High-sulfate, high-chemical oxygen demand wastewater treatment using aerated methanogenic fluidized beds

Many industrial wastewaters have both high organic pollution and sulfate (SO4-2) concentrations. Although biological conversion of organics to methane may be an economical chemical oxygen demand (COD) removal option, significant inhibition of methane production results from reduction of SO4-2 to hydrogen sulfide (H2S), which is inhibitory to methanogenic microorganisms. Therefore, sulfate containing wastewater is often not amenable to conventional anaerobic treatment. Recently, limited aeration of recycle flow to hybrid and baffled reactors has been used to treat this wastewater and has been shown to reduce aqueous H2S concentrations by causing production of uninhibitory sulfur (SD) and thiosulfate (S2O3-2) as well as gas stripping volatile H2S. In this study, directly aerated methanogenic fluidized bed reactors (FBRs) achieved increased methane production compared to strictly anaerobic FBRs treating high-sulfate wastewater. Oxygen transfer satisfying up to 28% of the COD load resulted in maximum specific oxygen utilization rates of 0.20 mg oxygen/g volatile solids.min, with significant, concomitant methane production. Under typically inhibitory SO4-2 loading, higher aeration caused increased effluent SO4-2, increased H2S mass in the offgas, and lower reactor H2S concentration. As a result, COD removal increased from 25% for a strictly anaerobic FBR to 87% for an aerated FBR. In addition, aerated systems required significantly less alkalinity supplementation to maintain a pH value of 7, ostensibly because of shipping of acidic carbon dioxide. The potential pH increase associated with aeration also shifts sulfide speciation to less toxic bisulfide. Direct, limited aeration of methanogenic FBRs is described as a method for increased COD removal when treating high-COD, high-sulfate wastewater.