Septic tank and soak pit, 15 users

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.