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The anaerobic effluent passing the packed bed stripping column contains ferric iron, which will <br />react with sulfide to form ferrous iron and colloidal-size elemental sulfur (Figure 4-2). After the <br />ferrous iron is generated by air-oxygen, the sulfur is sepazated from this liquor through gravity <br />settling. The chemical requirements in the process are minimal because the anaerobic effluent is <br />recycled in a closed loop. The sulfur from the settler is dewatered in a filter press or by other <br />means depending on the method of disposal. <br />Effluent from the anaerobic system is further treated using an aerobic biological treatment unit as <br />a polishing step for removal of residual organic compounds and to oxidize manganese for <br />removal by precipitation. The aerobic treatment unit may also reduce total dissolved solids in <br />the effluent. <br />4.3.1.2 Process Chemistry <br />In the anaerobic reactor, sulfate is reduced to sulfide according to the reaction: <br />COD + sulfate ~ hydrogen sulfide + bicazbonate <br />In the stripper, the hydrogen sulfide from the wastewater is transferred to the gas phase: <br />HZS~~ -~ HZSts~ <br />In the absorber, the hydrogen sulfide is absorbed in the redox liquor and oxidized by ferric iron <br />to elemental sulfur: <br />H,Sp~+ 2Fe" -~ Sa~~ + 2Fe'* + 2H' <br />Finally, the ferrrous ion is reoxidized to the ferric form by means of aeration: <br />2Fe" + %Z OZ + Hz0 ~ 2Fe" + 20H" <br />During this final step, manganese can also be oxidized to manganese oxide by air injection, <br />addition of a stronger oxidizing agent, or the addition of an aerobic biological treatment unit (see <br /> <br />Bnu[e Maunta~n Resources, /nc. <br />p: Il002671reportslmarchrp(IV26Mtrmrtglmnrch.doc 44 Morch 21. /999 <br />