- 55 views
- 274 downloads
Roles of Microbial Syntrophy, Extracellular Polymeric Substances, and Power Supply Schemes on Electro-methanogenesis
-
- Author / Creator
- Reda, Basem Z
-
The concept of electro-methanogenesis by combining the microbial electrolysis cell and
anaerobic digestion (MEC-AD) has become a promising method for improving methane
generation and improving the stability of digesters. Although the electro-methanogenesis process
is often featured as a simple process of coupling MEC with an anaerobic digester, several
fundamental and engineering bottlenecks are associated with their practical application. Most
importantly, a streamlined roadmap for establishing an active microbiome, process design,
optimization, and scale-up has not yet been achieved. Particularly, this doctoral thesis focuses on
understanding the roles of microbial syntrophy, extracellular polymeric substances, and power
supply schemes on electro-methanogenesis.
First, we reported an experimental investigation of extracellular polymeric substances (EPS),
reactive oxygen species (ROS), and the expression of genes associated with extracellular electron
transfer (EET) in methanogenic biocathodes electrodes. The MEC-AD systems were examined
using two cathode materials: carbon fibers and stainless-steel mesh. A higher methane generation
was attained in MEC-AD with stainless-steel mesh as a cathode electrode. A higher abundance of
hydrogenotrophic Methanobacterium sp. and homoacetogenic Acetobacterium sp. appeared to
play a major role in superior methanogenesis from stainless steel biocathode than carbon fibers.
Moreover, the higher secretion of EPS accompanied by the lower ROS level in stainless steel
biocathode indicated that higher EPS perhaps protected cells from harsh metabolic conditions
(possibly unfavorable local pH) induced by faster catalysis of hydrogen evolution reaction. In
contrast, EET-associated gene expression patterns were comparable in both biocathodes. Thus,
these results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These
results provide new insights into the efficient methanogenic biocathode development.
Second, previous studies for conventional anaerobic digestion systems have emphasized
maintaining an optimum propionate/acetate (HPr/HAc) ratio. To date, the detrimental ratio of
HPr/HAc concentrations towards the electro-methanogenesis process has not been examined yet.
Thus, this study focused on understanding the impact of different VFAs concentrations with varied
HPr/HAc ratios on the microbial community and methanogenesis process. The total cumulative
methane production remained almost the same after increasing HPr/HAc ratio from 0.5 to 1.5.
When HPr/HAc ratios further increased to 2.5 and 5, the total cumulative methane production
markedly decreased. EET-associated gene expression reduced under high HPr/HAc ratios (2.5 and
5) indicates the partial inhibition of biofilm electroactivity. Geobacter and Methanobacterium
species were abundant under lower HPr/HAc ratios, while their abundance decreased under higher
HPr/HAc ratios. Therefore, this study demonstrated that higher HPr/HAc ratios would adversely
impact methanogenesis rates in MEC-AD systems.
Third, from the perspective of energy saving in the operation of MEC-AD, we focused on
developing an intermittent power supply scheme. The applied potential was switched off for 12
and 6 hours/day during the operation of a laboratory-scale MEC-AD system fed with glucose. The
results from the operation under continuous applied potential served as the control. The overall
biomethane generation and net energy income from the process were unaffected when the applied
potential turned off for 6 hours/day. Both quantitative and qualitative analyses of microbial
communities suggested that a balanced microbiome could be maintained under short-term
switching-off the applied potential. However, performance substantially deteriorated when the
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
effectively achieved by intermittently powering the reactor. However, previous efforts to optimize
power supply schemes for MEC-AD systems were limited to the synthetic substrate only.
However, conventional digesters are typically operated with more complex substrates. Hence, we
investigated the impact of intermittent power supply in MEC-AD fed with mixed primary and
sewage sludge. Overall, the electrocatalytic activity of the anode biofilm demonstrated a higher
current density at 12 hrs ON mode. Also, the maximum methane generation attained when the
applied potential switched ON for 12 hrs/day. The extracellular electron transfer-associated genes
showed the highest expression at 12 hrs ON mode. Accordingly, the intermittent applied potential
for 12 hrs/day could provide an attractive opportunity to saving electrical energy input in MECAD systems, thereby its economic benefits. -
- Graduation date
- Spring 2022
-
- Type of Item
- Thesis
-
- Degree
- Doctor of Philosophy
-
- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.