Number and size of RSF, 1,20,000 population

Data

Population 1 20 000; 120 lpcd; filtration rate 3000 l/h/m² $=3$ m/h; $L:B=1.5$. Filters work 24 h/day (wash water is neglected, since the average supply rate is given).

Step 1: Flow

$$Q=120\,000\times120=14\,400\,000\ \text{l/day}=14400\ \text{m}^3/\text{day}=600\ \text{m}^3/\text{h}$$

Step 2: Total filter area

$$A=\frac{Q}{\text{rate}}=\frac{600}{3}=200\ \text{m}^2$$

Step 3: Number of units

$N=1.22\sqrt{Q}$ (Q in MLD) $=1.22\sqrt{14.4}=4.63$, so adopt 5 units.

Step 4: Size of each unit

$$A_{unit}=\frac{200}{5}=40\ \text{m}^2,\qquad 1.5B^2=40\Rightarrow B=5.16\ \text{m},\ L=1.5B=7.75\ \text{m}$$

Adopt $B=5.2$ m, $L=7.8$ m ($A=40.56$ m²).

Step 5: Check

Actual rate $=600/(5\times40.56)=2.96$ m/h. With one unit under backwash, the rate on the remaining 4 units is 3.70 m/h (acceptable).

Depth: free board 0.5 + water 1.5 + sand 0.7 + gravel 0.45 + under-drain 0.3 = 3.45 m.

Answer: provide 5 rapid sand filters, each 7.8 m long $\times$ 5.2 m wide (area 40.6 m²), total area 203 m².