This gas is often found in groundwater that contains iron and manganese. It is easily identified by its 'rotten egg' odor. In addition to causing serious taste and odor problems, sulfide also promotes the growth of sulfur bacteria. Hydrogen sulfide can cause corrosion problems with iron and concrete pipes and reservoirs.
Sulfides are formed by the anaerobic reduction of sulfates and organic matter into sulfides and bisulfides. This reaction is pH dependent.
- Hydrogen sulfide predominates at a pH of less than 7.
- Hydrogen bisulfide predominates in a pH range between 7 and 9.5.
- Sulfide is predominant at a pH above 9.5.
Standard treatment for hydrogen sulfide has been aeration and degasification or oxidation with chlorine to elemental sulfur. Hydrogen sulfide reacts with oxidants such as Cl2, O3 ClO2 and H202 to form polysulfides (HSm) and hydrogen polysulfide (H2Sm). These compounds cause a milky blue suspension of colloidal sulfur getting a 'musty or earthy' taste or an odor similar to a rubber tire and a taste similar to drinking from a rubber garden hose. This odor may not become apparent until the water is heated and the polysulfide odor is released.
Polysulfides are treated by oxidation and the addition of sulfur dioxide (SO2). The sulfur dioxide promotes the conversion of polysulfides and elemental sulfur to thiosulfate.
Oxidation of sulfides in the treatment process is extremely important in the removal of iron and manganese. The following factors affect water quality:
Treatment of all other substances that have an oxidation demand must be completed before the oxidant can effectively oxidize iron and manganese.
Organic complexes must be oxidized before the oxidant can effectively oxidize ironand manganese.
An excess of 0.5 mg/l of oxidant (chlorine) is required in this reservoir/distribution system to inhibit any growth of sulfur bacteria.