Integration of anaerobic digestion with bio-electrochemical technologies in cascading circular bioeconomy systems producing biofuel and chemicals

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Ning, Xue
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University College Cork
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Anaerobic digestion is a waste treatment technology, which can alleviate greenhouse gas emissions, reduce environmental pollution whilst simultaneously generating biomethane (a clean energy source which can be used as a direct replacement for natural gas), biofertilizer (which can reduce fossil fertiliser use), and biogenic CO2 (which can be further valorised). However, many anaerobic digestion systems are implemented as standalone systems without optimizing circularity in system design. Conventional anaerobic digestion systems can face challenges such as: low biomethane production rate (in particular from feedstocks with high portions of lignin and cellulose), and inefficient biogas upgrading to biomethane. In an endeavour to simultaneously increase biogas production and upgrading, the potential for integrating anaerobic digestion with three emerging bio-electrochemical technologies in a circular cascading bioeconomy was assessed, including for power to gas, microbial electrolysis cell, and microbial electrosynthesis. An energy balance assessment indicated that these three circular cascading bio-electrochemical systems could display positive energy outputs if the electricity used would have been otherwise curtailed or constrained. This drove the thesis to develop bio-electrochemical cascading anaerobic digestion systems for value-added biofuel and chemical production. Anaerobic digestion is a complex microbial process that involves multiple syntrophic interactions with interspecies electron transfer as a crucial factor influencing digestion efficiency. Biochar has been shown to support direct interspecies electron transfer between fermentative bacteria and methanogenic archaea, thereby increasing biomethane production and reducing reaction times. The first experimental work investigated the biomethane potential in batch two-stage co-digestion of grass silage and cattle slurry, with varying dosages of biochar supplementation. Biochar addition at the optimal dosage of 10 g/L in two-stage digesters led to the highest methane yield of 253 L per kilogram (kg) volatile solid (VS), which was 24% higher than that from two-stage digesters without biochar supplementation. Continuous single-stage and two-stage co-digestion of grass silage and cattle slurry with 10 g/L biochar supplementation were compared in the second experimental work. In continuous trials, operated at an organic loading rate of 4.0 g VS/L/d, the second-stage digester in two-stage digestion produced a methane yield of 237 L/kg VS with 10 g/L biochar addition; this was 7% higher than the second-stage digester without biochar addition. The incorporation of two-stage anaerobic digestion and the addition of biochar was shown to be a promising approach to enhance system stability and improve biomethane production. To expand on the application of conductive materials in improving biogas production in anaerobic digestion, biochar was added to an integrated microbial electrolysis cell and anaerobic digestion system (the MEC-AD system) in the third experimental work. The results demonstrated that the biomethane yield and methane content in biogas in the MEC-AD system (with plain graphite cathode) increased by 68% and 17%, respectively, compared to conventional anaerobic digestion when co-digesting grass silage and cattle slurry. Biochar supplement (10g/L) in the MEC-AD system was shown to further increase biomethane yield by 9% as compared to the MEC-AD system without biochar addition. The combination of an enhanced electric field and biochar addition in the MEC-AD system provides a pathway for effective in-situ bioconversion of carbon dioxide to biomethane and improved substrate utilisation. The overall carbon utilisation of biomass conversion in AD and MEC-AD can be limited by the presence of carbon dioxide (CO2; approximately 30–45%) in the off-gas. This residual CO2 can be upgraded into valuable chemical products (such as acetic acid and ethanol) in microbial electrosynthesis, a process by which microorganisms utilize electrical energy to convert CO2 into value-added compounds. A 3D cobalt and nickel coated carbon felt (CoNi-CF) cathode was developed in this last experimental work and applied in microbial electrosynthesis reactors. The highest acetate concentration obtained from the microbial electrosynthesis reactor was 18.4 mmol/L, with a carbon conversion efficiency (C in acetate) of 75.4%, while the maximum ethanol production achieved was 4.5 mmol/L, with a carbon conversion efficiency (C in ethanol) of 18.6%. This thesis explored the synergistic integration of anaerobic digestion and bio-electrochemical technologies in cascading circular bioeconomy systems, and demonstrated that simultaneously enhanced biogas production and CO2 upgrading can be achieved through efficient direct interspecies electron transfer by adding biochar and/or by imposing an external electricity supply. The results from this thesis can provide guidance on designing future cascading circular bio-systems to produce advanced biofuel and value-added chemicals.
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Circular bioeconomy system , Anaerobic digestion , Bio-electrochemical technology , Microbial electrosynthesis , Biochar , Direct interspecies electron transfer
Ning, X. 2023. Integration of anaerobic digestion with bio-electrochemical technologies in cascading circular bioeconomy systems producing biofuel and chemicals. PhD Thesis, University College Cork.