Roles of Microbial Syntrophy, Extracellular Polymeric Substances, and Power Supply Schemes on Electro-methanogenesis
Creators: Reda, Basem Z
Publisher: name: University of Alberta Library
Year: 2022

<p>The concept of electro-methanogenesis by combining the microbial electrolysis cell and<br>
anaerobic digestion (MEC-AD) has become a promising method for improving methane<br>
generation and improving the stability of digesters. Although the electro-methanogenesis process<br>
is often featured as a simple process of coupling MEC with an anaerobic digester, several<br>
fundamental and engineering bottlenecks are associated with their practical application. Most<br>
importantly, a streamlined roadmap for establishing an active microbiome, process design,<br>
optimization, and scale-up has not yet been achieved. Particularly, this doctoral thesis focuses on<br>
understanding the roles of microbial syntrophy, extracellular polymeric substances, and power<br>
supply schemes on electro-methanogenesis.<br>
First, we reported an experimental investigation of extracellular polymeric substances (EPS),<br>
reactive oxygen species (ROS), and the expression of genes associated with extracellular electron<br>
transfer (EET) in methanogenic biocathodes electrodes. The MEC-AD systems were examined<br>
using two cathode materials: carbon fibers and stainless-steel mesh. A higher methane generation<br>
was attained in MEC-AD with stainless-steel mesh as a cathode electrode. A higher abundance of<br>
hydrogenotrophic Methanobacterium sp. and homoacetogenic Acetobacterium sp. appeared to<br>
play a major role in superior methanogenesis from stainless steel biocathode than carbon fibers.<br>
Moreover, the higher secretion of EPS accompanied by the lower ROS level in stainless steel<br>
biocathode indicated that higher EPS perhaps protected cells from harsh metabolic conditions<br>
(possibly unfavorable local pH) induced by faster catalysis of hydrogen evolution reaction. In<br>
contrast, EET-associated gene expression patterns were comparable in both biocathodes. Thus,<br>
these results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These<br>
results provide new insights into the efficient methanogenic biocathode development.<br>
Second, previous studies for conventional anaerobic digestion systems have emphasized<br>
maintaining an optimum propionate/acetate (HPr/HAc) ratio. To date, the detrimental ratio of<br>
HPr/HAc concentrations towards the electro-methanogenesis process has not been examined yet.<br>
Thus, this study focused on understanding the impact of different VFAs concentrations with varied<br>
HPr/HAc ratios on the microbial community and methanogenesis process. The total cumulative<br>
methane production remained almost the same after increasing HPr/HAc ratio from 0.5 to 1.5.<br>
When HPr/HAc ratios further increased to 2.5 and 5, the total cumulative methane production<br>
markedly decreased. EET-associated gene expression reduced under high HPr/HAc ratios (2.5 and<br>
5) indicates the partial inhibition of biofilm electroactivity. Geobacter and Methanobacterium<br>
species were abundant under lower HPr/HAc ratios, while their abundance decreased under higher<br>
HPr/HAc ratios. Therefore, this study demonstrated that higher HPr/HAc ratios would adversely<br>
impact methanogenesis rates in MEC-AD systems.<br>
Third, from the perspective of energy saving in the operation of MEC-AD, we focused on<br>
developing an intermittent power supply scheme. The applied potential was switched off for 12<br>
and 6 hours/day during the operation of a laboratory-scale MEC-AD system fed with glucose. The<br>
results from the operation under continuous applied potential served as the control. The overall<br>
biomethane generation and net energy income from the process were unaffected when the applied<br>
potential turned off for 6 hours/day. Both quantitative and qualitative analyses of microbial<br>
communities suggested that a balanced microbiome could be maintained under short-term<br>
switching-off the applied potential. However, performance substantially deteriorated when the<br>
applied potential turned off for 12 hours/day. Overall, the results of this study suggest that MEC-AD operation does not need a continuous power supply, and higher energy efficiency can be<br>
effectively achieved by intermittently powering the reactor. However, previous efforts to optimize<br>
power supply schemes for MEC-AD systems were limited to the synthetic substrate only.<br>
However, conventional digesters are typically operated with more complex substrates. Hence, we<br>
investigated the impact of intermittent power supply in MEC-AD fed with mixed primary and<br>
sewage sludge. Overall, the electrocatalytic activity of the anode biofilm demonstrated a higher<br>
current density at 12 hrs ON mode. Also, the maximum methane generation attained when the<br>
applied potential switched ON for 12 hrs/day. The extracellular electron transfer-associated genes<br>
showed the highest expression at 12 hrs ON mode. Accordingly, the intermittent applied potential<br>
for 12 hrs/day could provide an attractive opportunity to saving electrical energy input in MECAD systems, thereby its economic benefits.</p>