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Department of Urban and Environmental Engineering (Environmental Science and Engineering)Anaerobic digestion (AD) is considered a viable method for treating food waste (FW) because it biologically converts organic waste into biogas. However, FW has unique characteristics that complicate its stable AD over the long term, for example, seasonal variations in the production and composition of FW. Korean FW is characterized by a high content of vegetables and fruits (approximately 55%) that are rich in dietary fiber, which can cause difficulties in hydrolysis and, thus, degradation of FW. The amount of FW increases dramatically during the ???Kimjang??? season that mainly Napa cabbage accounts for approximately 20% of the total FW produced in Korea. This significant seasonal variation in the composition of FW recurs annually, and it could influence the performance and stability for the AD of FW. Nonetheless, few studies have systematically investigated the effect of this variation on the performance of AD and methods to enhance the stability of FW digesters suffering the issue of seasonal variations in FW. Bioaugmentation is a method for improving the degradation of organic pollutants through the addition of exogenous microorganisms that can degrade the target compounds in situ. Therefore, the selection of appropriate microorganisms that can thrive and retain the desired metabolic properties in a given environment is an important factor in bioaugmentation. Bioaugmentation has been suggested as a promising strategy for enhancing the performance of AD at the microbial community level. Rumen provides a favorable environment for the formation and development of a naturally formed anaerobic microbial community consisting of metabolically versatile microorganisms, and rumen microorganisms are a good source of hydrolytic bacteria capable of decomposing complex matters, including fibers. Additionally, rumen microorganisms contain acidogens and methanogens, and they produce methane as the final product of biodegradation, similar to AD microbial communities. These characteristics make it feasible to use rumen microorganisms as an exogenous microbial source for bioaugmentation of AD processes. In this doctoral research, the bioaugmentation potential of rumen culture for enhancing the biomethanation of Korean FW was examined, with emphases on increase in substrate digestibility and long-term stability of the bioaugmented process. In study 1, the potential of rumen fluid (RF) as a bioaugmentation source was first examined in batch tests. RF and two cellulolytic Clostridium species were tested in different combinations and various seeding ratios to determine the optimal bioaugmentation source and ratio by using simulated Korean FW. Then, a continuous experiment employing the optimal bioaugmentation condition determined in the batch test (10% RF to the inoculated anaerobic sludge on a volatile suspended solids (VSS) basis) was performed using the same substrate. The experimental results indicated that bioaugmentation with RF effectively enhanced the biomethanation of FW in both batch and continuous modes. The microbial community structures, especially bacterial community structures, shifted significantly after the introduction of RF. Therefore, it was found to be possible to alter the composition and function of microbial communities and, thus, to enhance the biomethanation of FW through bioaugmentation with RF. In study 2, for comparison, the aforementioned bioaugmentation strategy was applied to single- and two-phase processes for treating real Korean FW. In the two-phase process, the amount of RF to be added to the acidogenic reactor was determined based on the VSS concentration in the reactor (i.e., smaller amount of RF compared to that used in the single-phase process) to test the possibility of reducing the consumption of RF, which is a relatively scarce resource. Both processes were operated at varying organic loading rates (OLRs, 0.5???6.0 g volatile solids (VS)/L??d) without pH control. Both processes showed comparable methanogenic performances at OLRs ???4.0 g VS/L??d, and the acidogenic reactor maintained stable production of volatile fatty acids (mainly lactate) and ethanol, despite the highly acidic pH ???3.4. However, the single-phase process achieved stable AD performance with an increased OLR 5.0 g VS/L??d, whereas the two-phase process failed. These results can be ascribed to the provision of a more favorable environment for syntrophic interactions between acidogens and methanogens and the addition of more amount of RF in the single-phase process. Consequently, the single-phase configuration was selected for the subsequent long-term experiment because achieving stable and robust performance is important for AD plants. Study 3 focuses on the feasibility of bioaugmentation with rumen culture (RC) as a strategy to enhance the biomethanation of FW in batch and long-term continuous experiments. Batch tests were conducted for three inocula (i.e., anaerobic sludge with FW, RC-inoculated RF with FW, mixed culture of anaerobic sludge and RC with FW) with Napa cabbage (i.e., simulated kimjang waste (KW)) and cellulose. The results of the three subculture cycles indicated that the mixed-culture inoculum provided a higher biogas yield than the other inocula, indicating that bioaugmentation with RC has the potential for enhancing the biomethanation of fiber-rich FW. Then, bioaugmentation with RC was examined in the continuous experiment with fluctuations by adding KW into FW (0???20% of the total substrate VS). The results demonstrated that bioaugmentation with RC effectively increased the biomethanation of FW (by 12.3% increase in methane yield compared to the control without bioaugmentation), especially after the addition of KW. Changes to the microbial community structure corresponding to bioaugmentation and adaptation to fluctuations in substrate composition, such as the emergence of hydrolytic/acidogenic bacteria originating from the RC and the dominant shift to hydrogenotrophic methanogenesis, were observed. Importantly, the bioaugmented microbial populations seemingly remained active and helped sustain the enhanced AD performance in the long-term experiment (>38 months). Therefore, the proposed bioaugmentation strategy proved to be effective for improving the robustness and resilience of an FW digester in terms of handling seasonal fluctuations in FW composition and loading. In conclusion, this study verified that bioaugmentation with RC is a practical tool for enhancing the AD of Korean FW in terms of energy production and process stability. Moreover, long-term effectiveness of the bioaugmentation strategy was demonstrated in the continuous mode with varying fractions of KW (i.e., simulated seasonal variations). The findings of this study will be useful for managing AD plants that treat FW and significantly add to the literature on this topic.clos

Enhancing anaerobic digestion of vegetable waste and cellulose by bioaugmentation with rumen culture

Anaerobic digestion (AD) has been widely used to valorize food waste (FW) because of its ability to convert organic carbon into CH4 and CO2. Korean FW has a high content of fruits and vegetables, and efficient hydrolysis of less biodegradable fibers is critical for its complete stabilization by AD. This study examined the digestates from different anaerobic digesters, namely Rs, Rr, and Rm, as the inocula for the AD of vegetable waste (VW) and cellulose (CL): Rs inoculated with anaerobic sludge from an AD plant, Rr inoculated with rumen fluid, and Rm inoculated with anaerobic sludge and augmented with rumen fluid. A total of six conditions (3 inocula x 2 substrates) were tested in serial subcultures. Biogas yield was higher in the runs inoculated with Rm than in the other runs for both VW (up to 1.10 L/g VS added) and CL (up to 1.05 L/g VS added), and so was biogas production rate. The inocula had different microbial community structures, and both substrate type and inoculum source had a significant effect on the formation and development of microbial community structures in the subcultures. The overall results suggest that the bioaugmentation with rumen microbial consortium has good potential to enhance the anaerobic biodegradability of VW, and thereby can help more efficiently digest high fiber-content Korean FW

Continuous treatment of dairy effluent in a downflow anaerobic filter packed with slag grains: Reactor performance and kinetics

A downflow anaerobic filter packed with blast-furnace slag (BFS) grains was evaluated for treating cheese whey, a high-strength dairy effluent. The reactor demonstrated effective and stable biomethanation of whey in continuous mode without pH control. The chemical oxygen demand (COD) removal remained at around 80%, with the reactor pH being maintained neutral without buffering, in the organic loading rate (OLR) range of 0.8-2.4 g COD/l∙d. Although the COD removal decreased down to below 60% at higher OLRs along with acid accumulation, the methane production rate continued to increase with increasing OLR. On the other hand, the methane yield was maintained at 0.28-0.34 l/g COD throughout the experiment. The substrate removal and methane production kinetics were successfully described by the modified Stover-Kincannon model. The overall results suggest that BFS grains could be used as economic filter media for anaerobic treatment of cheese whey and other easily acidifying waste streams.clos

Treatment of low-strength ammonia wastewater by single-stage partial nitritation and anammox using upflow dual-bed gel-carrier reactor (UDGR)

This study investigated the single-stage partial nitritation and anammox (S-PNA) treatment of low-strength ammonia wastewater (<= 140 mg NH4+-N/L). Upflow dual-bed gel-carrier reactor (UDGR) with polyvinyl al- cohol cryogel biocarriers, developed in this study, was employed for the anammox biomass enrichment from conventional activated sludge and subsequent S-PNA experiments. Anammox biomass was successfully enriched from conventional activated sludge. The enriched anammox carriers were inoculated together with gel carriers containing nitrifying sludge into the S-PNA reactors. S-PNA activity developed rapidly, and the nitrogen removal efficiency and rate reached up to 90.1% (with complete ammonia removal) and 0.15 kg N/m(3).d, respectively, under low nitrogen loading conditions (0.10-0.17 kg N/m(3).d). The microbial community structure changed significantly while adapting to anammox and S-PNA conditions. Anammox was likely driven solely by a Candidates Jettenia population accounting for <= 49.4% of bacterial 16S rRNA genes. The results demonstrate that the UDGR-based S-PNA is suitable for treating low-strength wastewater

A comparative study of single- and two-phase anaerobic digestion of food waste under uncontrolled pH conditions

This study compared single- versus two-phase systems for semi-continuous anaerobic digestion of food waste without pH control at varying organic loading rates (OLRs). The methanogenic reactors of both systems required trace element supplementation for stable operation at 3.0 g VS (volatile solids)/L.d or higher OLRs. Under trace-element supplemented conditions, both systems achieved stable and efficient performance at OLRs up to 4.0 g VS/L.d. The two-phase system outperformed the single-phase system at 1.0-4.0 g VS/L.d OLRs, but it failed at an OLR of 5.0 g VS/L.d. Meanwhile, the single-phase system maintained the stable performance and reached its maximum methane production at this OLR. These results suggest that a single-phase configuration is more advantageous for robust treatment of food waste without pH control at high organic and hydraulic loads. Hydrogenotrophic methanogens dominated the methanogen community throughout the experiment in both systems. Microbial community structure shifts correlated with reactor operation and performance characteristics