Septic tank and soak pits, 15 users, Terai

Assumptions: Terai has a high ground-water table (about 3 m) and flat land, so soak pits are kept shallow (effective depth 2.0 m) and infiltration rate for sandy soil is taken as 50 L/m²/d; detention 24 h; pit diameter at most 3 m.

Septic tank

• Sewage flow: $Q = 15 \times 45\ \text{L/c/d} = 675\ \text{L/d} = 0.675\ \text{m}^3\text{/d}$
• Sewage (settling) zone for 24 h detention: $V_1 = 0.675\ \text{m}^3$
• Sludge digestion zone: $0.0425 \times 15 = 0.638\ \text{m}^3$
• Digested sludge storage for 4 years: $(0.085/3) \times 4 \times 15 = 1.700\ \text{m}^3$

$$V = 0.675 + 0.638 + 1.700 = 3.01\ \text{m}^3$$

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

$$\begin{aligned} \text{Plan area} &= 3.01/1.5 = 2.01\ \text{m}^2 \\ B &= \sqrt{2.01/3} \approx 0.9\ \text{m}, \quad L = 2.01/0.9 \approx 2.3\ \text{m} \end{aligned}$$

Check: $2.3 \times 0.9 \times 1.5 = 3.11\ \text{m}^3 \ge 3.01\ \text{m}^3$ (safe).

Septic tank: internal size $2.3\ \text{m} \times 0.9\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $1.55$ m long (2/3), second $0.75$ 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 pits

• Required infiltration area: $A = Q/\text{rate} = 675/50 = 13.50\ \text{m}^2$ (only the side wall below the inlet is counted; the bottom is ignored as it clogs)
• Adopt effective depth $h = 2.0$ m

$$\pi D h = 13.50 \Rightarrow D = \frac{13.50}{\pi \times 2.0} = 2.15 \approx 2.2\ \text{m}$$

Check: $\pi \times 2.2 \times 2.0 = 13.82\ \text{m}^2 \ge 13.50\ \text{m}^2$.

Soak pit: one circular pit, 2.2 m internal diameter and 2.0 m deep, lined with dry brick/stone masonry with open joints (honeycomb), filled with brick bats/gravel, and covered with a slab.

Because of the high water table, keep the pit bottom at least 1 m above the highest ground-water level; if this cannot be done use dispersion trenches or an evapotranspiration mound instead.