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Volatile fatty acid (VFA) profile is an important parameter in anaerobic reactors because it enables the assessment of metabolic pathways. Volatile fatty acids were monitored during sulfate reduction in a UASB (upflow anaerobic sludge blanket) reactor treating 2g/L sulfate concentration and with the organic loading increasing from 3.5kgCOD/m³d to 5.9kgCOD/m³d, for a 1-day residence time. In the absence of recirculation, the best outcome (65% reduction) was noticed with the lowest organic loading (3.55kg/m³d). When recirculation was applied, sulfate reduction yields increased to 89%, corresponding to a sulfate removal rate of 1.94kgSO₄²⁻/m³d. The reactor performance was discussed in relation to microbial diversity and metabolic pathways. At high organic loading, two metabolic pathways account for lactate degradation: (i) lactate is oxidized to acetate and carbon dioxide by the incomplete-oxidizer SRB …