Sulfide is generated during anaerobic treatment of high sulfate wastewater. Some industries such as tannery petrochemical, coal gasification and viscose rayon also contribute direct sulfide to the waste stream. The prime objective of sulfide removal in anaerobic treatment system is to eliminate an unpleasant odor, which can be perceived at concentration of 0.05 – 0.2 mg/L.

Methods of Sulfide Removal

Different methods of sulfide removal are: chemical precipitation, gas stripping coupled with chemical precipitation, chemical oxidation and O2/air oxidation (biotic and abiotic).

Chemical Precipitation

Many of the divalent metals such as iron, zinc, copper etc. are capable of precipitating sulfide as insoluble metal sulfides. Although, iron sulfide is more soluble than sulfides of zinc, copper, nickel, cobalt etc. , iron salts, e.g. Fe2+ and Fe3+, are widely used because of economic and toxicity considerations.

Fe2+ + HS- ? FeS ? + H+ (2.8)

Ferrous sulfide is essentially insoluble, but continuous precipitation of FeS in the reactor could lead to serious consequences such as reduction in effective volume of the reactor, and clogging of voids in anaerobic filters etc.

Stripping and Precipitation by Iron Salts : The sulfide-laden biogas is allowed to enter into a tower of iron fillings or Fe2O3, where the sulfide is eliminated. The sulfide free biogas is then recirculated back to the reactor to strip out the additional gaseous sulfide from the reactor. The process is operated in continuous mode. The exhausted Fe2O3 can be regenerated by heating Fe2S3 in air/O2. Some of the demerits in using iron are high cost of chemicals, difficulty in exact dosing control etc.

Sulfide Oxidation by Oxygen or Air: Oxygen or air can both biologically (biotic) and chemically (abiotic) (with or without catalyst) oxidize sulfide. In comparison to the other methods of controlling sulfide toxicity, the use of oxygen has many merits such as eliminating cost of chemicals and excess chemical sludge treatment and disposal, causing no clogging or precipitation problems in the reactor, improving organic removal and process stability, and converting sulfide to non-obnoxious sulfur. Most importantly, the use of oxygen allows for process automation in controlling oxygen dosing. Due to these facts, oxygen is used to control sulfide toxicity in this study.

Biotic Sulfide Oxidation: The biotic (or biological) oxidation of sulfide has been comprehensively studied in recent years. Many of the studies were employed for sulfide elimination rather than to control the sulfide toxicity to MPB. The sulfide is primarily oxidized to elemental sulfur under oxygen limiting condition (< 0.1 mg/L) by aerobic sulfide oxidizing bacteria.

2HS- + O2 ? 2S0 + 2OH- (?G?? = -129.50 kJ/mol HS-)
2HS- + 4O2 ? 2SO42- + 2H+ (?G?? = -772.43 kJ/mol HS-)

The bacteria involved in sulfide oxidation belong to a group of colorless sulfur bacteria, of which Thiobacillus is the best known. Thiobacillus is mostly facultative autotrophic, utilizing reduced inorganic sulfur compounds as electron donors and carbon dioxide as a carbon source. However, some heterotrophic Thiobacilli were reported in a sulfide oxidizing reactor, when the sulfide-laden wastewater contained organic matter.

Abiotic Sulfide Oxidation: Oxygen or air can also abiotically eliminate sulfide in the aqueous phase. Chen and Morris (1972) reported that the sulfide-oxygen reaction is complex in nature and proceeds through chain reactions. The oxygenation of aqueous sulfide leads to the formation of polysulfides (Sx2-) as intermediates. At a high sulfide-to-oxygen ratio, sulfur precipitation occurs which limits the formation of polysulfides; but at a low ratio, direct oxidation to thiosulfate and other oxyanions occurs at a neutral pH range.

Biotic Versus Abiotic Sulfide Oxidation

Sulfide oxidation takes place both biologically and chemically during oxygenation/aeration. However, the study on simultaneous biotic and abiotic sulfide oxidation is limited. One of such studies was conducted by Buisman et al. (1990b) who evaluated the biological and chemical sulfide oxidation under identical conditions in a well-buffered synthetic sulfide solution. The authors reported that at a low sulfide concentration of about 10 mg/L, biological oxidation occurred 75 times faster than the non-catalyzed chemical oxidation. However, at a high initial concentration of 100 mg/L, the biological oxidation was only 7 times faster than the chemical oxidation.

For more information on this topic or other associated papers, please visit Prof. Khanal's website at: www.public.iastate.edu/~samirk.