CH4 production at moderate H2/CO2 pressures insights on the use of anaerobic granular sludge as biocatalyst

Abstract

Introduction: The continuous increase in energy consumption and the intensive use of fossil fuels, lead to the emission of greenhouse gases (GHG) and, in particular, to an increase in the concentration of CO2 in the atmosphere. In this context, the improvement in global awareness and the demand for sustainable technologies and products strongly contribute to laid plans to combat climate change. CO2-to-CH4 conversion represents a cutting-edge solution for CO2 capture and use, contributing to the reduction of GHG emission. Catalytic conversion of CO2-to-CH4 have been investigated, however, the high cost associated to the catalysts employed limits their use on a large scale. Biological CO2 methanation can overcome the significant technical and economic challenges of catalytic CO2 methanation. The biological production of CH4 using CO2-rich gases together with H2 is a promising strategy for the production of bioproducts. Hydrogenotrophic methanogens have a crucial role on the direct conversion of CO2+H2 into CH4, hence the importance to study the specific hydrogenotrophic methanogenic activity (SHMA). Methodology: In this work, the effect of initial substrate (H2/CO2) pressure, from 100 to 500 kPa, on the SHMA, on CH4 production rate and on developed microbial communities were evaluated. Two different pressurized bioreactors were studied using anaerobic granular sludge as the biocatalyst and H2/CO2 (80:20, v/v) as sole carbon and energy source. Gaseous compounds were analyzed by GC and archaeal diversity within granular sludge was monitored by 16S r-RNA based techniques. Results: The results showed an increase in the SHMA as well as in the CH4 production rate with the increase of the initial H2/CO2 pressure. This results are very interesting since no inhibitory effects were observed on the microbial activity, demonstrating the resistance of the anaerobic granular sludge. The Illumina results showed that Methanosarcinales, Methanobacteriales and Methanomicrobiales were the three orders that prevailed in the pressurized system, for all the pressures tested. However, hydrogenotrophic methanogens from Methanobacterium and Methanospirillum genera slightly increased their relative abundance, varying from 38% (100 kPa) to 41% (500 kPa) and from 8% (100 kPa) to 12% (500 kPa), respectively. Conclusions: In conclusion, the archaeal community seems to be very stable when submitted to increasing H2/CO2 pressures, highlighting the potential of the anaerobic granular sludge as an efficient microbial platform for the production of added-value compounds from gaseous carbon waste streams.