Department of Urban and Environmental Engineering (Environmental Science and Engineering)Anaerobic digestion (AD) is widely applied in the treatment of various organic wastes or wastewaters, and biogas (mainly CH4 and CO2) produced from AD is considered a promising renewable energy source. Producing methane rich biogas from waste biomass through AD is considered a viable option for sustainable energy supplies. Macroalgae, particularly harmful and/or inedible types, have recently emerged as an attractive feedstock for biogas production. Macroalgal biomass is rich in easily biodegradable organics and contains little lignin, which is advantageous for efficient bioconversion. Most previous studies on the AD of macroalgal biomass have tested edible seaweed species with high agricultural value, reducing the economic feasibility of using macroalgae as a feedstock for biogas production. Ulva is a macroalgal genus noted for causing harmful green tides worldwide. Green tides have significant environmental and economic consequences related to the accumulation and decay of bloomed algal biomass. Given that macroalgal blooms have become more frequent and severe with global warming, biomethanation of Ulva biomass is considered environmentally and economically appealing.
However, there are practical limitations for using Ulva biomass as a sole feedstock; these limitations are primarily related to its high nitrogen and sulfur contents relative to carbon content. A low C:N ratio can result in an accumulation of free ammonia, which severely inhibits methanogenesis. Particularly, the high sulfur content of Ulva biomass can cause an excessive production of hydrogen sulfide (H2S) through dissimilatory sulfate reduction by sulfate reducing bacteria (SRBs) during AD. H2S is toxic to methanogens and other microorganisms involved in AD, and additionally, SRBs compete directly with methanogens for common substrates (i.e., H2 and acetate). The rigid macroalgal structures and seasonal variation in production are also characteristics of Ulva biomass that limit its use as a biomethanation feedstock. In this doctoral thesis, the AD of sulfur rich Ulva biomass was investigated to achieve stable and robust methanogenic performance, with particular emphases on methanogenesis and sulfidogenesis.
In Study 1, the mono digestion of Ulva biomass was investigated with various reactor configurations of batch, repeated batch, continuous stirred tank reactor, and sequencing batch reactor. The results of these experiments demonstrated that Ulva had a high potential as feedstock for biogas production through AD in terms of organic removal and methane productivity. However, H2S production at high levels negatively affected methanogenesis, particularly during long term continuous operation. It was therefore suggested that sulfide control is important for stable Ulva AD
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in continuous mode.
In Study 2, co--digestion of Ulva biomass with cheese whey was investigated as an approach to reduce sulfide production and to mitigate the limitations of Ulva as a sole substrate (e.g., low C/N ratio and seasonality). The co--digestion experiments were performed in continuous reactors with the substrate mixing ratio implemented in various manners: one with gradual increase and the other with gradual decrease in the Ulva fraction in the substrate mixture. The co--digestion with cheese whey resulted in an enhancement of methanogenic performance with reduced sulfidogenesis compared to the mono--digestion of Ulva biomass. The optimal mixing ratio between Ulva biomass and cheese whey for co--digestion was determined based on the methane recovery and treatment capacity. However, a considerable amount of H2S was still produced in the co--digestion reactors, suggesting the need for further research on effective sulfide control.
In Study 3, promoting direct interspecies electron transfer (DIET) between electro--syntrophic partners involved in AD was attempted to further enhance the biomethanation of Ulva biomass with cheese whey in a mixture (at the optimal mixing ratio determined in Study 2). Recent studies have reported that electrically conductive material, such as magnetite (Fe3O4), serve as electrical conduit and stimulate DIET between electroactive exoelectrogenic fatty acid oxidizers and electrotrophic methanogens, thereby accelerating methanogenesis. This suggests that it may be possible, by adding conductive material, to increase electron flow to methanogenesis rather than to sulfate reduction using DIET. To examine such a possibility, the performances of the continuous co--digestion reactors with or without magnetite addition were comparatively analyzed. Interestingly, by adding magnetite, nearly complete removal of H2S was achieved in situ, whereas no significant effect was observed on methane production. It was experimentally proven that H2S was removed by the microbial oxidation to elemental sulfur (S0). Based on the microbial community analysis results and thermodynamic calculations, the newly found fate of sulfur under anaerobic conditions is proposed to be driven by a novel electro--syntrophic association between anaerobic sulfide--oxidizing bacteria and electrotrophic methanogenesis. This finding suggests the new possibility for in situ sulfide control and S0 recovery in continuous AD of sulfur--rich biomass.
In conclusion, this PhD study examined the biomethanation of sulfur--rich Ulva biomass and suggests different approaches for enhancing methanogenic performance and process stability in continuous systems. Furthermore, a novel strategy for in situ H2S control by magnetite--promoted DIET is proposed. The stimulation of anaerobic sulfide oxidation to S0 via DIET in the presence of magnetite (or other conductive materials) is a remarkable observation that has not been reported.
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The findings of this study has introduced a novel dimension in anaerobic sulfur metabolism and the possibility of in situ H2S control by promoting DIET in sulfur-rich methanogenic environments.clos
