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close this bookGuide to the Development of On-Site Sanitation (WHO, 1992)
close this folderPart II - Detailed design, construction, operation and maintenance
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View the documentChapter 6 Operation and maintenance of on-site sanitation
View the documentChapter 7 - Components and construction of latrines
View the documentChapter 8 - Design examples

Chapter 7 - Components and construction of latrines

Pits
Latrine floors
Slabs
Footrests and squat holes
Seats for latrines
Water seals and pans
Vent pipes
Superstructure

Many components of sanitation systems are common to different types of latrines. In this chapter, the technical details of the following components are considered:

- pits and pit linings;
- latrine floors, which may be cast directly on the ground where the pit or vault is offset;
- slabs, supported over direct or offset pits;
- footrests and squat holes;
- seats;
- water seals, pans, pipes and junction chambers;
- vent pipes;
- superstructures.

Pits

Excavation

Most pit latrines provide sanitation for a single household, usually necessitating a pit about 1 m across and 3 m or more in depth (although much larger pits are common in some areas), or two shallow pits of up to 1.5 m in depth. The pit may be circular, square or rectangular in plan. Circular pits are more stable because of the natural arching effect of the ground around the hole, with no sharp corners to concentrate the stresses (Fig. 7. 1). However, people often find that square or rectangular pits are easier to dig. The depth of the pits often follows local traditions. It is usually advantageous to dig the pit as deep as possible, but this depends on soil conditions, cost of lining and the level of the groundwater.

Pit linings

The need for a pit lining depends upon the type of latrine under construction and the condition of the soil. In septic tanks and aqua privies, for example, which require watertight compartments, the pit is always lined. However, in pit latrines it is only necessary to have a lining if the soil is likely to collapse during the life span of the latrine. It is not easy to decide in advance whether a soil will be self supporting. If other excavations in the locality (such as shallow wells) have proved to be self-supporting over a number of years, then it is probably safe to assume that a pit for a latrine can be dug without support. Granular soils such as sands and gravels normally require support. Cohesive soils, such as silts and clays, and soils with a high proportion of iron oxides, such as laterites, are often self-supporting. However, silts and clays may lose their self-supporting properties when wet, particularly where there is a varying water table.


Fig. 7.1. Strength of pit shapes

If there is any doubt about the conditions it is better to assume that the soil is not self-supporting. Increasingly it is recommended that all pits should be lined, especially where the design life is over five years. Failure of an unlined deep pit can be extremely hazardous for the person excavating it. If the failure occurs some years later it can be expensive for the owner and disturbing for the users. In all cases the top 300 500 mm should be lined and sealed to support the slab (and where necessary the superstructure) and to prevent contamination of the surface and entry of vermin.

The lining may be of any material that supports the soil and that will last as long as the design life of the pit. Commonly, materials such as fired bricks, concrete blocks, concrete, ferrocement and local stone are used, but stabilized soil blocks, old oil drums (though with a limited life in corrosive groundwater) and unglazed fired clay pipes have also been successful.

Quarried stone, where available cheaply, makes a satisfactory lining. The more regular blocks should be used for the top 500 mm with mortar joints. Less-regular stone can be used for the remainder of the lining without mortar in the vertical joints. The builders or masons must be skilled and experienced if the lining is to last a reasonable length of time. Where local stone is used, its durability must be confirmed. Some stone will deteriorate when exposed to air or water or to frequent changes between wet and dry conditions.

The use of timber or bamboo is not generally recommended, since they are subject to insect and fungal attack and often have a limited life. Some hard woods can be satisfactory provided they are treated with tar, creosote or other preservative to lengthen their life. Care must be taken to ensure that none of the preservatives leach into the groundwater as even low levels of some preservatives can be toxic (WHO, 1984). Woven cane and bamboo have been used for the lower part of a lining with stronger materials used for the top 500 mm. However, unless the pits are designed to have an extremely short life, cane and bamboo should be avoided.

Construction

Shallow pits

In almost all cases, pits of up to 1.5 m in depth can be excavated to their full depth and then lined from the bottom up. If the soil is very loose, the sides of the excavation may have to be sloped to prevent collapse. The space between the lining and the soil can then be backfilled, preferably with a granular material such as sand or gravel. Granular materials are used because they fill the space between the soil and the lining without leaving large voids. They also act as a filter to prevent soil particles being washed into the pit. Voids behind the lining produce locally increased loads on the lining which may cause collapse.

It is usual to provide a foundation for the lining similar to that provided for a domestic house. In most soils, a foundation width equal to twice the wall thickness is usually sufficient (Fig. 7.2). In very soft ground it may be necessary to construct wider foundations to prevent the weight of the lining itself forcing it into the soil (Fig. 7.3). Where the superstructure load is not directly applied to the lining, a widened foundation may not be required since the load applied to the ground at the base of the lining is small and considerable skin friction builds up between the sides of the lining and the ground.

Soakpits or leaching pits require a porous lining to allow the wastewater to escape into the ground. The method of achieving this depends upon the lining material used. With bricks, blocks or local stone, a proportion of the vertical joints are left unmortared. These unmortared joints may be confined to specific courses (e.g., every third or fourth course) rather than being spread throughout the lining. This enables the fully mortared courses to carry the load exerted by the soil on the lining. Where the ground is relatively strong, a more open, honeycomb technique is used, with only small dabs of mortar joining the masonry. Alternatively, specially manufactured bricks with angled ends to suit round pits and a central opening to allow for infiltration may be used (D. J. T. Webb, personal communication).


Fig. 7.2. Lining for a shallow pit in firm ground


Fig. 7.3. Lining for a shallow pit in soft ground

Concrete, ferrocement and fired clay ring linings are made porous by creating holes of 25-50 mm in diameter through the lining. Alternatively, the ring joints are held open by small stones or bricks. Additionally, concrete linings may be made of "no fines" concrete, that is, concrete without any fine aggregate (sand). A mix of one part of cement to four parts of clean gravel (with stones of 6-18 mm in diameter) is suitable. Where precast rings are used, the upper and lower 100 mm of the ring should be made of conventional concrete for extra strength.

Deep pits

The method of excavating deep pits depends upon the stability of the soil during the construction period. In soils that are self-supporting, the pit may be dug to its full depth and the lining installed afterwards. If the ground is not self-supporting, the lining must be constructed as the pit is dug.

Where a lining is not required for support during excavation, the pit is dug to the full depth, making allowance for the thickness of the lining to be installed subsequently. Accurate dimensions are maintained by using a plumb bob to ensure verticality and a template, either circular or rectangular to retain the horizontal dimensions. Ensuring correct dimensions minimizes the costs of lining and backfilling. Sometimes the soil near the surface is weathered and likely to collapse. In that case, the top metre of soil may be supported with a temporary lining (Fig. 7.4). If the finished lining is to be of precast concrete rings, then the top metre of soil will have to be excavated to a larger diameter so that the rings can pass inside the temporary lining.


Fig. 7.4. Excavation for a pit to be lined with precast concrete rings

When the hole has been excavated to the design depth, the bottom is levelled and cleaned. In firm ground, a foundation can be constructed by cutting a groove into the walls of the pit and building a ring beam. In exceptionally soft ground, where the lining is likely to sink into the floor of the pit, the ring beam foundation can be replaced by a floor slab of "no fines" concrete, 75-100 mm in thickness, covering the whole base of the pit. This will distribute the weight of the lining over a larger area of the pit base, thus reducing the load per unit area and preventing upwards heave of the soil (see Fig. 7.3).

Construction of linings

Precast rings

The use of precast concrete (Fig. 7.5) or fired clay rings for the lining of pits has the advantage that the lining can be prepared before excavation begins. This is particularly useful in weaker soils because it reduces the time the soil remains unsupported. The rings to be placed at the bottom of the hole may be porous, designed to allow the liquid wastes to seep into the surrounding soils or they may be sealed to create a wet tank, designed to increase the rate of sludge digestion. The ring nearest to the surface should be fully sealed to prevent entry of surface water and rodents and also contamination of the soil. As with shallow pits, any space between the back of the rings and the soil should be filled with sand or gravel.

Brick, blockwork and stone lining

These are built in a similar way to precast concrete linings, i.e., by building up from the foundations. With very deep pits it may be wise to allow time for the cement mortar to gain strength before filling any space behind the lining, to prevent the weight of the fill from deforming the lining. Except for the top 300-500 mm, the joints are left open as described above to ensure infiltration of liquid to the soil.


Fig. 7.5. Pit bottom lined with precast concrete rings

In situ concrete lining

In this method the hole is lined with concrete cast in the hole (Fig. 7.6). After excavation, shuttering is positioned to a convenient height allowing for compaction, and the space between is filled with concrete. Normally the concrete does not require steel reinforcement for structural strength. However, a small amount of steel may reduce shrinkage cracking. The lining can be made porous by leaving small holes in the concrete (short lengths of 25-SO mm of pipe fitted between the shuttering and the soil will be satisfactory). Alternatively "no fines" concrete can be used.


Fig. 7.6. Pit with concrete lining in situ

Ferrocement lining

When mortar is plastered over layers of fine wire mesh (such as chicken wire) the resulting material is called ferrocement. It is strong, light, requires no shuttering and is easy to construct. It is now widely used for such structures as water tanks and latrine slabs and can be adapted for use as a pit lining.

In some countries the term ferrocement refers to any cement-based material reinforced with steel. Specifically it now describes a material consisting of several layers of small-diameter steel mesh (usually hexagonal chicken wire, with wire of 0.7-1.3 mm in diameter and openings of 12 mm). The layers are tied together with fine wire at 150-mm intervals and then plastered with a rich cement mortar (one volume of cement to two volumes of sand) to give a finished thickness of about 25 mm.

After excavating the hole, as much loose material as possible is removed from the pit walls. Cement mortar is applied directly to the walls of the pit to give a layer approximately 12 mm thick. This layer is then covered with two or three thicknesses of steel mesh, held in place with long staples driven through the mortar into the soil. A second coat of mortar is then applied and pushed firmly into the holes in the wire mesh. On completion, the mortar covering the mesh should be at least 10 mm thick. Where a porous lining is required, holes can be punched through the mortar while it is still weak.

Ferrocement rings may also be precast on the surface and used in the same way as concrete rings.

Excavation in loose ground

Where the ground is very loose and liable to collapse if left unsupported, or where the excavation enters the water table, the most common method of construction is to prefabricate the lining on the surface, place it in a starter excavation, dig out the soil below and allow the lining to sink as the hole is dug. This method is called "caissoning" (Fig. 7.7).

A hole is excavated as deep as possible (experience of the local ground conditions will determine the depth). A precast concrete ring fitted with a cutting edge is then placed in the hole. Additional rings are placed on top until ground level is reached. Excavation now begins inside the rings. As the ground is dug away from under the cutting edge, the rings start to sink under their own weight. Additional rings are then placed on top until the required depth is reached.


Fig. 7.7. "Caissoning" a pit

This method may also be used for linings of bricks or blocks. However, the lining must be constructed sufficiently far above the ground to ensure that the mortar has fully set before the lining enters the ground. The honeycomb method of construction cannot normally be expected to have sufficient strength to be sunk as a caisson.

Where caissoning is employed because of a high groundwater table, excavation should take place towards the end of the dry season when the water table is at its lowest. As the lower ring enters the water it is possible to continue excavation for up to one metre by scooping material in a bucket or with a specially shaped shovel.

Backfilling

Any space around the outside of the lining should be backfilled with compacted earth taken from the pit or, where available, with sand and gravel. If the ground is particularly weak, the top of the pit may be backfilled with weak concrete or a soil-cement mixture to give additional strength. Strengthening may be important if the top of the pit has become overly enlarged during excavation.

Latrine floors

Floors of latrines, whether laid on the ground or supported over a pit, should be smooth and impervious so that they may be cleaned easily and have a satisfactory appearance to users. The upper surface should be at least 150 mm above the surrounding ground level (Fig. 7.8) to prevent rain and surface water entering the latrine.

The floor surface should slope gently to facilitate cleaning and to prevent surplus wash water from collecting in puddles. The slope is normally from the outer edge of the floor towards the squat hole or pan at the centre, so that the water used for cleaning flows into the pit and does not foul the area surrounding the slab. A fall of about 20 mm between the edge and the centre of a slab up to 1.5 m across is sufficient to prevent pools of liquid forming (Fig. 7.8). Where seats are used, the floor should slope away from the seat support so that any wash water flows towards the latrine entrance.

If a precast slab is smaller than the inside floor area of the superstructure, an impervious surface is normally provided to seal the area between the slab and the inside wall of the building. Any area around the slab which is left as bare earth could be fouled, thus becoming a possible site for hookworm infestation. However, in order to minimize costs, the space around the squatting area inside the superstructure should be limited. This reduces building costs for the superstructure as well as flooring materials. But the squat hole or pan should not be so close to the superstructure that users are forced to lean against the wall when they are trying to defecate. A minimum floor space of 80 cm in width and I m from front to back is normally acceptable (Mare, 1985b).

Slabs

Requirements

A latrine slab serves two main purposes, as a support and as a seal. It has to support the weight of the person using the latrine and, possibly, the weight of the superstructure. It also seals the pit, with the exception of the squat hole and, where required, the vent-pipe hole. This facilitates control of flies and smells and reduces the likelihood of rodents and surface water entering the pit. Where the slab has been made in sections (for ease of placing and emptying) or has a removable cover, the joints should be sealed with a weak mortar such as a lime or mud mortar.


Fig. 7.8. Requirements of slabs

To support the weight of a person over a latrine pit the suspended slab has to act structurally in the manner of a bridge. Where seats are provided, the extra weight has to be allowed for when designing the slab. Depending on the design of the slab, the materials may have to be able to resist forces in tension as well as in compression (Fig. 7.9). The materials needed to carry the tensile forces are often more expensive than those commonly used in low-cost buildings. The slab is often the most expensive individual component that has to be paid for by the user. It is therefore important to ensure that it is carefully designed to serve its purpose with a minimum of costly material.

The slab normally rests on a foundation or on the top of the pit lining (see Fig. 7.8). This ensures that the weight of the slab and the weight of the person using it are spread evenly on the soil. Particular care must be taken where the slab also has to carry part of the weight of the superstructure. If the ground is weak, the foundation prevents subsidence or collapse of the ground underneath the load. Any gaps between the slab and the pit lining should be sealed with earth or a weak mortar to prevent ingress of water. This seal also prevents small animals and insects getting into and out of the pit.

Where a pit is excavated to a larger diameter than planned, precast slabs are occasionally supported on timber poles. This practice is not advisable as the heavy load on the poles is likely to lead to early failure. However, small slabs (approximately 500 mm square), designed to provide a hygienic squat hole for existing latrines at minimum cost, will not overload a timber support (Fig. 7.10).


Fig. 7.9. Tension and compression forces in a slab


Fig. 7.10. Small slabs for upgrading timber and earth structures

The latrine slab should feel secure and should not deflect noticeably under the weight of a person using the latrine. It needs to be as clean and attractive as possible so that people feel comfortable using the latrine. There is then much less chance of the latrine being misused or fouled.

Offset pits used with pour-flush latrines require a cover slab to prevent entry of flies and rodents and to ensure safety, particularly of children. With the omission of a squat hole, the structural requirements are the same as for a latrine slab.

Shapes of direct pit slabs

The shape and size of the pit are the first factors to be considered when designing a supported slab. Latrine pits can be round, square or rectangular and it is usual to find that a particular shape becomes the accepted design for a particular area.

Borehole latrines have a small span and therefore require very simple slabs. The shape will depend on the users' needs for a clean hygienic area with correctly spaced footrests, rather than being controlled by the size of the hole to be covered. Larger, hand-dug pits 1-1.5 m in width require a shape designed to span and seal the pit. An exception to this is where the span of the slab is reduced by corbelling the top of the lining (Fig. 7. 11). This decreases the amount of material required in the slab and thus reduces the cost.


Fig. 7.11. Minimizing the span of the slab

Slabs may be precast or constructed in situ, which means that the slabs are built over the pit, exactly where they are to be used. In a large agency-assisted programme, slabs are often manufactured at a convenient construction site away from the latrines and then brought to the site and laid across the pits. Where slabs are to be moved, weight and shape are both significant factors.

The shape of the slab is also determined by the type of latrine. Water-seal latrines, aqua-privies, ventilated pits and pits sealed with hole covers all have different requirements. For example, the need for an extra hole close to the edge of the slab for a vent pipe makes the unreinforced dome slab unsuitable for ventilated latrines.

The slab normally overlaps the supporting pit lining or foundation by at least 100 mm on all sides to ensure that the load is adequately transferred. This overlap may have to be extended to 200 mm where the pit is unlined and the slab is resting directly on the soil (see Fig. 7.8).

Cement-based slabs and components

In most countries, concrete or cement-based slabs provide the most durable and economic method of covering latrine pits. There are many different ways of using cement. Its ability to bind with other materials and provide a clean watertight surface make it the obvious choice for the majority of programmes.

Concrete is a mixture of cement, sand, gravel and water. When set, it forms a hard dense material which is extremely strong in compression but weak in tension. Cast as a simple flat slab across a pit, its own weight and the weight of any person on it forces the concrete to deflect downwards in the centre. As the load increases, small tension cracks form on the underside of the beam. With heavy loads, these cracks may extend upwards through the concrete until the slab breaks. To prevent this happening, steel bars or other reinforcement may be placed in the concrete on the lower side of the slab to carry the tension load and prevent the cracks spreading.

Unreinforced concrete

Small slabs, such as those required for borehole latrines or to provide a hygienic platform for the squatting area of timber-supported slabs (see Fig. 7.10), do not need any reinforcement. Where an unreinforced span of greater than 0.5 m is required, the slab should be cast in the form of a "flat" arch. The weight of the load is then directed through the arch to the supporting area on the ground. The underside of the concrete remains in compression and no reinforcement is required. Using this principle, a shallow circular dome or arch can be constructed to cover a latrine pit. The dome is strong enough to support itself and the people using it without any expensive steel reinforcement. A slab using this principle has been developed by a team in Mozambique (Fig. 7.12) and has proved to be economical and popular. The slabs are about 40 mm thick and rise 100 mm in the centre to give the arch effect (International Development Research Centre, 1983).


Fig. 7.12. Dimensions of domed slab without reinforcement

Although such domed slabs fall away from the centre, a small inward slope about 100 mm wide immediately around the squat hole is incorporated to direct any waste into the pit. These slabs have been used most effectively in areas with sandy soil which quickly absorbs any surplus wash water.

The concrete slab is given the shape of a dome by mounding up earth to the required profile of the underside of the slab. The earth is compacted and smoothed. It may then be covered with plastic sheeting or old cement bags, or coated in old engine oil to break any bond between the earth and the fresh concrete. A circular iron strip made from an oil drum is used as the edge former or mould. The concrete around the centre hole is made slightly thinner so that a slope towards the hole can be made. Each slab has to be allowed to harden undisturbed for several days after casting.

To save space in the casting yard, up to five slabs may be cast on top of each other, using a lower, previously cast slab as a former for the next slab. Particular attention has to be given to the concrete mix of a thin unreinforced slab. A maximum aggregate size of 10 mm and slightly more cement than usual is required. The recommended mix is one part by volume of cement to two parts of sand and one and a half parts of 6-10 mm aggregate.

An unreinforced slab may also be produced in a rectangular mould with a flat upper surface and a dome on the underside (Fig. 7. 13). As an unreinforced dome slab cannot accept a second hole close to one edge for a vent pipe, flies, smells and cockroaches are prevented from leaving the pit by providing a tight-fitting cover over the squat hole. This is cast directly in the squat hole so that it fits exactly. A layer of cement bag paper may be used to prevent the fresh concrete sticking to the old.


Fig. 7.13. Semi-domed slab


Fig. 7.14. Arched brickwork lining and support

Bricks can be used to form an unreinforced arch across a rectangular pit (Fig. 7. 14) using a rough framework of bamboo, reeds or forest poles which is left in the pit. The space above the arch is levelled with river sand and topped with a 20-mm cement-sand screed sloping towards the centre. This technique requires very little cement and no steel. However, these structures have to be built by skilled masons and there is no opportunity for precasting. Emptying of the pit can only be carried out through the squat hole.

Reinforced concrete

Because of the weakness of concrete in tension it is often reinforced with other materials. Most commonly it is strengthened by the inclusion of steel bars. Details of the reinforcing steel required for common sizes of slab are shown in Table 7.1. Mild steel bars, 6 mm in diameter spaced at intervals of 150 mm, or 8 mm in diameter spaced at intervals of 250 mm in each direction, are normally sufficient for 80-mm thick slabs of up to 1.5 m in span. This span distance is measured at the point of minimum span, that is, the shortest distance between two points which fully support the slab. Where used correctly, reinforcement in a concrete slab will support at least six adults on a 1.5-m span slab. For the small spans illustrated, extra steel is not required for trimming around the pit opening.

Table 7.1. Spacing of steel reinforcement bars for concrete slabs a

Slab

Steel bar

Spacing of steel bars (mm) for minimum slab span of:

thickness

diameter






(mm)

(mm)

1 m

1.25 m

1.5 m

1.75 m

2 m

65

6

150

150

125

75

50


8

250

250

200

150

125

80

6

150

150

150

125

75


8

250

250

250

200

150

a The steel bars should be fixed on the lower side of the slab, with 12-mm cover or thickness of concrete beneath each bar. Steel to be laid at above spacings in both directions. Size and spacing of steel calculated for grade 20 concrete and mild steel reinforcement, with characteristic yield stress of 210 N/mm², or high-yield mesh, yield stress 485 N/mm².

The reinforcing steel is laid in both directions, that is, with one layer of bars perpendicular to the second layer (Fig. 7.15). Where the slab is rectangular, the bars parallel to the direction of the minimum span should be beneath the bars in the direction of the longer span. For the bars specified, a characteristic yield strength for the steel of 210 N/mm² is assumed. Care is required to ensure that the steel is of the required quality.


Fig. 7.15. Reinforced concrete rectangular slab (for details of reinforcement see Table 7.1)

When individual bars are used, some may be omitted by mistake. One way of avoiding this is to use steel mesh, which consists of smaller diameter bars welded together. This can be cut to the required shape but there is likely to be wastage of the off-cuts that have to be discarded. A mesh with 7-mm bars at 200-mm centres, with a cross-sectional steel area of 193 mm²/m (yield stress 485 N/mm²) is normally sufficient.

Care must be taken when reinforcing concrete with steel to ensure that the steel is completely surrounded by the concrete. There should be at least 12 mm of concrete under the steel bars and at the ends of all bars. This protects the steel from the corrosive effect of gases and moisture in the pit. When concrete is placed in a mould or former it has to be compacted by manual or mechanical vibration to remove any air bubbles and to ensure the durability of the completed slab. Simple wooden or steel moulds can be reused many times to give the required shape to the wet concrete if they are coated with a suitable release agent. There are many proprietary agents, but used engine oil painted on to the mould effectively prevents the concrete from sticking. Alternatively, plastic sheeting or empty cement bags may be used to prevent bonding. These materials may also be used between the ground and the underside of the slab. The squat hole is formed using a shaped wooden mould with a bevelled edge. A vent pipe opening may be created with an offcut of plastic pipe which is removed a few hours after casting so that it can be reused many times.

An alternative way of using steel for reinforcement is to precast a ferrocement slab. The method of construction is described under construction of linings, p. 89. A flat ferrocement slab is strong enough to carry the imposed load but is too flexible for the users' comfort. In order to ensure adequate stiffness, the ferrocement may be shaped as a dome or may be cast with ribs on the soffit (Fig. 7.16). Four layers of mesh are normally required for a slab with a I -m span. It is necessary to ensure that the cement mortar has been adequately pressed through all the layers of wire mesh and compacted to a dense material if it is to have adequate strength.

Steel reinforcement is used in various ways in different countries reflecting differences in price and availability. Because of the relatively high cost of steel, many techniques have been investigated in the search for cheaper alternatives. One approach is to reinforce concrete with small unconnected fibres with a low modulus of elasticity. These are either natural fibres, such as sisal, jute, coir, Manila hemp or kenaf, or man-made fibres such as fibrillated polypropylene. The fibres are chopped and added to the cement mix. Use of these low-modulus fibres does not reinforce the concrete in the conventional sense of carrying the tensile load, but is particularly beneficial in ensuring adequate curing of the concrete without the formation of minute shrinkage cracks (Parry, 1985). The resultant "unreinforced" concrete attains a much higher tensile strength than would otherwise be possible. Slabs made from fibre-reinforced cement should normally be given the shape of an arch or dome to minimize tensile forces in the soffit.


Fig. 7.16. Ferrocement slab

Slabs have also been reinforced with barbed wire, fencing wire, scrap steel from cars and broken machinery, redundant universal beams and almost anything that is available. Although a saving is made on reinforcement, these methods usually lead to a much greater use of concrete in order to cover the larger sections of steel and therefore are rarely economical.

Bamboo has a high strength-to-weight ratio and in certain parts of the world is widely available. Because of the low cost, bamboo strips have been used as an alternative to steel bars but it is important to ensure that the bamboo strips in a slab are completely covered by the concrete so that water and vapours cannot rot the bamboo. The strips should initially be treated with preservative. One recommended method (UNCHS, undated) is for the bamboo to be dipped in white lead and 10% varnish to inhibit water absorption from the freshly placed concrete. Even where treated, there is some doubt as to the longterm durability of bamboo as reinforcement.

Where cement is relatively expensive, a technique known as reinforced brickwork can be utilized, in which part of the concrete is replaced by whole or half bricks, leaving steel reinforced concrete ribs to support the bricks (Fig. 7.17). The whole slab requires a cement skimming over the surface to make it impervious to fouling by the users.


Fig. 7.17. Reinforced brickwork slab

Concrete mixes

Different concrete design mixes (that is, combinations of cement, sand, aggregate and water) are suitable for use in various circumstances. The concrete mix that is most often used is 1 :2 :4 (one unit by volume of cement with two units by volume of sand and four units by volume of aggregate). The sand should be clean and hard and may be sized by sieving through ordinary mosquito netting. Coarse aggregate comprises graded stones 6-18 mm in size and should be free of fine dust. This mix results in a finished volume of concrete which is approximately 70% of the total volume of the individual dry materials.

The cement, sand and coarse aggregate have to be mixed with a specific amount of water to give the optimum strength for the amount of cement used. For concrete mixed and placed by hand, there should normally be a water: cement ratio of 0.55 by weight, i.e., the weight of water is approximately half the weight of cement. Cement weighs 1400 kg/m3 and water 1000 kg/m3; a 50-kg bag of cement thus has a volume of 0.035 m³. A 1 :2:4 concrete mix using one 50-kg bag of cement therefore requires 0.070 m³ of clean sand, 0.140 m³ of aggregate and 0.027 m³ of water, which results in 0.17 m³ of finished concrete.

The volume of water is applicable where the aggregate and sand are "saturated, surface dry". In hot dry climates, the small pores in the aggregate, as well as the surface, are likely to be "oven dry" rather than saturated. To use the specified amount of water would then lead to an extremely stiff, unworkable concrete. The aggregate should therefore be thoroughly wetted with water before mixing begins. The correct wafer: cement ratio results in a relatively stiff but workable material which produces a skim of water on the surface of the concrete as it is worked flat with a trowel. When the mix has too much water, the strength is reduced considerably. An increase of only 50°,, in the water content decreases the finished concrete strength by half, which is the equivalent of wasting half the cement in the bag.


Fig. 7.18. Checking the water content of concrete with a slump cone

To check that the calculated amount of water is correct, a trial mix may be prepared and a slump test carried out. In this test, the concrete mix is compacted into a slump cone (Fig. 7.18), which is similar to an upturned bucket 300 mm high with the base removed. When the cone is removed, the concrete will slump, i.e., reduce in height; the maximum slump, for concrete that is to be reinforced, should be about 100 mm, and less for unreinforced concrete.

Caring for concrete

After it has been cast, concrete must be cured. It should be covered with either wet sand, straw, cement bags, jute sacks, plastic or palm leaves to keep the concrete moist and as cool as possible. The chemical reaction which causes the cement particles to bind is dependent upon the amount of water present. If the moisture has been sucked out from the surface of the concrete by the heat of the sun, the chemical reaction cannot take place and the surface of the slab will not be durable. In hot dry climates the concrete and its covering need to be watered twice a day for seven days after casting. If the concrete is not cured, it will have only 60% of its ultimate design strength; if cured for three days, it will attain only 80%, but if kept damp for seven days will reach almost 100% (Reynolds & Steedman, 1974).

A good guide for field workers is: "Make the concrete mixture as dry as you can; and then keep the cast concrete as wet as you can."

The most effective way of checking the strength of a slab is to test load it seven days after casting. As, normally, only one person at a time will use the latrine, to test load the slab with five or six people gives an adequate and convincing factor of safety. The slab should be supported at its edges by four or five bricks placed on flat ground, and the people should stand on the slab, avoiding areas directly over the bricks. Testing the strength of precast slabs by throwing them off the back of the delivery truck at the site, on the understanding that those that do not break are adequate, is not recommended.

The final concrete surface should be clean, dense and free of blemishes. The surface will absorb urine unless it is sealed effectively with, for example, proprietary sealant, alkali-resistant gloss paint, bitumastic paint, or two coats of a 25% solution of silicate of soda (Khanna, 1985).

A screed (a thin layer of cement mortar) is sometimes applied to a flat slab after casting to create the desired slope towards the squat hole. However, unless the screed is applied before the concrete has completely set there is a danger of its flaking off in use. Wherever possible the required slope should be cast in the original concrete, a dense surface being obtained by trowelling with a steel float as the concrete begins to set. Alternatively the slab may be cast upside down on plastic sheeting to ensure a good finish.

Footrests are normally cast separately, after the concrete of the slab has hardened. The area where the rests will be cast is roughened when the slab surface is being given its final trowelling. Formers for the footrests can be made out of any available material such as tin or wood, but the individual formers should be connected together and to fixed points on the edge of the slab to ensure that the rests are always cast in the same position (Fig. 7.19).


Fig. 7.19. Formwork for the casting of footrests

Weights of concrete slabs

If cement-based slabs are to be moved, weight is an important consideration. For example, a 65-mm-thick circular concrete slab, 1.5 m in diameter, weighs approximately 275 kg, while an 80-mm thick slab weighs 340 kg. A rectangular slab, 65 mm thick, designed to cover a pit of size 2.2 m x 1.1 m would weigh 360 kg, unless made in sections (Fig. 7.20). Circular slabs are not normally made in sections. When whole, a round slab can be moved by two or three people, rolling it on its edge (Fig. 7.21). This is particularly useful in the management of the construction yard and can sometimes even be used to transfer the slab to the household site without a vehicle.


Fig. 7.20. Rectangular slab In two sections


Fig. 7.21. Circular slab for ease of transport

Concrete for other components

Concrete for floors of latrines that are not directly above the pits is cast in situ. A slightly weaker concrete mix of 1 :3 :6 may be used but the curing requirements remain as described above. The outlet pipe and pan should be carefully laid to the desired level before the concrete is cast.

Cover slabs for offset pits and floors and cover slabs of septic tanks are normally also made of concrete. Walls of septic tanks are usually constructed from concrete blocks or cement-plastered fired bricks. The requirements for good quality concrete are identical to those for components discussed previously.

Other materials for slabs

Wood

The simplest slabs in rural areas are made from rough poles and tree branches laid closely together over the pit. A timber slab is always liable to deterioration because of fungal decay owing to the moist gases rising from the pit and also because of the threat from termites and boring insects in tropical climates. Durable timbers such as the heartwood of some tropical hardwoods are normally too expensive for use in latrines but, where available, may be expected to last satisfactorily for several years.

A thick layer of earth or mud is often spread over the poles or branches to bind them together and create a smooth surface (Fig. 7.22). In many places, people are skilled at making mud floors which are almost as hard as cement and quite smooth. They need not be rough or unsanitary. There are various methods of improving the mud with local materials, such as mixing the soil with a liquor obtained by soaking animal dung overnight. In some areas the mud is mixed with charcoal or other small aggregate, or with cow dung and then smeared with ashes. Alternatively, the mud from ant-hills has been found to make a hard, practically waterproof surface (Denyer, 1978). If the surface is not kept in good condition, however, there is a danger of hookworm larvae penetrating the feet of users.

The life of a rough timber slab can be extended by using a mixture of soil and cement to plaster and protect the wood. Alternatively, a thin cement mortar screed can be laid over the surface of the earth to protect against hookworm and to improve hygiene. However, it is usually more cost-effective to use the cement to provide a permanent concrete slab which can be transferred to a new pit when the first is filled. Where more than half a bag of cement is needed to stabilize the earth, a concrete slab is likely to be a cheaper alternative.

In an area where timber is abundant, hewn or sawn logs supporting a platform of wooden planks make a floor that is preferable to the mud and pole version (Fig. 7.23). The surface can be kept clean, and signs of imminent collapse are normally apparent to the adult user. The durability of timbers may be improved by some form of treatment. The effectiveness of these treatments depends upon the amount of preservative that the timber can be made to absorb, which is a function of the permeability of the timber and the process used. Suitable preservatives include ordinary tar, tar-oils such as creosote, water-based preservatives such as copper/chrome/arsenic, and specialized organic solvents (Tack, 1979). Each type of preservative has its own characteristics and particular uses. Where treated timber is not available and the cost of using preservatives on a small scale is high, other more durable alternatives may be cheaper in the long run.


Fig. 7.22. Timber and earth slab

Simple timber slabs are often considered to be unsuitable for sanitation projects, since people are less likely to use the latrine if they are afraid that the slab may collapse under them. However, the danger of collapse is usually less than the dangers associated with not having any appropriate system of sanitation. If no other materials are available at reasonable cost, a rough pole slab that has to be renewed every few years is to be preferred to no latrine at all.


Fig. 7.23. Sawn timber slab

Scrap iron and steel

In urban areas where sanitation is most urgently required, supplies of even the cheapest materials, such as rough poles, are usually limited and relatively expensive. The simplest alternative used by householders on an informal basis is to lay parts of discarded vehicles or any other scrap materials across the pit opening to provide support, with flattened containers, oil drums or galvanized iron roofing sheet to make a surface. Such materials do not seal the pit but they enable the user to excrete into a relatively safe hole rather than at the side of the street. However, where there are significant dangers, especially for children, these methods cannot be recommended.

Miscellaneous materials

Slabs have been made in a variety of other materials. Glass-reinforced plastics, polyvinyl chloride (PVC), ceramics and glass fibre have all been used to meet particular needs and situations. Plastic floors tend to flex under the weight of the user unless they are deeply ribbed. Some of these materials can also be used to give a special surface finish to concrete slabs.

Footrests and squat holes

Footrests are required to lift the users' feet off the slab in case it is already fouled and also to position the users so that they are less likely to dirty the slab or the edge of the squat hole. The positions and sizes of footrests must be determined to suit the needs of the people in each area. Fig. 7.24 indicates a typical layout. Different people in different societies with different-sized bodies and varying flexibility of tendons may excrete between their feet or behind their ankles. Their feet may be parallel or angled. It is therefore advisable to check with young and old, and with male and female in a community before assuming a particular layout. McClelland & Ward (1976) reported that in one sample of 140 people, the distance from heel to anus in a squatting adult varied from 0 to 0.25 m with a mean of 0.13 m for men and 0.10 m for women.

Excreta enter a pit either by falling through a squat hole or by passing through a water seal. Details of seals are given later. Squat holes have to be large enough to limit fouling of the edges but not so large that children are frightened of using the latrine. The hole can either be rectangular, elliptical, pear-shaped or circular with a straight extension as in a keyhole (Fig. 7.25). The maximum width should be 180 mm and the length at least 350 mm. In a concrete slab, the edge of the former used to make the hole should be angled to ease its withdrawal after casting.


Fig. 7.24. Possible footrest positions

Seats for latrines

In many parts of the world, people prefer to sit to defecate. To make a latrine seat, a support or pedestal is built or mounted on top of the slab. The seat level should be at a position that is comfortable for the majority of the users (Fig. 7.26); this is normally about 350 mm above the top of the slab.

The seat support can be made on site from brick, concrete, mud block or timber and should be designed to minimize the load on the slab. A heavy type of construction adds weight to the slab which then requires more expensive reinforcement to carry the load. Commercially available or project-manufactured pedestals made of ceramic, glass reinforced plastic (GRP), PVC or ferrocement can also be used where people can afford them.


Fig. 7.25. Squat hole shapes and former


Fig. 7.26. Latrine seat


Fig. 7.27. Pedestal seat liner

The inside of the pedestal should be designed to prevent constant fouling by excrete, which leads to increased odour and fly breeding. One approach is to use a large-diameter opening of 250 mm or more, but this might discourage use by children who are frightened by the large opening. An alternative is to have a 1 80-mm diameter hole through the pedestal which is lined with a smooth material such as cement mortar or an insert of glass fibre (Fig. 7.27) or ceramic. A third alternative is a tapered hole, increasing from an opening size of about 180 mm at seat level to 300 mm at the slab. If possible the pedestal should overhang slightly so that the seat can be used with the feet tucked under to mimic the squatting position.

Shapes of locally made pedestals vary from a rectangular box, where the user sits on one side but can also sit across a corner with one foot on either side, to a circular or oval design. It is important to obtain a good seal between the pedestal and the slab.

A seat cover may be fitted to seal off an unventilated pit. Where a vent pipe is fitted, an adequate flow of air to the pit can be obtained by raising the seat cover slightly above the seat, as is the case with conventional flush pedestals.

A special fitment with a small opening can be made to encourage children to use the latrine. Alternatively the pedestal top can be enlarged to accommodate a second seat with a smaller opening, possibly at a lower level, for the use of children.

Water seals and pans

A pour-flush latrine utilizes a water seal to prevent odour and insects entering the latrine from the pit. This water seal may be part of the pan unit (Fig. 7.28) or may be connected immediately below the pan (Fig. 7. 29). For on-site sanitation, flushing is normally carried out by the wash-down method where the force of the flush water thrown into the pan is enough to drive the excrete through the water seal.

The pan may be of the squatting type or of the pedestal variety where the user can sit. The amount of water needed for flushing


Fig. 7.28. Combined pan and water seal for direct pour-flush latrine depends on the design of the pan or pedestal, the depth and volume of the water seal, and the minimum passage size through the seal. For a water seal directly above the pit about 1 litre of water is normally sufficient for flushing. Two litres may be required for an offset pit and a minimum of 3 litres for an improved pedestal pan and offset pit.


Fig. 7.29. Pan and seal for offset pour-flush latrine

The depth of the water seal is measured as the depth of water that would have to be removed from a fully filled trap to allow the passage of air (Fig. 7.28). The seal volume is the amount of water held within the trap when the unit is not being used, and the minimum passage size is the opening through which the water must flow and which may be of a smaller diameter than the connecting pipe. The depth of the seal in a conventional WC is approximately 50 mm. However, the deeper the seal the more water is required for flushing. In pour-flush latrines, the depth of the seal is normally reduced to the minimum compatible with maintaining the seal in hot weather. The seal volume will be reduced by evaporation, the water loss being proportional to the time between consecutive flushings and the degree of exposure to direct sunlight and air movement. A minimum seal depth of 20 mm is considered reasonable with an optimum passage size of 70 mm (Mare, 1985b).

Water seals that can be removed during the dry season to minimize water usage are not recommended. It is likely that the seal would not be replaced at the beginning of the wet season and therefore the latrine would not work effectively.

Types of water seal

Where the water seal services a direct pit, the pan and the seal should be made as a single piece with a hemispherical bowl known as a "gooseneck" trap. This is designed to discharge into the centre of the pit and not against the pit lining where it might cause damage. These types of seal can easily be damaged by users trying to clear blockages with a rod where the thin cement of the seal is unsupported. As direct pits have become less popular because of emptying difficulties, the use of the gooseneck trap has also declined.

In many countries, the pan is made separately from the seal to facilitate manufacture and to give the installer greater freedom as to where the offset pit is located in relation to the pan. The normal system, which has an inclined outlet, is known as a P-trap, while the system with a vertical outlet is called an S-trap.

Water-seal materials

Pans and water seals may be produced by manufacturers or by project staff to standard specifications in a variety of materials. Ceramics, such as white vitreous china or other glazed earthenware, have traditionally been used for pans and pedestals. However, such items may be expensive to purchase and require careful attention to packing if they are to be transported safely. They may also be heavy and require a strengthened slab for a direct pit. Particularly because of the problems of transport and handling, the use of plastics for pans and water seals is becoming more common. Glass-fibre pans and high-density polyethylene (HDPE) water seals are light and easily transportable, even by bicycle, and are often preferred by users, even when more expensive than the cement-based systems described below.

The cheapest pans and seals are made from cement mortar (10-30 mm thick) close to the point of sale or delivery. They can be produced on a large scale without factory facilities, and can be repaired easily when damaged. Such units are likely to be rougher than manufactured pans and seals, and a reaction between urine and the cement normally leads to some staining of the surface and some odour from the trap. This can be minimized by the addition of marble dust and chippings to the cement mortar. When dry, the surface can then be rubbed down with carborundum stones to provide an attractive mosaic finish. Colourings may also be added to the mortar to give a more attractive appearance.

An alternative method of production uses casting boards to cast the pan and seal in two halves with a 1 :2:2 concrete mix pressed around the form. After 24 hours the two sections can be removed from the moulds and joined together with neat cement, the inner surface also being smoothed off with neat cement. One disadvantage of having the pan and water seal in one piece is that the trap cannot be rotated in the direction of the offset pit.

The Thai model, which is now in use in about 3 million rural homes, employs a two-part mould and is cast in a single step, including the platform, without the need for grouting pieces together. The depth and angle of the seal are uniform. Large numbers of moulds can be cast quickly, thus facilitating production of pans and seals so that large numbers of households can have pour-flush latrines in a very short period of time (J. T. Visscher, personal communication).

Making the pan and trap separately enables very simple forms to be used. These may be built up from clay and husk or plastered brick or concrete which can be reused many times. A release agent is needed to break the bond between the mould and the new concrete. Proprietary agents are available, though used engine oil or even cow-dung wash have proved to be cheap and effective.

Pedestals designed for pour-flushing with small quantities of water (about 3 litres) are normally made of ceramic to ensure a smooth finish. Less efficient units may be made using cement-based methods with ferrocement, fibre-reinforced cement and concrete with marble chippings.

Pipes and junction chambers

The water seal may be connected to the offset pit by conventional pipework (see Fig. 6.9) or by a covered drain (see Fig. 6.10). Where double pits are in use, a junction chamber or inspection chamber (see Fig. 6.15) is required whereby the flow can be directed into one pit or the other.

The pipe or channel should be not less than 75 mm wide and should be as smooth and direct as possible. Any roughness or sharp bends will tend to slow the passage of excrete, eventually leading to a build-up of deposits and a blockage. The cheapest available non-pressure pipes will be adequate, whether in fired clay, plastic or asbestos cement. The minimum slope should be I in 30 for smooth pipes and I in 15 for rougher pipes or hand-shaped channels. If the slope is too steep there is a danger of solids being deposited in the pipe.

Special care must be taken where the pipe passes through the superstructure wall (see Fig. 6.12 and 6.13). If possible, some degree of flexibility is required at the pipe joints or in the channel so that differential settlement of the latrine superstructure or the pit lining will not cause damage. There is unlikely to be significant loading on the ground above the connecting pipe, but where there is any possibility of vehicles crossing the area between latrine and pit, conventional pipe bedding and protection should be used.

The pipe or drain should extend some distance into the pit so that the wastewater discharges directly towards the centre and does not dribble down the pit walls, with a consequent build up of deposits.

Where a covered drain is used to connect a double-pit system, a simple Y-junction can be constructed to divert the flow. The junction in a pipework connection between pits and latrine requires a chamber which should be of sufficient size to allow for ease of construction of the concrete benching. It must also allow for the flow to be diverted from one pit to another with a temporary blockage in one or other arm of the Y-junction. A minimum internal dimension of 250 mm is recommended (Roy et al., 1984). The chamber cover slab needs to be removable to allow for access to divert the flow, but also has to be heavy enough and fixed in such a way that it is difficult for children to remove.

Vent pipes

The vent pipe, i.e., the tube connecting the latrine pit to the open air above the pit, serves two purposes: ( I ) to create a draught of air from the superstructure, through the squat hole and out of the pit, passing up the vent; (2) to act as a light source which will attract flies to the screen trap which is attached to the top of the vent. Normally the vent pipe is straight and rises vertically above the pit so that the daylight at the top can be seen directly by any flies in the pit (Fig. 7.30). A straight pipe also maximizes the air flow; bends in the vent absorb part of the energy in the air movement.

With certain types of slab, or where existing slabs require upgrading with a vent, there may be a need to bring the pipe out horizontally underneath the slab before turning to the vertical. In this situation an ancillary light source is required in the form of a glass or perspex window at the bend (Fig. 7.31). Flies in the pit are first attracted to the light source at the window. They cannot escape from the vent at that point so, following the air flow upwards, they then go towards the light at the top of the vent.


Fig. 7.30. Straight vent pipe


Fig. 7.31. Angled vent pipe with window

The draught through the vent is created primarily by the movement of wind across the top of the pipe. This air movement creates a suction effect, sucking air out of the pit and up the vent. To achieve satisfactory air movement, the top of the vent should be at least 500 mm above the highest part of the roof, except where the roof is conical, in which case the pipe should reach at least the height of the roof apex. However, if the pipe can be extended even higher, a stronger updraught will be created in the vent. Wind speed increases even at slightly higher elevations above the ground, which creates a stronger suction effect. Also, the higher the vent, the less likely it is to be shielded by buildings or other obstructions which may cause air turbulence and reduce or even reverse the updraught in the vent. Any large trees or overhanging branches close to the vent may significantly affect air movement and thus reduce the effectiveness of the ventilated latrine. Similarly, a rain cowl should not be placed on top of the vent, as it will reduce the air flow; the amount of rain entering the pit is not likely to be significant.

The vent should therefore be located in the best position to catch any air movements across the upper end of the pipe. Vent pipes are normally placed outside the superstructure, particularly where the building materials available make it difficult to construct a watertight joint where the pipe would pass through the roof. Free-standing pipes may be secured to the wall of the superstructure using standard pipe fittings, strips of galvanized steel, galvanized wire or other noncorrosive material. Where possible, the vent should be located on the side of the building which faces the equator, that is the side which receives most sunlight. The warming of the surface of the vent pipe, raises the temperature of the air in the pipe, increasing the upward draught. Painting the vent black aids this thermal effect. However, the air movement over the top of the vent is the most significant factor in causing updraught and a vent placed inside the building will still work effectively.

The updraught may also be increased by using a spiral design for the superstructure, which funnels the air into the structure. If there are no other ventilation holes, this produces a positive pressure inside the structure, thus forcing air through the squat hole and the pit and up the vent. However, where the winds are particularly variable and often blow from a direction away from the superstructure opening, a negative pressure may be created which will suck foul air out of the pit and into the building (Fig. 7.32).

Dimensions of the vent pipe

Vents may be square or round and can be constructed from a wide variety of materials. Circular vent pipes should normally have an internal diameter of at least 150 mm for smooth materials (PVC or asbestos cement) or 230 mm for rough surfaces (such as locally produced cement-rendered pipes), although in exposed places with high wind speeds a smaller diameter may be sufficient. It is normally advantageous to enlarge the top of the vent pipe by about 50 mm to account for the head losses, that is, the reduction in energy and therefore in updraught caused by the air passing through the fine mesh of the flyscreen (Fig. 7.33). There is a danger that cobwebs, dirt or insect matter may build up on the screen, restricting air flow. Belling the top of the pipe can serve to balance these restrictions.


Fig. 7.32. Layouts for superstructures, vent pipes and pits


Fig. 7.33. Belled vent with fly screen

Materials

Materials suitable for vent pipes include asbestos cement, unplasticized PVC, bricks, blocks, hollowed-out bamboo, ant-hill soil, cement rendered reeds or bamboo, and cement-rendered hessian (Ryan & Mara, 1983). The choice of material will need to take into account durability, availability of materials and skills, cost, and availability of funds. Ordinary PVC becomes brittle when exposed to strong sunlight, so material with a special stabilizer should be used if possible. Because galvanized steel corrodes in a humid atmosphere, the use of thin sheets is not recommended for vent pipes except in very dry climates.

Brick and block chimneys

Vent pipes may be made from bricks or blocks with cement mortar joints in the form of a chimney that is at least 230 mm² internally. The flyproof screen should be stretched over the top surface of the highest bricks. If it is built into the course joint one brick down, a receptacle is created which catches leaves and other debris. The chimney may be free-standing or built into the corner of the superstructure. Morgan & Mara (1982) suggested that thermal updraught in such chimneys continues well into the night because the brickwork retains heat which is released slowly to the air over a period of several hours.

Locally made vent pipes

Reeds, poles, thin bamboo or strips of 10 20 mm of large bamboo can be tied together with wire or string to make a mat which forms a base for cement mortar. The mat, about 2.5 x 1.0 m, is rolled round rings made of green sticks to form a tube about 300 mm in diameter. Flyproof netting is fixed over one end of the tube, which is then laid on the ground. The upper part of the pipe is covered with a layer of cement mortar made with one part of cement to three parts of sand. When the mortar has dried the tube is put in position with the mortared part against the wall of the latrine. Then the outer part of the pipe is plastered with cement mortar. Alternatively the pipe may be rotated on the ground and completely plastered before erection.

A vent pipe can also be made with hessian. First, a 250-mmdiameter tube is formed of spot-welded steel mesh made of 4-mm bars at 100-mm centres (100 mm apart, centre to centre). Hessian or jute cloth is stitched tightly round the outside of the tube and flyproof netting is stitched over one end. Cement mortar, made of one part of cement to two parts of sand, is then brushed over the tube in several layers until a total thickness of about 10 mm is formed. The vent pipe is then fixed in place. Alternatively, a pipe may be made from ferrocement with three or four layers of mesh plastered with cement mortar and without any hessian.

Fly screens

Fly screens should be made of material that will not be affected by temperature, sunlight, or the corrosive gases that are vented from the pit. Stainless steel or aluminium are considered to be best. Their comparatively high cost may be justified by their long life, especially as the screen accounts for a very small proportion of the total cost of the latrine. PVC-coated glass-fibre netting is relatively cheap and has lasted for more than seven years in Zimbabwe (Morgan & Mara, 1982). However, it tends to become brittle after about five years and is likely to tear at the point where it passes over the edge of the pipe. Ordinary plastic screens deteriorate quickly in sunlight. Painted mild steel mesh, commonly sold as window screening against mosquitos, and galvanized mild steel mesh last only a few months before corrosion by the pit gases renders them ineffective. Gases and sunlight weaken the screens but the actual tearing of the material is assumed to be caused by birds alighting or possibly by lizards which frequent the top of rough-walled vent pipes or simply by the tension within the flexing screen (P. R. Morgan, personal communication).

A mesh size of 1.2-1.5 mm is recommended. If the apertures are larger small flies can pass through. If the apertures are smaller there is too much resistance to the updraught of air. The screen should be firmly fixed to the top of the pipe. Netting may be fitted over the top of brick and block chimneys during building and on locally made vent pipes during fabrication. Screens may be glued to PVC pipes with epoxy resin or tied on with a piece of wire. Where there is a particular problem with mosquitos breeding in wet pits, it may be necessary to install removable traps over the squat hole or pedestal (Curtis & Hawkins, 1982).

Netting should be inspected regularly (at least once a year) to ensure that it is still in place and that it remains in good condition. Part of routine maintenance is to pour a bucketful of water through the screen and down the pipe to wash away cobwebs and other material.

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.

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. 7.34. Ventilation In a pour-flush latrine


Fig. 7.35. Ventilation In a VIP latrine

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).

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.


Fig. 7.36. Privacy screens made from cheap locally available materials

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

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 halfbrick 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 onebrick 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.