Enhanced biogas production from swine manure anaerobic digestion via in-situ formed graphene in electromethanogenesis system

The aim of this work was to enhance methane production from swine manure anaerobic digestion (AD) with in-situ formed graphene in microbial electrolysis cell (AD-MEC). The AD-MEC for in-situ graphene exfoliation incorporated a carbon felt anode and a titanium mesh cathode with an applied voltage of 2.5 V. Atomic force microscopic and Raman analyses confirmed that the graphene was in-situ formed. Voltage stimulation (1 h per day) and graphene production enhanced AD efficiency by promoting direct interspecies electron transfer between AD microorganisms. The increased biomass owing to enrichment of AD microorganisms on the carbon felt could also enhance AD efficiency. The voltage-graphene group (VGG) greatly enhanced the biogas yield (356.49 m³/t dry swine manure) and methane yield (222.17 m³/t dry swine manure), which were 41.49% and 60.89% higher than that of the control group, respectively. Microbial community analysis identified the dominant methanogens in the VGG system as Methanosarcina and Methanobrevibacter, while only Methanosarcina was dominant in the control system. Network analysis showed that the VGG system established a narrower niche than the control system, and microorganisms trended to gather more in the module.