Question

8 rapid sand filters are used to treat 35 MLD of water with rate of filtration $4000lit/hr/m^2$. If back washing rate is 4 times rate of filtration and duration of back wash is 15 minutes operated once in every 24 hrs, then the percentage of filtered water required to back wash the filter is

• A. 3.9 %
• B. 4.21 %
• C. 3.46 %
• D. 4.566 %

Answer 1

The correct option is B 4.21 %

$\text{The volume of water used in back wash =}$

$\text{ROB}\times \text{DOB}\times \text{Area of each filter}$

$\text{The volume of water filtered =}$

$\text{ROF}\times \text{DOF}\times \text{Area of each filter}$

$\text{\% of water used in back wash =}$

$\frac{\text{Volume of water in back wash}}{\text{Volume of water filtered}}\times 100$

$=\frac{\text{ROB}\times \text{DOB}\times \text{Area of each filter}}{\text{ROF}\times \text{DOF}\times \text{Area of each filter}}\times 100$

$=\frac{4\times 4000\times \frac{15}{60}}{4000\times (24-\frac{15}{60})}\times 100=4.210\%$