Septic tank and soak pit, hostel of 400

Assumptions (flow and soil data not given): sewage 100 L/person/day, detention 24 h, infiltration 50 L/m²/d (sandy loam), digestion 0.0425 m³/person and storage 0.085 m³/person for 3 years as given. Soak pit effective depth 3 m, pit diameter 3 m.

Septic tank

• Sewage flow: $Q = 400 \times 100\ \text{L/c/d} = 40000\ \text{L/d} = 40.000\ \text{m}^3\text{/d}$
• Sewage (settling) zone for 24 h detention: $V_1 = 40.000\ \text{m}^3$
• Sludge digestion zone: $0.0425 \times 400 = 17.000\ \text{m}^3$
• Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 400 = 34.000\ \text{m}^3$

$$V = 40.000 + 17.000 + 34.000 = 91.00\ \text{m}^3$$

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

$$\begin{aligned} \text{Plan area} &= 91.00/2.0 = 45.50\ \text{m}^2 \\ B &= \sqrt{45.50/3} \approx 3.9\ \text{m}, \quad L = 45.50/3.9 \approx 11.7\ \text{m} \end{aligned}$$

Check: $11.7 \times 3.9 \times 2.0 = 91.26\ \text{m}^3 \ge 91.00\ \text{m}^3$ (safe).

Septic tank: internal size $11.7\ \text{m} \times 3.9\ \text{m} \times 2.3\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $7.80$ m long (2/3), second $3.90$ 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: $A = Q/\text{rate} = 40000/50 = 800.00\ \text{m}^2$ (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 = $\pi \times 3.0 \times 3 = 28.27\ \text{m}^2$).

$$N = \frac{800.00}{28.27} = 28.29 \Rightarrow 29\ \text{pits}$$

Soak pits: 29 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.

Such a large number of soak pits is impractical on a small site; in practice a hostel of this size uses several septic tanks in parallel or a series of dispersion trenches / a small package treatment plant followed by disposal.