Impact of iron oxide nanoparticles on the anaerobic digestion process
Date of Issue2018
School of Civil and Environmental Engineering
Nanyang Environment and Water Research Institute
Anaerobic digestion (AD) is a practical option to address the energy depletion and pollution mitigation problems. The anaerobic digestion process is a multi-step reaction where complex organic substrates are converted to a mixture of gases such as methane and carbon dioxide. This conversion process is dependent on the concerted exchange of metabolites among five physiologically different microbial communities. Efforts to improve the anaerobic digestion process have included changes in reactor configurations as well as expensive physico-chemical interventions. Since the core of an AD process is its microcosm, changes in the microbial communities is thought to effect significant changes in the performance of anaerobic digestion. Hydrolysis is frequently the rate - limiting step in the anaerobic digestion of organic substrates and cellulose is frequently used as the model substrate in the study of the hydrolysis reaction. The key biochemical activities involved in the conversion of cellulose to methane and carbon dioxide include: hydrolysis, acidogenesis, acetogenesis and methanogenesis, which also simulates the entire anaerobic digestion process. The first phase of this study involved a preliminary investigation on the bio-augmentation effect of a pure culture of Dissimilatory Iron reducing Bacteria (DIRB) + iron oxide nanoparticles on the anaerobic digestion of feed sludge. Results obtained from this study led to the hypothesis that the hydrolysis event could be largely influenced by the addition of nanoparticles. This led to the next phase of the study which investigated the augmentation effects of specifically colloidal Iron Oxide Nanoparticles (IONp) on the anaerobic digestion of cellulose primarily to evaluate the hydrolysis potential as well as to study the effects of the IONp on the overall anaerobic digestion process. Metaproteomic analysis revealed increased Eubacteria metabolic activity in response to the IONp augmentation, which resulted in increased enzyme/protein secretion into the mixed liquor. This biochemical event was accompanied by a large increase in soluble COD (837.5 mg/L in the IONp augmented test reactor and 358.4 mg/L within the control) in the early phase of the anaerobic digestion process. Relative and absolute quantification of the secreted proteins revealed a collection of hydrolytic enzymes such as Endoglucanase, Hydrolases, Peptidases and Proteases had been significantly expressed by microorganisms associated with hydrolytic activity such as Kluveromyces lactis, Bacillus cereus, Pseudomonas sp., Salmonella sp, Yeasts and Fungi in the IONp augmented system in comparison to the control. Increased acidogenic activity could result in metabolic imbalances among the microbial communities which may further manifest as an accumulation of reduced fatty acids and well as hydrogen leading to process AD instability and failure. However, the results of this study indicated that in the presence of IONp, a well-balanced microbial syntrophy among the fatty acid oxidizing bacteria and the methanogens was promoted as evidenced by the comparatively reduced amounts of propionic acid (on day 7, 238.5mg/L in the IONp test reactor and 540mg/L in the control and on day 23, 14.8mg/L in the IONp test reactor and 579.7 mg/L in the control reactor) as well as negligible amount of hydrogen gas detected in the IONp augmented test reactors. Improved hydrolysis together with a well-balanced microbial syntrophy led to the overall improvement of methane production in the IONp augmented test reactors. A significant increase in the Eubacteria 16S rRNA gene copy numbers (p < 0.001) had also been noted in the early phase of the anaerobic digestion process. Among the Methanogen communities, it was observed that Methanomicrobiales and Methanosaetaeceae were the dominant methanogens. This indicated that both hydrogenotrophic and acetoclastic methanogenesis had been facilitated in the presence of the nanoparticles. In this study, it was also noted that the addition of IONp led to the establishment of three distinct microbial phases: microbial stress and cell death of approximately one order of magnitude primarily due to oxidative stress and nanoparticle deposition of the cell surface, followed by microbial rewiring via the production of microbial nanowires and intercellular nanotubes, and recovery. This study presents evidence of intercellular nanotube communication within an environmental sample – i.e., anaerobic microbial consortium subjected to stress. This observation suggested a mode of microbial communication within the anaerobic process not previously explored and which could have implications on anaerobic bioprocess design. Taken together, these results suggest a promising potential for the use of iron oxide nanoparticles as a relatively simple and economically viable intervention to improve both the rate-limiting hydrolysis as well as microbial syntrophy in the anaerobic digestion process.