Septic tank and drain field, 22 persons

Given: 22 persons, 135 L/c/d, infiltration 35 L/m²/d, desludging every 3 years. Detention 24 h assumed.

Septic tank

• Sewage flow: $Q = 22 \times 135\ \text{L/c/d} = 2970\ \text{L/d} = 2.970\ \text{m}^3\text{/d}$
• Sewage (settling) zone for 24 h detention: $V_1 = 2.970\ \text{m}^3$
• Sludge digestion zone: $0.0425 \times 22 = 0.935\ \text{m}^3$
• Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 22 = 1.870\ \text{m}^3$

$$V = 2.970 + 0.935 + 1.870 = 5.78\ \text{m}^3$$

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

$$\begin{aligned} \text{Plan area} &= 5.78/1.5 = 3.85\ \text{m}^2 \\ B &= \sqrt{3.85/3} \approx 1.2\ \text{m}, \quad L = 3.85/1.2 \approx 3.3\ \text{m} \end{aligned}$$

Check: $3.3 \times 1.2 \times 1.5 = 5.94\ \text{m}^3 \ge 5.78\ \text{m}^3$ (safe).

Septic tank: internal size $3.3\ \text{m} \times 1.2\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $2.20$ m long (2/3), second $1.10$ 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.

Drain field (dispersion trenches)

• Infiltration area: $A = 2970/35 = 84.86\ \text{m}^2$ (trench bottom area)
• Trench width $w = 0.6$ m, so total length $= 84.86/0.6 = 141.4$ m
• The maximum length per trench is about 30 m, so adopt 5 trenches, each 28.5 m long (85.5 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.