Enhancing anaerobic digestion of complex substrates by powdered activated carbon: System performance, kinetic modeling and ecological analysis

Adams, Mabruk, 2025. Enhancing Anaerobic Digestion of Complex Substrates by Powdered Activated Carbon: System Performance, Kinetic Modeling and Ecological Analysis, University of Galway.

Abstract

Anaerobic digestion (AD) is a remarkable biotechnology that utilizes facultative anaerobic bacteria to decompose biodegradable materials, potentially aiding in the offset of global warming via renewable energy (biogas CH4) generation. Complex substrates are predominant constituents in most waste and wastewater streams used in anaerobic digestion systems. That is, unlike simple or single-substrate systems, complex substrates encompass a diverse range of organic materials that better reflect real-world waste conditions. However, this diversity indicates heightened complexity with regards to microbial interactions and metabolic pathways. As such, the AD of complex substrates is often faced with substrate induced microbial activity inhibition, volatile fatty acid (VFA) accumulation and process instability. In this regard, a variety of physicochemical and biological techniques including temperature, pH and carbon-to-nitrogen (C/N) ratio regulation, substrate pre-treatment, microbial immobilization and bioaugmentation, etc have been proposed and tested. The incorporation of conductive materials (CMs), such as powdered activated carbon (PAC), represents an innovative strategy for optimizing anaerobic digestion of complex substrates. PAC supplementation aids in the mitigation of prolonged retention times, biomass loss, and poor microbial syntrophy, which are often encountered during complex substrate digestion. Research focusing on complex substrates is, therefore, essential for developing scalable and efficient waste-to-energy solutions that accommodate the variability and heterogeneity inherent in industrial and municipal waste streams, ultimately enhancing the practical applicability of anaerobic digestion technologies. Moreover, elucidation of the stimulatory mechanisms that underpin PAC-assisted metabolism of complex substrates, encompassing microbial activities and specific metabolic pathways and metagenomics, is crucial. The objectives of this study were: i) to investigate biomethanation of complex substrate and evaluate the regulation of syntrophic bacteria enrichment via operational mode variation and PAC addition; ii) to elucidate the influence of carbon type and PAC addition on rate-limiting biokinetics (hydrolysis and methanogenesis) for single versus complex substrates, using different kinetic models; and iii) to provide a detailed analysis of the synergistic influence of operational modes and PAC addition on microbial ecology in relation to electron transport, functional genes regulation and energy metabolism. Operational mode and PAC are key factors facilitating microbial syntrophy and interspecies electron transfer during anaerobic digestion, consequently benefiting process stability and efficient methanogenesis. In this study, continuous-flow reactors (CFR) and sequencing batch reactors (SBR), with and without the addition of PAC, respectively, were operated to examine their effects on system performance and methanogenic activity. Based on the cycle-test result, the PAC-amended CFR (CFRPAC) demonstrated enhanced methanogenic activities, while SBRs exhibited slow methanogenic rates. However, activity assays indicated that SBRs were beneficial for organics removal in batch experiments fed with peptone. Taxonomic and functional analysis confirmed that CFRs were optimal for proliferating oligotrophs (e.g., Geobacter) and SBRs were more suitable for copiotrophs (e.g., Desulfobulbus). Metagenomic analysis revealed that CFRs had efficient acetate metabolic pathways from propionate and ethanol, whereas SBRs did not, resulting in the buildup of propionate. This study confirmed the enhancement of microbial syntrophy via PAC addition as well as the acclimation of electroactive bacteria (e.g., Geobacter) with complex substrates. Kinetic analysis of the potential rate limiting subprocesses was employed to simulate the anaerobic biodegradation of complex substrate and analyse their response towards differential substrate characteristics and PAC integration. Moreover, owing to the varying degrees of precision when using different kinetic models for the same dataset, multiple kinetic model examinations were conducted to improve the description and reliability of biodigestion kinetics, especially for complex substrates. Similarly, a variety of metrics, in addition to the commonly used correlation coefficient (R2) were also applied to compare the relative performances of these models. Complex substrate AD systems exhibited inter-composition inhibition; hence, poor hydrolytic rates compared to the single substrate systems, irrespective of substrate type and reactor configuration. It was inferred that the initiation of biomethanation was primarily influenced by the operational mode, rather than by PAC supplementation, even though PAC addition led to consistent shortening of the lag phases. Moreover, the kinetic characterization of the compositionally distinct substrates in this study indicates that PAC addition tended to linearize the usually sigmoidal cumulative CH4 curves, thereby reducing model fitness. Additionally, the higher R² values and minimal error function values observed in most experimental scenarios support the conclusion that employing the Modified Bertalanffy model (BTM) for estimating performance parameters exhibited a more precise simulation of both single and complex substrate biomethanation kinetics. Furthermore, the significant recurring under- and overestimation of lag phase (λ) and maximum CH4 production potential (Pmax) by the Modified Transference Model (MRM), despite its high goodness of fit, underscores the inadequacy of relying solely on goodness of fit as an assessment method. Essentially, even though no direct correlation could be established between kh/Rm′ and the methanation performance indicators (Pmax, Rmax and λ), the recorded ≤ 1 kh/Rm′ values, indicated hydrolysis was the rate limiting step except for peptone-fed SBRCON (kh/Rm′ of 1.128). High-resolution ecological analysis revealed that the majority of the front-end microbiome displayed significant sensitivity to PAC, whereas the back-end anaerobic digestion subprocesses of acetogenesis and methanogenesis were effectively facilitated. Moreover, notable rare taxa exhibiting characteristics and functionalities similar to those of the core microbiome in the respective reactors were identified and classified. The primary pathway in mesophilic complex substrate AD, particularly in CFRs, involved complex substrate hydrolysis followed by the successive acidogenesis of polysaccharides and amino acids, syntrophic acetate oxidization coupled with hydrogenotrophic methanogenesis (SAO-HM), and homoacetogenesis, ultimately producing acetate, H2, CO2, and CH4 as final products. Furthermore, methanogens were the overwhelming contributors of ATPase genes in CFRs while bacteria were primarily responsible for energy metabolism in SBRs. The metagenomic analysis further showed that both direct interspecies electron transfer (DIET) and acetoclastic pathways existed in the PAC-supplemented reactors. Genes involved in the CO2 reduction pathway were assigned to Methanothrix, Methanobacterium and Methanosarcina, indicating their possible involvement in the DIET process. Taken together, the results obtained suggested robust homoacetogenesis and SAO-HM activities despite the predominance of aceticlastic methanogenesis. This was particularly evident in CFRs and to a lesser extent PAC amended SBR. This study underscores the pivotal role of anaerobic digestion as a transformative biotechnology for the effective valorization of organic waste, contributing to renewable energy generation and climate change mitigation. By elucidating the mechanistic foundations of PAC-assisted biodegradation of complex substrates, this research advances the sustainability and efficiency of anaerobic digestion technologies. The variation of operational modes and the introduction of PAC have been demonstrated as effective strategies for regulating microbial activities, enriching functional microorganisms, promoting efficient electron transfer through ethanol oxidation, and directing key metabolic pathways. Moreover, the implementation of practical optimization strategies, such as a two-stage anaerobic system that spatially and temporally separates hydrolysis and acidogenesis from subsequent acetogenesis and methanogenesis, emerges as a feasible solution for enhancing PAC-assisted biodegradation of complex substrates. Overall, the findings of this study present significant implications for improving anaerobic digestion processes, reinforcing its role in sustainable waste management and renewable energy production.

Funder

Sustainable Energy Authority of Ireland

Publisher

University of Galway

Rights

CC BY-NC-ND

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University of Galway Theses (PhD Theses)