria begin to degrade the fatty acids absorbed in the previous stage and convert ATP into ADP to obtain energy. The storage of this energy requires the adsorption of excess phosphorus from the water. The reaction of adsorbing phosphorus requires the ORP in the aerobic pool to be between +25 and +250mV for biological phosphorus removal to occur.
Therefore, the staff can control the treatment effect of the phosphorus removal process section through ORP to improve the phosphorus removal effect.
When the staff does not want denitrification or nitrite accumulation to occur in a nitrification process, the ORP value must be maintained above +50mV. Similarly, managers prevent the generation of odor (H2S) in the sewer system. Managers must maintain an ORP value of more than -50mV in the pipeline to prevent the formation and reaction of sulfides.
Adjust the aeration time and aeration intensity of the process to save energy and reduce consumption. In addition, staff can also use the significant correlation between ORP and dissolved oxygen in water to adjust the aeration time and aeration intensity of the process through ORP, so as to achieve energy saving and consumption reduction while meeting the biological reaction conditions.
Through the ORP detection instrument, staff can quickly grasp the sewage purification reaction process and water pollution status information based on real-time feedback information, thereby realizing the refined management of sewage treatment links and efficient management of water environment quality.
In wastewater treatment, many redox reactions occur, and the factors affecting ORP in each reactor are also different. Therefore, in sewage treatment, staff also need to further study the correlation between dissolved oxygen, pH, temperature, salinity and other factors in water and ORP according to the actual situation of the sewage plant, and establish ORP control parameters suitable for different water bodies.