Sulfatereducing Bacteria

During dissimilatory sulfate reduction relatively large quantities of sulfate are removed from the bulk solution by sulfate-reducing bacteria (SRB). The principal SRB belong in the genera Desulfovibrio and Desulfotomaculum. Sulfate is removed cBOD + SO42- O2

FIGURE 13.3 Assimilatory sulfate reduction. During aerobic conditions, aerobic bacteria remove sulfate from the bulk solution and use sulfate as the sulfur nutrient. Here, sulfate is reduced intracel-lularly to sulfide (-SH), and the sulfide is incorporated into new cellular material (MLVSS).

cBOD SO42-

cBOD SO42-

FIGURE 13.4 Dissimilatory sulfate reduction. During dissimilatory sulfate reduction, sulfate-reducing bacteria remove sulfate from the bulk solution in order to degrade soluble cBOD. Sulfate removes from the cell the electrons that are released from the degraded cBOD. Here, the sulfur in the sulfate is not incorporated into new cellular material but is released from the cell in the form of hydrogen sulfide (H2S) or sulfide (HS

FIGURE 13.4 Dissimilatory sulfate reduction. During dissimilatory sulfate reduction, sulfate-reducing bacteria remove sulfate from the bulk solution in order to degrade soluble cBOD. Sulfate removes from the cell the electrons that are released from the degraded cBOD. Here, the sulfur in the sulfate is not incorporated into new cellular material but is released from the cell in the form of hydrogen sulfide (H2S) or sulfide (HS

from the bulk solution under strictly anaerobic conditions and is used to oxidize hydrogen (Equation 13.2) and soluble organic compounds such as succinate (HOOCCH2CH2COOH). SRB produce hydrogen sulfide. Some of the hydrogen sulfide escapes to the atmosphere.

The quantity of hydrogen sulfide that escapes to the atmosphere depends upon the pH of the wastewater, initial dissolved hydrogen sulfide concentration, and temperature. At pH 7, hydrogen sulfide represents 50% of the dissolved sulfides in the wastewater. The concentration of dissolved sulfides as hydrogen sulfide increases as the pH decreases or temperature decreases.

Dissimilatory sulfate reduction also is known as respiratory sulfate reduction. It occurs in waterlogged soils, stagnant ponds, the intestinal tract of ruminant animals, and wherever a strictly anaerobic condition occurs in the presence of sulfate, SRB, and hydrogen or the presence of soluble cBOD. Often, these anaerobic conditions occur in sanitary sewers and wastewater and sludge treatment and holding tanks.

In wastewater treatment plants, dissimilatory sulfate reduction is of concern in anaerobic digesters and treatment units where oxygen and nitrate are absence. SRB are known for their production of malodors and corrosion of iron pipes. SRB reduce sulfate when soluble organic compounds are available and free molecular oxygen and nitrate (NO3-) are not available.

When sulfur-containing compounds such as proteins are degraded with the use of sulfate, numerous sulfur-containing, volatile malodorous compounds are produced. Significant malodorous compounds are the mercaptans (Table 13.2). Other sulfur-containing, volatile malodorous compounds include thiocresol (CH3-C6H4-SH), thiophenol (C6H5SH), polysulfides (dimethyl disulfide (CH3-S-S-CH3), methylethyl disulfide (CH3-S-S-CH2CH3), organic sulfides, and sulfur dioxide (SO2). Examples of organic sulfides include dially sulfide, dimethyl sulfide, methyl iso-propyl sulfide, diethyl sulfide, and methyl pentyl sulfide.

Sulfur dioxide may dissolve in wastewater to produce sulfurous acid (H2SO3), hydrogen sulfite (HSO3-), and sulfite (SO32-). Sulfurous acid dissociates to form hydrogen sulfite, which reacts with free chlorine and combined chorine resulting in the formation of chloride and sulfate. The formation of sulfite produces an increased chlorine demand. Sulfite also is an oxygen scavenger (Equation 13.3) and is found in some industrial wastewaters. Sodium sulfite (Na2SO3) often is added to boiler feedwater as a corrosion inhibitor.

TABLE 13.2 Mercaptans Commonly Produced During Anaerobic Degradation of Sulfur-Containing Compounds

Mercaptan

Formula

Odor

Allyl

CH2=CHCH2SH

Garlic

Amyl

CH3CH2CH2CH2SH

Putrid

Croty1

CH3CH=CHCH2SH

Skunk

Ethyl

CH3CH2SH

Decayed cabbage

Methyl

CH3SH

Decayed cabbage

t-butyl

(CH3HCSH

Skunk

FIGURE 13.5 Interface between oxygen and sulfate; interface between sulfur-oxidizing and sulfate-reducing bacteria. On the interface of a floc particle or biofilm where dissolved oxygen is absent and sulfate is present, numerous sulfur-oxidizing bacteria and sulfate-reducing bacteria may be found. On the outside of the interface where dissolved oxygen is present, sulfur-oxidizing bacteria oxidize reduced forms of sulfur to sulfate. On the inside of the interface where dissolved oxygen is absence and sulfate is present, sulfate-reducing bacteria reduce sulfate to sulfide.

FIGURE 13.5 Interface between oxygen and sulfate; interface between sulfur-oxidizing and sulfate-reducing bacteria. On the interface of a floc particle or biofilm where dissolved oxygen is absent and sulfate is present, numerous sulfur-oxidizing bacteria and sulfate-reducing bacteria may be found. On the outside of the interface where dissolved oxygen is present, sulfur-oxidizing bacteria oxidize reduced forms of sulfur to sulfate. On the inside of the interface where dissolved oxygen is absence and sulfate is present, sulfate-reducing bacteria reduce sulfate to sulfide.

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