---
title: "VIP latrine: working and design procedure"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:vip-latrine-working-design"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/vip-latrine-working-design
site: BCT Engineering
---

# VIP latrine: working and design procedure

Sanitary Engineering · Chapter 9: Disposal of Sewage from Isolated Buildings

- Asked in: 2071 Bhadra, 8 marks; 2070 Magh, 8 marks (asked 2 times)
- Video: [Design criteria + Procedure of VIP or PIT lattrine l|5thsem|Sanitary Engineering|CTEVT|Prashant YT|](https://www.youtube.com/watch?v=pdrROcvvDlU) — Prashant YT
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/vip-latrine-working-design
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

With a neat sketch describe the working and design procedure of ventilated improved pit latrine.

## Short answer

Excreta fall into the pit and decompose anaerobically; wind and warm air in the vent pipe create suction, drawing odours out while the dark superstructure and fly screen trap insects. Pit volume is designed at 0.04–0.06 m³/person/year plus 0.5 m free space; depth 1.5–3 m; vent pipe 150 mm Ø, 0.5 m above roof.

## Answer

A **ventilated improved pit (VIP) latrine** is a pit latrine with a vent pipe that removes odours and traps flies, so it is practically free of smell and insect nuisance.

#### Working

1. Excreta fall into the pit through the squat hole in the slab and are digested anaerobically in the pit.
2. Wind blowing across the top of the vent pipe creates suction, and warm air rising in the pipe (heated by the sun) draws air from the pit. Fresh air enters through the squat hole, passes down into the pit and up the vent, carrying the smell out above the roof.
3. The superstructure is kept fairly dark inside. Flies attracted by the odour at the top of the vent cannot enter because of the fly screen. Flies that breed in the pit move towards the light at the top of the vent pipe, are stopped by the screen, and are trapped there until they die.
4. Liquid seeps into the soil; solids decompose and reduce in volume.

```text
          wind -->
      fly screen  _
         |       | |  vent pipe, 0.5 m
      ___|_______|_|  above roof
     |  dark superstructure |
     |  O squat hole        |
 ____|______________________|__ slab
 |                           |
 |   pit (lined, open joints) |
 |___________________________|
```

#### Design procedure

1. Find the number of users $n$ and the design period $T$ (years) between emptying (single pit: 5-10 years; double pit: 1-2 years each).
2. Sludge accumulation rate $r$: 0.04 m³/person/year (wet pit) or 0.06 m³/person/year (dry cleansing material).
3. Pit volume $V = n \times r \times T$, plus 0.5 m free space above the sludge level.
4. Choose the plan (circular 1.0-1.5 m dia or rectangular) so that the depth is within 1.5-3 m, with the pit bottom at least 1.5 m above the ground-water level.
5. Pit lining: brick or stone with open joints, with the top 0.3 m in cement mortar (watertight), and the slab on a raised collar to keep out surface water.
6. Vent pipe: internal diameter 150 mm (min. 110 mm), 0.5 m above the highest point of the roof, on the sunny side, fitted with a fly screen.
7. Superstructure: 1.0 m $\times$ 1.2 m in plan, with walls of local material and a roof; the inside is kept fairly dark, and the door faces the prevailing wind.

**Example:** for 5 users, 3 years, wet pit: $V = 5 \times 0.04 \times 3 = 0.6$ m³; using a 1 m dia circular pit (area 0.785 m²), depth $= 0.6/0.785 + 0.5 = 1.3$ m (adopt 1.5 m).

## Check yourself

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

1. How is smell removed from a VIP latrine?
   A. The pit is flushed with water
   B. Wind across the vent top and warm rising air draw pit air up the vent, with fresh air entering via the squat hole
   C. Chemicals neutralise odour
   D. Air is pushed down the vent pipe
   Answer: B. Suction at the vent top draws air down the squat hole, through the pit and up the vent, out above the roof. (Key point: Odour removal)

2. Why is the superstructure kept dark inside?
   A. To keep the pit warm
   B. To reduce evaporation
   C. So flies are attracted to the light at the top of the vent, where the screen traps them
   D. To stop users seeing the pit
   Answer: C. Flies seek light, are blocked by the mesh and die. (Key point: Fly trapping)

3. Pit volume is calculated as
   A. V = n + r + T
   B. V = n x r / T
   C. V = r x T only
   D. V = n x r x T, plus 0.5 m free space
   Answer: D. Users x accumulation rate x design period, with 0.5 m free space above sludge level. (Key point: Pit volume)

4. Sludge accumulation rate r is
   A. 0.04 m3/person/year for a wet pit and 0.06 for dry cleansing material
   B. 0.06 for wet and 0.04 for dry
   C. 0.4 for both
   D. 0.004 for both
   Answer: A. Dry materials like paper add more bulk, so 0.06 is used for dry pits. (Key point: Accumulation rate)

## Related in Sanitary Engineering

- [Double pit VIP latrine for 15 users](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Avip-double-pit-15-users) — Sanitary Engineering, chapter 9, asked 2×
- [Design a VIP latrine, household of 10](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Avip-latrine-design-10-persons) — Sanitary Engineering, chapter 9
- [VIP latrine; septic tank for 15 users](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Avip-septic-tank-15-users-80lpcd) — Sanitary Engineering, chapter 9
- [VIP latrine and septic tank, family of 10](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Avip-latrine-septic-tank-10-family) — Sanitary Engineering, chapter 9
- [Septic tank and soak pit design in Nepal](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-design-considerations-nepal) — Sanitary Engineering, chapter 9, asked 2×