Bio-methanation enhancement of anaerobic digestion with urine-modified biochar: Insights into pyrolysis optimization, methanation mechanism, and techno-economic potential

Organic matters in biomass can be converted into biomethane via anaerobic digestion (AD), enabling the resource utilization and energy conversion of biomass. This study innovatively utilized urine to prepare urine@biochar to enhance AD. The urine@biochar prepared through mixing urine and bio-waste directly and following by pyrolysis had the maximal biomethane yield, with an increase ratio of 33.6% compared to control. This urine@biochar had the maximal specific surface area, providing more active sites for microbial enrichment and substance diffusion; also, a large number of C-N bonds were formed, and nitrogen forms like oxidized-N, pyridinic-N, pyrrolic-N, and graphitic-N were introduced, promoting electrochemical activity and redox activity. It increased the activities of α-glucosidase, protease, electron transfer, Coenzyme F420, and cytochrome C by 7.1%, 25.5%, 21.7%, 26.5%, and 43.7%, and enriched Methanobacterium and Clostridium_sensu_stricto_13, which might form direct interspecies electron transfer. The techno-economic analysis indicated that this strategy significantly reduced carbon footprint from biomass and urine treatment, advancing waste treatment industry toward low-carbon but high-efficiency operation goal.