Recovering the sulfidogenic activity under harsh sulfide-rich fermentative conditions in a thermophilic structured-bed reactor: Performance and microbial aspects

Abstract

Anaerobic treatment of sulfate-rich wastewaters is challenging due to the inevitable biological sulfate reduction to sulfide, which limits biogas use and can negatively impact the treatment performance due to the toxic character of sulfide. Establishing sulfidogenesis during fermentation has recently been highlighted as a novel application for acidogenic systems. Different strategies were applied to recover the efficiency of a thermophilic (55 °C) ethanol-fed anaerobic structured-bed reactor operated under acetogenic/sulfidogenic conditions after stress events: [i] the progressive re-insertion of liquid phase recirculation, [ii] the decrease in the sulfate loading rate (SLR) towards the minimization of residual sulfate levels, and [iii] the decrease in the organic loading rate (OLR). Long-term monitoring (631 d) divided into 13 operating phases (7 of which are detailed herein) showed a gradual improvement in the removal efficiency of sulfate, achieving 93.7% when applying suitable OLR (20.0 kg-COD m−3 d−1), SLR (3.2 kg-SO42- m−3 d−1), hydraulic retention time (15.0 h, theoretical value) and liquid upflow velocity (3.34 m h−1). A relatively restricted number of key microbial groups mediating both the sulfate reduction and acetate production from ethanol was identified, including the genera Coprothermobacter, Thermacetogenium, Fervidobacterium, Anaerobaculum and Thermotoga. This pattern most likely resulted from using ethanol as the only carbon source and explains the relatively long-term period (ca. 80 d) required to optimize the sulfidogenic activity. From a practical perspective, the operating strategies studied herein could be efficiently applied in industrial wastewater treatment plants, such as sulfate-rich sugarcane vinasses.