Movement Of Sulfur Through An Activated Sludge Process

Sulfur enters a wastewater treatment plant in inorganic and organic forms (Figure 13.6). Sulfides as H2S and HS- are present in the sewer system as the result of two biochemical events. First, thiol groups (-SH) are released from the sulfur amino acids and sulfur-containing proteins (organic sulfur compounds) when these compounds are degraded by bacteria in the biofilm and sediment of the sewer system. Second, H2S and HS- are produced in the sewer system when sulfate (SO42-) is reduced through dissimilatory sulfate reduction by SRB when they degrade soluble cBOD.

The anaerobic degradation of organic sulfur compounds in the sewer system also results in the production of several malodorous, sulfur-containing organic compounds. Some of these compounds may be released to the atmosphere.

Sulfate is present in raw wastewater. The significant sources of sulfate are urine and groundwater. Sulfite (SO32-) may be present in raw wastewater, if it is discharged from boiler feedwater or cooling tower water.

The primary clarifier influent contains inorganic and organic forms of sulfur. Inorganic forms consist of H2S, HS-, SO42-, and perhaps SO32-, while organic forms consist of the sulfur amino acids, sulfur-containing proteins, and malodorous sulfur-containing compounds. Some of the influent proteins may be removed in the primary clarifier, if they are adsorbed to solids that settle out. Some hydrogen sulfide and some malodorous sulfur-containing compounds may be released from the primary clarifier to the atmosphere.

SO42-SO32-HS-

Organic S

SO32- ---> HS-/H2S Organic S --- > HS-/H2S + VSC

Sewer system

SO42-

SO32-HS-

Organic S

Primary clarifier

Aeration tank

FIGURE 13.6 Movement of sulfur in the activated sludge process. Sulfur compounds enter the sewer system as sulfate in groundwater and urine, sulfide and sulfite as oxygen scavengers, and organic sulfur. In the sewer system under anaerobic conditions, sulfate and sulfite are reduced to sulfide through the degradation of cBOD. Some sulfide escapes the sewer system as hydrogen sulfide, while sulfur-containing malodorous compounds from the degradation of amino acids and proteins also escape the sewer system. In the primary clarifier, additional hydrogen sulfide and sulfur-containing malodorous compounds may be released; with increasing HRT in the primary clarifier, sulfate may be used to degrade cBOD in the sludge, and fermentation may produced sulfur-containing malodorous compounds. In the aeration tank, sulfide and sulfite may be biologically and chemically oxidized to sulfate. Sulfide also may be oxidized to elemental sulfur by sulfur-oxidizing filamentous organisms such as type 021N. VSC produced in the sewer system, primary clarifier, and secondary clarifier may be stripped to the atmosphere; with increasing HRT in the aeration tank, organic sulfur compounds are degraded to release sulfide that are oxidized to sulfate.

Aeration tank

FIGURE 13.6 Movement of sulfur in the activated sludge process. Sulfur compounds enter the sewer system as sulfate in groundwater and urine, sulfide and sulfite as oxygen scavengers, and organic sulfur. In the sewer system under anaerobic conditions, sulfate and sulfite are reduced to sulfide through the degradation of cBOD. Some sulfide escapes the sewer system as hydrogen sulfide, while sulfur-containing malodorous compounds from the degradation of amino acids and proteins also escape the sewer system. In the primary clarifier, additional hydrogen sulfide and sulfur-containing malodorous compounds may be released; with increasing HRT in the primary clarifier, sulfate may be used to degrade cBOD in the sludge, and fermentation may produced sulfur-containing malodorous compounds. In the aeration tank, sulfide and sulfite may be biologically and chemically oxidized to sulfate. Sulfide also may be oxidized to elemental sulfur by sulfur-oxidizing filamentous organisms such as type 021N. VSC produced in the sewer system, primary clarifier, and secondary clarifier may be stripped to the atmosphere; with increasing HRT in the aeration tank, organic sulfur compounds are degraded to release sulfide that are oxidized to sulfate.

Most sulfur compounds that enter the primary clarifier can be found in the primary clarifier effluent, because they are either soluble or suspended. These compounds then enter the aeration tank of the activated sludge process.

In the aeration tank, numerous biological and chemical changes occur with the sulfur compounds. Sulfides in the aeration tank may be biologically and chemically oxidized to sulfate. These oxidations result in dissolved oxygen consumption. Some sulfides may be removed by the sulfur filamentous organisms (type 021N, Beggia-toa, and Thiothrix) and oxidized to elemental sulfur (S0)

Malodorous sulfur-containing compounds may be stripped to the atmosphere or degraded. Their degradation results in the release of sulfides. With sufficient hydraulic retention time (HRT) the sulfur amino acids that are absorbed and degraded and the sulfur-containing proteins that are adsorbed, solubilized, and degraded release thiol groups. The released sulfides and thiol groups also undergo biological and chemically oxidation to sulfate.

Bacteria in the aeration tank remove sulfate as their sulfur nutrient. Here, the sulfate is reduced intracellularly to sulfide and incorporated into cellular material (MLVSS) or organic sulfur.

Activated sludge solids and nondegraded cBOD that are transferred to a secondary clarifier or thickener and remain for a relatively long retention time can experience anaerobic activity, if free molecular oxygen and nitrate (NO3-) are not available for bacterial activity. Anaerobic activity results in the use of sulfate and fermentative pathways to degrade cBOD. As a consequence of anaerobic activity, H2S, HS-, and malodorous sulfur-containing compounds are produced. Soluble forms of sulfur leave the secondary clarifier, while soluble, colloidal, and particulate forms of sulfur are returned from the thickener overflow to the activated sludge process.

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