Slow sand filter for 3500 population

The consumption "45 lps" is read as 45 lpcd (45 lps for 3,500 people is impossible). The population of 3,500 is taken as the design population. A filtration rate of 0.2 m³/m²/h is assumed.

Step 1: Design flow

• Design population = 3,500 (given)
• Daily demand $Q = 3,500 \times 45/1000 = 157.5$ m³/day = 6.56 m³/h

Step 2: Area and number of units

• Filtration rate = 0.2 m³/m²/h (range for slow sand filters 0.1–0.4)
• Total area $A = Q/(\text{rate}) = 6.56/0.2 = 32.8$ m²
• Keep each unit below about 100 m² and use at least 2 working units: working units n = 2, plus 1 standby unit for cleaning, so 3 units in all
• Area of one unit $= 32.8/2 = 16.4$ m²
• Take $L = 2B$: $B = \sqrt{16.4/2} = 2.86$ m. Adopt B = 2.9 m, L = 5.8 m (area 16.8 m² each)
• Check: rate with 2 working units = 0.195 m/h (OK)

Step 3: Depth of unit

Sand is scraped 15–25 mm every 1–2 months; when its depth falls to 0.5 m it is topped up to 0.9 m. Inlet and outlet chambers, a filtered-water weir, a drain valve and a filter-to-waste connection complete each unit.

Step 4: Section

 inlet -> |  free board 0.3 m             |
          |~~~~ supernatant water 1.0 m ~~|
          |-------------------------------|
          |  filter sand 0.9 m            |
          |-------------------------------|
          |  gravel 0.3 m (3 layers)      |
          |==o==o== under-drain 0.2 m ==o=|
                       |
          outlet weir -> filtered water tank

Answer: 2 working + 1 standby slow sand filters, each 2.9 m × 5.8 m (16.8 m²), total depth about 2.7 m.