By Bob Dabkowski Hach Wastewater Specialist
Oxidation Reduction Potential (ORP) is commonly a misunderstood science. Applying ORP to wastewater treatment is often a considered a mystery and sometimes called "black magic". It is easy to overlook this simple sensor and the complex data it brings to the wastewater treatment professional. The following application note details the theory of ORP, a background into common biological nutrient removal (BNR) systems, and the basic application of ORP sensors into these common BNR systems. Unlike dissolved oxygen control in aeration basins, ORP control in BNR systems focuses on optimization of the process and not necessarily cost savings, although some cost savings may be realized.
Oxidation Reduction Potential theory is best summarized as an indication of the solution's ability to oxidize or reduce another solution. It can also be defined as the sum of all the potentials in the water. Every ion, compound, and element in water exhibits a potential—the sum of these potentials is the ORP value of the water. The ORP value of the water tells the treatment professional if the environment of the water is oxidative (positive millivolt values), or reductive (negative millivolt values).
A simple way to think of ORP is through dechlorination. Chlorine is a strong oxidant—adding chlorine to the water changes the ORP to a positive value, indicating an oxidative solution. The chlorinated solution oxidizes organics and also kills microorganisms. Since this strongly oxidative solution would be harmful to the environment, it needs to be neutralized by adding a strong reductant, such as sodium bisulfite. Sodium bisulfite reduces the chlorine present to harmless chloride where it can be discharged to a receiving body. This is a simplified view of dechlorination to help understand the nature of ORP, since in the real world there are other compounds exerting a background ORP on the solution.