The acidogenic phase of a two-stage anaerobic digestion process using distillery molasses slops effluent with high sulfate concentrations (4.2-5.1 g/l) was investigated. Removal of sulfate was studied at pH 5.8, 6.2, 6.6 and in two different reactors: continuous stirred tank reactor and an upflow fixed film fixed bed reactor. Batch experiments were carried out to obtain the maximum specific growth rates of sulfate reducing bacteria (SRB) at the above mentioned pH values. The biological sulfate removal increased with pH and so did the acetic acid production from the fermentative bacteria and SRB. For the same pH and hydraulic retention time the sulfate reduction was more efficient in the fixed film reactor than in the CSTR. The soluble sulfides from the sulfate reduction presented at the acidogenic reactor effluent were precipitated before the methanogenic phase to avoid biogas contamination and methanogenic bacteria inhibition; under such conditions, sulfide concentrations in the biogas thus produced were very low and high methane volumetric rates of production were achieved.

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