subject: Paddlewheels and their Role in Algae Fuels [print this page] Open ponds, typically raceway ponds in which algae are grown, require some form of mixing to maintain cells in suspension, to prevent thermal stratification, and to disperse nutrients. Hence, for a proper mixing of the ponds, paddle wheels are used.
Paddlewheels consist of simple, partial depth blades and high-speed rotors. With paddlewheels providing the flow, algae are kept suspended in the 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.
Paddlewheels (PW) consist of simple, partial depth blades and high-speed rotors. With paddlewheels providing the flow, algae are kept suspended in the water, and are circulated back to the surface on a regular frequency. A PW consists of three structural units: paddle blades, motor and gear box. A paddle blade is made of aluminium, stainless steel or fiber glass, based on the requirements.
However, these PWs do present some challenges. They are large relative to other types of mixers, especially at higher heads. Another major drawback is that these PWs are very expensive unless a low- shear pump is used.
Though PW s have a few issues, there are a few reasons why PW has emerged as a preferred method for mixing algae in oxidation/ high- rate ponds. First of all, they are well matched to the pumping requirements of high-rate ponds in that they are high-volume and low-head devices. Moreover, they are mechanically simple, requiring minimum maintenance and their drive train can easily be designed to accommodate a wide range of speeds without drastic changes in efficiency.
