Effect of hydraulic retention time on sulfate reduction in a carbon monoxide fed thermophilic gas lift reactor

Thermophilic hydrogenogenic carbon monoxide (CO) converting microorganisms present in anaerobic sludge play a crucial role in the application of CO as electron donor for sulfate reduction. Hydrogenogenic CO conversion was investigated in a gas lift reactor (55 °C) at different hydraulic retention times (HRT). Operation at a HRT>9 h resulted in predominant consumption of CO-derived H2 by methanogens (up to 90%) and thus in a poor sulfate reduction efficiency of less than 15%. At HRTs<4 h, consumption of the CO-derived H2 was dominated by sulfate-reducers, i.e. up to 95% of H2 was used for sulfate reduction. Sulfate reduction rates of 17 mmol L−1 d−1 were achieved at a HRT of 3 h, with over 87% of the H2 produced used for sulfate reduction. Methane production, however, persisted when operating under these conditions and increasing the HRT by returning it to values >5.5 h resulted in a dominance of methanogenesis over sulfate reduction. The sulfate reduction rates were limited by the amount of CO supplied and its conversion efficiency (about 85%) at higher CO loads likely resulting from a low biomass retention.