While designing a circular primary sedimentation tank, if the surface overflow rate (SOR) is decreased while keeping the discharge constant, what will be the effect on its functioning?
- AIts efficiency will increaseCorrect
- BIts efficiency will decrease
- CIts efficiency remains unchanged
- DMost likely the tank will overflow
Solution
Concept
Effect of Surface Overflow Rate on sedimentation efficiency
Formula
SOR = Q / A; Fractional Removal efficiency η = vs / SOR (for vs < SOR)
Step-by-step solution
1. Surface overflow rate is defined as SOR = Q / As, where Q is flow rate and As is surface area. 2. If SOR is decreased while Q is constant, surface area As must be larger. 3. A lower SOR means the critical settling velocity vc decreases, allowing finer particles to settle. 4. Therefore, the overall particle removal efficiency of the sedimentation tank increases. Answer: A
Explanation
Surface Overflow Rate (SOR = Q / A) represents the settling velocity of particles that are 100% removed in an ideal sedimentation tank. Decreasing SOR at constant flow Q requires increasing the surface area A. When SOR decreases, particles with smaller settling velocities can settle successfully, thereby increasing the solids removal efficiency of the sedimentation tank.
Why the other options are wrong
B) Efficiency increases because slower-settling particles are captured. C) Efficiency is inversely related to overflow rate, so it does not remain constant. D) Lower SOR means a larger tank, reducing any risk of hydraulic overflow.
Shortcut
Lower SOR = Larger surface area = Finer particles settle = Higher removal efficiency.
Common mistake
Thinking that a lower rate means poorer performance or that tank overflow rate relates to liquid overflowing walls.
Correct answer: A) Its efficiency will increase