Raceway ponds: The only practicable method  of large-scale production of micro algae are raceway ponds. In these ponds, the algae, water and nutrients circulate around a racetrack. With paddle wheels providing the flow, algae are kept suspended in water, and are circulated back to the surface on a regular frequency. The ponds are usually kept shallow because the algae need to be exposed to sunlight, and sunlight can only penetrate the pond water to a limited depth. The ponds are operated in a continuous manner, with CO₂ and nutrients being constantly fed to the ponds, while algae-containing water is removed at the other end.