Conductive biochar promotes oxygen utilization to inhibit greenhouse gas emissions during electric field-assisted aerobic composting

Abstract

The insufficient oxygen supply in partial materials commonly results in significant greenhouse gas emissions during composting, which is essentially attributed to the poor electron transfer in the composting systems. Electric field-assisted aerobic composting (EAC) is considered effective in mitigation of greenhouse gas emissions, but the poor conductivity of composting materials hampers its efficiency and applicability. In this study, conductive biochar was added in the EAC system to investigate its effects on the performance and greenhouse gas emissions during the composting processes. In the system of EAC with biochar, the electrochemical properties, O₂ utilization and composting performance were improved compared to the systems without biochar or assisted electric field. The maximum current of EAC with biochar was 0.32 A, higher than that without biochar (0.28A). Particularly, the peak concentrations of CH₄ and N₂O in the EAC system with biochar were 0.86 mg·kg^-1 and 1.43 mg·kg^-1, which were 45 % and 27 % lower than those in the EAC without biochar, respectively. The direct global warming potential attributed to CO₂, CH₄, and N₂O was 3.96 g CO₂-equivalent·kg^-1 dry mass, providing a 31.6 % reduction compared to conventional composting. Microbial analyses revealed that biochar increased the relative abundance of electroactive bacteria including Bacillus, Tepidimicrobium and Corynebacterium. In contrast, the abundances of potential nitrifying and denitrifying bacterial species of Pseudomonas, Corynebacterium, Acinetobacter, and Bacillus were significantly lowered in the biochar-assisted EAC system (11.35 %). The results showed that the addition of biochar was able to promote the electrical conductivity of composting materials and accelerate the organic oxidation process by increasing O₂ consumption, and accordingly change the dominant microbial community on both composting and biochar particles. This study verified the mechanism of the effectiveness of biochar in greenhouse gas control in composting processes, and thus provided evidence for facilitating the sustainable development of composting technologies.

Keywords: Composting; Conductivity; Electric field-assisted; Electroactive bacteria; Oxygen utilization.