Septic tank for 15 users

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.