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Direct Interspecies Electron Transfer in advanced anaerobic digestion system for gaseous transport biofuel production

Periodic Reporting for period 1 - DIET (Direct Interspecies Electron Transfer in advanced anaerobic digestion system for gaseous transport biofuel production)

Reporting period: 2018-05-01 to 2020-04-30

Biogas can accelerate the decarbonisation of the European energy sector, and is suggested to play significant roles in future transport systems. Producing biogas from algae and other renewable substrates may be effected through anaerobic digestion (AD), which converts biodegradable components into biogas (a mixture of approximately 60% CH4 and 40% CO2) through different communities of syntrophic bacteria and archaea. The upgraded “green” gas containing over 97% biomethane can be used to as an advanced transport biofuel for heavy goods vehicles and bus fleets. However, existing AD technologies can suffer from two major drawbacks: (1) Efficiency of the AD process is sensitive to many factors (such as substrate hydrolysis, pH and microbial activity), which can result in instability and inefficiency of biogas production; (2) the anaerobic digestate generated after AD still possess a significant amount of energy and may require significant land area to assimilate nutrient load. The inefficiency of AD fundamentally arises from the interspecies electron transfer between syntrophic bacteria and methanogenic archaea. Therefore, the challenges on how to improve electron transfer efficiency and overall energy recovery of AD are significant and must be overcome to enhance biogas yield and optimise the third-generation biofuel system.
The overall research objective is to propose a future AD-based circular economy system, which produces renewable gaseous transport biofuel. The research explores the mechanisms of microbial electron transfer in the presence of different conductive materials, such as highly conductive but expensive graphene and more cost effective biochar including those derived from digestate for ultimate system circularity. The biomethane production rate in the proposed system can be enhanced by between 20 and 40% as compared to existing AD technology without addition of conductive materials.
The work on the future AD-based circular cascading system was divided into three work packages: (1) Modelling direct/indirect interspecies electron transfer in methanogenesis, (2) Developing strategies to improve microbial electron transfer and AD performance, and (3) Developing a future AD-based system for gaseous biomethane production in a circular economy. The fellow has developed models to illustrate the electron transfer from a thermodynamic view based on the Fick’s diffusion law and the Nernst equation. For example, in syntrophic methanogenesis through indirect hydrogen transfer, propionic acid oxidation is thermodynamically unfavourable and can occur only when a low hydrogen pressure is maintained by hydrogen-scavenging methanogens. Propionic acid oxidation through direct interspecies electron transfer (DIET) is thermodynamically favourable, which implies a more efficient start-up for methanogenesis. This project has further investigated the effect of different parameters (such as temperature, substrate, and graphene addition) on the performance of DIET enhanced AD. The optimised conditions have resulted in the increase in biomethane production rate by ca. 25% using ethanol or glycine as the model substrate. Furthermore, key microbial communities of fermentative bacteria and methanogenic archaea have been revealed. Cost-effective biochar can be potentially produced through pyrolysis of solid digestate from AD. This project has outlined an advanced AD system by incorporating conductive biochar to improve AD efficiency and overall energy conversion efficiency. Biochar addition in AD increased methane yield by 17% and peak production rate by 29% from the seaweed Laminaria digitata. Integrating AD with pyrolysis improved methane and bio-oil yield whilst decreasing quantities of anaerobic digestate by 26%.
An advanced AD-based cascading circular bioenergy system can increase biomethane production rate by ca. 30%, implying a reduced process time as compared to traditional AD. The enhanced performance is accompanied by rapid conversion of CO2 to methane, suggesting this process not only enhances energy production, but also promotes the concept of carbon neutrality. The research has resulted in the development of a cascading circular bioenergy system incorporating AD and pyrolysis, where biochar can be used to enhance DIET in AD or used as a soil amendment to increase soil organic content and enhance carbon sequestration. The work from this project supports technological competitiveness through excellence in science and engineering specifically in relation to renewable bioenergy. The developed technology can use a wide range of wet organic substrates such as agricultural feedstocks and algae for production of advanced transport biofuels. The concept is embedded in a circular bioeconomy approach which can lead to decarbonisation of agriculture (through for example reduced fugitive methane emissions from open slurry tanks), production of advanced gaseous biofuels (ideally suited to the hard to decarbonise sectors of trucks and ferries), enhanced carbon sequestration (through increased soil organic carbon) and as such is an important strategy to pursue to promote sustainability within the EU.
Microbial electron transfer between bacteria and archaea