- Biological hydrogen production from renewable feedstocks was reckoned as a promising method for sustainable energy production. Bioelectrochemical hydrogen production using microbial electrolysis cells (MECs) demonstrated superiorities over the conventional methods for hydrogen production. The introduction of the membrane-less single chamber design further improved the feasibility of MECs for practical application, by offering superior electrochemical performance and decreased capital cost. However, as the hydrogen produced via the cathodic electrochemical process became accessible to the microbial communities, the presence of hydrogen scavengers was observed along with significantly decreased hydrogen production. Hydrogen consumptions via methanogenesis was noted as the prevalent hydrogen scavenging process. Though methanogenic hydrogen consumption was well studied, there was still a lack of practical inhibitory methods for the practical applications, as the existing approaches demonstrated limited effectiveness and/or increased cost and operational difficulty. On the other hand, homoacetogenesis was considered as a newly identified hydrogen scavenging process in MECs, which could result in a hydrogen production-consumption loop. However, the negative impact of such a loop remained controversial and no effective inhibitory method against homoacetogenesis was reported. Furthermore, due to the presence of hydrogen scavengers along with inefficient designs, previously constructed up-scaled MECs yielded unsatisfactory performance, hindering the potential for the practical application of MECs.
The present dissertation aims to develop a practical method against methanogenesis, to investigate and quantify the negative impact of homoacetogenesis on hydrogen production performance and the critical factor determining the homoacetogenic hydrogen consumption rate, to develop an approach to cease homoacetogenic hydrogen production in single chamber MECs, and to investigate the hydrogen production performance using real feedstocks in up-scaled single chamber MECs with the developed inhibitory methods against hydrogen scavengers.
The results in the present dissertation demonstrated significant progress toward understanding and inhibiting hydrogen scavengers to benefit the application of using single chamber MECs for hydrogen production. In the attempt to develop a more practical inhibitory method against methanogenesis, low concentration acetylene (0.1-5%, v/v) was examined as an effective methanogen inhibitor with only periodical injection needed. Current generation and the synergy between fermentative bacteria and exoelectrogens were not negatively affected by acetylene. These results demonstrated the great potential of using acetylene as a cost-effective inhibitor against methanogenesis in MECs. In terms of further understanding the impact of homoacetogenesis, single chamber MECs were operated under various conditions and the hydrogen production performance was monitored. Hydrogen partial pressure was determined as the most critical factor affecting homoacetogenic hydrogen consumption rate, while acetate concentration had little impact. At higher hydrogen partial pressures, hydrogen yield and energy efficiency decreased to as low as 66% and 48%, respectively, indicating the significant impact of the hydrogen production-consumption loop. In the attempt to cease the homoacetogenic hydrogen consumption, low concentration chloroform (0.005-0.03%, v/v) was tested as highly effective against homoacetogenesis, enhancing hydrogen yield and cathodic hydrogen recovery from 21% and 14% to 94% and 90%, respectively. The inhibitory effect was more specific against homoacetogens, as the electrochemical performance was not significantly affected. With the promising inhibitory effects of the developed method, a 10 L single chamber MEC with a high electrode surface area to volume ratio (66 m2/m3) was constructed. In the 10 L MEC, 0.02% (v/v) chloroform enhanced the hydrogen production rate from 0 L/L/D to 4.9 L/L/D and the current density from 12-16 A/m2 to 18-21 A/m2, demonstrating promising hydrogen production performance in the up-scaled MEC. To further examine the potential practical application of the up-scaled MEC for hydrogen production, the hydrogen yield and the hydrogen production rate were examined in the 10 L MEC using glucose, and validated using real lignocellulosic hydrolysate and brewery wastewater, respectively. The hydrogen yield using lignocellulosic hydrolysate was as high as 91% based on the dosed substrate, though the energy efficiency based on the input electricity was determined to be low (<62%). The hydrogen production rate using brewery wastewater was determined as high as 33.9 L/L/D, yet primarily from dark fermentation instead of the MEC process. Further investigations are still warranted for improving the efficiency of the up-scaled single chamber MECs.