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Hydrogen sulfide (H 2 S) is an odorous and poisonous gas present in biogas (the main product of anaerobic digestion), in sour gas (i.e. natural gas with a high content of H 2 S) or in general in the gases coming as a concomitant with crude oil extraction. H 2 S can also be produced abiotically, e.g. through the reaction of anhydrite and hydrocarbons as reported in deep carbonate gas reservoirs (Worden & Smalley, 1996). In environmental engineering applications, H 2 S is most often generated during anaerobic fermentation of substrates containing sulfur compounds. Sulfate-reducing bacteria (SRB) with a respiratory metabolism use sulfate as an electron acceptor while producing sulfide, which is toxic both in its liquid as well as in its gaseous form. Gaseous hydrogen sulfide causes the corrosion of concrete and steel digesters (see Chapter 5) and other equipment such as pipes, burners and combined heat and power (CHP) units, which increases the operational costs since the engine oil is acidified and its replacement interval is significantly shortened. While burning, the acid rain precursors SO 2 and SO 3 are emitted to the environment. Last but not the least, gaseous hydrogen sulfide is extremely toxic and causes health risks for the operators of fermenters and sewer maintainers. The olfactory nerve responding to the smell of hydrogen sulfide is paralyzed after a few inhalations of 100-150 ppm,