The oxidation-reduction potential (ORP) generally is a measurement of a sub­stance’s affinity to either gain or lose electrons. In AD it reflects the availability of oxidants, such as oxygen or nitrate ions or of reductants such as hydrogen. A high ORP (>50 mV) indicates the presence of free oxygen in the anaerobic environment. An ORP between 50 to -50 mV is characteristic of an anoxic environment with nitrates and nitrites, the most favorable electron acceptors. At ORP lower than -50 mV, the environment in the digester is strongly reducing. If sulfate ions are present and the ORP is in the range from -50 to -100 mV, sulfate reducing micro­organisms can outperform methanogens for hydrogen and acetate since sulfate is a more thermodynamically favorable electron acceptor. The most favorable ORP for fermentation and acid production is from -100 to -300 mV, which indicates that the strongest oxidant available is found in different organic compounds that can be reduced to a mix of acids and alcohols. Methanogenesis requires the ORP <-300 mV when carbon dioxide is used as an electron acceptor and methane is formed [76].