---
title: "Septic tank and soak pit, 15 users"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:septic-tank-soak-pit-15-users-100"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-soak-pit-15-users-100
site: BCT Engineering
---

# Septic tank and soak pit, 15 users

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

- Asked in: 2079 Chaitra, 8 marks
- Video: [How To Design A Septic Tank In 5 Minutes!](https://www.youtube.com/watch?v=EyXRB92HXi4) — Engr Suliman Khan
- 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-15-users-100
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

Design a septic tank and soak pit for 15 users of a house at a place where infiltration capacity of soil is 100 liters per square meter per day. The peak sewage flow rate 110 lpcd and the detention time in septic tank is 1 day and sludge is cleaned at an interval of 3 years. The ground water table lies 10 m below the ground level.

## Short answer

Septic tank: flow 1650 L/d, total volume = 1.650+0.638+1.275 = 3.56 m³, tank 2.7×0.9×1.5 m. Soak pit: required side wall area 1650/100 = 16.5 m²; effective depth 3 m, diameter = 16.5/(π×3) ≈ 1.8 m, lined with open-jointed masonry, bottom 1.5 m above groundwater.

## Answer

**Given:** 15 users, 110 L/c/d, detention 1 day, desludging every 3 years, infiltration 100 L/m²/d, ground-water 10 m below ground (so a deep pit is safe).

#### Septic tank

- Sewage flow: $Q = 15 \times 110\ \text{L/c/d} = 1650\ \text{L/d} = 1.650\ \text{m}^3\text{/d}$
- Sewage (settling) zone for 24 h detention: $V_1 = 1.650\ \text{m}^3$
- Sludge digestion zone: $0.0425 \times 15 = 0.638\ \text{m}^3$
- Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 15 = 1.275\ \text{m}^3$

$$
V = 1.650 + 0.638 + 1.275 = 3.56\ \text{m}^3
$$

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

$$
\begin{aligned} \text{Plan area} &= 3.56/1.5 = 2.38\ \text{m}^2 \\ B &= \sqrt{2.38/3} \approx 0.9\ \text{m}, \quad L = 2.38/0.9 \approx 2.7\ \text{m} \end{aligned}
$$

Check: $2.7 \times 0.9 \times 1.5 = 3.65\ \text{m}^3 \ge 3.56\ \text{m}^3$ (safe).

**Septic tank:** internal size $2.7\ \text{m} \times 0.9\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $1.80$ m long (2/3), second $0.90$ 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: $A = Q/\text{rate} = 1650/100 = 16.50\ \text{m}^2$ (only the side wall below the inlet is counted; the bottom is ignored as it clogs)
- Adopt effective depth $h = 3$ m

$$
\pi D h = 16.50 \Rightarrow D = \frac{16.50}{\pi \times 3} = 1.75 \approx 1.8\ \text{m}
$$

Check: $\pi \times 1.8 \times 3 = 16.96\ \text{m}^2 \ge 16.50\ \text{m}^2$.

**Soak pit:** one circular pit, 1.8 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.

The ground-water table is 10 m deep, far below the pit bottom (3 m), so the 1 m clearance is satisfied.

## Check yourself

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

1. Total septic tank volume is
   A. 1.65 m3
   B. 3.56 m3
   C. 2.29 m3
   D. 4.5 m3
   Answer: B. 1.650 + 0.638 + 1.275 = 3.56 m3. (Key point: Septic volume)

2. At 1.5 m depth and 3:1 L:B, the dimension is found by
   A. Area = V x d
   B. B = V/L only
   C. Area = V/d = 2.38 m2, B = sqrt(A/3) = 0.9 m, L = 2.7 m
   D. L = B/3
   Answer: C. Area from volume over depth, then L = 3B gives B = sqrt(A/3). (Key point: Plan dimensions)

3. In soak pit sizing, which area is counted as infiltrating?
   A. Bottom only
   B. Side wall plus bottom
   C. The top slab
   D. Only the side wall below the inlet; the bottom clogs and is ignored
   Answer: D. A = 1650/100 = 16.5 m2 is provided by pi D h on the sides. (Key point: Soak pit area)

4. With h = 3 m and A = 16.5 m2, the diameter is
   A. 1.75 m, adopt 1.8 m
   B. 5.5 m
   C. 0.58 m
   D. 3.5 m
   Answer: A. D = 16.5/(pi x 3) = 1.75 m; 1.8 m gives 16.96 m2. (Key point: Soak pit size)

## Close variants set in other exams

- [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 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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- [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 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
- [VIP latrine; septic tank for 15 users](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Avip-septic-tank-15-users-80lpcd) — Sanitary Engineering, chapter 9

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