In a particle bed the equilibrium between the gravitational and frictional forces defines a velocity limit that we are going to call V_mf. 

If the rising velocity of the fluid is less than the V_mf  the particles has a tendency to settle whereas if the rising velocity is higher the particles are carried upward by the fluid.

In fact, the fluidized bed is not made up of merely one particle, but rather of a group of particles, moreover the particles are not all the same size. In practice, when a granular mass inside a column is subjected to increasing rising velocity, the results summarized on the graph below are attained.

Find below the minimum fluidization velocities for various filtering media ( T = 20 °C ) : 

NES : Nominal effective size

MES : Measured effective size

In biological treatment process the materials used are those which have a small effective size and develop a large specific area. In order to allow the fixation and development of a large bacterial mass.

APPLICATION IN WATER TREATMENT:

Resource: Water treatment Handbook Degrémont