---
title: "Septic tank and soak pit, 10 users, 85 l/m²/d"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:septic-tank-10-users-85"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-10-users-85
site: BCT Engineering
---

# Septic tank and soak pit, 10 users, 85 l/m²/d

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

- Asked in: 2072 Asoj, 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/septic-tank-10-users-85
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

Design a septic tank and soak pit for 10 number of users of a house at a place where infiltration capacity of soil is 85 liters per square meter per day. The peak sewage flow rate 80 lpcd and the septic tank is cleaned at an interval of 2 years. The ground water table lies 6 m below the ground level.

## 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 1.79 m³. Soak pit: one circular pit of 1.0 m diameter and 3 m effective depth, providing 9.41 m² infiltration area at 85 L/m²/d rate.

## Answer

**Given:** 10 users, 80 L/c/d (peak), desludging every 2 years, infiltration 85 L/m²/d, ground-water 6 m deep. Detention 24 h assumed; soak pit depth 3 m keeps 3 m clearance above ground-water.

#### Septic tank

- 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 2 years: $(0.085/3) \times 2 \times 10 = 0.567\ \text{m}^3$

$$
V = 0.800 + 0.425 + 0.567 = 1.79\ \text{m}^3
$$

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

$$
\begin{aligned} \text{Plan area} &= 1.79/1.5 = 1.19\ \text{m}^2 \\ B &= \sqrt{1.19/3} \approx 0.7\ \text{m}, \quad L = 1.19/0.7 \approx 1.8\ \text{m} \end{aligned}
$$

Check: $1.8 \times 0.7 \times 1.5 = 1.89\ \text{m}^3 \ge 1.79\ \text{m}^3$ (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: $A = Q/\text{rate} = 800/85 = 9.41\ \text{m}^2$ (only the side wall below the inlet is counted; the bottom is ignored as it clogs)
- Adopt effective depth $h = 3$ m

$$
\pi D h = 9.41 \Rightarrow D = \frac{9.41}{\pi \times 3} = 1.00 \approx 1.0\ \text{m}
$$

Check: $\pi \times 1.0 \times 3 = 9.42\ \text{m}^2 \ge 9.41\ \text{m}^2$.

**Soak pit:** one circular pit, 1.0 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. How much sludge digestion volume is provided for 10 users?
   A. 0.425 m^3
   B. 0.8 m^3
   C. 0.567 m^3
   D. 4.25 m^3
   Answer: A. Digestion zone is 0.0425 m^3 per person, so 0.0425 x 10 = 0.425 m^3. (Key point: Sludge digestion)

2. Total septic tank volume for 10 users, 80 lpcd, 2 years?
   A. 2.08 m^3
   B. 1.79 m^3
   C. 1.23 m^3
   D. 0.8 m^3
   Answer: B. V = 0.800 + 0.425 + 0.567 = 1.79 m^3. (Key point: Total volume)

3. What infiltration area is required for the soak pit?
   A. 85/800 = 0.11 m^2
   B. 800 x 85 m^2
   C. 800/85 = 9.41 m^2
   D. 1790/85 = 21 m^2
   Answer: C. Area = daily flow (800 L/d) divided by infiltration rate (85 L/m^2/d). (Key point: Soak pit area)

4. With h = 3 m and area 9.41 m^2, what diameter is found from pi D h?
   A. 0.5 m
   B. 2.0 m
   C. 3.0 m
   D. 1.0 m
   Answer: D. D = 9.41/(pi x 3) = 1.0 m. (Key point: Pit diameter)

## Close variants set in other exams

- [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×

## Related in Sanitary Engineering

- [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×
- [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×
- [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 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
- [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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