Sterilization in hydroponic recycling system using visible light-reactive titanium dioxide photocatalysts

This research was aimed at developing a hydroponie recycling system using a visible light- reactive titanium dioxide (TiO₂) photocatalyst. We made a sterilization system for the nutrient solution by utilizing the features of the filters coated with visible light-reactive titanium dioxide photocatalyst. The system is composed of a photocatalyst filter (300×300×20 mm), a 4-tier stair type processing channel (305 mm wide, 2,100 mm long, 30 mm high), a supply tank, a catchment tank, a water pump, a circulation control facility, an insolation sensor, and a data log. We also evaluated the sterilization performance using a number of filters, the initial density of spores in the nutrient solution, the species of spores, flow rates, and the amount of insolation. In the experiment with 20 filters, sterilization performance per 1 filter was 20.3% higher than one per 5 filters. Sterilization performance at a low initial density of spores (26 counts/0.9 mm3) is 23.9% higher than at a high initial density of spores (440.3 counts/0.9 mm3). Colletotrichum was more sensitive to the photocatalyst than Fusarium, and the slow flow rate (0.5 L/min) has more sterilization effects (9.0%) than the fast flow rate (1.0 L/min). The logarithmic relation of the average insolation and lethal intensity of unit spore density is correlated as 0.926 of the coefficient of determination (R2). These results provide valuable information for TlO ₂ photocatalytic treatment of waste nutrient solutions under solar light irradiation.