What is the effect on settling velocity as temperature increases?

• A. decreases
• B. remains the same
• C. varies unpredictably
• D. increases

Answer: (D)

Settling velocity in a liquid is governed by how hard the fluid resists the particle’s downward motion. In the common laminar, small-particle regime, it follows that velocity is proportional to the buoyancy-driven force (ρp − ρf) divided by the fluid’s viscosity μ. As temperature rises, water becomes less viscous, so the drag opposing settling drops. At the same time, water also becomes slightly less dense, which increases the density difference (ρp − ρf) a bit since the particle density is essentially constant. Put together, these changes make settling faster. That’s why the correct answer is that the settling velocity increases with temperature. The other options don’t fit because increasing temperature does not slow settling, keep it unchanged, or cause unpredictable changes under typical wastewater conditions.