Performance improvement of a thermophilic sulfate-reducing bioreactor under acidogenic conditions: Effects of diversified operating strategies

The establishment of a sulfidogenic environment under thermophilic (55 °C) acidogenic conditions was assessed in an innovative structured-bed bioreactor to enhance sulfate removal and acetate production prior to methanogenesis. Diversified operating strategies, i.e., variations in the hydraulic retention time (HRT; 6-12 h), sulfate loading rate (SLR; 8-16 kg SO42− m−3 day−1) and liquid phase recirculation ratio (0.0-57.0) were assessed to both enable the establishment of sulfate-reducing conditions and remove H2S from the liquid phase. Ethanol was used as the only carbon source. Applying a low HRT (6 h) as the initial operating strategy severely hindered the establishment of sulfate-reducing bacteria (SRB) populations within the system (sulfate removal < 27.5%). In turn, applying effluent recirculation had a positive impact on the system (sulfate removal ∼ 60%) by providing an adequate buffer control along the entire height of the system, as well by displacing over 70% of the H2S to the gaseous phase. The maintenance of pH values above 6.1 proved to be adequate for the sulfidogenic activity, whereas enhanced acidic conditions (pH < 6.0) at the basal portion of the reactor comprised a determining factor to hinder sulfate reduction. SRB were able to handle H2S and acetate concentrations as high as 232 mg L−1 and 3111 mg L−1, respectively, after establishing an effective acidogenic/sulfidogenic environment, indicating that the proposed system has the potential to be used as the first stage in the anaerobic processing of sulfate-rich wastewater streams.