Deciphering anaerobic ethanol oxidation for better recovery of renewable energy from wastewater
Du, Bang
Du, Bang
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Publication Date
2024-03-28
Type
Thesis
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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.
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NUI Galway