---
title: "VIP latrine; septic tank for 15 users"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:vip-septic-tank-15-users-80lpcd"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/vip-septic-tank-15-users-80lpcd
site: BCT Engineering
---

# VIP latrine; septic tank for 15 users

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

- Asked in: 2081 Chaitra, 3+5 marks
- Video: [Design Of Septic Tank For 20 Users | What is Septic Tank](https://www.youtube.com/watch?v=2v5IvMzqK1o) — Civil Engineering Global
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/vip-septic-tank-15-users-80lpcd
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

Describe about VIP latrine. Design a septic tank for 15 users. The rate of sewage flow is 80 lpcd. Assume sludge is cleaned from septic tank once in a three year.

## Short answer

A VIP latrine uses a vent pipe and dark interior to remove odours and trap flies. For septic tank: flow 15×80=1200 L/d; volume = 1.200 (settling) + 0.638 (digestion) + 1.275 (3‑yr storage) = 3.11 m³; internal dimensions 2.4×0.9×1.5 m liquid depth, two chambers.

## Answer

#### VIP latrine

A **VIP (ventilated improved pit) latrine** is a pit latrine in which a vertical vent pipe (150 mm dia, 0.5 m above the roof, with fly screen at top) removes smell and traps flies.

- Wind blowing across the pipe top and warm air rising in it draw air from the pit through the squat hole, carrying odour out above the roof.
- The superstructure is dark, so flies are attracted to the light at the top of the pipe, are blocked by the screen and die.
- The pit (lined with open-jointed brick/stone) is 1.5-3 m deep, designed at 0.04 m³/person/year; it has a slab with squat hole and a raised collar against surface water.
- Double-pit versions allow one pit to rest and decompose while the other is used.

```text
        vent (screen)
          |  ___________
          | | dark room |
   _______|_|_O_________|__ slab
   |        pit            |
```

#### Septic tank for 15 users

**Given:** 80 L/c/d, desludging every 3 years; detention 24 h assumed.

- Sewage flow: $Q = 15 \times 80\ \text{L/c/d} = 1200\ \text{L/d} = 1.200\ \text{m}^3\text{/d}$
- Sewage (settling) zone for 24 h detention: $V_1 = 1.200\ \text{m}^3$
- Sludge digestion zone: $0.0425 \times 15 = 0.638\ \text{m}^3$
- Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 15 = 1.275\ \text{m}^3$

$$
V = 1.200 + 0.638 + 1.275 = 3.11\ \text{m}^3
$$

Adopt liquid depth $d = 1.5$ m and $L:B \approx 3:1$.

$$
\begin{aligned} \text{Plan area} &= 3.11/1.5 = 2.07\ \text{m}^2 \\ B &= \sqrt{2.07/3} \approx 0.9\ \text{m}, \quad L = 2.07/0.9 \approx 2.4\ \text{m} \end{aligned}
$$

Check: $2.4 \times 0.9 \times 1.5 = 3.24\ \text{m}^3 \ge 3.11\ \text{m}^3$ (safe).

**Septic tank:** internal size $2.4\ \text{m} \times 0.9\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $1.60$ m long (2/3), second $0.80$ m (1/3), separated by a partition wall with an opening near mid-depth. Inlet and outlet through T-pipes dipping 0.3 m below water level.

## Check yourself

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

1. In a VIP latrine, which feature traps flies?
   A. A water seal in the pan
   B. A fly screen on the vent pipe top with a dark superstructure
   C. A chlorine dose
   D. A sloping floor
   Answer: B. Flies are drawn to the light at the vent's top, are blocked by the mesh, and die. (Key point: VIP principle)

2. Total septic tank volume is made of
   A. Sewage zone only
   B. Digestion zone + free board
   C. Sewage zone + sludge digestion zone + digested sludge storage
   D. Sewage zone x 3 years
   Answer: C. V = 1.200 + 0.638 + 1.275 = 3.11 m3. (Key point: Septic volume components)

3. Digested sludge storage for 15 users, 3 years, with 0.085 m3 per person for 3 years, is
   A. 0.425 m3
   B. 3.825 m3
   C. 0.638 m3
   D. 1.275 m3
   Answer: D. (0.085/3) x 3 x 15 = 1.275 m3. (Key point: Sludge storage)

4. With a 2.4 m long tank in two chambers, the first chamber is about
   A. 1.6 m (2/3 of length)
   B. 1.2 m
   C. 0.8 m
   D. 2.0 m
   Answer: A. First chamber 2/3 L = 1.6 m; second 0.8 m. (Key point: Chambers)

## Related in Sanitary Engineering

- [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, 15 users](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-soak-pit-15-users-100) — Sanitary Engineering, chapter 9, asked 2×
- [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×
- [Septic tank and soak pits, 15 users, Terai](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-15-users-terai) — Sanitary Engineering, chapter 9
- [Septic tank and soak pit, 10 users, 85 l/m²/d](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-10-users-85) — Sanitary Engineering, chapter 9, asked 2×
- [Septic tank and soak pit, household of 80](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-soak-pit-80-persons) — Sanitary Engineering, chapter 9, asked 2×

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