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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
fermer ce répertoireChapter 5. Technical factors affecting excreta disposal
Afficher le documentHuman wastes
Afficher le documentGround conditions
Afficher le documentInsect and vermin problems
ouvrir ce répertoire et afficher son contenuChapter 6. Operation and maintenance of on-site sanitation
ouvrir ce répertoire et afficher son contenuChapter 7. Components and construction of latrines
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
 

Ground conditions

Ground conditions affect the selection and design of sanitation systems, and the following five factors should be taken into consideration:

 

- bearing capacity of the soil;
- self-supporting properties of the pits against collapse;
- depth of excavation possible;
- infiltration rate;
- groundwater pollution risk.

Bearing capacity of the soil

All structures require foundations, and some soils are suitable only for lightweight materials because of their poor load-carrying capacity - marshy and peaty soils are obvious examples. In general, it is safe to assume that if the ground is suitable for building a house it will be strong enough to support the weight of a latrine superstructure made of similar materials, providing the pit is appropriately lined.

Self-supporting properties of the pits

Many types of latrine require the excavation of a pit. Unless there is specific evidence to the contrary (i.e., an existing unlined shallow well that has not collapsed), it is recommended that all pits should be lined to their full depth. Many soils may appear to be self-supporting when first excavated, particularly cohesive soils, such as clays and silts, and naturally bonded soils, such as laterites and soft rock. These self-supporting properties may well be lost over time owing to changes in the moisture content or decomposition of the bonding agent through contact with air and/or moisture. It is almost impossible to predict when these changes are likely to occur or even if they will occur at all. It is therefore safer to line the pit. The lining should permit liquid to percolate into the surrounding soil.

Depth of excavation

Loose ground, hard rock or groundwater near to the surface limit the depth of excavation possible using simple hand tools. Large rocks may be broken if a fire is lit around them and then cold water poured on the hot rock. Excavation below the water table and in loose ground is possible by "caissoning" (see Chapter 7), but it is expensive and not usually suitable for use by householders building their own latrines.

Infiltration rate

The soil type affects the rate at which liquid infiltrates from pits and drainage trenches. Clays that expand when wet may become impermeable. Other soils such as silts and fine sands may be permeable to clean water but become blocked when transmitting effluent containing suspended and dissolved solids.

Opinions vary regarding the areas through which infiltration takes place. For example, Lewis et al. (1980) recommended that only the base of pits or drainage trenches should be considered and that lateral movement (the sidewall influence) be ignored. Mara (1985b) and others have assumed that infiltration takes place only through the side walls as the base rapidly becomes blocked with sludge. Until more evidence is available, it is recommended that the design of pits and trenches should be based on infiltration through the side walls up to the maximum liquid level. For trenches, the area of both walls should be used.

The rate of infiltration also depends on the level of the groundwater table relative to the liquid in the pit or trench. In the unsaturated zone, the flow of liquid is induced by gravity and cohesive and adhesive forces set up in the soil. Seasonal variation may produce a change in the amount of air and water in the soil pores and this will affect the flow rate. Conditions at the end of the wet season should normally be used for design purposes as this is usually the time when the groundwater level is at its highest. In the saturated zone all pores are filled with water and drainage depends on the size of the pores and the difference in level between the liquid in the pit or trench and the surrounding groundwater.

Soil porosity also affects infiltration. Soils with large pores, such as sand and gravel, and rocks such as some sandstones and those containing fissures, drain easily. Silt and clay soils, however, have very small pores and tend to retain water. Soils containing organic materials also tend to retain water but the roots of plants and trees break up the soil, producing holes through which liquids can drain quickly.

The rate of groundwater flow in unsaturated soils is a complex function of the size, shape and distribution of the pores and fissures, the soil chemistry and the presence of air. The speed of flow is normally less than 0.3 m per day except in fissured rocks and coarse gravels, where the speed may be more than 5.0 m per day, with increased likelihood of groundwater pollution.

Pore clogging

Soil pores eventually become clogged by effluent from pits or drainage trenches. This may reduce or even stop infiltration through the soil. Clogging may be caused by:

 

- blockage of pores by solids filtered from the liquid;
- growth of microorganisms and their wastes;
- swelling of clay minerals; and
- precipitation of insoluble salts.

When liquid first infiltrates into unsaturated soil, aerobic bacteria decompose much of the organic matter filtered from the liquid, keeping the pores clear for the passage of air as well as effluent. However, once organic matter builds up so that air cannot pass through the pores, the rate of decomposition (now by anaerobic bacteria) is slower, and heavy black deposits of insoluble sulfides are built up.

Clogging of the pores can be minimized by ensuring that infiltration occurs uniformly over the whole system. Poorly designed infiltration systems (particularly trenches) often cause the liquid to converge on a small section of the system. This produces localized high infiltration rates and clogging in that area. Clogging can sometimes be reduced by a regime of alternate "resting" and "dosing" of the soil. The infiltration area is allowed to rest, i.e., to become fully drained of liquid for a period before infiltration recommences. During the resting period, air reaches the soil surface and the anaerobic bacteria causing the clogging die off, allowing the surface to become unclogged.

Determining infiltration rates

It is rarely possible to measure accurately the rate of flow of effluent from pits and drainage trenches, especially as the flow often decreases as soil pores become clogged. Consequently various empirical rules are used. Some recommendations are based on the rate of percolation of clean water from trial holes dug on the site of a proposed pit or drainage field using various design criteria to allow for differences in infiltration rates (US Department of Health, Education, and Welfare, 1969; British Standards Institution, 1972). Laak et al. (1974) found that, for a wide range of soils, the infiltration rates of effluent were 10-30 litres per m2 per day. A conservative rate of 10 litres per m2 per day was recommended for general application. On the other hand, rates of up to 200 litres per m2 per day are considered applicable in practice in the United States of America (US Department of Health, Education, and Welfare, 1969), and Aluko (1977) found that, in Nigeria, designs with a maximum of 294 litres per m2 per day have proved satisfactory. The infiltration capacities given in Table 5.4 (US Environmental Protection Agency, 1980) are recommended as a basis for the sizing of pits and drainage trenches where information about actual infiltration rates is not available. The capacities given for coarse soils are restricted to prevent possible groundwater pollution and therefore may be unnecessarily conservative in areas where this is not a problem. Gravel is capable of much higher infiltration rates, which may be a problem in areas where shallow groundwater is used for human consumption.

Table 5.4. Recommended infiltration capacitiesa

Type of soil

Infiltration capacity, settled sewage
(l per m2 per day)

Coarse or medium sand

50

Fine sand, loamy sand

33

Sandy loam, loam

25

Porous silty clay and porous silty clay loam

20

Compact silty loam, compact silty clay loam and non-expansive clay

10

Expansive clay

<10

 

a Source: US Environmental Protection Agency, 1980.

Groundwater pollution risk

This section summarizes the likely effects of on-site sanitation systems on groundwater and the ways in which pollution can be minimized. Lewis et al. (1980) have carried out detailed reviews of these aspects.

The effluent from pits and drainage trenches may contain pathogens and chemical substances that could contaminate drinking-water supplies. Because of their comparatively large size, protozoa and helminths are rapidly removed by the straining action of the soil, but bacteria and viruses are more persistent. The bacterial and viral pathogens that may be carried in water are discussed in Chapter 2.

Of the chemical substances generally present in domestic wastes, only nitrates present serious health dangers. Young babies bottle-fed with milk made from water with a high nitrate concentration may develop "blue baby disease", methaemoglobinaemia, which can be fatal if untreated. There is conflicting evidence suggesting that low nitrate concentrations may contribute to gastric cancer (Nitrate Coordination Group, 1986).

The usual means by which effluents affect drinking-water supplies is through pollution of groundwater that feeds wells and boreholes. A further danger is when effluent infiltrates the ground at shallow depth near to water pipes in which there is intermittent flow or in which the pressure is at times very low. Just as poor joints, cracks and holes in the pipe walls allow water to leak out when the pipes are full, so effluent leaks into the pipes when they are empty or under reduced pressure. Recommendations for allowable levels of pollutants in drinking-water are given in Guidelines for drinking-water quality (WHO, 1984).

Purification in unsaturated soil

Effluent passing through unsaturated soil (that is, soil above the groundwater table) is purified by filtration and by biological and adsorption processes. Filtration is most effective in the organic mat where the soil pores are clogged. In sandy soils, Butler et al. (1954) found a dramatic reduction in coliforms in the first 50 mm. The passage of pollutants from a new pit or drainage trench reduces as the pores become clogged.

Viruses, because of their small size, are little affected by filtration and their removal is almost entirely by adsorption on to the surface of soil particles; this is greatest where the pH is low (Stumm & Morgan, 1981). Adsorption of both viruses and bacteria is greatest in soils with a high clay content, and is favoured by a long residence time - that is, when flow rates are slow. Because the flow is much slower in the unsaturated zone than below the groundwater table, there is longer contact between soil and effluent there, increasing opportunities for adsorption. Adsorbed microorganisms can be dislodged, for example by flushes of effluent or following heavy rainfall, and may then pass into lower strata of the soil.

Viruses, whether they have been removed or remain in effluent, live longer at lower temperatures (Yeager & O'Brien, 1979). Both viruses and bacteria live longer in moist conditions than in dry conditions, and therefore in soils with a good water-holding capacity than in sandy soils. Bacteria live longer in alkaline than in acid soils. They also survive well in soils containing organic material, where there may be some regrowth.

Generally there is little risk of groundwater pollution where there is at least 2 m of relatively fine soil between a pit or drainage trench and the water table, providing the rate of application is not greater than 50 mm/day (equivalent to 50 litres per m2 per day). This distance may have to be increased in areas subject to intense rainfall, as the increased infiltration rate produced by the percolating rainwater may carry pollution further.

Fissures in consolidated rock may allow rapid flow of effluent to underlying groundwater with little removal of microorganisms. Holes in soil caused by tree roots or burrowing animals can act in the same way as fissures.

Purification in groundwater

There is little information about survival of either viruses or bacteria in groundwater, although it appears that low temperature favours long survival times. Enteric bacteria may survive in cool groundwater for more than three months (Kibbey et al., 1978). Field experiments indicate that the maximum distance that viruses and bacteria travel in groundwater before being destroyed is equal to the distance travelled by the groundwater in about ten days (Lewis et al., 1980).

In fine-grained soils and pollution sources surrounded by a mature organic mat, the distance travelled may be as little as 3 m, whereas a new source in fast-flowing groundwater may cause pollution up to 25 m downstream (Caldwell, 1937). The pollution extends from the source in the direction of groundwater flow, with only limited vertical and horizontal dispersion. However, this does not apply to pollution in fissured ground, where the pollution may flow through the fissures for several hundred metres, often in an unpredictable direction.

In most cases the commonly used figure of a minimum of 15 m between a pollution source and a downstream water abstraction point will be satisfactory. Where the abstraction point is not downstream of the pollution, i.e., to the side or upstream, the distance can be reduced provided that the groundwater is not abstracted at such a rate that its direction of flow is turned towards the abstraction point (Fig. 5.2). This is particularly useful in densely populated communities, where shallow groundwater is used as a water supply.

If it is not possible to provide sufficient space between the latrine and the water point, consideration should be given to extracting water from a lower level in the aquifer (Fig. 5.3). The predominant flow of groundwater (except fissured flow) is along the strata, with very little vertical movement. Provided the extraction rate is not too great (handpump or bucket extraction is acceptable), and the well is properly sealed where it passes through the pollution zone, there should be little or no risk of pollution.


Fig. 5.2. Zone of pollution from pit latrine

 

WHO 91417


Fig. 5.3. Protecting a hand pump from the pollution from a pit latrine

 

WHO 91418

Significance of pollution

While faecal pollution of drinking-water should be avoided, the dangers of groundwater pollution from on-site sanitation should not be exaggerated. A depth of two metres of unsaturated sandy or loamy soil below a pit or drainage trench is likely to provide an effective barrier to groundwater pollution and there may be little lateral spread of pollution. Where the groundwater is shallow, artificial barriers of sand around pits can control pollution (Fig. 5.4).


Fig. 5.4. Reducing the pollution from a pit latrine with a barrier of sand

 

WHO 91419

Unless water is extracted locally for domestic purposes, pollution of groundwater from on-site sanitation does no harm and is to be preferred to the considerable risks associated with defecation in the open. Where on-site sanitation would result in pollution of wells used for drinking-water, it is generally cheaper and easier to provide water from outside than to build sewers or use vacuum tankers to remove excreta.

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