Strategies for enhancing propionate degradation in anaerobic digestion systems

Anaerobic digestion (AD) is a sustainable biotechnology for converting organic waste into renewable bioenergy in the form of methane. However, the accumulation of volatile fatty acids—particularly propionate—often causes process instability and even reactor failure. Propionate degradation depends on the interaction between syntrophic propionate-oxidizing bacteria (SPOB) and methanogens operating under thermodynamic constraints. The slow growth rate and environmental sensitivity of SPOB hinder their enrichment, and their population dynamics could be influenced by operational conditions such as solids retention time (SRT), operational mode, and organic loading rate (OLR). Understanding how operational strategies shape the ecological selection and activity of SPOB is therefore critical for linking microbial community assembly to the optimisation of propionate degradation in anaerobic systems. This study aimed to develop an integrated ecological and engineering framework to enhance propionate degradation and methanogenic stability in AD systems. Specifically, the objectives were to: (i) establish a novel AD mathematical model incorporating the r/K selection theory to describe microbial population dynamics and propionate removal under varying operational conditions; (ii) experimentally investigate how operational mode (continuous-flow reactors (CFRs) vs. sequencing batch reactors (SBRs)), SRT, and organic carbon compositions (mixed vs. sole propionate) influence propionate degradation, syntrophic interactions, and microbial community assembly; and (iii) explore the effects and mechanisms of granular activated carbon (GAC) in mitigating propionate accumulation under elevated OLR conditions. A mathematical model integrating the r/K selection theory was first established to mechanistically link microbial life-history strategies with operational parameters. The model distinguished r- and K-strategist functional guilds based on their kinetic and stoichiometric characteristics, providing an ecological perspective on microbial succession in AD systems. Simulation results indicated that K-strategists predominated under extended SRTs in completely mixed reactors, whereas r-strategists dominated under shorter SRTs and SBRs. The interplay between SRT and substrate availability was identified as a key driver of microbial competition and reactor performance. Sensitivity analysis further highlighted SRT and the first-order decay rate as dominant parameters affecting the growth of SPOB and methanogens. Guided by these theoretical insights, reactor-scale experiments were conducted to evaluate how operational mode, SRT, and organic carbon composition affect propionate degradation and microbial community structure. Long SRTs and SBRs markedly enhanced methane yield and chemical oxygen demand (COD) removal efficiency, whereas reactors with short SRTs showed limited propionate removal across all conditions. Reactors fed with mixed carbon sources (ethanol/acetate/propionate) exhibited the shorter lag phases and higher maximum methane production rates compared with those using sole propionate. SBRs enriched Syntrophobacter (16.0%–22.8%) and Desulfobulbus (4.1%–5.0%), whose relative abundances in CFRs were only 4.1%–13.5% and 1.0%–1.2%, respectively. Based on these findings, SRT and feeding mode were strategically manipulated to enrich SPOB according to the r/K selection theory. Extended SRTs enabled complete COD removal in both SBRs and CFRs, compared with limited efficiencies under short SRTs (21.6%–39.1%). Microbial community analysis revealed a shift from the putative r-strategist Desulfobulbus (up to 3.3%) under short SRTs to the putative K-strategist Syntrophobacter (up to 2.2%) under extended SRTs. Notably, the combination of long SRTs and SBR mode facilitated co-enrichment of Pelotomaculum (5.1%) and Syntrophobacter, forming a complementary syntrophic consortium that enhanced propionate oxidation. To further improve system resilience under high-load conditions, the effects of conductive materials (CMs) supplementation were examined. The addition of GAC increased methane yield and propionate removal by 24.9% and 16.2%, respectively, at an OLR of 7 g COD/L/d. GAC addition maintained higher microbial diversity and enriched electroactive syntrophs such as Smithella and Mesotoga. Functional prediction based on PICRUSt2 suggested that GAC may enhance interspecies interactions and stress tolerance by promoting extracellular electron transfer and reinforcing bacterial membrane stability. Predicted genes associated with electron transfer processes increased by 14.3–65.8% in the GAC-amended systems, indicating that CMs may facilitate robust syntrophic metabolism under environmental stress. Overall, this research establishes a comprehensive ecological and engineering framework for mitigating propionate inhibition in AD. By integrating the r/K selection theory, operational control, and CMs facilitation, this study advances the mechanistic understanding of SPOB–methanogen cooperation, provides an ecological basis for adaptive reactor design, and offers practical strategies to achieve stable, high-rate, and resilient anaerobic systems for sustainable waste-to-energy conversion.

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University of Galway

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CC BY-NC-ND

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University of Galway Theses (PhD Theses)