Operation Strategy Of Biofilm-Based And Granule-Based Suspanded Bed Hydrogen-Producing Reactors

Hydrogen is a clean, effetive and renewable energy, and is a satisfactory alternative for fossile hydrocarbon fuels in the future. Biohydrogen from waste is attactive due to energy recovery and environmental cleanup at the same time. Improving hydrogen-producing capacity and reducing the cost are the key factor to realize industrialization. One of the most effective ways is to employ high effecient reactor systems and obtain higher active biomass concentration. As compared to cell suspended growth systems, cell attached growth expanded bed system has been noted more efficient in obtaining higher biomass concentration and mass transfer in low HRT. Nevertheless, the research and application of this reactor is scarecely reported. Accordingly, deep understanding of the principles and applying techniques of the cell attached growth expanded bed hydrogen production reactor is quite necessary. The present research attempted to investigate the strategy of rapid startup and stable operation mainly on biofilm-based and granule-based expanded bed hydrogen production processes. Moreover, higher hydrogen production ability and biomass concentration are desired as well as stable operation.Aiming at solving the above problems, this thesis focused on startup and operation strategy and the factors, which affected the processes of biofilm-based and granule-based hydrogen-producing expanded bed reactors. The results showed organic loading rate (VLR) was a significant parameter in reactor control and hydrogen production, especially in fermentation types formation. When VLR was controled at 4 kgCOD/m3·d and 8 kgCOD/m3·d, the reactors achieved successful startup. Ethanol-type fermentation formed under the VLR of 8 kgCOD/m3·d.In order to enrich hydrogen production bacteria and establish better communities in mixed culture reactors, inoculation at start-up stage need pretreatment. Four pretreatment methods, this is, heat-shock, acid, alkaline and repeated-aeration pretreatment, were conducted. The heat-shock, acid and repeated-aeration pretreatment operation had effectively suppressed the methanogenic activity in the mixed culture, and the alkaline pretreatment was unsuccessful at methanogenesis repression. Different pretreatment methods could induce different fermentation types. Ethanol-type fermentation was observed by repeated-aeration pretreatment. The different pretreatment methods could affect the formation of microbial communities by the evaluation of denaturing gradient gel electrophoresis (DGGE) profiles. The results indicated that the pretreatment methods had probably led to the differences in the initial microbial communities, which were directly responsible for different fermentation types and hydrogen yields.Inoculated by repeated aeration pretreatment sludge, started at VLR of 8 kgCOD/m3·d, the biofilm-based expanded bed reactor performed well in 150 d operation, and the average hydrogen production rate was 0.71 L/L·h.Studies showed that hydrogen-producing granule formation is key factor to successful startup of granule-based expanded bed reactor. Inoculation source, the size and amount of the initial carrier, and VLR all affected microbial granulation. Mixture of anoxic sludge and anaerobic sludge was better choice for inoculation. VLR increase was favorable for granulation. It was also observed that quick decrease of HRT was effective in rapid granulation which increase of substrate concentration. The granulation period was shortened by 66.7 % using quick HRT decrease technique. Central composite design (CCD) using response surface methodology was applied to study the interactive effects of carrier size and amount on hydrogen producing bacteria granulation. The optimized results showed when granule size of 0.68 mm and inoculation ratio of 5.92 %, the predicted hydrogen production rate was 13.47 L/d, and the average biomass concentration was 15.82 g/L. According to the results of the statistical design, the verified tests were conducted in triplicate tests. The results showed that the excellent correlation between predicted and measured values verifies the model validation and existence of an optimal point.A granule-based expanded bed reactor was operated according to the optimized conditions obtained. Results showed that hydrogen-producing granule was observed at the 20 d from startup, and the maximum hydrogen production rate was 1.07 L/L·h with the average biomass concentration of 24.1 g/L. The settling velocity increased with granule size in the reactor. The granule size and the settling velocity was in an quadratic polynomial relationship, which could be expressed as Y = 33.93x2 - 8.453x + 16.58. The fitting index, R2 = 0.998, which suggested that the model was quite accurate to reflect the relationship of hydrogen producing granule size and the granule settling velocity.Glucose as substrate, startup at respectively optimized operating conditions, comparison tests were conducted to biofilm-based and granule-based expanded bed hydrogen-producing reactors. The results showed that when HRT of 1.0 h, the corresponding glucose concentration of 40 g/L, the biofilm-based expanded bed achieved maximum hydrogen production rate of 6.54 L/L·h; when HRT was 1.5 h, the corresponding glucose concentration of 60 g / L, the granule-based expanded bed reactor achieved maximum hydrogen production rate of 6.85 L/L·h. Moreover, when HRT of 1.0 h and the corresponding glucose concentration of 40 g/L, the two reactors obtained the maximum hydrogen yield of 1.69 mol/mol-glucose and 1.54 mol/mol-glucose, respectively. There was no significant difference observed in the average biomass in the two reactors. The average biomass concentration was all in a range of 25~29 g/L. The microbial communities were investigated by means of DGGE and Confocal Laser Scanning Microscope (CLSM) techniques. Results showed that the polysaccharide was unevenly distributed in the biofilm, and the concentration was less. However, in the hydrogen-producing granule, the polysaccharide was more evenly distributed with a high concentration. The microcosm was well established in the biofilm and the hydrogen-producing granule. There were more active hydrogen-producing bacteria detected in the outer section of the granule compared to the inner section, which was responsible for high effective hydrogen production.For long-term operation of the biofilm-based reactors, the regular supplement of the large amount of carrier should be taken into consideration. In contrast, the granule-based reactors would not to consider these shortcomings. As a result, based on the consideration of lower the operating costs and simplifying operational complexity, granule-based expanded bed reactor was recommended as better choice for a continuous flow reactor for large scale production of hydrogen.