Septic tank and soak pit, 8 persons

Given: 8 persons, 100 L/c/d, desludging every 3 years, infiltration 50 L/m²/d. Detention 24 h assumed; soak pit depth 3 m.

Septic tank

• Sewage flow: $Q = 8 \times 100\ \text{L/c/d} = 800\ \text{L/d} = 0.800\ \text{m}^3\text{/d}$
• Sewage (settling) zone for 24 h detention: $V_1 = 0.800\ \text{m}^3$
• Sludge digestion zone: $0.0425 \times 8 = 0.340\ \text{m}^3$
• Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 8 = 0.680\ \text{m}^3$

$$V = 0.800 + 0.340 + 0.680 = 1.82\ \text{m}^3$$

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

$$\begin{aligned} \text{Plan area} &= 1.82/1.5 = 1.21\ \text{m}^2 \\ B &= \sqrt{1.21/3} \approx 0.7\ \text{m}, \quad L = 1.21/0.7 \approx 1.8\ \text{m} \end{aligned}$$

Check: $1.8 \times 0.7 \times 1.5 = 1.89\ \text{m}^3 \ge 1.82\ \text{m}^3$ (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: $A = Q/\text{rate} = 800/50 = 16.00\ \text{m}^2$ (only the side wall below the inlet is counted; the bottom is ignored as it clogs)
• Adopt effective depth $h = 3$ m

$$\pi D h = 16.00 \Rightarrow D = \frac{16.00}{\pi \times 3} = 1.70 \approx 1.7\ \text{m}$$

Check: $\pi \times 1.7 \times 3 = 16.02\ \text{m}^2 \ge 16.00\ \text{m}^2$.

Soak pit: one circular pit, 1.7 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.