---
title: "Septic tank for 15 users"
subject: "Sanitary Engineering"
chapter: 9
key: "semester_6/sanitary_engineering:septic-tank-15-users-suitable-data"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-15-users-suitable-data
site: BCT Engineering
---

# Septic tank for 15 users

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

- Asked in: 2073 Magh, 4 marks
- Video: [Design of Septic Tanks  II Easy step-by-step calculations for onsite wastewater treatment](https://www.youtube.com/watch?v=VcRbM0MhTPw) — Water Academy
- 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-15-users-suitable-data
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9

## Question

Design a septic tank for a household having average users of 15. Assume suitable data suitably.

## Short answer

With 80 L/c/d sewage and 3-year desludging, required volume is 3.11 m³. Provide a two-chamber septic tank of internal size 2.4 m × 0.9 m × 1.5 m liquid depth plus 0.3 m freeboard.

## Answer

**Assumptions:** water supply 100 L/c/d, sewage 80% = 80 L/c/d, detention 24 h, desludging every 3 years, digestion 0.0425 m³/person, storage 0.085 m³/person for 3 years.

- Sewage flow: $Q = 15 \times 100\times0.8\ \text{L/c/d} = 1200\ \text{L/d} = 1.200\ \text{m}^3\text{/d}$
- Sewage (settling) zone for 24 h detention: $V_1 = 1.200\ \text{m}^3$
- Sludge digestion zone: $0.0425 \times 15 = 0.638\ \text{m}^3$
- Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 15 = 1.275\ \text{m}^3$

$$
V = 1.200 + 0.638 + 1.275 = 3.11\ \text{m}^3
$$

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

$$
\begin{aligned} \text{Plan area} &= 3.11/1.5 = 2.07\ \text{m}^2 \\ B &= \sqrt{2.07/3} \approx 0.9\ \text{m}, \quad L = 2.07/0.9 \approx 2.4\ \text{m} \end{aligned}
$$

Check: $2.4 \times 0.9 \times 1.5 = 3.24\ \text{m}^3 \ge 3.11\ \text{m}^3$ (safe).

**Septic tank:** internal size $2.4\ \text{m} \times 0.9\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $1.60$ m long (2/3), second $0.80$ 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.

## Check yourself

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

1. Sewage flow for 15 users with 100 L/c/d water supply?
   A. 1.5 m^3/d
   B. 1.2 m^3/d
   C. 0.8 m^3/d
   D. 15 m^3/d
   Answer: B. Sewage = 80% of supply: 15 x 80 = 1200 L/d. (Key point: Assumed flow)

2. Total septic tank volume?
   A. 2.5 m^3
   B. 1.2 m^3
   C. 3.11 m^3
   D. 4.0 m^3
   Answer: C. V = 1.200 + 0.638 + 1.275 = 3.11 m^3. (Key point: Total volume)

3. How is the tank divided?
   A. Two equal chambers
   B. Three equal chambers
   C. One chamber
   D. Two chambers: first 2/3 and second 1/3 of the length
   Answer: D. First chamber is 2/3 of length (1.6 m), second 1/3 (0.8 m). (Key point: Chambers)

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