With small suspended particles the calculated settling velocities become so small that sedimentation is clearly impracticable, and for most purposes sedimentation is not an appropriate treatment process for particles smaller than about 50μm. If particles are flocculent it is possible to utilize this property and encourage agglomeration of fine material so that settleable solids are produced. Such agglomeration is brought about largely by the creation of velocity gradients in the suspension which cause collisions between particles. The number of collisions in a suspension is proportional to the velocity gradient, so that within limits the degree of agglomeration increases with the level of mixing to which the suspension is exposed.
As flocculation produces larger and larger particles, the suspension then becomes more sensitive to the effects of shear so that high velocity gradients will eventually result in breakup of large floc particles.
Velocity gradients can be created either by hydraulic turbulence caused by baffles or by mechanically driven paddles. The latter are more flexible in use since the degree of flocculation can be easily adjusted.
By considering the behaviour of an element of fluid exposed to a velocity gradient it can be shown that the relationship between velocity gradient G and power input per unit volume Ρ is
P=µG2
For a paddle flocculator
power = drag on paddles x velocity of paddles relative to fluid
P=CD A ν3 /2V
where
ν = velocity of paddle relative to fluid
V= volume of tank
For most flocculent, suspensions in water and wastewater treatment, an appropriate value of G is usually 20-75 s-1