Question

A rapid sand filter is to be provided in a water treatment plant to process the water for a town with a population of 3,45,000. The water demand is 225 liter/ capita/ day. The rate of filtration is $15m^3/m^2/hour$. Allow 5% of filtered water storage time to the backwash requirements. Each back washing period is 30 minute. The number of filters required including one as a standby unit is [Assume the available surface area configuration of one filter unit is $50m^2$]

• A. 7
• B. 4
• C. 5
• D. 6

Answer 1

The correct option is D 6

Daily water demand of filtered water
= 105% of water required daily
(5% more for backwashing)
$=\frac{105\times 345000\times 225}{100}=81.5MLD$
Effective time for working of filter units = 24 - backwashing time (0.5 hours)
= 23.5 hours
$\therefore$ Water filtration required per hour $=\frac{81.5}{23.5}=3.468ML/h$
Now, filtration rate = $15m^3/m^2/h$
$\therefore$ Area of filter required $=\frac{\left[\frac{3.468\times {10}^0}{{10}^3}\right]}{15}=231.2m^2$
Area of one filter unit$=50m^2\therefore Numberofunitrequired=\frac{231.2}{50}=4.624\approx 5unit$