High Efficiency Induced Air flotation using Hydrodynamic Cavitation to Dewater Algal Solutions

Development of algae as a source of renewable chemicals and fuels is hindered by the difficulty of recovering the algae from the growth solutions. Air flotation can separate solids and liquids by having gas bubbles attach to the solid particles to increase buoyancy and lift them to the surface. The small particle sizes of algae require that small bubble sizes be used. Conventionally very fine bubbles are created using dissolved gas flotation DAF). DAF requires high pressures to supersaturate gases into water and has poor energy efficiency. However, this can be overcome by using bubble generators based on local controlled hydrodynamic cavitation to generate large volumes of very small bubbles with much less energy. These will have long residence time in the water, rise slowly in the gravity field while attaching to the algae cells and lifting them to the free surface. In this proposed Phase I SBIR project, we will investigate the feasibility of using a bubble generator concept based on controlled cavitation to remove water and concentrate the algae from the growth media and recover the intact cells. The objectives of the project will be to design and construct a bench- scale algal growth media dewatering loop with the bubble generator capable of producing large numbers of bubble of diameters less than 50 microns, and to then move to pilot scale systems after addressing R&D issues. A controlled cavitation bubble generator will be tested. The bubble sizes and numbers produced will be diagnosed by high speed video, image analysis, and acoustic techniques. The effects of operating parameters pressure drop, air and water flow rates) on bubble size distributions will be determined. The performance of the solid- liquid separation system with different algae species, algae concentrations, and air-liquid void fractions will be determined. The percent recovery of algae form solution, percentage of solids in the recovered slurry, and algae quality will be measured in these experiments. If necessary to increase the percentage of solids to 20% secondary concentration methods such as vacuum filtration will be tested. An energy efficient method for dewatering algae solutions will reduce the production costs for renewable chemicals and fuels produced from algae, and reduce the barriers to bringing this technology to commercial scale. The separation technology developed in this SBIR would also have applications in other fields such as mining, wastewater treatment, and petroleum process water treatment.