What Is ORP?

If the ORP value is low, it indicates that the wastewater treatment system has a high content of reducing substances or organic pollutants, a low concentration of dissolved oxygen, and a favourable reducing 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 oxidising substances is high, and the oxidising environment is favourable.
 
The traditional redox water treatment technology has shortcomings such as lack of precise control conditions, waste of chemicals, unfriendly to the environment, etc. However, with the help of ORP measuring instruments, using the electrical signal of ORP as a means of detection and control, the precise control level of the redox water treatment technology can be greatly improved, so as to improve the treatment effect.
 
Its detection principle is similar to that of pH, and many pH on-line detectors have a two-channel detection method, in which there is a channel for ORP detection.
 
In conclusion, ORP is an important direction for the development of automatic control technology and anaerobic precise control in wastewater treatment plant, which is of great significance for saving energy, controlling the metabolic pathway of anaerobic microorganisms and improving the treatment effect.

 
The difficulty of ORP and the factors affecting it
Due to the numerous redox reactions occurring in wastewater treatment and the different factors affecting ORP in each reactor, it is difficult to judge which one of the factors is mainly responsible for the ORP change.
 
For example, in the activated sludge treatment system there are a lot of organic substances, and a larger change in the concentration of organic substances causes a smaller change in ORP, but it is difficult to judge which kind of organic substances are mainly responsible for the change in ORP.
 
Therefore, before studying the indicative role of ORP change on wastewater treatment, it is important to understand what factors influence its change.
 
1.Dissolved Oxygen (DO)
As we all know, DO indicates the content of oxygen dissolved in water, in aerobic tank, DO should be controlled at 2mg/l out of the outlet, if it is pure oxygen aeration should be 4mg/l. anoxic denitrification tank DO should be 0.5mg/l. In anaerobic tank, molecular oxygen is basically absent, and nitrate nitrogen is better than 0.2mg/l.
 
DO, as an oxidising agent in wastewater treatment, is the most direct cause of elevated system ORP. In pure water, ORP is linearly related to the logarithm of DO, with ORP rising with DO.

 
2.PH
In wastewater treatment, pH is an important control factor. Aerobic microorganisms and fermentation acid-producing bacteria optimal growth pH value of 6.5 ~ 8.5, anaerobic methanogenic bacteria optimal pH of 6.8 ~ 7.2. In order to control the appropriate pH value, generally through the addition of alkali adjustment method of control.
 
The metabolic activities of microorganisms pollutants have a great influence on pH. In the acid production stage, acid-producing bacteria decompose macromolecular organic matter to produce fatty acids and carbon dioxide have the effect of lowering pH, but in the process of decomposing proteins to produce ammonia has the effect of raising the pH; in the methanogenic stage, methanogens use acetic acid to produce methane which can raise the pH value of the system. pH is an important factor that causes the rise and fall of the ORP, and the higher the pH value, the lower the ORP value. The higher the pH, the lower the ORP; the lower the pH, 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 because ORP is also affected by microbial activities, dissolved oxygen and other factors.
 
3. Temperature
Temperature is a very important indicator in the wastewater treatment process. Aerobic microorganisms are active at 15-30?, and the optimal temperature for anaerobic microorganisms is near 35? and 55?.
 
In the process of anaerobic wastewater treatment, the change of temperature has an important effect on the composition and proliferation of microorganisms, the rate of methane production, and the sedimentation performance of sludge, etc. Therefore, in order to ensure the stability of the anaerobic tank operation, the wastewater is generally cooled down by cooling towers and heated by water vapour to regulate the temperature of the wastewater to 35 ? or 55 ? before it enters the anaerobic tank.
 
Research practice shows that the higher the solution temperature, the lower the ORP of the solution; in the wastewater treatment process, the effect of temperature is also the same. In addition the higher the temperature of the water treatment process, the lower the ORP, but also with the increase in temperature leads to smaller water molecule clusters.