Canals within ash basins increase sedimentation by increasing which factor?

• A. Turbidity
• B. Surface area
• C. Particle size
• D. Retention time

Answer: (D)

The key concept is that sedimentation is driven by how long the water spends in the settling area. Canals inside ash basins slow the flow and create a longer path, which increases the hydraulic detention time. With more time in the basin, gravity has more opportunity to pull suspended ash particles out of suspension, so sedimentation improves. Turbidity is a measurement of how cloudy the water is, not something that causes settling when you add canals. Particle size affects how quickly particles settle, but the canals don’t change particle size—they change how long the particles can settle. Surface area of the basin or contact surfaces doesn’t directly drive gravity settling in this context. Therefore, the factor that increases sedimentation when canals are present is retention time.