---
title: "Slow sand filter, 10000 population, base year 2068"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:ssf-bk68"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/ssf-bk68
site: BCT Engineering
---

# Slow sand filter, 10000 population, base year 2068

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2068 Baisakh (old course), 8 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-bk68
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Average water consumption rate is 150 lpcd in an urban area. Design a slow sand filtration unit for a community having the population of 10000 at the base year 2068.

## Short answer

Design flow is 1500 m³/day (62.5 m³/h) for 10,000 people at 150 lpcd. Using filtration rate 0.2 m³/m²/h, total filter area 312.5 m². Provide 4 working + 1 standby units, each 6.3m × 12.6m (79.4 m²), total depth 2.7m including sand, gravel, under-drain and freeboard.

## Answer

The population of 10,000 at the base year is taken as the design population (no growth rate is given). A filtration rate of 0.2 m³/m²/h is assumed.

#### Step 1: Design flow
- Design population = 10,000 (given)
- Daily demand $Q = 10,000 \times 150/1000 = 1,500.0$ m³/day = 62.50 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 $A = Q/(\text{rate}) = 62.50/0.2 = 312.5$ m²
- Keep each unit below about 100 m² and use at least 2 working units: working units **n = 4**, plus **1 standby** unit for cleaning, so 5 units in all
- Area of one unit $= 312.5/4 = 78.1$ m²
- Take $L = 2B$: $B = \sqrt{78.1/2} = 6.25$ m. Adopt **B = 6.3 m, L = 12.6 m** (area 79.4 m² each)
- Check: rate with 4 working units = 0.197 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: 4 working + 1 standby slow sand filters, each 6.3 m × 12.6 m (79.4 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. What population is used when no growth is given?
   A. 10 000 at the base year
   B. 20 000
   C. 5000
   D. 2068
   Answer: A. 2068 is a BS year, not population. (Key point: Population)

2. Total area for 62.5 m^3/h at 0.2 m/h is
   A. 31.25 m^2
   B. 312.5 m^2
   C. 12.5 m^2
   D. 3125 m^2
   Answer: B. A = Q/rate. (Key point: Total area)

3. What are the sand and gravel depths?
   A. 0.3 m sand and 0.9 m gravel
   B. 0.7 m sand, 0.45 m gravel
   C. 0.9 m sand and 0.3 m gravel in 3 layers
   D. 1.5 m sand, none gravel
   Answer: C. Slow filters have deeper, finer sand (ES 0.2 to 0.3 mm). (Key point: Layers)

4. How many units?
   A. 4 working and 1 standby, 6.3 × 12.6 m each
   B. 1 unit
   C. 8 units
   D. 4 units with no standby
   Answer: A. 312.5/4 = 78.1 m^2, B = 6.25, adopt 6.3. (Key point: Answer)

## Related in Water Supply Engineering

- [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 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, 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, 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, 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 construction and design](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Assf-construction-design) — Water Supply Engineering, chapter 6