Formation And Hydrogen Production Performance Of Photosynthetic Bacterial Biofilm On Solid Surface

The rapidly diminishing reserves of fossil fuels and resulting global climate change, environmental pollution and health problems by their combustion make the current energy structure face a significant challenge. Hydrogen, which is of high combustion efficiency with high caloric value and can be directly used in fuel cells, is considered as one of the most promising alternatives to fossil fuels. Bio-hydrogen production from organic pollutants leads a solution for the confliction of environmental protection and energy requirements and has a bright future due to its moderate reaction condition and low energy consumption. Compared with hydrogen production by dark-fermentation, the photo-fermentation process has attracted intensive attention due to high purity of the produced hydrogen, high theoretical substrate conversion efficiency and no O2-evolving activity which causes O2 inactivation in green algae hydrogen production systems. The biofilm method, as one of the cell immobilization technologies, not only effectively improves the biomass amount per unit volume, the tolerance ability of bacteria and the hydrogen production rate, but also bear the advantages of high mass transfer rate, good light penetration and simple operation.In a typical biofilm-based bioreactor, substrate diffuses from bulk liquid into biofilm, while end-products transports inversely into the bulk liquid. It can be expected that the biofilm structure is a critical factor affecting the mass transport substrates and products as well as the overall performance of the reactor. Focusing on photo-hydrogen production by biofilm technology, visualization flat-panel photobioreactors with indigenous Rhodoseudomonas palustris CQK 01 were constructed in the present study for observation on biofilm structure and hydrogen production performances, and the effect of the structure of photosynthetic bacterial (PSB) biofilm formed under different illumination intensities, illumination wavelengths, influent flow rates and substrate concentrations were then discussed on the substrate transport and hydrogen production performance in the bioreactor. Based on the experimental works, a two-dimensional model with the diffusion-reaction equations and cellular automata rules combined with the previously obtained growth kinetics parameters of PSB was established to simulate PSB growth and biofilm formation on the solid carrier. As a result, the effects of illumination intensity, substrate concentration, operation temperature, pH and initial inoculation were numerical predicted on the biofilm growth and structure on the solid surface. The main outcomes of the present study were shown as following:1. The predominant morphology of the bacteria in the biofilm was short rods, while a few long rods were also observed. The biofilm structures formed under different star-up conditions significantly affected hydrogen production performance of the photobioreactor during steady operation process.2. The PSB biofilm formed under illumination wavelength 590 nm and illumination intensity 5000 lx had larger bacterial size, higher porosity, higher biomass dry weight (0.915 mg/cm2) and biofilm thickness (18.7μm), resulting in the best hydrogen production performance during steady operation process. The biofilm formed under illumination wavelength 470 nm gained the highest biomass dry weight (1.013 mg/cm2) and biofilm thickness (27.8μm); however, it led to inferior hydrogen production performance during steady operation process due to the lowest porosity.3. The PSB biofilm structure turned to be dense with the increase in influent flow rate. The biofilm formed under influent flow rate 38 ml/h obtained the highest thickness (19.1μm) due to low shear force, while the biomass was insufficient. The biofilm formed under 228 ml/h gained larger bacterial size and moderate porosity, leading to the highest hydrogen production performance during the steady operation process.4. The PSB biofilm turned to loose with increasing influent substrate concentration. The biofilm formed under 110 mmol/l of substrate concentration had the largest porosity, while the highest biomass dry weight and biofilm thickness were achieved by the biofilm formed under 60 mmol/l of substrate concentration inducing the best hydrogen production performance during steady operation process.5. During the steady operation process, the hydrogen production rate of the PSB biofilm increased with increasing illumination wavelength, illumination intensity, influent flow rate and substrate concentration to achieve a peak value and then dropped when these parameters were further increased. The maximum hydrogen production rate was 11.2 mmol/m2/h. The hydrogen yields of biofilms continuously dropped with the increase in influent substrate concentration and the maximal hydrogen yield was 0.66 mol H2/mol glucose. The light conversion efficiency of the reactor decreased with enhancing illumination intensity and the maximum value achieved to 28.1%.6. The simulation results from the PSB biofilm growth model indicated that the biofilm porosity decreased during the biofilm formation process, while the surface roughness reached a stable value after a certain time of growth, and the biofilm thickness increased as time progressed. The optimal growth condition for the PSB biofilm formation was 5000 lx of illumination intensity, 10.0 g/l of substrate concentration, 30℃of temperature, 7.0 of pH and 500 of initial inoculation.