---
title: "Septic tank and soak pit, hostel of 400"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:septic-tank-soak-pit-hostel-400"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-soak-pit-hostel-400
site: BCT Engineering
---

# Septic tank and soak pit, hostel of 400

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

- Asked in: 2077 Chaitra, 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-soak-pit-hostel-400
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

If you are asked to design septic tank and soak pit for a hostel with 400 users, calculate sizes. For septic tank, consider sludge digestion rate of 0.0425 m³/person and volume required for storage of digested sludge 0.085 m³/person for 3 years cleaning period.

## Short answer

Septic tank internal dimensions 11.7 m × 3.9 m × 2.3 m (including 0.3 m freeboard) with two chambers. Total liquid volume 91 m³. Soak pits: 29 circular pits of 3 m diameter and 3 m effective depth, providing 800 m² infiltration area at 50 L/m²/d rate.

## Answer

**Assumptions** (flow and soil data not given): sewage 100 L/person/day, detention 24 h, infiltration 50 L/m²/d (sandy loam), digestion 0.0425 m³/person and storage 0.085 m³/person for 3 years as given. Soak pit effective depth 3 m, pit diameter 3 m.

#### Septic tank

- Sewage flow: $Q = 400 \times 100\ \text{L/c/d} = 40000\ \text{L/d} = 40.000\ \text{m}^3\text{/d}$
- Sewage (settling) zone for 24 h detention: $V_1 = 40.000\ \text{m}^3$
- Sludge digestion zone: $0.0425 \times 400 = 17.000\ \text{m}^3$
- Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 400 = 34.000\ \text{m}^3$

$$
V = 40.000 + 17.000 + 34.000 = 91.00\ \text{m}^3
$$

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

$$
\begin{aligned} \text{Plan area} &= 91.00/2.0 = 45.50\ \text{m}^2 \\ B &= \sqrt{45.50/3} \approx 3.9\ \text{m}, \quad L = 45.50/3.9 \approx 11.7\ \text{m} \end{aligned}
$$

Check: $11.7 \times 3.9 \times 2.0 = 91.26\ \text{m}^3 \ge 91.00\ \text{m}^3$ (safe).

**Septic tank:** internal size $11.7\ \text{m} \times 3.9\ \text{m} \times 2.3\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $7.80$ m long (2/3), second $3.90$ 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: $A = Q/\text{rate} = 40000/50 = 800.00\ \text{m}^2$ (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 = $\pi \times 3.0 \times 3 = 28.27\ \text{m}^2$).

$$
N = \frac{800.00}{28.27} = 28.29 \Rightarrow 29\ \text{pits}
$$

**Soak pits:** 29 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.

Such a large number of soak pits is impractical on a small site; in practice a hostel of this size uses several septic tanks in parallel or a series of dispersion trenches / a small package treatment plant followed by disposal.

## Check yourself

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

1. Flow 100 L/c/d for 400 users and 24 h detention gives a sewage zone of
   A. 40 m3
   B. 4 m3
   C. 400 m3
   D. 16.7 m3
   Answer: A. 400 x 100 = 40,000 L/d = 40 m3. (Key point: Sewage zone)

2. Digested sludge storage for 400 users is
   A. 17 m3
   B. 34 m3 (0.085 x 400)
   C. 11.3 m3
   D. 3.4 m3
   Answer: B. 0.085 m3/person for 3 years x 400 = 34 m3; total 40 + 17 + 34 = 91 m3. (Key point: Storage volume)

3. Required infiltration area at 50 L/m2/d is
   A. 80 m2
   B. 8000 m2
   C. 800 m2
   D. 2000 m2
   Answer: C. A = 40000/50 = 800 m2. (Key point: Soak pit area)

4. Why is the answer noted as impractical on a small site?
   A. One pit is too large to build
   B. Septic tanks cannot treat hostels
   C. Soak pits need a pump
   D. It needs about 29 large pits, so trenches or package treatment are used
   Answer: D. 800/28.27 gives 29 pits; for this size parallel tanks, trenches or a small treatment plant are preferred. (Key point: Practical limit)

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