VIP latrine and septic tank, family of 10

1. VIP latrine for 10 users

Assumptions: wet pit, sludge accumulation 0.04 m³/person/year; double pit, each pit lasts 2 years; free space 0.5 m.

$$V = 10 \times 0.04 \times 2 = 0.8\ \text{m}^3 \text{ per pit}$$

Adopt pit $1.0\ \text{m} \times 1.0\ \text{m}$ plan: effective depth $0.8$ m + 0.5 m free space = 1.3 m (adopt 1.5 m deep). Provide two such pits, a 150 mm vent pipe 0.5 m above roof with fly screen, RCC squat slab and a $1.0 \times 1.2$ m superstructure with a dark interior. Pits are lined with open-jointed brick/stone and the pits are at least 0.5 m apart.

2. Septic tank for 10 users

Assumed sewage rate 80 L/person/day (80% of 100 lpcd water supply), detention 24 h, desludging every 3 years.

• Sewage flow: $Q = 10 \times 80\ \text{L/c/d} = 800\ \text{L/d} = 0.800\ \text{m}^3\text{/d}$
• Sewage (settling) zone for 24 h detention: $V_1 = 0.800\ \text{m}^3$
• Sludge digestion zone: $0.0425 \times 10 = 0.425\ \text{m}^3$
• Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 10 = 0.850\ \text{m}^3$

$$V = 0.800 + 0.425 + 0.850 = 2.08\ \text{m}^3$$

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

$$\begin{aligned} \text{Plan area} &= 2.08/1.5 = 1.38\ \text{m}^2 \\ B &= \sqrt{1.38/3} \approx 0.7\ \text{m}, \quad L = 1.38/0.7 \approx 2.0\ \text{m} \end{aligned}$$

Check: $2.0 \times 0.7 \times 1.5 = 2.10\ \text{m}^3 \ge 2.08\ \text{m}^3$ (safe).

Septic tank: internal size $2.0\ \text{m} \times 0.7\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $1.35$ m long (2/3), second $0.65$ 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.