Role of exogenous hydrogen in methane yield during anaerobic digestion of waste fat, oil, and grease by coupling carbon cloth with exogenous hydrogen

Carbon cloth (CC) coupled with exogenous hydrogen (EH2) was used to promote direct interspecies electron transfer (DIET) during anaerobic digestion of waste fat, oil, and grease (FOG). However, whether the change of EH2 injection pattern would further enhance DIET and FOG conversion for methane production is unknown, and the underlying mechanism is not fully understood. In this study, three EH2 injection pressures (0.2/0.4/0.6 atm) and frequencies (once/every 8 days/every 4 days) were chosen to collaborate with CC for investigating the role of EH2 in methane yield during FOG digestion. The results show that, combined with CC, increased EH2 injection pressure/frequency resulted in gradually enhanced DIET to increase FOG conversion rate (from 69.9% to 96.7%), leading to the highest cumulative methane production (1204.9 mL/gVSadded) 58.9% higher than that of Control. During the digestion, the secretion of extracellular polymeric substances was increased by EH2 to promote bioaggregation and electron transfer between cells for enhanced biological DIET in the digestate. The influence of EH2 on the microbial communities on CC surface was bigger than that in suspended sludge, leading to enhanced syntrophic acetate oxidation-hydrogenotrophic methanogenesis (SAO-HM) for acetate conversion on CC. As the amount of EH2 increased, the acetate conversion by SAO-HM via DIET was improved, which simultaneously promoted the syntrophic metabolisms between acidogens/acetogens and methanogens to achieve more efficient FOG conversion. Together, this work provides new insight into the role of EH2 in the enhancement of DIET and methane yield during FOG digestion.