Septic tank and soak pits, hotel of 80

Assumptions: mid-southern zone (Terai/inner Terai) with sandy loam soil, infiltration rate 50 L/m²/d, ground-water at least 5 m below the surface; detention 24 h, soak pit depth 3 m and diameter 3 m.

Septic tank

• Sewage flow: $Q = 80 \times 210\ \text{L/c/d} = 16800\ \text{L/d} = 16.800\ \text{m}^3\text{/d}$
• Sewage (settling) zone for 24 h detention: $V_1 = 16.800\ \text{m}^3$
• Sludge digestion zone: $0.0425 \times 80 = 3.400\ \text{m}^3$
• Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 80 = 6.800\ \text{m}^3$

$$V = 16.800 + 3.400 + 6.800 = 27.00\ \text{m}^3$$

Adopt liquid depth $d = 2.0$ m and $L:B \approx 3:1$.

$$\begin{aligned} \text{Plan area} &= 27.00/2.0 = 13.50\ \text{m}^2 \\ B &= \sqrt{13.50/3} \approx 2.2\ \text{m}, \quad L = 13.50/2.2 \approx 6.2\ \text{m} \end{aligned}$$

Check: $6.2 \times 2.2 \times 2.0 = 27.28\ \text{m}^3 \ge 27.00\ \text{m}^3$ (safe).

Septic tank: internal size $6.2\ \text{m} \times 2.2\ \text{m} \times 2.3\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $4.15$ m long (2/3), second $2.05$ 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: $A = Q/\text{rate} = 16800/50 = 336.00\ \text{m}^2$ (only the side wall below the inlet is counted; the bottom is ignored as it clogs)
• Adopt effective depth $h = 3$ m

A single pit would be too large, so adopt pits of $D = 3.0$ m (area of each = $\pi \times 3.0 \times 3 = 28.27\ \text{m}^2$).

$$N = \frac{336.00}{28.27} = 11.88 \Rightarrow 12\ \text{pits}$$

Soak pits: 12 circular pits, each 3.0 m diameter and 3 m deep, honeycomb-lined and filled with brick bats/gravel, spaced at least 3 m (about 2 times the diameter) apart and connected through a distribution box. Soak-pit bottoms must stay at least 1 m above the highest ground-water level.

The large number of pits can be replaced by dispersion trenches or by two tanks in parallel if space is limited.