Induced air flotation (IAF) was developed using a rotating-flow microbubble generator for water treatment. This newly-developed generator can produce fine microbubbles with diameters on the order of micrometers, whereas it has to use a large quantity of water as propulsion, especially when surfactant-free water is used. That is to say, if the microbubble generator is utilized in flotation, the recycle ratio in IAF will be very high. To overcome this problem, we applied a centrifugal pump and altered the air-induced way of the generator. Microbubble size, microbubble concentration and particle removal efficiency of the improved microbubble generating method were compared with those of traditional ones.
The best performance occurred in the improved method where gas and water were simultaneously induced and mixed by the pump, and dispersed through the rotating-flow microbubble generator. Microbubble size was decreased, while microbubble concentration was increased by the improved method. Particle removal efficiency of over 95% was achieved by recycling only 10% of treated water.
Batch experiments were carried out under various flocculation and flotation conditions to evaluate the improved IAF process. A slight overdose achieved better removal efficiency and no or less flocculation time performed very well. Lower water flow rates achieved better removal efficiency due to the increase of the bubble concentration and the decrease of the bubble size. The air flow rate showed little effect on removal efficiency.