VIP latrine; septic tank for 15 users

VIP latrine

A VIP (ventilated improved pit) latrine is a pit latrine in which a vertical vent pipe (150 mm dia, 0.5 m above the roof, with fly screen at top) removes smell and traps flies.

• Wind blowing across the pipe top and warm air rising in it draw air from the pit through the squat hole, carrying odour out above the roof.
• The superstructure is dark, so flies are attracted to the light at the top of the pipe, are blocked by the screen and die.
• The pit (lined with open-jointed brick/stone) is 1.5-3 m deep, designed at 0.04 m³/person/year; it has a slab with squat hole and a raised collar against surface water.
• Double-pit versions allow one pit to rest and decompose while the other is used.

        vent (screen)
          |  ___________
          | | dark room |
   _______|_|_O_________|__ slab
   |        pit            |

Septic tank for 15 users

Given: 80 L/c/d, desludging every 3 years; detention 24 h assumed.

• Sewage flow: $Q = 15 \times 80\ \text{L/c/d} = 1200\ \text{L/d} = 1.200\ \text{m}^3\text{/d}$
• Sewage (settling) zone for 24 h detention: $V_1 = 1.200\ \text{m}^3$
• Sludge digestion zone: $0.0425 \times 15 = 0.638\ \text{m}^3$
• Digested sludge storage for 3 years: $(0.085/3) \times 3 \times 15 = 1.275\ \text{m}^3$

$$V = 1.200 + 0.638 + 1.275 = 3.11\ \text{m}^3$$

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

$$\begin{aligned} \text{Plan area} &= 3.11/1.5 = 2.07\ \text{m}^2 \\ B &= \sqrt{2.07/3} \approx 0.9\ \text{m}, \quad L = 2.07/0.9 \approx 2.4\ \text{m} \end{aligned}$$

Check: $2.4 \times 0.9 \times 1.5 = 3.24\ \text{m}^3 \ge 3.11\ \text{m}^3$ (safe).

Septic tank: internal size $2.4\ \text{m} \times 0.9\ \text{m} \times 1.8\ \text{m}$ (including 0.3 m free board), in two chambers: first chamber $1.60$ m long (2/3), second $0.80$ 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.