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fermer ce livreA Guide to the Development of on-site Sanitation (WHO; 1992; 246 pages)
Afficher le documentPreface
ouvrir ce répertoire et afficher son contenuPart I. Foundations of sanitary practice
fermer ce répertoirePart II. Detailed design, construction, operation and maintenance
ouvrir ce répertoire et afficher son contenuChapter 5. Technical factors affecting excreta disposal
ouvrir ce répertoire et afficher son contenuChapter 6. Operation and maintenance of on-site sanitation
fermer ce répertoireChapter 7. Components and construction of latrines
Afficher le documentPits
Afficher le documentLatrine floors
Afficher le documentSlabs
Afficher le documentFootrests and squat holes
Afficher le documentSeats for latrines
Afficher le documentWater seals and pans
Afficher le documentVent pipes
Afficher le documentSuperstructure
ouvrir ce répertoire et afficher son contenuChapter. 8 Design examples
ouvrir ce répertoire et afficher son contenuPart III. Planning and development of on-site sanitation projects
Afficher le documentReferences
Afficher le documentSelected further reading
Afficher le documentGlossary of terms used in this book
Afficher le documentAnnex 1. Reuse of excreta
Afficher le documentAnnex 2. Sullage
Afficher le documentAnnex 3. Reviewers
Afficher le documentSelected WHO publications of related interest
Afficher le documentBack Cover
 

Superstructure

The building or superstructure of any latrine is required to give privacy and protection to the user. From the health point of view the superstructure is less important than the pit and slab. However, as most people initially desire sanitation because of the convenience and privacy of having their own facilities, it is important that the superstructure meets the users' needs. Many sanitation projects leave the design and construction of the superstructure to the user. Although there may be some benefit in having a uniform design, it is advantageous to involve the owner or user in the construction. A properly built superstructure should conform to certain guidelines, the most important of which are outlined below.

Size

The size of the building should be such that people are encouraged to use the facility properly, without its becoming an oversized status symbol. If the floor area is much larger than the pit slab, people may be tempted to defecate on the floor, particularly if the squat hole has been fouled by previous users. The height should accommodate a person standing upright without his or her feeling oppressed by the roof. However, if people are used to stooping when going into buildings, a lower entrance may be acceptable or even preferred. Where latrines are also being used as wash rooms or bath houses, a larger area should be allowed for.

Shape

Where the superstructure is not attached to the dwelling, there are two possible basic shapes (see Fig. 7.32): (1) a simple round or rectangular box, with or without a privacy wall; (2) a spiral, which may be round or rectangular. Although the spiral design uses more wall materials (while saving on the possibly more expensive door and hinges), it has the advantage of keeping the inside of the building partially dark and is therefore more suitable for ventilated pit latrines.

If there is a door in a spiral design the functioning of the latrine is not affected by its being left open. The design automatically incorporates a privacy screen. However, if the pit has only a short life and the superstructure will need to be moved to a new location when the pit is full, then a simpler structure may be more suitable.

In some cultures there may be a prohibition on facing in a particular direction when defecating. This must obviously be taken into account when the latrine is being positioned.

Location

The latrine may be built as a free-standing unit within the compound or may be attached to the house. If it is reached from inside the house there is a greater likelihood that it will be properly maintained. It also has the advantage that access can be controlled more easily by the householder. However, greater care has to be taken of the pit lining because of its proximity to the house foundations and the pit must be accessible from outside the house for emptying. Offset pour-flush latrines have the advantage that the pit or pits may be sited in any convenient space, even in the most cramped urban conditions. The pits may even be under the footpath access to the latrine.

Ventilation

It is desirable to provide openings in the superstructure or around the door to ensure adequate ventilation of the latrine. The inlet vents are most effective when they face the prevailing wind and should preferably be at a different height from the outlet vents to improve the efficiency of air change (Fig. 7.34). A minimum requirement of about six complete air changes per hour (10 m3/hour) has been recommended by Ryan & Mara (1983). An opening of at least 0.15 m2 should be adequate in most climates.


Fig. 7.34. Ventilation in a pour-flush latrine

 

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With a ventilated pit, the air movement is required to clear the superstructure of stale air by passing into the pit for exhaust through the vent pipe. Where there is a fairly constant prevailing wind, any openings should be on one side of the structure only, facing the wind, so that there is no through draught and to ensure maximum air movement through the pit (Fig. 7.35). However, where the prevailing wind is variable, it may be necessary to have other openings in the superstructure to prevent a suction effect when the wind blows from a different direction. This can lead to foul air being sucked out of the pit through the superstructure, to the discomfort of the users.


Fig. 7.35. Ventilation in a VIP latrine

 

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The superstructure must be strong enough to support a vent pipe extending 500 mm above the roof line. Alternatively it may be found that a block or brick vent adds rigidity to the superstructure.

Lighting

In general a latrine that is bright and light is more attractive to its users. A ventilated pit requires a partially darkened superstructure so that any flies in the pit are attracted by the daylight at the top of the vent pipe rather than light from the inside of the latrine. However, the internal walls of the superstructure may be whitewashed and some light allowed through ventilation openings.

Where possible, the opening spiral or door of a ventilated pit latrine should not face east or west as the low sun in the morning or evening would light up the inside of the structure and encourage the movement of flies out of the pit.

Access

Contrary to normal building practice, the door is usually designed to open outwards to increase the usable space inside the building and to avoid hitting any footrests. This may not be practicable in grass-roofed structures with low eaves. In some cultures a privacy wall is required to screen the door. If a spiral design is used, no door is required (though one may be fitted if desired), which is an advantage where wood and other material for making doors are expensive or in short supply.

Cleanliness

A superstructure that is left dirty and in a constant state of disrepair will soon be unused as a latrine and abandoned. It is therefore important that the building can be cleaned and maintained easily.

Materials

The design of the superstructure and the materials employed normally depend upon the style and construction methods of other buildings in the area. It is to be expected that people will build their latrine out of the same materials as their dwelling - although perhaps to a slightly lower standard. The temptation for projects to produce structures in a grand style should be avoided. If the latrine buildings promoted by a project are of more expensive construction than local housing (even if they are temporarily subsidized) they cost more than people can ordinarily afford. This acts as a disincentive for new households to construct sanitation systems when the initial promotion is finished. Similarly, the introduction of new materials and methods should normally be avoided in a latrine programme as this diverts attention from the real purpose of the sanitation system. It is better to use local skills and materials which local tradesmen understand how to use and, most importantly, how to maintain.

Many different types of materials can be used and the most common of these are described below.

Screens and fences

The superstructure does not necessarily have to be a roofed building, although there are obvious advantages in providing protection from the rain and sun. However, in some cultures people have become used to defecating in the open and find it objectionable to have to go into a small building. Also, where funds are limited the overall cost of the latrine is considerably reduced by erecting a simple fence made out of the cheapest locally available "waste" materials (such as grass, grain stalk, woven palm) to meet the need for privacy (Fig. 7.36).


Fig. 7.36. Privacy screens made from cheap locally available materials (A)

 

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Fig. 7.36. Privacy screens made from cheap locally available materials (B)

 

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In periurban areas, agricultural byproducts may not be available. Other waste products such as cardboard or beaten tin cans or sacking suspended on poles can provide the required privacy at very little cost.

It should be noted that a ventilated pit design needs a roofed and darkened superstructure.

Mud and wattle

In many parts of the world the housing consists of mud and wattle, that is upright poles, with the bark removed, interwoven with small branches, the whole being plastered with mud. Such a system can be readily adapted to the needs of a small latrine, whether round or spiral, with a thatched roof made from palm leaves or grass thatch. Mud and wattle may be improved by nailing bamboo strips to straight upright poles and filling the gaps with small stones before plastering with mud. A more regular, longer-lasting structure is obtained. This can be roofed with thatch or with beaten tin or even galvanized corrugated iron to provide a strong weatherproof structure (Fig. 7.37).


Fig. 7.37. Reinforced mud and wattle superstructure

 

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Bamboo

Shelters can be made from larger-diameter bamboo poles forming the main frame with smaller bamboos nailed or strapped to them to form the walls. Alternatively palm leaves or bamboo matting can be used to fill in the walls of the bamboo frame.

Sawn timber

Increasingly, sawn timber is becoming an expensive and rare commodity in low-income areas, but if off-cuts are available from a saw mill, these can be used to clad a simple timber-framed structure.

Sun-dried bricks

Known as adobe, modagadol, kacha or by other local names, these bricks are simply made from a mixture of well-puddled and tempered clay. Moulded in simple wooden formers, they are allowed to dry slowly, out of direct sunlight. They can be strengthened with the addition of natural fibres such as fine grasses or coconut fibres. The superstructure is erected slowly using mud mortar, and where necessary the walls can be strengthened with the addition of fencing wire on alternate horizontal joints. Care must be taken to ensure that the walls are not made too thick if the superstructure is built above a pit. A great weight of walling can exert undue pressure on the foundations and sides of the pit and may lead to collapse.

Machine-pressed blocks

This technique employs a portable steel press to compact prepared soils in order to produce regular blocks. The blocks may be stabilized with up to 8% of cement or lime depending upon the character of the soils used and the degree of exposure of the finished wall. The blocks are laid in mud mortar and can be plastered externally with mud mortar which requires attention every couple of wet seasons. However, as is the case with the sun-dried bricks, care has to be taken to ensure that walls are not made too thick and heavy.

Fired bricks

Where also used for housing, these make an excellent material for latrine construction. To exert minimum pressure on the ground, a half-brick wall (112 mm thick) built in cement mortar is used with pillars at the corners. If mud is used as the mortar to reduce costs then a one-brick wall (225 mm thick) should be constructed.

Concrete blocks

Where a more expensive standard is acceptable, or if firewood for brick firing is restricted, concrete blocks can be made by hand on site or purchased from a local manufacturer. The blocks are usually 150 mm thick but to reduce materials 65-mm blocks can be made. However, greater skill is required in the laying of these blocks and it is unlikely that a householder would be able to build without skilled assistance.

Stone

Traditional building techniques with stones are sometimes used for latrine construction. This is normally to be avoided over direct pits as the thickness of the walls (often 450 mm or more) exerts a high load, requiring a strong pit lining for support. Stone buildings are quite acceptable, however, for offset pits.

Ferrocement

A strong cement mortar pressed into three or four layers of wire mesh forms a strong, reasonably stiff membrane known as ferrocement. This material has been used successfully for spiral superstructures but can only be used where cement costs are low and the people are willing to accept a new technology along with their new latrines.

Other materials

Plasticized materials, corrugated asbestos cement, galvanized iron and aluminium sheets are also used.

Roofing

Materials such as thatch, palm leaves, clay tiles, fibre-cement tiles, wood shingles, corrugated iron, corrugated aluminium, asbestos cement, ferrocement and precast concrete can all be used for roofing the latrine superstructure. An important point to note is that the roof must be adequately tied into the wall structure and the walls must be strong enough to resist the uplift of high winds. Some materials, for example, galvanized corrugated iron, lead to greatly increased temperatures inside the latrine which may increase odour and make the building less pleasant to use.

Doors

A door is not required for efficient functioning of most latrines. However, for various reasons, users often wish to have a sawn timber door. Where possible it is advisable to mount the door on self-closing hinges. Doors can also be made from beaten tins or corrugated iron on a wooden frame, bamboo strips or anything else that is available. Simple curtains may suffice where timber is scarce. A door is not necessarily required for privacy of the user. Where spiral designs have become common it is normal for people to knock on the outside of the structure before entering to warn anybody using the latrine of their approach.

Hinges do not have to be manufactured in steel; strips of old car tyres or leather from old shoes can equally well be used.

Conclusion

In conclusion it may be emphasized that a superstructure is usually required first for privacy and secondly as a shelter for the user from the wind and rain. Brandberg (1985) asked the question, "Why should a latrine look like a house?" to demonstrate that the poorest people need not be excluded from the benefits of sanitation because they cannot afford the superstructure. A simple screen for privacy can adequately serve as a first phase while funds are found for a building. At a later stage materials in common use for house construction in the area will be suitable for building the latrine superstructure.

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