Hydrogen sulphide (H2S) is a toxic and lethal gas produced in sewers and digesters under microbial anaerobic decomposition of organic matter. This organic matter is decomposed using oxygen that comes from the reduction of sulphate (SO42-). The resulting sulphide ion (S2-) reacts with hydrogen ions (H+) in the water and forms H2S.
H2S is a highly toxic and flammable gas. Hydrogen sulphide is heavier than air (density of atmospheric air 1.2 g/L, H2S 1.4 g/L) and thus tends to accumulate at the bottom of poorly ventilated spaces. H2S has a very characteristic odour and smells strongly like rotten eggs and flatulence.
It is detectable in low concentrations (ppm) and deadens the sense of smell in higher concentrations or after prolonged exposure. Respiratory paralysis and death may occur quickly at concentrations as low as 0.07 % by volume in air.
H2S is a potent corrosive4 which is responsible for, for example, corrosion of concrete sewer pipes. gas. H2S renders some steels brittle and might lead to sulphide stress cracking. In contact with oxygen, aerobic bacteria oxidises H2S into sulphate ions (SO42-). The sulphate ion reacts with free hydrogen ions (H+) in damp areas forming H2SO.