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close this bookA Guide to the Development of on-site Sanitation (WHO; 1992; 246 pages)
View the documentPreface
open this folder and view contentsPart I. Foundations of sanitary practice
close this folderPart II. Detailed design, construction, operation and maintenance
open this folder and view contentsChapter 5. Technical factors affecting excreta disposal
close this folderChapter 6. Operation and maintenance of on-site sanitation
View the documentPit latrines
View the documentSimple pit latrines
View the documentVentilated pit latrines
View the documentVentilated double-pit latrines
View the documentPour-flush latrines
View the documentOffset pour-flush latrines
View the documentDouble-pit offset pour-flush latrines
View the documentRaised pit latrines
View the documentBorehole latrines
View the documentSeptic tanks
View the documentAqua-privies
View the documentDisposal of effluent from septic tanks and aqua-privies
View the documentComposting latrines
View the documentMultiple latrines
View the documentOther latrines
open this folder and view contentsChapter 7. Components and construction of latrines
open this folder and view contentsChapter. 8 Design examples
open this folder and view contentsPart III. Planning and development of on-site sanitation projects
View the documentReferences
View the documentSelected further reading
View the documentGlossary of terms used in this book
View the documentAnnex 1. Reuse of excreta
View the documentAnnex 2. Sullage
View the documentAnnex 3. Reviewers
View the documentSelected WHO publications of related interest
View the documentBack Cover
 

Ventilated pit latrines

These are also known as ventilated improved pit (VIP) latrines (Fig. 6.5). The major nuisances that discourage the use of simple pit latrines - smell and flies - are reduced or eliminated through the incorporation of a vertical vent pipe with a flyscreen at the top (Morgan, 1977). Wind passing over the top of the vent pipe causes a flow of air from the pit through the vent pipe to the atmosphere and a downdraught from the superstructure through the squat hole or seat into the pit. This continuous flow of air removes smells resulting from the decomposing excreta in the pit and vents the gases to the atmosphere at the top of the vent pipe rather than through the superstructure. The flow of air is increased if the doorway of the superstructure faces the prevailing wind (Mara, 1984). If a door is fitted it should be kept shut at all times (except when entering or leaving) to keep the inside of the latrine reasonably dark, but there should be a gap, normally above the door, for air to enter. The area of this gap should be at least three times the cross-sectional area of the vent pipe.


Fig. 6.5. Ventilated improved pit (VIP) latrine

 

WHO 91424

The superstructure can be constructed in the form of a spiral (Fig. 6.6). This excludes most of the light whether a door is fitted or not. The defecation hole must be left open to allow the free passage of air. The vent pipe should extend at least 50 cm above the latrine superstructure except where the latter has a conical roof, in which case the pipe should extend as high as the apex. Air turbulence caused by surrounding buildings or other obstructions may cause reverse air flow, leading to foul odours and flies in the superstructure. If mean wind speeds are about 2 m/s, as is fairly common in rural areas, air speeds in the vent pipe are about 1 m/s (Ryan & Mara, 1983). Air flow may also occur at lower wind speeds because of solar radiation heating the air in the vent pipe, causing the air to rise. The vent pipe should then be placed on the equator side of the superstructure. It may be painted black to increase solar absorption, if the material of the pipe is not itself black.


Fig. 6.6. Spiral construction for the superstructure

 

WHO 91425

In latrines relying on solar radiation for ventilation, foul odours are sometimes experienced in the superstructure at certain times of the day (usually early morning). This occurs where the outside air temperature is colder than the air in the pit, which may prevent the air circulating. Very little can be done to prevent this, other than sealing the defecation hole at night.

In addition to removing odours from the pit, the screened vent pipe significantly controls flies. In Zimbabwe, Morgan (1977) compared the number of flies leaving the squat hole of a VIP latrine with the number leaving a simple pit latrine. The results are shown in Table 6.1.

Flies are attracted to the pit by the odour coming from the vent pipe but are unable to enter because of the screen. A few flies enter the pit through the squat hole or seat, and lay eggs in the pit. New young flies attempt to leave the pit by flying towards the light. If the latrine superstructure is kept sufficiently dark, the major source of light is at the top of the vent pipe, but the screen prevents the flies from escaping there and they eventually fall back into the pit to die.

Well-constructed and maintained VIP latrines combat all the problems associated with simple pit latrines, except mosquitos. However, they are considerably more expensive than simple pits, since a ventilation pipe and full superstructure are required. Because the defecating hole is directly over the pit they accept any form of anal cleaning material without blocking. Routine operation is limited to keeping the superstructure clean, ensuring that the door (where fitted) is kept closed, occasionally checking that the fly-proof netting on top of the vent pipe is not blocked or broken, and pouring water down the vent pipe once a year to remove spiders' webs.

Table 6.1. Comparison of the numbers of flies leaving the squat holes of a simple pit latrine and a VIP latrinea

Period of trapping

No. trapped in unvented privy

No. trapped in vented privy

8 October-5 November

1723

5

5 November-3 December

5742

20

3-24 December

6488

121

 

a Source: Morgan, 1977.
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