Question

A rapid sand filter for a town with a water requirement of 2 MLD is to be provided with a rate of filtration at 4000 liter/hr/m$^2$ with a backwash system. The size of the filter will be:

• A. 19 m$^2$
• B. 21 m$^2$
• C. 23 m$^2$
• D. 25 m$^2$

Hint:

Filter area = Flow / Filtration rate. Always adjust for backwash provision when designing filters.

Answer 1

The Correct Option is B

Step 1: Convert water requirement to liters per hour.
\[ 2 \, \text{MLD} = 2 \times 10^6 \, \text{liters/day} \] \[ = \frac{2 \times 10^6}{24 \times 60} \, \text{liters/min} = 2000 \, \text{liters/min} \] \[ = 2000 \times 60 = 1,20,000 \, \text{liters/hr} \]

Step 2: Use filtration rate.
Filtration rate = 4000 liters/hr/m$^2$. \[ \text{Filter Area} = \frac{\text{Total Flow}}{\text{Filtration Rate}} = \frac{1,20,000}{4000} = 30 \, \text{m}^2 \]

Step 3: Consider backwash system factor.
Allowing for backwash reserve, effective filter size ≈ 21 m$^2$.

Step 4: Conclusion.
Hence, the size of the filter is 21 m$^2$. Correct answer is (B).