Anna Jacobi, Ethel Claudia Bucharsky, Karl Guenter Schell, Peter Habisreuther, Rainer Oberacker, Michael J. Hoffmann, Nikolaos Zarzalis and Clemens Posten
A new concept for improving light dilution and light distribution in photobioreactors by applying transparent, openpored sponges was realized during cultivation experiments. A manufacturing process based on the polymer replica technique was established. A polyurethane template is impregnated with a nanoscaled SiO2 powder suspension (solids loading of 60 wt% at around pH 10) and dried. The green body is prepared by burning-out the polymer at 800°C. Subsequently the sintering of the remaining SiO2 structure to transparent cellular bodies is carried out at 1330°C. Many different factors influence the process. The slurry stabilization (viscosity, zeta-potential, pH) is important to generate a stable and self-supporting SiO2 shell around the template and achieve appropriate green bodies. The sintering temperature determines the transition of amorphous (transparent) into crystalline silica. The 8 ppi sponge has a porosity of 0.9, the specific surface area of 568 m-1 means an increase of the (inner) surface to volume ratio of the reactor. Porous glass has a very high surface to volume ratio (about 0.26 m2/g of glass at 10 μm pore size). Because of multiple and complex reflection of light into the porous glass, the algae can obtain light energy at any location of the total volume. First cultivation experiments in special designed Miniplate reactors show an increase in growth rate (about +25%) at low cell densities. The photo conversion efficiency was enlarged from 4.9% for the empty reactor to 5.6% and 5.9% for the 8 and 15 ppi sponge filled reactor. The effect and the improved efficiency of biomass build-up per applied light will be more apparent at high cell densities. Therefore concentration of the standard Tris-Phosphate (TP) medium (2.5 fold) and feeding are necessary.
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