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Slow sand filter, 15,000 population, 150 L/h/m2

Short answer

Design flow 1500 m³/d at0.15 m³/m²/h. Use5 working plus1 standby beds, each6.5 m ×13.0 m (84.5 m²), total depth2.7 m (free board0.3 m, water1.0 m, sand0.9 m, gravel0.3 m, underdrain0.2 m).

Design slow sand filter for treating water with 15,000 populations in the community considering water demand of 100 litres per capita per day and filtration rate of 150 litres/hour per square meter. After designing, sketch with all components and their dimension.

The population of 15,000 is taken as the design population (no growth rate is given).

Step 1: Design flow

  • Design population = 15,000 (given)
  • Daily demand Q=15,000×100/1000=1,500.0Q = 15,000 \times 100/1000 = 1,500.0 m³/day = 62.50 m³/h

Step 2: Area and number of units

  • Filtration rate = 0.15 m³/m²/h (given: 150 L/h/m²) (range for slow sand filters 0.1–0.4)
  • Total area A=Q/(rate)=62.50/0.15=416.7A = Q/(\text{rate}) = 62.50/0.15 = 416.7 m²
  • Keep each unit below about 100 m² and use at least 2 working units: working units n = 5, plus 1 standby unit for cleaning, so 6 units in all
  • Area of one unit =416.7/5=83.3= 416.7/5 = 83.3 m²
  • Take L=2BL = 2B: B=83.3/2=6.45B = \sqrt{83.3/2} = 6.45 m. Adopt B = 6.5 m, L = 13.0 m (area 84.5 m² each)
  • Check: rate with 5 working units = 0.148 m/h (OK)

Step 3: Depth of unit

ComponentDepth
Free board0.3 m
Supernatant water1.0 m
Filter sand (effective size 0.2–0.3 mm, UC 3–5)0.9 m
Gravel in 3 layers (3–6, 6–12, 12–40 mm)0.3 m
Under-drain space (central drain and laterals)0.2 m
Total2.7 m

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: 5 working + 1 standby slow sand filters, each 6.5 m × 13.0 m (84.5 m²), total depth about 2.7 m.

✓ Answer checked by Claude Opus 5.5 (high effort)

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