Bacterial cells need soluble sulfur as a growth nutrient and satisfy this need by using soluble sulfide (HS ). However, excessive concentrations of sulfides or dissolved hydrogen sulfide gas (H2S) cause toxicity.
Hydrogen sulfide is one of the compounds most toxic to anaerobic digesters. The methane-forming bacteria are the bacteria that are most susceptible to hydrogen sulfide toxicity. Hydrogen-consuming methane-forming bacteria are more susceptible to hydrogen sulfide toxicity than acetoclastic methane-forming bacteria. Acid-forming bacteria also are susceptible to hydrogen sulfide toxicity.
Soluble hydrogen sulfide toxicity occurs because sulfide inhibits the metabolic activity of anaerobic bacteria. Although the mechanism by which sulfide inhibits anaerobic bacteria is not completely understood, toxicity can occur at concentrations as low as 200mg/l at neutral pH. Because diffusion through a cell membrane is required to exert toxicity and non-ionized hydrogen sulfide diffuses more rapidly across a cell membrane than sulfide, hydrogen sulfide toxicity is pH dependent (Figure 17.1).
Hydrogen sulfide is formed in anaerobic digesters from the reduction of sulfate and the degradation of organic compounds such as sulfur-containing amino acids and proteins. The amino acids cystine, cysteine, and methionine that are incorporated into many proteins contain sulfur in a thiol group (-SH) that is released during the degradation of the amino acids (Figure 17.2).
Sulfate is relatively non-inhibitory to methane-forming bacteria. Sulfate is reduced to hydrogen sulfide by sulfate-reducing bacteria (SRB). For each gram of chemical oxygen demand (COD) degraded by SRB 1.5 grams of sulfate are reduced to hydrogen sulfide.
Several genera of anaerobic bacteria reduce sulfate or sulfur to hydrogen sulfide. The genus name of these bacteria begins with the prefix "Desulf.The genera include Desulfuromonas, Desulfovibrio, and Desulfomonas. SRB are similar to methane-forming bacteria with respect to habitat and cellular morphology or structure.
The presence of hydrogen sulfide also can be due to the reduction of elemental sulfur. An additional source of sulfides is sulfate salts present in wastewaters from metallurgical industries.
Sulfide in an anaerobic digester may be in the soluble or insoluble form. In the insoluble form such as lead sulfide (PbS) and iron sulfide (Fe2S3), sulfide does not exert toxicity. Insoluble sulfide cannot enter bacterial cells. A common operational practice to prevent sulfide toxicity in anaerobic digesters is to add iron. This practice precipitates the sulfide as iron sulfide, which gives the treated sludge a black color. Dissolved sulfide can react with any heavy metal except chromium.
Although some of the sulfide leaves the digester sludge as free hydrogen sulfide gas, and some is precipitated as heavy metal salts, a portion of the sulfide remains dissolved. Concentrations of dissolved hydrogen sulfide above 200mg/l are toxic and should be reduced.
Free hydrogen sulfide gas can be removed from digester sludge by the rapid production of carbon dioxide, hydrogen, and methane. Treatment measures that can be used to reduce soluble hydrogen sulfide include 1) diluting the sulfides, 2) separating and treating the sulfate/sulfide waste stream, 3) precipitating the sulfide as a metal salt, and 4) scrubbing and recirculating digester biogas.
Sulfide toxicity is most likely to occur under low organic loadings. Under these conditions, insufficient biogas is produced. This deficiency in biogas production results in poor stripping of sulfide from the sludge.
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