Optimization of electromethanogenesis: effect of operating condition under mesophilic conditions

Abstract

In 2019, over 90% of the global population live exposed to poor air quality, which represents a significant threat to public health worldwide. In indoor spaces, where humans spend most of their time, can reach higher concentrations of pollutants such as CO2, a greenhouse gas contributing to climate change. Therefore, there is an increasing need to develop technologies to mitigate atmospheric CO2 and enhance indoor air quality, which has great relevance for human health, but also technologies capable of transforming the captured CO2 into value-added products to be used on-site or to be introduced into the market, generating economic benefits. To address these challenges, the MICRO-BIO process is proposed as a comprehensive platform to capture CO2 from indoor air and transform it into valuable carbon-neutral chemicals by coupling CO2 direct air capture to a bioelectrochemical system (BES). Microbial electrosynthesis technologies (MEST) reactors rely on chemolithoautotrophic microorganisms that can reduce CO2 by the H2-mediated archaeal-type Wood-Ljungdahl pathway. The production rates in MES reactors are widely variable and dependent on several factors such as reactor materials, inoculum sources, and operation parameters. This final degree project focuses on studying the operating conditions of a BES bioelectrochemical transformation of indoor CO2 into CH4. For that purpose, several experiments are established changing the hydraulic residence time (HRT) and empty bed residence time (EBRT). The acquired results suggest that the studied BES had a 50-day adaptation period of the biomass to adjust their metabolism to the switch between fed-batch and continuous mode operation. Also, HRT and EBRT effects on CH4 conversion and productivity rate were evaluated. During experimentation, the biofilm CH4 productivity rate was underestimated, so to avoid washout, the use of pearls containing the biomass is proposed for future optimization. Furthermore, the obtained results demonstrate that the limiting reagent for CO2 reduction is H2, so increasing the applied voltage is proposed for future experimentation to increase water hydrolysis, providing more H2 to the MES system, and subsequently increasing the conversion and productivity rate. CO2 mass transfer of CO2 from gas to the liquid phase was also limiting the CH4 conversion. To improve the system and avoid mass transfer limitations, capillary module assembly is required for future experimentation