The vent pipe, i.e., the tube connecting the latrine pit to the open air above the pit, serves two purposes: (1) 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.
Fig. 7.30. Straight vent pipe
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. 7.31. 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 non-corrosive 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).
Fig. 7.32. Layouts for superstructures, vent pipes and pits
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. 7.33. Belled vent with fly screen
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 × 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-mm-diameter 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 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.