The fluidisation of the particles in the reactor leads to the surface of the particles being continuously turned over. This also increases the mass transfer rate. Fluidised beds are typically categorized as either being a 2 phase system which are not aerated and 3 phase system which is aerated by sparging Fluidized bed reactors Fluidized bed bioreactors are used widely in wastewater treatment. Fluidized bed bioreactors are also used for animal cell culture. Animal cells are trapped in gels or on the surface of special particles known as "microcarriers". Fluidized bed reactors are one example of perfusion culture technology used for animal cell culture. Comparing fluidised bed and fixed reactors Fluidised bed reactors are considerably more efficient than fixed bed reactors for the following reasons: (1) A high concentration of cells can be immobilized in the reactor due to the larger surface area for cell immobilization is available (2) Mass transfer rates are higher due to the larger surface area and the higher levels of mixing in the reactor. (3) Fluidised bed reactors do not clog as easily as fixed bed reactors