Question

an engineer proposes increasing sedimentation efficiency by:

i. increasing tank surface area
ii. reducing flow rate
iii. increasing depth only

which combination is correct?

• i and ii only
• i only
• ii and iii only
• i, ii and iii

Explanation:

🆕 New Concept Discovered: Sedimentation Tank Design
Factors affecting particle removal efficiency in settling basins.

Step 1: Analyze the governing equation for sedimentation efficiency

In ideal settling basin theory (Camp-Hazen model), the removal efficiency of particles with a settling velocity \( v_s \) is determined by comparing it to the overflow rate (surface loading rate) \( v_0 \):

$$ v_0 = \frac{Q}{A_s} $$

where:

• \( Q \) is the influent flow rate.
• \( A_s \) is the surface area of the settling tank.

The removal efficiency \( \eta \) for particles with \( v_s < v_0 \) is given by:

$$ \eta = \frac{v_s}{v_0} = \frac{v_s \cdot A_s}{Q} $$

Step 2: Evaluate each proposal

• I. Increasing tank surface area (\( A_s \)):

Increasing \( A_s \) decreases the overflow rate \( v_0 \), which increases the removal efficiency \( \eta \). (Correct)

• II. Reducing flow rate (\( Q \)):

Reducing \( Q \) decreases the overflow rate \( v_0 \), allowing more time for particles to settle, which increases the removal efficiency \( \eta \). (Correct)

• III. Increasing depth only (\( H \)):

According to ideal settling theory, the depth of the tank \( H \) does not affect the removal efficiency of discrete particles because both the detention time and the required settling distance scale proportionally with depth. Therefore, increasing depth only does not increase efficiency. (Incorrect)

Step 3: Select the correct combination

Since proposals I and II increase efficiency, while proposal III does not, the correct combination is I and II only.

Answer:

I and II only