---
title: "VIP latrine and septic tank, family of 10"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:vip-latrine-septic-tank-10-family"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/vip-latrine-septic-tank-10-family
site: BCT Engineering
---

# VIP latrine and septic tank, family of 10

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

- Asked in: 2075 Baisakh, 8 marks
- 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-septic-tank-10-family
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

Design VIP latrine and septic tank for a family of 10 users. The detention time for septic tank is 24 hr. Sludge is cleaned in every three years.

## Short answer

VIP latrine: two pits each 1.0 m × 1.0 m × 1.5 m deep, 150 mm vent pipe with fly screen. Septic tank: flow 800 L/d, volume 2.08 m³, size 2.0 m × 0.7 m × 1.5 m liquid depth plus 0.3 m freeboard, in two chambers.

## Answer

#### 1. VIP latrine for 10 users

Assumptions: wet pit, sludge accumulation 0.04 m³/person/year; double pit, each pit lasts 2 years; free space 0.5 m.

$$
V = 10 \times 0.04 \times 2 = 0.8\ \text{m}^3 \text{ per pit}
$$

Adopt pit $1.0\ \text{m} \times 1.0\ \text{m}$ plan: effective depth $0.8$ m + 0.5 m free space = **1.3 m** (adopt 1.5 m deep). Provide two such pits, a 150 mm vent pipe 0.5 m above roof with fly screen, RCC squat slab and a $1.0 \times 1.2$ m superstructure with a dark interior. Pits are lined with open-jointed brick/stone and the pits are at least 0.5 m apart.

#### 2. Septic tank for 10 users

Assumed sewage rate 80 L/person/day (80% of 100 lpcd water supply), detention 24 h, desludging every 3 years.

- Sewage flow: $Q = 10 \times 80\ \text{L/c/d} = 800\ \text{L/d} = 0.800\ \text{m}^3\text{/d}$
- Sewage (settling) zone for 24 h detention: $V_1 = 0.800\ \text{m}^3$
- Sludge digestion zone: $0.0425 \times 10 = 0.425\ \text{m}^3$
- Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 10 = 0.850\ \text{m}^3$

$$
V = 0.800 + 0.425 + 0.850 = 2.08\ \text{m}^3
$$

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

$$
\begin{aligned} \text{Plan area} &= 2.08/1.5 = 1.38\ \text{m}^2 \\ B &= \sqrt{1.38/3} \approx 0.7\ \text{m}, \quad L = 1.38/0.7 \approx 2.0\ \text{m} \end{aligned}
$$

Check: $2.0 \times 0.7 \times 1.5 = 2.10\ \text{m}^3 \ge 2.08\ \text{m}^3$ (safe).

**Septic tank:** internal size $2.0\ \text{m} \times 0.7\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $1.35$ m long (2/3), second $0.65$ 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. How is each VIP pit volume found?
   A. Users x sludge rate x years = 10 x 0.04 x 2 = 0.8 m^3
   B. 10 x 0.04 x 3 = 1.2 m^3
   C. 10 x 80 L
   D. 0.04 x 2
   Answer: A. Volume = n x r x T with sludge accumulation 0.04 m^3/person/year. (Key point: VIP pit volume)

2. Why is 0.5 m added to the 0.8 m sludge depth?
   A. Allowance for vent pipe
   B. Free space above sludge
   C. Foundation depth
   D. Water table margin
   Answer: B. 0.8 m for sludge plus 0.5 m free space gives 1.3 m, adopted 1.5 m. (Key point: Pit depth)

3. What is the purpose of the vent pipe with fly screen?
   A. Fill the pit with water
   B. Support the slab
   C. Remove odour and trap flies
   D. Drain leachate
   Answer: C. The vent carries gases above roof level and the screen traps flies; the dark interior draws them to the vent. (Key point: Vent pipe)

4. Total tank volume for 10 users?
   A. 1.5 m^3
   B. 0.8 m^3
   C. 3.2 m^3
   D. 2.08 m^3
   Answer: D. V = 0.800 + 0.425 + 0.850 = 2.08 m^3. (Key point: Septic tank volume)

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