Raceway ponds are a modified version of the open pond system that has a different flow pattern compared to that of the simple pond. In raceways, the water flow direction is con­trolled by the rotation speed of paddlewheels, in contrast to only coaxial mixing in conven­tional open ponds. Therefore, in the raceway systems, the microalgae, water, and nutrients are continuously circulated around a racetrack, following the same direction as a paddlewheel. In this way, the circulation rate around the racetrack can be adjusted by the paddle speed. With paddlewheels providing the driving force for liquid flow, the microalgae are kept suspended in the water and are circulated back to the surface on a regular frequency.

Despite their diversified appearance, the most common raceway cultivators are driven by paddlewheels and are usually operated at a water depth of 15-20 cm. The raceways are usu­ally operated in a continuous mode with constant feeding of CO2 and nutrients into the sys­tem while the microalgae culture is removed at the end of the racetrack. This operation is quite similar to that of plug-flow reactors (PFRs) used in the chemical industry.

The same drawbacks observed in the operation of open ponds are also found in raceways. Furthermore, the requirement of large areas for microalgae cultivation is considered the bar­rier for commercialization of microalgae processes. Nevertheless, control of environmental factors (such as mixing) in raceways is easier than in conventional open ponds, making the use of raceways for the cultivation of microalgae more attractive.