Simultaneous production of biogas and high-purity hydrogen from expired rice wine by combining anaerobic digestion and microbial electrolysis

Abstract

Microbial electrolysis has gained increasing attention as a promising approach for producing green hydrogen, and utilizing organic waste streams as feedstock is widely accepted as a viable option for enhancing its sustainability and applicability. However, high concentrations of suspended particles and the compositional complexities of real organic waste streams make their direct utilization for microbial electrolysis challenging, particularly in dual-chamber microbial electrolysis cells (MECs) for high- purity hydrogen production. For sustainable application of MECs, it is important to secure an economical and readily available substrate, such as organic waste dark fermentation effluent (DFE), that is rich in simple organic matters and relatively low in suspended particles. Makgeolli, a popular traditional rice wine in Korea, is produced at a rate of approximately 400 million liters per year. Thus, the disposal of makgeolli after its expiration date is challenging. Because makgeolli is rich in ethanol, a utilizable electron source, it is a potential sustainable feedstock for MECs. However, the high concentration of suspended solids (SS) in makgeolli makes it unsuitable for direct use in MECs. By separating expired makgeolli (EM) into supernatant separated from EM by gravity settling for 24 h (MS) and concentrate separated from EM by gravity settling for 24 h (MC) via gravity sedimentation, we aim to improve the bioavailability of the substrate and reduce operational problems associated with high concentrations of SS in dual-chamber MECs for high-purity hydrogen production. To achieve this goal, we conducted the following three steps: In Study Ⅰ, we focused on the utilization preference between different synthetic substrates to gain a better understating of the effect of substrate characteristics on MEC performance before real wastewater application. Few studies have investigated the use of ethanol as a substrate for dual-chamber MEC, although ethanol is a major fermentation product that is reported to facilitate exoelectrogen electron transfer in anaerobic digestion (AD). Six synthetic DFEs, respectively containing acetate, propionate, lactate, butyrate, ethanol, and their equal mixture at 1.5 g chemical oxygen demand (COD)/L, were compared for hydrogen production in dual-chamber MECs. The overall hydrogen recovery was highest when fed with acetate, followed by lactate, propionate, butyrate, mixture, and ethanol. Although the overall hydrogen yield was relatively low for ethanol, possibly due to the consumption of elections by methanogens (mainly Methanobacterium) and homoacetogens (mainly Acetobacterium), the ethanol- fed MECs achieved the highest hydrogen production rate. The findings of this study suggest the potential use of ethanol-rich real wastewater as a suitable substrate for hydrogen production via microbial electrolysis. In Study ⅠⅠ, substrate potential was evaluated in MECs of ethanol-rich EM during batch operation. Additionally, the technical and economic feasibility of hydrogen production from EM by combining the MEC of MS with the AD of MC were examined. Highly soluble and ethanol-rich MS demonstrated significantly greater hydrogen production and exoelectrogenic bacterial enrichment in the MECs than EM. In contrast, in the AD tests, MC with its higher alkalinity potential exhibited superior and more stable methane production than EM. The combined approach to separately process MS and MC could potentially increase total energy recovery (H₂ + CH₄) by 18–21% and product revenue by 6–180% compared to processing whole EM using either MEC or AD alone without solid–liquid separation. These findings present a sustainable and economical strategy for valorizing EM, a high-strength organic waste stream, warranting further research for long-term continuous operation. In Study ⅠⅠⅠ, for a more practical application, an approach was designed to operate dual-chamber MECs and continuous stirred tank reactors (CSTRs) for AD in long-term continuous mode (>60 days for MEC and >448 days for AD). Both EM-fed MECs and MS-fed MECs showed a superior hydrogen production rate in the range of 86–121 mL H2/d anolyte·d for an organic loading rate (OLR) of 1–3 g COD/L·d but did not increase further, possibly due to limitation in the electrode surface area. The AD system indicated a much more stable methane production rate and yield in MC-fed AD than in EM-fed AD. This stability was attributed to the high protein content (i.e., high ammonia alkalinity potential) in MC, which is more than 6-fold greater than that in EM. These findings suggest that the combined AD–MEC process provides better daily energy recovery and product revenue than a single process at the high-strength organic concentration used for treatment during long-term continuous operation. To capitalize on this potential, strategies must be devised to address the limitations inherent in each system, accompanied by a comprehensive technical and economic assessment. In conclusion, my Ph.D. research demonstrated that EM can serve as an effective substrate for dual-chamber MECs. The integration of AD and MEC, with a solid–liquid separation strategy, has shown potential for considerably enhancing bioenergy production. This study provides insight into the importance of developing novel substrates that are suitable for AD and MEC. Additionally, the study provides valuable information about the impact and economic benefits of utilizing EM in a combined AD–MEC system compared to separate AD and MEC systems. These aspects suggest that further studies on topics such as long-term continuous operation, high-intensity organic enrichment treatment, and reactor modification for scale-up, would have considerable value for overcoming the limitations of the MEC and AD systems used in this experiment.