---
title: "Double pit VIP latrine for 15 users"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:vip-double-pit-15-users"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/vip-double-pit-15-users
site: BCT Engineering
---

# Double pit VIP latrine for 15 users

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

- Asked in: 2071 Magh, 4 marks; 2068 Bhadra (old course), 8 marks (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/vip-double-pit-15-users
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

Design a double pit VIP latrine for a family of 15 users. Assume the necessary data suitably.

## Short answer

Each pit volume for 15 users at 0.04 m³/person/yr for 2 years = 1.2 m³. Pit dimensions 1.2 m × 1.0 m × 1.5 m depth. Two pits used alternately, vent pipe 150 mm with fly screen, superstructure with dark interior, 15 m from water sources.

## Answer

A **double pit VIP (ventilated improved pit) latrine** has two pits side by side, used one at a time. When one pit is full it is closed and the second is used; the contents of the first pit decompose for a year or more and are then safely emptied and reused as manure.

#### Design data (assumed)

- Users: 15 (family/joint use).
- Sludge accumulation rate (wet pit, water used for anal cleansing): 0.04 m³/person/year.
- Each pit is designed to last 2 years, so while one is in use the other rests for 2 years.
- Free space above sludge: 0.5 m.

#### Pit size (each pit)

$$
V = 15 \times 0.04 \times 2 = 1.2\ \text{m}^3
$$

Adopt a pit plan of 1.2 m $\times$ 1.0 m = 1.2 m², so effective depth = 1.2/1.2 = 1.0 m, plus 0.5 m free space = **1.5 m total depth**.

#### Provisions

- **Two pits** of 1.2 m $\times$ 1.0 m $\times$ 1.5 m, with a clear gap of 0.5 m or more between them; lined with brick/stone in cement mortar with open joints in the lower part, and a watertight upper 0.3 m collar.
- **Slab:** RCC slab with a squat hole over each pit; the hole over the resting pit is sealed with a concrete cover, so excreta drop only into the pit in use.
- **Vent pipe:** 150 mm dia (not less than 110 mm) PVC/GI/bamboo pipe, extending at least 0.5 m above the roof, fitted with a fly screen (mesh) at the top; one vent per pit. Place the vent pipes on the sunny side of the superstructure.
- **Superstructure:** about 1.0 m $\times$ 1.2 m in plan, 2 m high, with a fairly dark interior so that flies move towards the light at the top of the vent; the door should face the prevailing wind, and the roof slopes to the back.
- Located at least 15 m from water sources and downhill of wells.

```text
  vent pipe (fly screen)
        |   superstructure
   _____|______
  | [door]  O  |   O = squat hole
  |____________|__ slab
  | Pit A | Pit B |   1.2 x 1.0 x 1.5 m each
  | (use) | (rest)|
```

## Check yourself

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

1. Each pit for 15 users, 0.04 m3/person/year, 2 years, needs
   A. 0.6 m3
   B. 1.2 m3
   C. 2.4 m3
   D. 0.3 m3
   Answer: B. V = 15 x 0.04 x 2 = 1.2 m3. (Key point: Pit volume)

2. With a 1.2 m2 plan, why is the total depth 1.5 m?
   A. 1.2 m plus 0.3 m collar
   B. 2 x 0.75 m for two pits
   C. 1.0 m effective depth for sludge plus 0.5 m free space
   D. Depth equals plan length plus width
   Answer: C. Effective depth = 1.2/1.2 = 1.0 m; add 0.5 m free space above sludge. (Key point: Total depth)

3. What is the advantage of a double pit latrine?
   A. Both pits are used together to double capacity
   B. Faecal matter is never emptied
   C. The vent pipe is not required
   D. One pit rests and decomposes while the other is used, then is safely emptied
   Answer: D. Resting for a year or more makes contents safe to remove as manure. (Key point: Why two pits)

4. How does the superstructure help control flies?
   A. A dark interior makes flies move to the light at the screened vent top
   B. A bright interior attracts flies out the door
   C. The roof slopes toward the front
   D. The door faces the vent
   Answer: A. Flies go toward the vent's light, are blocked by the fly screen and die. (Key point: Fly control)

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