ROLE OF FLOW AND MASS TRANSFER IN PHOTOBIOREACTOR: FROM BACTERIA LOCOMOTION TO HYDROGEN PRODUCTION
Photo fermentation by photosynthetic bacteria (PSB) in biofilm photobioreactors (PBRs) is a green route to produce hydrogen energy; however, it faces many thermodynamic challenges such as flow, heat, mass and light transfer in the growth and metabolism of microbes, significantly limiting the PBR performance. In PBRs, photosynthetic bacteria in suspension are absorbed and attached onto surface of support materials to form stable porous biofilms. As liquor flow over the biofilm surface, the organic substrates transport from the bulk liquid into the biofilm, and subsequently are degraded by PSB to generate hydrogen and CO2 as well as metabolic heat. The metabolic products inversely diffuse to the bulk flow. Such processes are associated with combined biochemical reactions and multi-scale transport steps. Facing the interdisciplinary challenges, we have focused our attention on transport mechanisms and characteristics in the PBRs, including the movement and adsorption behaviors of bacteria, the mechanisms of PSB biofilm formation on the surface of solid materials, and the characteristics of fluid flow and mass transfer coupling with biochemical reactions. In addition, we proposed novel approaches for PBR performance enhancement based on the fundamental research. The main findings are listed below:
(1) The locomotion behaviors of suspended bacteria on support material surface and the chemotaxis under a non-uniform chemical concentration were explored by a theoretical model that takes account of the interaction between flagellum and liquid, Brownian motion and the tumble motion. The effects of flow field, substrate concentration and support material structure were discussed comprehensively. Based on the theoretical study, a new unsaturated operation mode and a novel biomaterial containing SiO2-chitosan-medium coating glass slide were proposed to improve the adhesive capacity of PSB by 8.1 times.
(2) The mechanisms of the growth and biofilm formation of PSB on structured surfaces were investigated by systematic experiments and a theoretical model with diffusion-reaction equations and cellular automata rules. The evolutions of biofilm morphology and characteristic parameters were obtained at various operating conditions. The results indicated that substrate limitation is the most critical factor affecting the biofilm activity. In addition, the optical fiber and light guide plate with surface modification were firstly employed to enhance the light transport and biofilm formation, resulting in a 4.4 times increase in PSB biomass accumulation.
(3) The transport characteristics and hydrogen production performance in various PBRs were studied by visual and numerical experiments to discover the desirable PBR structure and operating conditions for performance enhancement. A novel optical fiber bundle biofilm PBR and a light guide plate PBR with multi-layer decoration by photothermal biomaterial were proposed to improve hydrogen production by 2.6 and 5.1 times, respectively.