Anaerobic reduction of sulfate in immobilized cell bioreactors, using a microbial culture originated from an oil reservoir

The kinetics of anaerobic sulfate reduction was studied in continuous immobilized cell bioreactors, using a consortium of sulfate reducing bacteria (SRB) enriched from the produced water of a Canadian oil reservoir. The effects of carrier matrix, concentration of sulfate in the feed and sulfate volumetric loading rate were investigated. The bioreactor performance, as assessed by volumetric reduction rate of sulfate, was dependent on the total surface area provided by the immobilization carrier matrix. Among the three tested matrices, sand displayed a superior performance with a maximum reduction rate of 1.7 g/L h was achieved at the shortest residence time of 0.5 h. This reduction rate was 8- and 40-fold faster than those obtained with biomass support particles (0.2 g/L h; residence time: 5.3 h) and glass beads (0.04 g/L h; residence time: 28.6 h), respectively. Further kinetics studies with sand as a carrier matrix indicated that the extent of sulfate reduction rate was dependent on the feed sulfate concentration and volumetric loading rate of sulfate. At a constant feed sulfate concentration, increases in sulfate volumetric loading rate caused the reduction rate to pass through a maximum. Contrary to the pattern reported for the freely suspended SRB cells, the increases in feed sulfate concentrations led to lower reaction rates in the immobilized cell bioreactors. The maximum volumetric reduction rate with feed sulfate concentrations of 1, 2.5 and 5 g/L were 1.7, 0.8 and 0.7 g/L h, observed at residence times of 0.5, 2.7 and 2.7 h, respectively. The SRB consortium used in this study was dominated by incomplete oxidizers. The experimental data for lactate utilization and acetate production rates were in close agreement with those calculated based on the stoichiometry of the involved reactions.