VIP latrine: working and design procedure

A ventilated improved pit (VIP) latrine is a pit latrine with a vent pipe that removes odours and traps flies, so it is practically free of smell and insect nuisance.

Working

1. Excreta fall into the pit through the squat hole in the slab and are digested anaerobically in the pit.
2. Wind blowing across the top of the vent pipe creates suction, and warm air rising in the pipe (heated by the sun) draws air from the pit. Fresh air enters through the squat hole, passes down into the pit and up the vent, carrying the smell out above the roof.
3. The superstructure is kept fairly dark inside. Flies attracted by the odour at the top of the vent cannot enter because of the fly screen. Flies that breed in the pit move towards the light at the top of the vent pipe, are stopped by the screen, and are trapped there until they die.
4. Liquid seeps into the soil; solids decompose and reduce in volume.

          wind -->
      fly screen  _
         |       | |  vent pipe, 0.5 m
      ___|_______|_|  above roof
     |  dark superstructure |
     |  O squat hole        |
 ____|______________________|__ slab
 |                           |
 |   pit (lined, open joints) |
 |___________________________|

Design procedure

1. Find the number of users $n$ and the design period $T$ (years) between emptying (single pit: 5-10 years; double pit: 1-2 years each).
2. Sludge accumulation rate $r$: 0.04 m³/person/year (wet pit) or 0.06 m³/person/year (dry cleansing material).
3. Pit volume $V = n \times r \times T$, plus 0.5 m free space above the sludge level.
4. Choose the plan (circular 1.0-1.5 m dia or rectangular) so that the depth is within 1.5-3 m, with the pit bottom at least 1.5 m above the ground-water level.
5. Pit lining: brick or stone with open joints, with the top 0.3 m in cement mortar (watertight), and the slab on a raised collar to keep out surface water.
6. Vent pipe: internal diameter 150 mm (min. 110 mm), 0.5 m above the highest point of the roof, on the sunny side, fitted with a fly screen.
7. Superstructure: 1.0 m $\times$ 1.2 m in plan, with walls of local material and a roof; the inside is kept fairly dark, and the door faces the prevailing wind.

Example: for 5 users, 3 years, wet pit: $V = 5 \times 0.04 \times 3 = 0.6$ m³; using a 1 m dia circular pit (area 0.785 m²), depth $= 0.6/0.785 + 0.5 = 1.3$ m (adopt 1.5 m).