Redox potential or ORP is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. In wastewater biology, redox potential's importance concerns electron donors (often organic compounds measured as BOD5) and electron acceptors. This movement of electrons generates the energy used by the organisms. The redox potential in the water is directly related to the available electron acceptors. The most common wastewater electron acceptors are listed below:
- Oxygen - when oxygen is the electron acceptor we have aerobic conditions. At neutral pH, this is represented by ORP > 0. Aerobic metabolism provides more energy for microbial growth and reproduction.
- Nitrate/nitrite - many organisms have the ability to use nitrate/nitrite as an alternative electron acceptor when dissolved oxygen is not present (although some will continue to use nitrate/nitrite even under aerobic conditions). Nitrate/nitrite as a terminal electron acceptor provides more energy than acceptors lower on the "electron tower".
- Sulfate - as redox potentials drop below -125 mV at pH of 7, microbes capable of utilizing sulfate as a terminal electron acceptor start to appear. The sulfate is converted to reduced S= and increasingly to H2S as pH drops. Energy from sulfate reduction is lower than for Oxygen, Nitrate, and Iron.
- Organics - when the electron acceptor is an organic compound, you have fermentative respiration. This is the first step in anaerobic digester microbial processes, but also occurs in anoxic systems with low redox potential. Fermentative respiration produces short chain fatty acids (also odorous) including acetic, butyric, and propionic acids.
- Methane production happens when organic acids and hydrogen are used by archaea organisms into methane. In this case, the low redox potential <-400 mV, allows for energy production by combining CO2, H+, and COOH (organic acids) with methane being the final product.
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