Synergistic combination of anaerobic digestion and gasification

Biochar from pyrolysis has been proposed as one of the solutions to enhance methane production and improve stability in anaerobic digestion. However, there is a lack of study on the use of biochar from the gasification process. Biochar from the gasification process is typically considered as waste output and has no market due to potential higher ash content and heavy metals. However, the combination of anaerobic digestion and gasification has the potential of larger energy recovery from waste materials compared to the pyrolysis process. The aim of this study is to evaluate the performance of the application of biochar produced from the gasification process in an anaerobic digester to enhance methane production and its impact during high organic loading. Tests were performed under different amount of biochar ranging from 4.0 g to 12.0 g. The 1 L reactors were fed semi-continuously with food waste at 3 g-VS L⁻¹ day⁻¹ and subsequently at 6 g-VS L⁻¹ day⁻¹. The optimal biochar loading was determined to be 4 g and improved methane yield by 27%. 12 g biochar did not improve methane yield at 3 g-VS L⁻¹ day⁻¹ organic loading rate but provided better stability during high organic loading. The biochar’s ash content released into the reactor did not reach the inhibitory level, therefore are not the contributing factor for lower performance. Methane content of the reactors without biochar dropped below 10% within 4 days of organic loading shock, while reactors with biochar, sustained production above 10% methane content. The application of biochar from the gasification process enhanced biogas production and process stability. This provides a possible pathway for a synergistic combination of anaerobic digestion and gasification of waste for energy recovery and reduction in ash/waste disposal.