Double pit VIP latrine for 15 users

A double pit VIP (ventilated improved pit) latrine has two pits side by side, used one at a time. When one pit is full it is closed and the second is used; the contents of the first pit decompose for a year or more and are then safely emptied and reused as manure.

Design data (assumed)

• Users: 15 (family/joint use).
• Sludge accumulation rate (wet pit, water used for anal cleansing): 0.04 m³/person/year.
• Each pit is designed to last 2 years, so while one is in use the other rests for 2 years.
• Free space above sludge: 0.5 m.

Pit size (each pit)

$$V = 15 \times 0.04 \times 2 = 1.2\ \text{m}^3$$

Adopt a pit plan of 1.2 m $\times$ 1.0 m = 1.2 m², so effective depth = 1.2/1.2 = 1.0 m, plus 0.5 m free space = 1.5 m total depth.

Provisions

• Two pits of 1.2 m $\times$ 1.0 m $\times$ 1.5 m, with a clear gap of 0.5 m or more between them; lined with brick/stone in cement mortar with open joints in the lower part, and a watertight upper 0.3 m collar.
• Slab: RCC slab with a squat hole over each pit; the hole over the resting pit is sealed with a concrete cover, so excreta drop only into the pit in use.
• Vent pipe: 150 mm dia (not less than 110 mm) PVC/GI/bamboo pipe, extending at least 0.5 m above the roof, fitted with a fly screen (mesh) at the top; one vent per pit. Place the vent pipes on the sunny side of the superstructure.
• Superstructure: about 1.0 m $\times$ 1.2 m in plan, 2 m high, with a fairly dark interior so that flies move towards the light at the top of the vent; the door should face the prevailing wind, and the roof slopes to the back.
• Located at least 15 m from water sources and downhill of wells.

  vent pipe (fly screen)
        |   superstructure
   _____|______
  | [door]  O  |   O = squat hole
  |____________|__ slab
  | Pit A | Pit B |   1.2 x 1.0 x 1.5 m each
  | (use) | (rest)|