Implications of volatile fatty acid profile on the metabolic pathway during continuous sulfate reduction

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 2 g/L sulfate concentration and with the organic loading increasing from 3.5 kg COD/m3 d to 5.9 kg COD/m3 d, for a 1-day residence time. In the absence of recirculation, the best outcome (65% reduction) was noticed with the lowest organic loading (3.55 kg/m3 d). When recirculation was applied, sulfate reduction yields increased to 89%, corresponding to a sulfate removal rate of 1.94 kg SO42−/m3 d. 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 (sulfate-reducing bacteria) Desulfomonas, Desulfovibrio, Desulfolobus, Desulfobulbus and Desulfotomaculum spp.; (ii) lactate is converted to acetate by fermenting bacteria such as Clostridium sp. High propionate concentrations imply that there are low sulfate reduction efficiencies.

► In UASB reactors improved mass transfer enhances SRB activity.
► Clostridium competes with SRB for lactate at high COD/sulfate ratios.
► High propionate levels suggest fermentation as main mechanism of lactate oxidation.
► Recirculation can improve the performance of UASB reactors treating sulfate.
► Control of the COD/sulfate ratio is key for sulfate reduction in UASB reactors.