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: Determine filter area using filtration rate.
Filtration rate is 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: Adjust for backwash system.
After accounting for backwash reserve, the effective filter size is approximately 21 m$^2$.

Step 4: Finalize filter size.
Therefore, the required filter size is 21 m$^2$. The correct option is (B).