---
title: "Slow sand filter, 15,000 population, 150 L/h/m2"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:ssf-a75"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ssf-a75
site: BCT Engineering
---

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

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2075 Asoj, 4+4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ssf-a75
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

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.

## 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).

## Answer

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 $Q = 15,000 \times 100/1000 = 1,500.0$ 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 $A = Q/(\text{rate}) = 62.50/0.15 = 416.7$ 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 $= 416.7/5 = 83.3$ m²
- Take $L = 2B$: $B = \sqrt{83.3/2} = 6.45$ 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
| Component | Depth |
|---|---|
| Free board | 0.3 m |
| Supernatant water | 1.0 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 |
| **Total** | **2.7 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

```text
 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.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. No growth rate is given, so what is used?
   A. The present 15 000
   B. Population doubled
   C. Population after 20 years at 2 %
   D. Population halved
   Answer: A. The given population is the design population. (Key point: Design population)

2. What filtration rate is used?
   A. 15 m^3/m^2/h
   B. 0.15 m^3/m^2/h (150 L/h/m^2)
   C. 1.5 m^3/m^2/h
   D. 0.0015 m^3/m^2/h
   Answer: B. 150 litres per hour per m^2 equals 0.15 m^3/m^2/h. (Key point: Rate)

3. The total area is
   A. 41.7 m^2
   B. 4167 m^2
   C. 416.7 m^2
   D. 62.5 m^2
   Answer: C. 62.5 m^3/h divided by 0.15 gives 416.7. (Key point: Total area)

4. How many units are provided?
   A. 1 working only
   B. 5 working and no standby
   C. 50 working plus 1 standby
   D. 5 working plus 1 standby, each 6.5 m × 13.0 m
   Answer: D. Area per unit is 83.3 m^2, with a standby for cleaning. (Key point: Units)

## Related in Water Supply Engineering

- [Slow sand filter, 10000 population, base year 2068](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-bk68) — Water Supply Engineering, chapter 6
- [Slow sand filter for 3500 population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-c70o) — Water Supply Engineering, chapter 6
- [Slow sand filter, 10000 pop, 1.5% growth](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-a74) — Water Supply Engineering, chapter 6
- [Slow sand filter, 35,000 in 2019 for 2039](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-c75) — Water Supply Engineering, chapter 6
- [Slow sand filter, 15000 present population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-c76) — Water Supply Engineering, chapter 6
- [Rapid sand filter design, 50,000 population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-c72) — Water Supply Engineering, chapter 6

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