Modified biochar promotes the direct interspecies electron transfer between iron-reducing bacteria and methanogens in high organic loading co-digestion.

High organic loading (HOL) could reduce substrate degradation and methane production. The objective of this study was to investigate the promotion mechanism of iron-modified biochar in HOL co-digestion. The results showed that the specific surface area of iron-modified biochar prepared at 500 ⁰C (500Fe@BC) was 131.7 m/g. In 12% (w/w) of HOL co-digestion, 500Fe@BC addition enhanced methanogenesis by both aceticlastic and hydrogenotrophic pathways and showed the best methane yield performance. Compared with the non-biochar addition group, an increase of 56.6% and 11% in average methane content and cumulative methane yield was observed in the presence of 500Fe@BC during 25 days of hydraulic retention time. Furthermore, the buffer capacity of HOL co-digestion has been intensified, which attributed to the 500Fe@BC accelerated the hydrolysis of substrates and promoted the consumption of the volatile fatty acids. Moreover, 500Fe@BC promoted the enrichment of iron-reducing bacteria (Clostridium_sensu_stricto_1, Romboutsia) and methanogens (Methanosarcina, Methanobacterium).