The settling velocity of a particle is affected by which of the following factors?

• A. Flow rate only
• B. Flow rate and viscosity of the water and the settling characteristics of the particle
• C. Viscosity of the water only
• D. Settling characteristics of the particle only

Answer: (B)

Settling velocity reflects the balance between the downward pull of gravity on the particle and the resistance from the water. The resistance comes from drag, which depends on how viscous the water is, and on how easily the particle moves through it, which is set by the particle’s size, density (settling characteristics), and shape. In a real tank, the water is not still—the flow rate and any turbulence or upward motion oppose settling. Higher flow or turbulence can carry particles upward or hinder their downward movement, while higher water viscosity increases drag and slows settling. Larger, denser, or more streamlined particles with less drag settle faster, all else equal. So the settling velocity is influenced by flow rate, viscosity, and the particle’s own settling characteristics.