The maximum demand at a water purification plant has been estimated as 12 million litres per day. For the raw supplies, a rectangular sedimentation tank is to be designed with mechanical sludge removal arrangement. Consider depth of the tank as 4 m, detention period as 6 hours, and velocity of flow as 0.003 m/s. The width (in m) of the detention tank is________ (rounded off to two decimal places).

The correct option is 11.57.

To find the width of the rectangular sedimentation tank, we can follow these step-by-step calculations:
First, let us list the given parameters:
- Maximum daily water demand ($Q$) = 12 million litres per day (MLD)
- Depth of the tank ($H$) = 4 m
- Detention period ($t_d$) = 6 hours
- Velocity of flow ($V$) = 0.003 m/s

Step 1: Calculate the Volume of the Tank
The volume of the tank is determined by multiplying the rate of water flow by the detention period.
First, convert the flow rate from million litres per day (MLD) to cubic meters per day ($m^3/\text{day}$). Since 1 million litres is equal to 1000 cubic meters ($m^3$):
$Q=12\times 1000=12000m^3/\text{day}$
Next, convert this daily flow rate into an hourly flow rate:
$Q=\frac{12000}{24}=500m^3/\text{hour}$
Now, calculate the total volume ($V_{\text{tank}}$) required for the detention period of 6 hours:
$V_{\text{tank}}=Q\times t_d=500m^3/\text{hour}\times 6\text{hours}=3000m^3$

Step 2: Calculate the Length of the Tank
The length of the tank ($L$) is the distance the water travels during the detention period at the given velocity of flow.
Convert the detention period from hours to seconds:
$t_d=6\times 3600=21600\text{seconds}$
Now, calculate the length of the tank:
$L=V\times t_d=0.003\text{m/s}\times 21600\text{s}=64.8\text{m}$

Step 3: Calculate the Width of the Tank
The volume of a rectangular tank is given by:
$V_{\text{tank}}=\mathrm{Length}\times \mathrm{Width}\times \mathrm{Depth}=L\times B\times H$
Substitute the known values into the equation to solve for the width ($B$):
$3000=64.8\times B\times 4$
$3000=259.2\times B$
$B=\frac{3000}{259.2}\approx 11.574\text{m}$
Rounding to two decimal places, the width of the detention tank is:
$B=11.57\text{m}$