| Open ponds, typically raceway ponds in which algae | | | | algae-containing water is removed at the other end. |
| are grown, require some form of mixing to maintain | | | | Paddlewheels (PW) consist of simple, partial depth |
| cells in suspension, to prevent thermal stratification, and | | | | blades and high-speed rotors. With paddlewheels |
| to disperse nutrients. Hence, for a proper mixing of the | | | | providing the flow, algae are kept suspended in the |
| ponds, paddle wheels are used. | | | | water, and are circulated back to the surface on a |
| Paddlewheels consist of simple, partial depth blades | | | | regular frequency. A PW consists of three structural |
| and high-speed rotors. With paddlewheels providing the | | | | units: paddle blades, motor and gear box. A paddle |
| flow, algae are kept suspended in the water, and are | | | | blade is made of aluminium, stainless steel or fiber |
| circulated back to the surface on a regular frequency. | | | | glass, based on the requirements. |
| The ponds are usually kept shallow because the algae | | | | However, these PWs do present some challenges. |
| need to be exposed to sunlight, and sunlight can only | | | | They are large relative to other types of mixers, |
| penetrate the pond water to a limited depth. The | | | | especially at higher heads. Another major drawback is |
| ponds are operated in a continuous manner, with CO2 | | | | that these PWs are very expensive unless a low- |
| and nutrients being constantly fed to the ponds, while | | | | shear pump is used. |