Deciphering anaerobic ethanol oxidation for better recovery of renewable energy from wastewater
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Date
2024-03-28Embargo Date
2026-03-27
Author
Du, Bang
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Abstract
Anaerobic digestion is a promising technology relying on complex microbial
interactions to convert organics into methane, achieving sustainable energy recovery.
Ethanol is an important low-molecular intermediate during anaerobic digestion, and
ethanol-type fermentation is a major fermentation type in mixed cultures of
acidogenesis, alongside butyric acid-type and propionic acid-type fermentations.
Furthermore, anaerobic ethanol degradation has been identified through various
metabolic pathways: (i) conventional ethanol degradation to acetate and hydrogen via
interspecies hydrogen transfer (IHT), (ii) ethanol consumption leading to propionate
production, and (iii) a newly suggested mechanism wherein ethanol releases electrons
for electron acceptors via direct extracellular electron transfer (EET) or direct
interspecies electron transfer (DIET). These metabolic pathways illustrate the
intricacy and diverse possibilities of reactions when ethanol serves as a substrate
during anaerobic digestion. Therefore, it is imperative to elucidate the regulatory
strategies governing ethanol metabolism, encompassing aspects such as the
degradation rate and the specific metabolic pathways, within anaerobic digestion
ecosystems.
The objectives of this study were: (i) to interpret and regulate the participation of EET
pathway in ethanol metabolism to favour the overall ethanol oxidation; (ii) to enrich
syntrophic bacteria and modulate ethanol metabolic pathways through the
manipulation of operational parameters, including solids retention times (SRT) and
operational modes; and (iii) to regulate syntrophic relationships among
microorganisms and microbial activities by adjusting operational parameters and
introducing powdered activated carbon (PAC).
A thermodynamic approach was employed to analyse IHT and EET pathways in
ethanol consumption. The effects of the fraction of EET pathway in ethanol
degradation, product feedback, and the redox potential of redox-active mediator on
biomass yields and biogas production were evaluated. The involvement of EET makes
it thermodynamically favourable for ethanol oxidation. It was found that the EET
fraction played a crucial role in maintaining biomass yields, and ethanol oxidation
occurred when the redox potential was above -0.408 V through EET or when the
product concentration was below the threshold value. Moreover, strategies for the
application of one-reactor and zone-separation systems were proposed to optimize
system performance and bioenergy recovery with the appropriate redox potential
range.
Different operational modes (sequencing batch reactors, SBRs, and continuous-flow
reactors, CFRs) and SRTs (25 days and 10 days) were employed to regulate syntrophic
interactions in four reactors. Microorganisms with high half-saturation constants were
enriched in reactors with a 25-day SRT. SBRs favoured the acclimation of ethanol
oxidizing bacteria and acetotrophic methanogens with high half-saturation constants.
In SBRs, Syner-01 and Methanothrix dominated, and a low SRT of 10 days increased
the relative abundance of Geobacter to 38.0% for possible performing DIET. In CFRs,
a low SRT of 10 days increased the relative abundance of Desulfovibrio among
syntrophic bacteria in mediating IHT.
Two operational modes (SBRs and CFRs) with or without the addition of PAC were
adopted as the regulatory approach to modulate microbial activities and drive
metabolic pathways towards acetate or propionate. The operational mode of SBR and
the presence of CO2 facilitated ethanol metabolism towards propionate production,
while CFRs with an extended SRT enriched high relative abundances of Geobacter,
reaching 71.7% and 70.4% under conditions with and without the addition of PAC,
respectively. Although both long-term and short-term PAC additions increased sludge
conductivity and reduced the methanogenic lag phase, only the long-term PAC
addition resulted in enhanced rates of ethanol degradation and propionate
production/degradation.
This study advances anaerobic digestion technology by unravelling the complex
ethanol metabolic pathways. The study could offer insights into the EET pathway with
a novel approach to favouring ethanol oxidation, further affecting overall system
performance. Manipulating operational parameters, especially operational modes and
SRT, and introducing PAC emerge as effective strategies for regulating microbial
activities, enrich functional microorganisms, and directing metabolic pathways.
Proposed strategies for system optimization, including one-reactor and zone separation systems, present practical solutions for practical applications. The findings
not only contribute to the improvement of bioenergy recovery and wastewater
treatment but also provide insights for guiding reactor design based on different
operational modes and SRT.