Microbial Electrolysis Cell-Assisted Anaerobic Digestion for Enhancing Biomethane Recovery from High-strength Wastewater
Creators: Huang, Qi
Publisher: name: University of Alberta Library
Year: 2024

<p>Anaerobic digestion (AD) is a sustainable method for treating high-strength wastewater while concurrently recovering biomethane. Nevertheless, factors such as low temperature and complex organic substrates can limit digestion kinetics, with hydrolysis and methanogenesis as the primary rate-limiting steps. Additionally, inhibitory compounds like sulfate/sulfide and ammonia in some high-strength feedstocks can disrupt methanogenic activity. Microbial electrolysis cell-assisted anaerobic digestion (MEC-AD) is a novel technology to enhance methane recovery. This research primarily focuses on mitigating OLR limitations, overcoming hydrolysis constraints, and alleviating sulfate inhibition in the AD of high-strength wastewater through MEC-AD systems, using source-separated blackwater as representative high-strength wastewater. <br>
     Firstly, the research chain started by pushing the organic loading rates (OLRs) of electrochemically assisted anaerobic digestion of vacuum toilet blackwater at ambient temperature. The MEC-AD system sustained a high OLR of 3.03 g COD/L-d, achieving significantly higher methane yield under closed-circuit operation compared to open-circuit operation. Microbial community analysis revealed the enrichment of specific electroactive bacteria and their syntrophic interactions with electrotrophic methanogens.<br>
     Secondly, a comprehensive screening of applied voltages (0 - 1.6V) for vacuum toilet blackwater digestion in a MEC-AD reactor at ambient temperature revealed that 1.2 V provided optimal performance, improving chemical oxygen demand (COD) removal, methane yield, and hydrolysis/acidogenesis efficiency. Enrichment of hydrolytic and syntrophic bacteria and an increase in genes encoding complex organics metabolism were observed.<br>
     Thirdly, this thesis also explored a novel MEC-AD scheme incorporating granular activated carbon (GAC) and extended sludge retention time, which mitigated performance deterioration due to increased OLR and temperature drop. GAC addition significantly improved methane yield and hydrolysis, and biomarker analysis highlighted the roles of GAC biofilms and settled sludge in promoting methanogenesis and hydrolysis.<br>
     Efforts were made to evaluate the performance of a MEC-AD system under different COD/sulfate ratios. The MEC-AD reactor demonstrated greater resistance to H2S toxicity, indicating higher specific methanogenic activities. Proposed mechanisms for the superior performance included lower free sulfide concentrations and promoted syntrophic partnerships between sulfate-reducing bacteria (SRB), hydrogenotrophic methanogens, and electroactive bacteria. These insights aid in the development of efficient AD systems for handling sulfate stress.<br>
     The final aspect of the thesis assessed the performance of three configurations of MEC-AD systems under sulfate-reducing conditions. The single-chamber MEC-AD reactors exhibited significantly higher methane yield than the control reactor, and conversion to a dual-chamber configuration using an anion exchange membrane (AEM) further increased methane production. In contrast, the MEC-AD reactor with a cation exchange membrane (CEM) experienced a reduction in methane yield due to anolyte acidification.<br>
     The results of this thesis have advanced our understanding of MEC-AD systems&#39; potential to enhance the anaerobic digestion of high-strength wastewater, with valuable insights into optimization and performance under various conditions.</p>