Settling Velocity
Particle Physics of Clarification
Settling velocity fundamentals — Stokes’ law, floc characteristics, and how they drive plate settler performance.
Settling Velocity
Settling velocity is the terminal downward speed of a particle in water, governed by its size, density, and the fluid viscosity. It is the physical property that clarifier design must accommodate.
For discrete particles in the laminar regime, Stokes’ law relates settling velocity to the square of particle diameter and the density difference. This is why coagulation and flocculation — which grow small particles into larger, denser flocs — are so effective ahead of plate settlers.
In practice, most clarification involves flocculent settling, where particles aggregate and accelerate as they descend. Well-conditioned flocs settle rapidly across the short vertical distance between inclined plates.
v_s = g·(ρ_p − ρ_f)·d² / (18·μ)Stokes’ law: v_s = settling velocity; d = particle diameter; ρ_p, ρ_f = particle & fluid density; μ = dynamic viscosity.
Design Reference
Design Notes
Grow the Floc
Doubling particle size quadruples settling velocity (Stokes).
Short Travel
Particles need only settle to the nearest plate surface.
Temperature Effect
Cold water raises viscosity and slows settling — account for it in design.
Request an Engineering Assessment
Our engineers will review your application, flow rates, and water quality data to recommend the optimal plate settler configuration.