Deciphering the role of biochar type and particle size in anaerobic co-digestion of food waste and waste activated sludge under thermophilic conditions

Abstract

Biochar amendment has been proposed to enhance thermophilic anaerobic digestion (AD), yet the influence of biochar physicochemical properties on co-digestion performance and microbial restructuring remains unknown. In this study, three biochars derived from bamboo, hog manure, and hickory shell were evaluated during thermophilic (55 °C) co-digestion of food waste and waste activated sludge. Compared with the control (310.5 ± 7.6 mL CH₄ /g volatile solids), biochar supplementation significantly increased cumulative methane yield, with hickory shell biochar achieving the highest production (393.4 ± 9.8 mL CH₄ /g volatile solids), corresponding to a 26.7% increase. Kinetic analysis showed that biochar increased the maximum methane production rate by up to 34.6% and shortened the lag phase by approximately one-third. Propionate accumulation was suppressed with hickory shell biochar, accompanied by enhanced soluble chemical oxygen demand removal and improved pH buffering. Biochar particle-size variation at the millimeter scale had a negligible effect. Microbial analysis revealed enrichment of hydrolytic Clostridium_sensu_stricto_1 and versatile methanogens such as Methanosarcina and Methanoculleus in biochar-amended systems. Functional prediction indicated an increase in carbohydrate metabolism, acetate conversion (ackA, pta), and methanogenesis genes (mcrA/B/G). Overall, biochar enhanced thermophilic AD through coordinated improvements in hydrolysis, syntrophic metabolism, and methanogenic activity.

Graphical abstract

Keywords

Thermophilic co-digestion / Biochar / Microbial community / Organic solid waste / Hickory shell biochar

Highlight

	● Different types of biochar and particle size on thermophilic AD were investigated.
	● Hickory shell biochar achieved the highest methane yield of 393.4 ± 9.8 mL/g VS.
	● Particle size distribution at millimeter scale had negligible impact on AD.
	● Biochar enriched syntrophic fermenters and versatile methanogens.
	● Metabolic analysis revealed biochar enhanced hydrolysis and DIET-related pathways.

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