Sulphide (sulphide)

Sulphide as an effluent parameter is the sulphide ion (S²⁻) and its associated hydrogen sulphide (H₂S), measured as S. It is corrosive, foul-smelling (the rotten-egg odour), and toxic to aquatic life and to humans. In confined spaces, H₂S released from sulphide-bearing water is acutely dangerous — it is one of the leading causes of fatal workplace gassing incidents. The effluent limit is 2.0 mg/L for inland surface water and 5.0 mg/L for marine coastal discharge.

Sulphide forms wherever sulphur compounds are reduced under anaerobic (oxygen-free) conditions by bacteria — in stagnant effluent, sludge, anaerobic digesters, and sewers. It also enters effluent from tanneries (sulphide is used in dehairing), petroleum and chemical processing, and any stream where sulphate-reducing bacteria act on sulphur-bearing organic matter.

For recyclers, sulphide is most relevant to the CBG/biogas sector, where it is the same chemistry as the hydrogen sulphide that must be scrubbed from biogas. Anaerobic digestion of sulphur-containing feedstock produces H₂S in the gas and sulphide in the digestate liquor; the gas-side H₂S is a corrosion and safety problem, and the liquid-side sulphide is an effluent parameter. Tyre pyrolysis and any anaerobic handling of sulphur-bearing waste also generate sulphide.

The practical points are safety and treatment. Safety first: H₂S from sulphide-bearing effluent and digestate is lethal in confined spaces — digester pits, sumps and tanks require gas monitoring, ventilation and confined-space entry protocols, as H₂S deaths in such spaces are a recurring tragedy. For compliance, sulphide is removed by oxidation (aeration, chemical oxidants) or precipitation, and on the gas side by the H₂S scrubbing the CBG process already requires. Controlling sulphide protects workers, equipment (from corrosion) and the receiving water at once.