Mutual effects of CO2 absorption and H2-mediated electromethanogenesis triggering efficient biogas upgrading

Abstract

Anaerobic digestion coupled with bioelectrochemical system (BES) is a promising approach for biogas upgrading with low energy input. However, the alkalinity generation from electromethanogenesis is invariably ignored which could serve as a potential assistant for CO2 removal through the transformation into dissolved inorganic carbon (DIC). Herein, a novel bioelectrochemical CO2 conversion in the methanogenic BES was proposed based on active CO2 capture and in-situ microbial utilization. It was found that the BES using a stainless steel/carbon felt hybrid biocathode (BES-SSCF reactor) achieved a CH4 yield of 0.33 ± 0.03 LCH4/gCODremoval and increased CH4 production rate by 28.3% of BES-CF reactor at 1.0 V applied voltage. As the experiment progressed, CH4 content increased to 93.1% and CO2 content in the upgraded biogas maintained at below 3%. The continuous proton consumption from H2 evolution reaction in the hybrid biocathode was capable of creating a slightly alkaline condition in the BES-SSCF reactor and thereby the CO2 capture as bicarbonate was enhanced through endogenous alkalinity absorption. Microbial community analysis revealed that significant enrichment of Methanobacterium and Methanosarcina at the BES-SSCF cathodic biofilm was favorable for bicarbonate reduction into CH4 via establishment of H2-mediated electron transfer. Consequently, the remained CO2 and DIC only accounted for 12% of total carbon in the BES-SSCF reactor and the high conversion rate of CO2 to CH4 (82.3%) was achieved. These results unraveled an innovative CO2 utilization mechanism integrating CO2 absorption with H2-mediated electromethanogenesis.