Movement Of Sulfur Through An Anaerobic Digester

Sulfur compounds that are transferred to an anaerobic digester also undergo numerous biological and chemical events (Figure 13.7). Sulfides that enter the digester or released in the digester through anaerobic degradation of sulfur-containing compounds or dissimilatory sulfate reduction may be

• Removed from solution by anaerobic bacteria as their sulfur nutrient,

• Bonded and precipitated from solution by soluble metals such as cadmium, iron, and zinc,

• Sequestered (chelated) and held in solution by fatty acids that are produced in the digester through fermentative processes, or

• Released to the biogas as H2S from the sludge.

Organic sulfur compounds (amino acids and proteins) are degraded through anaerobic activity. Their degradation results in the release of thiol groups that experience the same fates in the digester as sulfide.

Sulfate in the anaerobic digester is reduced to sulfide through dissimilatory sulfate reduction. However, an excessive quantity of sulfate represents three poten-

FIGURE 13.7 Movement of sulfur In the anaerobic digester. Sulfur compounds enters the anaerobic digester as sulfate, sulfide, and organic sulfur. The anaerobic degradation of organic sulfur compounds results in the release of sulfide and VSC. Some of the VSC are degraded, while some VSC are collected in the biogas. Sulfide can be used as the sulfur nutrient for bacteria during anaerobic activity resulting in increased sludge production and may be chelated to organic acids or precipitated in metal salts. Sulfate is reduced to sulfide by sulfate-reducing bacteria.

TABLE 13.7 Guideline ORP and Anaerobic Digester Activity

ORP (mV)

Anaerobic Digester Activity

<-100

Sulfate reduction of substrate

<-200

Fermentation, mixed acid and mixed alcohol

production

<-300

Methane production

tial operational concerns with respect to digester performance. These concerns are (1) an increase in oxidation-reduction potential, (2) production of toxic hydrogen sulfide, and (3) a decrease in available substrate for methane-producing bacteria.

The presence of sulfates in an anaerobic digester increases the oxidation-reduction potential (ORP) of the digester sludge (Table 13.7).With increasing ORP, methane-producing bacteria become less active and fatty acids, one of primary substrates for methane-forming bacteria, accumulate. Although methane-forming bacteria become less active with increasing ORP, fermentative bacteria and SRB do not. These two bacterial groups continue to produce fatty acids. The accumulating fatty acids destroy alkalinity and lower pH, and the digester becomes "sour."

Although H2S/HS- at a low concentration is beneficial to an anaerobic digester, because it serves as the sulfur nutrient for anaerobic bacteria including methane-forming bacteria, at a high concentration it is toxic. With an increasing quantity of sulfides from anaerobic activity and decreasing pH (<7), toxic H2S is formed.

SRB and methane-forming bacteria have similar characteristics. Both are active under strict anaerobic conditions with similar pH and temperature growth ranges. Like methane-forming bacteria, some SRB are able to oxidize hydrogen (H2) and acetate (CH3COOH). Therefore, SRB compete with methane-forming bacteria for these substrates. Competition results in a loss of substrate for methane-forming bacteria and a decrease in methane production.

Part IV

Floc Formation

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