In a hospital, used water is collected in a cylindrical tank of diameter 2 m and height 5 m.

Diameter of cylinder (d) = 2 m
Radius of cylinder (r) = 1 m
Height of cylinder (H) = 5 m

Volume of cylindrical tank, $\mathrm{V}_{c}=\pi r^{2} H=\pi \times(1)^{2} \times 5=5 \pi \mathrm{m}$

Length of the park (l) = 25 m
Breadth of park (b) = 20 m
height of standing water in the park = h​

Volume of water in the park $=l b h=25 \times 20 \times h$

Now water from the tank is used to irrigate the park. So, 
Volume of cylindrical tank = Volume of water in the park

$\Rightarrow 5 \pi=25 \times 20 \times h$

$\Rightarrow \frac{5 \pi}{25 \times 20}=h$

$\Rightarrow h=\frac{\pi}{100} \mathrm{~m}$

$\Rightarrow h=0.0314 \mathrm{~m}$

Through recycling of water, better use of the natural resource occurs without wastage. It helps in reducing and preventing pollution. 
It thus helps in conserving water. This keeps the greenery alive in urban areas like in parks gardens etc.