Enhancing energy efficiency and by-product quality of anaerobic co-digestion of food and garden waste: hybridisation with thermochemical conversion to create a sustainable circular economy

Food and garden organics (FOGO) account for up to 60 % of domestic waste in Australia. Landfilling FOGO leads to significant environmental issues, including methane (CH 4 ) production and soil and water contamination. While methane capture is undertaken at sanitary landfills, significant losses result in greenhouse gas (GHG) emissions. This study assesses the integration of AD with pyrolysis/gasification to enhance energy and resource recovery from digestate. Results showed that biochar derived from FOGO digestate exhibited high carbon content (32 %–37 %) and fixed carbon (25 %–44 %), indicating that a substantial portion of the total carbon was in a stable, non-volatile form, suitable for long-term carbon sequestration. However, the FO: GO ratio did not significantly affect biochar properties, as AD stabilized feedstocks and reduced volatile matter content prior to thermal treatment. Furthermore, pyrolysis at a 65FO:35GO ratio produced biochar with lower ash and higher fixed carbon and heating value than gasification. Also, when the gasification temperature of the 65FO:35GO feed increased from 700 °C to 900 °C, fixed carbon rose from 22.2 % and volatile matter decreased from 21.8 %. PFAS in biochar were undetectable, and PAHs remained below the detectable limits of 0.0002. The mass and energy balances indicate that energy recovery from biogas and biochar is 481.5 MJ and 39.1 MJ, respectively. In the current study, the circular economy framework is defined through closed-loop resource and energy recovery and is demonstrated using mass and energy balances rather than a technoeconomic assessment. Overall, this study's findings demonstrate the potential of integrating AD with thermal treatments for sustainable resource recovery. • Coupling anaerobic digestion (AD) with pyrolysis/gasification would promote circular bioeconomy. • Systematic integration can be seen as a potential solution to organic waste disposal. • Biochar derived from digestate exhibited high carbon content, no PFAS and negligible PAH. • Detailed mass and energy balance of integrated unit is reported.