The application of ORP in wastewater treatment

There is a good linear positive correlation between the ORP value and the *ao acid nitrogen concentration. The activity of denitrification decreases as the redox potential increases. DO also inhibits the denitrification process. When a low ORP value is measured at the end of the anoxic section, it can be considered that *ao acid nitrogen has been effectively removed, allowing for the full utilization of the organic carbon in the influent water for denitrification. A high DO level inhibits the denitrification process; as a result, nitrate cannot be removed adequately, which in turn causes the ORP to increase. Dissolved oxygen inhibits the denitrification process; this dissolved oxygen refers to the DO concentration within the biofilm or flocs, rather than the liquid-phase DO concentration that we usually measure. Typically, when the DO concentration in the liquid phase is measured at 0.5 mg/L, there is virtually no DO inside the biofilm or flocs. In conventional suspended activated sludge processes, the denitrification DO is maintained at 0.5 mg/L; in attached growth systems, however, the biofilm imposes a greater resistance to oxygen transfer, allowing for higher dissolved oxygen concentrations

I have grasped some of the underlying meaning: ORP corresponds to VFA, just as it is the case with depth anaerobic testing for VFA, since what we measure is the residue remaining after the biochemical reactions have reached equilibrium. ORP reflects the residue remaining after denitrification; the higher the ORP, the less complete the denitrification process, indicating that the reaction has proceeded poorly. I completely agree. I think people regard the ORP parameter as similar to indicators such as COD and BOD; the value of ORP merely reflects whether denitrification is taking place thoroughly, rather than having any impact on the denitrification process itself. It would be better to clarify this relationship before discussing ORP. There is some truth to that, but if the nitration reaction does not proceed thoroughly – I mean, if the concentration of *ao acid returning back is very low and the ORP value is also low – then how can denitrification occur? *A low concentration of AO acids and a low ORP do not necessarily indicate that denitrification has taken place thoroughly. One might say I’m making excuses, but this is the reality... ORP is not a measure of the extent of denitrification; it is the result of redox reactions involving various oxidizing and reducing substances, and it serves as a comprehensive indicator of the overall redox condition. The response of ORP to the concentration of *ao acids is summarized by plotting relevant curves; unlike monitoring instruments such as BOD, COD, and PH, whose readings represent the actual values of BOD, COD, and PH in the water. Many factors can affect the ORP value, so each case must be analyzed on its own. It is very necessary to focus on the site.

The ORP detector should also be installed at the end of the anoxic section. At both the beginning and the end of this anoxic section, the concentration of *ao alkalinity is definitely higher at the beginning and lower at the end. The *ao alkalinity concentration in the recycled digestate is high as it enters the initial part of the section, and it decreases as it flows toward the end. As you mentioned, when the concentration of the backflow *ao acid salt is high, the ORP value also increases. It depends on where the ORP sensor is installed; I don’t think it’s possible to install such a sensor at the beginning of the system. Theoretically, ORP minimizes retrodigestion completely. ORP shows incomplete denitrification. At present, ORP meters are not yet widely used, and much of the data is obtained based on experiments. The ORP in the aerobic zone ranges from +100 to +400, while it is between -100 and -200 in the anoxic zone; theoretically, digestion and antidigestion can proceed normally within these ranges.