Solving the bottlenecks of sugarcane vinasse biodigestion: Impacts of temperature and substrate exchange on sulfate removal during dark fermentation

Abstract

Removing sulfate prior to methanogenesis and maintaining year-round operation of the plant are the two main bottlenecks to be solved prior to implementing highly efficient bioenergy recovery from sugarcane vinasse via anaerobic digestion. Sulfidogenesis establishment and optimization during vinasse fermentation at two temperature levels (30 °C and 55 °C) were assessed using continuous fixed-film reactors. Simulating the off season through replacing vinasse by molasses was also assessed. Performance results and the distribution of microbial communities were correlated in all operating conditions. Sulfidogenesis was not efficiently established at 30 °C, because sulfate-reducing bacteria (SRB) did not grow favorably during vinasse natural fermentation. Acetate accumulation (∼3 g L−1) resulting from homoacetogenic activity (most likely by Lachnospirales) was observed in this condition. Conversely, sulfate removal (driven by Desulfohalotomaculum and Desulfotomaculum at much lower abundances compared to fermenters, such as Clostridia) was successfully established (84–97 %) under varied hydraulic retention times (18, 12 and 6 h) at 55 °C. Replacing vinasse by molasses negatively impacted SRB due to a sharp drop in pH (<6.0) triggered by lactate accumulation (up to 16.8 g L−1) mainly by Bacillus. However, SRB were resilient to such severe acidic and thermophilic conditions, and sulfate removal was maximized once vinasse was re-introduced. These results strategically represent the best operating strategy for the compositional conditioning of sugarcane vinasse prior to methanogenesis, because the high temperature in which vinasse is released from distillation columns (85–90 °C) facilitates the implementation of thermophilic reactors by eliminating any energy demands related to system heating.