The (relative) insignificance of G revisited to include nanoparticles

The collision efficiency functions, α ( i , j ), of two colliding particles for the three operative transport mechanisms in flocculation (Brownian motion [Br], fluid shear [Sh], and differential sedimentation [DS]) were numerically calculated over a broad size range (including nanoparticles). All computations include the hydrodynamic force and van der Waals (vdW) attraction, while the effects of electrical double‐layer repulsive forces were investigated by their exclusion or inclusion. Flocculation of nanoparticles should be extremely rapid if they are uncharged but will be dramatically reduced when substantially charged. For small neutral particles, α ( i , j ) values could be greater than those obtained during Sh and DS as vdW attraction outweighs the (negative) hydrodynamic effects. Overall, the results confirm the findings of an earlier article, that Br is the dominant flocculation mechanism when at least one particle is small (<1 μm in diameter), Sh is only dominant when both particles are greater than 1 μm and the ratio of the two particle sizes is less than 10, and DS is dominant in all other cases. Flocculators should be operated with low‐velocity gradient ( G ) values (in the range of 10–20 s −1 ), with only sufficient mixing to keep particles in suspension, but proper particle destabilization is essential for effective flocculation.