---
title: "Slow sand filter for 3500 population"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:ssf-c70o"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ssf-c70o
site: BCT Engineering
---

# Slow sand filter for 3500 population

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2070 Chaitra (old course)
- 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-c70o
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Average water consumption rate is 45 lps in the village. Design a slow sand filtration unit for a community having the population of 3500 at the base year 2048 AD. Assume necessary data suitably.

## Short answer

Design flow is 157.5 m³/day (6.56 m³/h) for 3,500 people at 45 lpcd. Using filtration rate 0.2 m³/m²/h, total filter area 32.8 m². Provide 2 working + 1 standby units, each 2.9m × 5.8m (16.8 m²), total depth 2.7m including sand, gravel, under-drain and freeboard.

## Answer

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
| 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: 2 working + 1 standby slow sand filters, each 2.9 m × 5.8 m (16.8 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. Daily demand for 3500 people at 45 lpcd is
   A. 1575 m^3/day
   B. 15.75 m^3/day
   C. 157.5 m^3/day
   D. 45 m^3/day
   Answer: C. 3500 × 45 L = 157 500 L. (Key point: Demand)

2. Total filter area at 0.2 m/h is
   A. 32.8 m^2
   B. 3.28 m^2
   C. 328 m^2
   D. 6.56 m^2
   Answer: A. 6.56/0.2 = 32.8. (Key point: Area)

3. What is provided?
   A. 1 only
   B. 2 working + 1 standby, each 2.9 × 5.8 m
   C. 10 working
   D. 3 working with none spare
   Answer: B. Minimum two working plus one standby. (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, 15000 present population](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-c76) — Water Supply Engineering, chapter 6
- [Slow sand filter, 15,000 population, 150 L/h/m2](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-a75) — 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
- [Rapid sand filter, 45,000 population in 2021](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Arsf-a76) — 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