Septic tank and dispersion trenches, 20 users

Assumptions (Nepal practice): water supply 100 L/c/d, sewage 80% of this = 80 L/c/d; detention 24 h; desludging every 3 years; infiltration rate 50 L/m²/d (sandy loam).

Septic tank

• Sewage flow: $Q = 20 \times 100\times0.8\ \text{L/c/d} = 1600\ \text{L/d} = 1.600\ \text{m}^3\text{/d}$
• Sewage (settling) zone for 24 h detention: $V_1 = 1.600\ \text{m}^3$
• Sludge digestion zone: $0.0425 \times 20 = 0.850\ \text{m}^3$
• Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 20 = 1.700\ \text{m}^3$

$$V = 1.600 + 0.850 + 1.700 = 4.15\ \text{m}^3$$

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

$$\begin{aligned} \text{Plan area} &= 4.15/1.5 = 2.77\ \text{m}^2 \\ B &= \sqrt{2.77/3} \approx 1.0\ \text{m}, \quad L = 2.77/1.0 \approx 2.8\ \text{m} \end{aligned}$$

Check: $2.8 \times 1.0 \times 1.5 = 4.20\ \text{m}^3 \ge 4.15\ \text{m}^3$ (safe).

Septic tank: internal size $2.8\ \text{m} \times 1.0\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $1.90$ 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.

Dispersion trenches

• Infiltration area: $A = 1600/50 = 32.00\ \text{m}^2$ (trench bottom area)
• Trench width $w = 0.6$ m, so total length $= 32.00/0.6 = 53.3$ m
• The maximum length per trench is about 30 m, so adopt 2 trenches, each 27.0 m long (32.4 m² provided), 0.6 m wide and 0.6-1.0 m deep, laid at a grade of 1 in 200 to 1 in 300, at 2 m centre-to-centre spacing.

Each trench has a 0.3 m deep layer of gravel (20-40 mm) with a 100 mm perforated (or open-jointed) distribution pipe laid in it, covered with 50 mm of coarse sand/gravel, a filter layer such as geotextile or straw, and then soil. Effluent is fed from a distribution box.