| Global growing concerns about energy shortages and the environmental pollution have led to worldwide use of renewable energy. Hydrogen is an efficient and environmentally friendly energy carrier, which could play an important role in the reduction of emissions of greenhouse gases. Photo-fermentative hydrogen production have been studied extensively as one of the most promising processes, due to the solar energy utilization, organic pollution reduction and renewable energy production. In spite of high theoretical hydrogen yield, the actual hydrogen yield in continuous photobioreactor especially in large scale photobioreactor is very low. Therefore, enhacing hydrogen production efficiency and operation stability of photobireactor are the bottlenecks to commercialize photofermentative hydrogen production process.In this paper, material flow analysis of feedstock in continuous photobioreactor showns that due to the poor aggregation, photofermentative bacteria (PFB) cannot be efficiently separated from supernatant and rushed out with effluent continuously. To replenish the biomass washout, most of feedstock was continuously utilized for cell growth rather than hydrogen production, which caused low feedstock conversion efficiency. Consequently, the maximum hydrogen yield was1.89mol H2/mol acetate. The results showed that poor aggregation of PFB resulting in low biomass retention capacity of photobioreactor was the origin of low feedstock conversion efficiency. In order to enhance separation efficiency of PFB and effluent, a novel photofermentative membrane bioreactor was developed to retain bacteria with high hydrogen productivity in reactor through membrane separation. In first four cycles of operation, total hydrogen yield reached a higher value of3.02mol H2/mol acetate. However, with the increase in running time, the PFB enter the death phase, so hydrogen yield decreased rapidly. In addition, a significant portion of the energy had to be used for membrane separation, which decreased the net energy production. Moreover, with the occurrence of membrane fouling, energy consumption by photofermentative membrane bioreactor increased further.Based on shortcomings of traditional cell immobilization technology and special requirements of PFB on solid carrier, Activated carbon fibers (ACF) were firstly applied as fluidized solid carrier to immobilize photo-fermentative bacteria for hydrogen production. The maximum yield (3.05mol H2/mol acetate) and rate (33.01mL-H2/L-medium/h) of hydrogen production were obtained, using specific surface area (1500m2/g), length (1mm) and amount (0.8g/L) of ACF. In contrast with the conventional solid carriers, the ACF have been fluidized in the culture during the stirrer operation and each bacteria immobilized on the ACF could well absorb the light energy and convert substrate into H2. According to the fluidization and precipitation characteristics of ACF, photo-fermentative sequencing batch reactor (PFSBR) process was designed to continuously produce hydrogen. The PFSBR reached a maximum hydrogen yield (3.12mol H2/mol acetate) and hydrogen production rate (690mL-H2/L-medium/d), under the optimum HRT of144h and influent substrate concentration of60mmol/L. To further enhance the performance of solid carrier, surface modified ACF were developed by HNO3oxidation followed by steam explosion. The surface morphology and functional groups of activated carbon fibers were modified by HNO3oxidation followed by steam explosion, which greatly increased the immobilization capacity and allowed attachment of more photo-hydrogen producer on the surface. In the continuous operation, hydrogen production performance was enhanced to722mL-H2/L-medium/d and3.24mol H2/mol acetate by PFB immobilized on surface modified ACF.Aggregation plays a vital role in the separation and settling of microorganism. To enhance PFB aggregation ability, L-cysteine and Ca2+were screened from various factors influencing the microbial aggregation ability to trigger the aggregation formation of PFB. The conditions for hydrogen production by aggregation were optimized at different carbon source, nitrogen source, substrate concentration and carbon to nitrogen ratio. Through formation of disulfide bonds, L-cysteine not only promoted production of EPS, in particular the secretion of protein, but also stabilized the final confirmation of protein in EPS. Cell surface covered by EPS have been changed by L-cysteine, thus absolute Zeta potential decreased with L-cysteine, and total interaction energy barrier decreased and reached minimum at1.0g/L. Consequently the aggregation ability of PFB reached40.86%at1.0g/L of L-cysteine. Through reducing the absolute Zeta potentials of PFB, Ca2+greatly decreased total interaction energy barrier of PFB based on DLVO theory, thus promoted the aggregation of Rhodopseudomonas faecalis RLD-53. The aggregation ability of PFB reached28.85%at6mmol/L of Ca2+. With the increase of stirring rate, the effective collision between bacterial cells increases, which is conducive to the formation of floc, while Excessive stirring will lead to intense hydraulic shear, which will destroy the structure of floc and decrease the aggregation ability. Based on these findings, a novel process for hydrogen production by aggregation of photo-fermentative bacteria was developed. Under the optimum conditions at HRT of96h, organic loading rate of15mmol/L/d and light intensity of200W/m2, the maximum hydrogen production rate and yield were1043mL-H2/L-medium/d and3.35mol H2/mol acetate.Combining dark-photo fermentation with complementary capabilities could increase the hydrogen production and achieve the cascade utilization of biomass. To overcome complicated operation conventional coupling mode and imbalance of cell growth and metabolic rate of two types bacteria, integrated dark-photo fermentative bioreactor was designed to make dark and photo fermentative bacteria relatively isolation in spatial niche but metabolic substrate free pass. In the ratio of dark to photo chamber volume at1:4, phosphate concentration at20mmol/L, substrate concentration of8g/L and dark and photo fermentative bacteria inoculation ratio1:20, volatile acids accumulation decreased and hydrogen yield of4.96mol H2/mol glucose. The results provide scientific basis to further enhance the biological hydrogen production and cascade utilization efficiency of biomass. |
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