Transport Characteristics In Biofilm Photobioreactor With Photosynthetic Bacteria And Enhancement Of Hydrogen Production Performance

Hydrogen is one of the ideal energy carriers because it has numerous advantages ofexcellent combustion performance, cleanness and high efficiency. Photosyntheticbacteria can convert solar energy into hydrogen using water and simple organiccompounds as the hydrogen sources, and enable carbon dioxide reduction and wastetreatment, therefore, the technology of hydrogen production by photosynthetic bacteriaas an emerging bio-energy technology. At present, however, this technique is still in thestage of laboratory research and lags behind the industrial scale due to the slowerhydrogen production rate and lower light energy conversion efficiency. In order toimprove the performance of hydrogen production by photosynthetic bacteria, thecombination of biofilm technique and hydrogen production by photosynthetic bacteriais an effective way. In the process of the degradation of organic matter to producehydrogen by biofilm with photosynthetic bacteria, firstly, the organic substrate in theculture solution diffuses into the biofilm from the bulk liquid, and then, the organicsubstrate are degraded by biofilm with photosynthetic bacteria, at last, the generatedmetabolites, such as hydrogen and carbon dioxide, are is transmitted back to the bulkliquid. Thus, the process of mass transfer within the biofilm has a very significant effecton the hydrogen production performance of the biofilm with photosynthetic bacteria.In this study, focusing on the technology of hydrogen production by biofilm withphotosynthetic bacteria, the effect of different operational conditions including growthstages, flow rate and substrate concentration on the substrate distribution andmorphology characteristics of biofilm with photosynthetic bacteria were investigatedthrough the developed system with flat plate biofilm photobioreactor for hydrogenproduction. Base on the process of mass transfer in flat plate biofilm photobioreactor,the distribution of the pores and the cells were obtained through the treatment ofmicrostructure images of biofilm with photosynthetic bacteria obtained by the confocallaser scanning microscopy, and then the mass transfer and biochemical reaction kinetictheory were utilized for the development of the transport model of the flat plate biofilmphotobioreactor to predict the characteristics of the substrate degradation of thephotobioreactor and the substrate distribution within the biofilm under differentoperational conditions. In addition, in order to improve the hydrogen production performance of the biofilm with photosynthetic bacteria, in this work, the lightdistribution characteristics, biomass concentration and the biofilm formation propertieswere enhanced through the introduction of optical fiber technology, the development ofcomposite surface and the applications of biofilm formation under the nonsaturatedliquid phase. The main results were summarized as follows:①The change of morphology of biofilm with photosynthetic bacteria wasinvestigated in the biofilm formation stage, and the effect of the operational conditionsincluding the substrate concentration and flow rate on the morphology of biofilm withphotosynthetic bacteria was explored. The experimental results showed that: with theincrease of start-up time, the porosity in the biofilm with photosynthetic bacteriareduced and the structure of biofilm became denser. Moreover, the undersizeconcentration of the substrate limited the growth of biofilm with photosyntheticbacterial, reducing the biomass concentration of the biofilm; the oversize concentrationof the substrate led to looser structure of the biofilm. In addition, due to the influence ofthe shear force, biofilm with photosynthetic bacteria is easy to partial loss inhigh flowvelocity②Based on the process of mass transfer in the flat-panel biofilm photobioreactor,the distribution of pores and cells in the biofilm were found out by the treatment ofmicrostructure images of biofilm with photosynthetic bacteria obtained using confocallaser scanning microscopy, thereby the effects of operational conditions including lightintensity, temperature and pH on the transport and degradation characteristics ofsubstrate in flat-panel biofilm photobioreactor were analyzed using the transport modelof flat-panel biofilm photobioreactor constructed based on theory of mass transfer andbiochemical kinetics. Results showed that the modeling consequence could fitexperimental value well. The biofilm activity of photosynthetic bacterial was the highestand the degradation performance was the best at pH of7, illumination intensity of6000lx and temperature of30℃, at which condition, the substrate concentration wasminimum and hydrogen concentration was maximum.③An optical fiber bundle biofilm photobioreactor was developed. In this design,the optical fiber was used as the light conduit to increase the uniformity of light in thephotobioreactor, enhancing the hydrogen production performance of photobioreactor.The experimental results revealed that the hydrogen production performance of theoptical fiber bundle biofilm photobioreactor first increased and then decreased with the increases of hydraulic retention time and substrate concentration. At the hydraulicretention time of12h and the substrate concentration of50mmol/L, the maximumhydrogen production rate and the light conversion efficiency were12.1mmol/m~2/h and23.3%, respectively.④An optical fiber with additional rough surface was developed to enhance thespecific surface area of photobioreactor, enhancing the biomass per unit volume of thephotobioreactor. The experimental results revealed that the development of additionalrough surface enhanced the performance of hydrogen production of biofilmphotobioreactor with photosynthetic bacteria. Moreover, it was also confirmed that thedevelopment of the additional rough surface in the hydrogen production system causedan excellent stable operation of by the continuous operation experiments. In addition,when the substrate concentration was maintained at60mmol/L，flow rate was at30mL/h，the pH value was at7.0，and the temperature was30℃, the maximum hydrogenproduction rate, substrate degradation rate, substrate degradation efficiency and lightconversion efficiency were1.75mmol/L/h,10.8mmol/L/h,75.0%and9.3%,respectively.⑤The biofilm with photosynthetic bacterial formed in an unsaturated liquidphase condition was proposed to enhance the characteristics of biofilm formation andshorten the time of biofilm formation using the notion that the probability of themicroorganism migration from liquid region to the surface of solid matrix is inverselyproportional to the thickness of the liquid region. The experimental results verified thatthe biofilm with photosynthetic bacteria formed in the unsaturated liquid phasecondition was feasible and effective. In addition, compared with the biofilm withphotosynthetic bacterial formed in the saturated liquid phase condition, the biofilmformed in the unsaturated liquid phase condition possessed the more dense structure andthe relatively short time required for the biofilm formation. When the performance testwas performed in saturated liquid phase conditions, due to the relatively dense structure,the transfer resistance of the substrate and products (hydrogen, carbon dioxide andvolatile organic acids) in the biofilm formed in the unsaturated liquid phase condition islarger, lowering the performance of hydrogen production, however, when theperformance test was performed in unsaturated liquid phase conditions, the relativelyporous structure of the substrate in the biofilm formed in the saturated liquid phasecondition led to the lower biomass concentration, lowering the performance of hydrogen production.