Comparison of suspended and fixed photocatalytic reactor systems

Photocatalysis is a promising technology for the purification of pretreated wastewaters in sun-rich countries if an economically applicable reactor system is available. Within this project the catalyst separation as an essential process step of suspended reactor systems was investigated. For the separation of suspended catalyst a sedimentation basin with and without lamella and a membrane filtration were investigated. The sedimentation was found to be very sensitive to the kind of the ion background of wastewater, the pH, the TiO2 influent concentration as well as the hydrodynamics in the clarifier. Under optimized conditions effluent concentrations of less than 5 mg SS/L and a clear water without turbidity could be reached with a specific flow rate of up to 0.7 m3/m2/h. The best performance for P25 was achieved with a TiO2 influent concentration of 5 g/L. Membrane filtration was the only method to guarantee a complete retention of the TiO2 as well as a rejection of microorganisms and high molecular compounds. With cross-flow velocities of 3 m/s and a transmembrane pressure of 100 kPa flux rates up to 1,200 L/m2/h were achieved. A flow-film-reactor (FFR) was operated with the model compound DCA under identical conditions with fixed and suspended TiO2. Whereas the fixed system has the advantage that no separation step is necessary and a simple construction can be used, suspended systems offer a three times higher reaction velocity for a catalyst concentration of 10 g/L, but are also characterized by higher investment costs.