The ORP Potential

Oxidation-reduction potential or ORP has been used for many years in facilities that process wastewater generated by metal finishing plants, but only recently has it become prominent in municipal wastewater treatment plants. When using a typical ORP device, an operator inserts a probe directly into a plant’s tank or waste stream (usually two feet below the surface level). The probe contains a sensor that measures electrical charges from particles called ions, and these charges are converted to millivolts (mV) that can be either negatively or positively charged. Unlike “wet Chemistry” analysis that can be time-consuming and complex, ORP readings are instantaneous and easy to perform. And like all sampling measurements taken by operators, they are snapshots in time that can indicate process efficiency and identify treatment problems before they affect effluent quality.

Important oxidation-reduction reactions in wastewater treatment systems include nitrification, denitrification, biological phosphorus removal, biological malodor production, and the removal of cBOD (carbon and hydrogen-containing compounds).

By monitoring the ORP of wastewater, an operator can determine what biological reaction is occurring and if operational conditions should be changed to promote or prevent that reaction.

Let’s take a look at each of these reactions and their relation to ORP values in greater detail.

Nitrification
To satisfy discharge limits for total nitrogen or ammonia, wastewater treatment plants must nitrify. Nitrification is the oxidation of ionized ammonia (NH4+) to nitrate (NO3-) and is performed by nitrifying bacteria.

Denitrification
Denitrification is performed to satisfy total nitrogen discharge limits or destroy undesired filamentous organism growth. Denitrification is the reduction of nitrate (NO3-) to molecular nitrogen (N2) and is performed by denitrifying bacteria.

Biological Phosphorus Removal
Wastewater plants conduct biological phosphorus removal to meet total phosphorus discharge limits. The process consists of two treatment steps – first, biological phosphorus release and, second, biological phosphorus removal.

In the second step – biological phosphorus removal – the phosphorus accumulating bacteria degrade the absorbed acids in an aerobic tank and store the energy that was obtained from the degraded acids in phosphorus granules. This storage of energy requires the removal of large quantities of phosphorus from the bulk solution.

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