(417g) G-Value: Is it genuinely useful in sizing Flocculators and Rapid Mixers?

G-value is a shear rate which is used for the sizing of mixing equipment for flocculant dispersion and floc growth for water treatment. It has several advantages in that it is easily calculated and provides a gross representation of mixing in the vessel which is fairly well correlated with performance. But it also has a major disadvantage in that it does not take spatial variations of shear rate into account. The local G-value measured at the tip of the impeller blade will be higher than that measured near the vessel wall. Research has shown that it is the local G-value at the impeller tip which determines the floc size that is generated by the mixing equipment.

There are two steps in the flocculation process. In the first a flocculant is added which is mixed with the suspended solids. This is the Rapid Mixing step, and the flocculant eliminates the electrostatic forces between the particles which prevent them from colliding and adhering. Once the flocculant is mixed, the particles must be transferred to an environment in which they are allowed to collide and for the flocs to grow. This is the Flocculation step and there must be sufficient fluid motion to promote particle-particle collisions but not so much that the growing flocs are broken. Ultimately the balance between flow and shear determines how successful the flocculation process will be.

The common method for defining the intensity of mixing by specifying a G-value first proposed by Stein & Camp (J. Bos, Soc. Civ. Engrs., 1943). G is dependent on the power input per mass of fluid by the agitator’s impeller and the kinematic viscosity of the fluid. It can also be expressed in terms of the power input by the impeller and the volume and dynamic viscosity of fluid. Its value is specified by the consulting engineer. A high value of G is chosen for Rapid Mixing, typically 500 - 1000 s^-1, while a low value is chosen for Flocculation, typically 50 - 100 s^-1 (Waste Water Engineering, Fifth Edition, McGraw-Hill Education, 2014).

Impellers in agitated vessels can be characterized in terms of their ability to generate flow and shear. This understanding can be applied to determine which geometry is most efficient for rapid dispersion of flocculant and which is most efficient for promotion of floc growth. The shear generated by a small diameter hydrofoil will be higher than the shear generated by, for example, a large diameter pitched-blade turbine at the same G-value and power input. But a small hydrofoil will operate at a higher speed to deliver the required power input, based on the specified G-value. It is attractive to equipment vendors to quote the higher speed agitator because the shaft torque will be smaller making the equipment size and cost lower and their bid more attractive commercially, but this will not necessarily be the best option for the process. If the shear is too high the operator can compensate by using more flocculant adding to their running costs.