---
title: "Septic tank and soak pit, household of 80"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:septic-tank-soak-pit-80-persons"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-soak-pit-80-persons
site: BCT Engineering
---

# Septic tank and soak pit, household of 80

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

- Asked in: 2076 Baisakh, 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=JXnJRgaiGTA) — 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-soak-pit-80-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 80 persons. The sewage is generated at the rate of 120 lit/person/day. Assume that septic tank is cleaned once in 2 years and infiltration rate of soil is 80 lit/m²/day.

## Short answer

Septic tank internal dimensions 4.9 m × 1.8 m × 2.3 m (including 0.3 m freeboard) with two chambers. Total liquid volume 17.53 m³. Soak pits: 5 circular pits of 3 m diameter and 3 m effective depth, providing 120 m² infiltration area at 80 L/m²/d rate.

## Answer

**Given:** 80 persons, 120 L/c/d, desludging every 2 years, infiltration 80 L/m²/d. Assume 24 h detention and soak pit effective depth 3 m (ground-water table deep).

#### Septic tank

- Sewage flow: $Q = 80 \times 120\ \text{L/c/d} = 9600\ \text{L/d} = 9.600\ \text{m}^3\text{/d}$
- Sewage (settling) zone for 24 h detention: $V_1 = 9.600\ \text{m}^3$
- Sludge digestion zone: $0.0425 \times 80 = 3.400\ \text{m}^3$
- Digested sludge storage for 2 years: $(0.085/3) \times 2 \times 80 = 4.533\ \text{m}^3$

$$
V = 9.600 + 3.400 + 4.533 = 17.53\ \text{m}^3
$$

Adopt liquid depth $d = 2.0$ m and $L:B \approx 3:1$.

$$
\begin{aligned} \text{Plan area} &= 17.53/2.0 = 8.77\ \text{m}^2 \\ B &= \sqrt{8.77/3} \approx 1.8\ \text{m}, \quad L = 8.77/1.8 \approx 4.9\ \text{m} \end{aligned}
$$

Check: $4.9 \times 1.8 \times 2.0 = 17.64\ \text{m}^3 \ge 17.53\ \text{m}^3$ (safe).

**Septic tank:** internal size $4.9\ \text{m} \times 1.8\ \text{m} \times 2.3\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $3.30$ m long (2/3), second $1.60$ 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: $A = Q/\text{rate} = 9600/80 = 120.00\ \text{m}^2$ (only the side wall below the inlet is counted; the bottom is ignored as it clogs)
- Adopt effective depth $h = 3$ m

A single pit would be too large, so adopt pits of $D = 3.0$ m (area of each = $\pi \times 3.0 \times 3 = 28.27\ \text{m}^2$).

$$
N = \frac{120.00}{28.27} = 4.24 \Rightarrow 5\ \text{pits}
$$

**Soak pits:** 5 circular pits, each 3.0 m diameter and 3 m deep, honeycomb-lined and filled with brick bats/gravel, spaced at least 3 m (about 2 times the diameter) apart and connected through a distribution box. Soak-pit bottoms must stay at least 1 m above the highest ground-water level.

## Check yourself

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

1. Which three volumes are added to get the total septic tank capacity?
   A. Settling zone, sludge digestion zone and digested sludge storage
   B. Settling zone, soak pit volume and free board
   C. Sludge digestion zone, free board and inlet chamber
   D. Settling zone, scum zone and distribution box
   Answer: A. V = sewage (settling) zone + sludge digestion zone + digested sludge storage for the desludging period. (Key point: Capacity components)

2. For 80 persons at 120 L/c/d and 24 h detention, what is the settling-zone volume?
   A. 4.8 m^3
   B. 9.6 m^3
   C. 17.5 m^3
   D. 2.4 m^3
   Answer: B. Q = 80 x 120 = 9600 L/d = 9.6 m^3/d, and 24 h detention gives V1 = 9.6 m^3. (Key point: Settling zone volume)

3. Why is only the side wall of the soak pit counted as infiltration area?
   A. The bottom is always in ground water
   B. The side wall has higher infiltration rate
   C. The bottom is ignored because it clogs with solids
   D. The bottom is covered by the slab
   Answer: C. The pit bottom clogs quickly, so only the side wall below the inlet is taken as effective infiltration area. (Key point: Soak pit area)

4. Required area is 120 m^2 and each 3 m dia x 3 m deep pit gives 28.27 m^2. How many pits are adopted?
   A. 4
   B. 6
   C. 3
   D. 5
   Answer: D. 120/28.27 = 4.24, which is rounded up to 5 pits. (Key point: Number of pits)

## Close variants set in other exams

- [Septic tank and soak pit, 8 persons](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-8-persons) — Sanitary Engineering, chapter 9, asked 2×

## Related in Sanitary Engineering

- [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×
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- [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, hotel of 80](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aseptic-tank-hotel-80-users) — Sanitary Engineering, chapter 9

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