Value-added soluble metabolite production from sugarcane vinasse within the carboxylate platform: An application of the anaerobic biorefinery beyond biogas production

Currently, wastewater processing through anaerobic digestion can generate not only biofuels such as bio-hydrogen and methane, but also value-added soluble-phase intermediates (e.g. carboxylic acids), which have numerous applications. This study investigates the potential of using sugarcane vinasse, the main byproduct from ethanol production, as substrate in dark fermentation to recover soluble metabolites through the biorefinery approach. The impacts of temperature (30-60 degrees C) and initial pH (5.0-10.0) were initially assessed in batch tests using microbial consortia obtained from the natural fermentation of vinasse. The yield (401 mg-CODorganic acids g(-1)CODt(initial)) and productivity (653 mg-CODorganic acids L-1 d(-1)) of organic acids were maximized at alkaline/ mesophilic (pH = 8.8-10.0; 40 degrees C) conditions. Acetic-type fermentation prevailed at 30-40 degrees C, whilst butyrate was the primary metabolite at a higher temperature (60 degrees C). Further chain elongation-based experiments were conducted by adding ethanol and lactate as exogenous carbon sources in vinasse fermentation, also using vinasse-derived microbial consortia as the inoculum. Lactate was added as both chemical reactant and fermented cassava flour wastewater (fCFW). Ethanol addition was irrelevant to the acidogenic activity. Conversely, lactate addition directly increased the production of propionic, butyric, (iso)valeric, and caproic acids, with a pre-dominance of butyrate. Chain elongation was particularly favored when adding fCFW with pH adjusted to 7.0, boosting the generation of caproic acid from lactate and butyrate and (iso)valeric acid from acetate and propionate. These results highlight the potential for producing organic acids from vinasse as an alternative to gaseous fuels, expanding the suitability of dark fermentation targeting bioresource recovery from sugarcane.