---
title: "Septic tank and soak pit, 8 persons"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:septic-tank-8-persons"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-8-persons
site: BCT Engineering
---

# Septic tank and soak pit, 8 persons

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

- Asked in: 2069 Bhadra, 8 marks
- Video: [Design of Septic tank and Soak pit | Construction of Septic tank and soak pit | septic tank design](https://www.youtube.com/watch?v=4Iza6IkOqo0) — Civil Engineering Exam
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-8-persons
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

Design a septic tank and soak pit to dispose the sewage generated from a household of 8 persons. The sewage is generated at the rate of 100 liters/person/day. Assume that septic tank is cleaned once in 3 years and infiltration rate of soil is 50 liters/m²/day.

## Short answer

Septic tank internal dimensions 1.8 m × 0.7 m × 1.8 m (including 0.3 m freeboard) with two chambers. Total liquid volume 1.82 m³. Soak pit: one circular pit of 1.7 m diameter and 3 m effective depth, providing 16 m² infiltration area at 50 L/m²/d rate.

## Answer

**Given:** 8 persons, 100 L/c/d, desludging every 3 years, infiltration 50 L/m²/d. Detention 24 h assumed; soak pit depth 3 m.

#### Septic tank

- Sewage flow: $Q = 8 \times 100\ \text{L/c/d} = 800\ \text{L/d} = 0.800\ \text{m}^3\text{/d}$
- Sewage (settling) zone for 24 h detention: $V_1 = 0.800\ \text{m}^3$
- Sludge digestion zone: $0.0425 \times 8 = 0.340\ \text{m}^3$
- Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 8 = 0.680\ \text{m}^3$

$$
V = 0.800 + 0.340 + 0.680 = 1.82\ \text{m}^3
$$

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

$$
\begin{aligned} \text{Plan area} &= 1.82/1.5 = 1.21\ \text{m}^2 \\ B &= \sqrt{1.21/3} \approx 0.7\ \text{m}, \quad L = 1.21/0.7 \approx 1.8\ \text{m} \end{aligned}
$$

Check: $1.8 \times 0.7 \times 1.5 = 1.89\ \text{m}^3 \ge 1.82\ \text{m}^3$ (safe).

**Septic tank:** internal size $1.8\ \text{m} \times 0.7\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $1.20$ m long (2/3), second $0.60$ 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.

#### Soak pit

- Required infiltration area: $A = Q/\text{rate} = 800/50 = 16.00\ \text{m}^2$ (only the side wall below the inlet is counted; the bottom is ignored as it clogs)
- Adopt effective depth $h = 3$ m

$$
\pi D h = 16.00 \Rightarrow D = \frac{16.00}{\pi \times 3} = 1.70 \approx 1.7\ \text{m}
$$

Check: $\pi \times 1.7 \times 3 = 16.02\ \text{m}^2 \ge 16.00\ \text{m}^2$.

**Soak pit:** one circular pit, 1.7 m internal diameter and 3 m deep, lined with dry brick/stone masonry with open joints (honeycomb), filled with brick bats/gravel, and covered with a slab.

## Check yourself

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

1. For 8 persons at 100 L/c/d, what is Q?
   A. 0.8 m^3/d
   B. 8 m^3/d
   C. 0.08 m^3/d
   D. 1.0 m^3/d
   Answer: A. Q = 8 x 100 = 800 L/d = 0.8 m^3/d. (Key point: Flow)

2. What is the total volume for 3-year cleaning?
   A. 1.14 m^3
   B. 1.82 m^3
   C. 2.4 m^3
   D. 0.8 m^3
   Answer: B. V = 0.800 + 0.340 + 0.680 = 1.82 m^3. (Key point: Total volume)

3. Why is the check 1.8 x 0.7 x 1.5 = 1.89 m^3 done?
   A. To compute free board
   B. To find the soak pit size
   C. To confirm the rounded size is not smaller than the required volume
   D. To fix the infiltration rate
   Answer: C. Rounded dimensions must give volume at least equal to the required 1.82 m^3. (Key point: Safety check)

4. Area 16 m^2 and depth 3 m: what diameter?
   A. 3.4 m
   B. 0.85 m
   C. 5.3 m
   D. 1.7 m
   Answer: D. D = 16/(pi x 3) = 1.70 m. (Key point: Soak pit diameter)

## Close variants set in other exams

- [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×

## Related in Sanitary Engineering

- [Septic tank and drain field, 22 persons](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-drain-field-22) — 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×
- [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, hostel of 400](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-soak-pit-hostel-400) — 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×
- [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

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