Enhancing energy recovery via two stage co-fermentation of hydrothermal liquefaction aqueous phase and crude glycerol

Highlights: • Two stage HTL-AP fermentation boosted biogas production by 25.5% from single stage. • Two stage co-fermentation with crude glycerol further enhanced biogas yield by 85%. • Initial pH value affected intermediate metabolism and fermentation efficiency. • Organic removal and energy generation were enhanced by 48.6% and 84.9%. • Synergism of two stage, glycerol addition, and pH control improved energy recovery. Hydrothermal liquefaction (HTL) is a promising method to convert wet biomass into biocrude oil which can further be upgraded into transportation fuel. Approximately 20–40% of the total energy still remains in the aqueous phase after the HTL process. While conventional anaerobic digestion has demonstrated a limited conversion efficiency, two stage co-fermentation with crude glycerol was developed in this study to process HTL aqueous phase (HTL-AP) into hydrogen and methane, aiming to enhance biogas generation and energy recovery. Compared with single stage operation, two stage HTL-AP fermentation improved the biogas production by 25.5%. Subsequently, the addition of co-substrate crude glycerol helped relieve the acidic stress, adjusted the nutrient supply, and diluted the toxic concentration of chemicals in HTL-AP within the reactors. The biogas production was further enhanced by 1.85 times from single stage when the HTL-AP to crude glycerol ratio was 1:1. The initial pH value of the two stage operation was also controlled to optimize the metabolic pathways during the first stage of hydrogen production and to provide desirable intermediates for methanogenesis. Results showed that an initial pH of 5.5 resulted in the highest hydrogen production in this study. Accompanied with the enhanced biogas yield, the organic conversion, energy generation, and energy recovery from two stage co-fermentation were improved by 48.6%, 84.9%, and 40.1% compared to single stage fermentation, respectively. The enhanced biogas production, especially the hydrogen generation, provided a promising direction for wet biomass conversion. Specifically, downstream two stage treatment of HTL-AP could be integrated with upstream HTL by utilizing the produced hydrogen for upgrading biocrude oil via hydrocracking, and the methane could be used as a heating source for the HTL process.