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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
 

Human wastes

Volume of fresh human wastes

The amount of faeces and urine excreted daily by individuals varies considerably depending on water consumption, climate, diet and occupation. The only way to obtain an accurate determination of the amount at a particular location is direct measurement. Table 5.1 shows some reported average quantities of faeces excreted by adults (grams per person per day).

Even in comparatively homogeneous groups there may be a wide variation in the amounts of excreta produced. For example, Egbunwe (1980) reported a range of 500-900 g of faeces per person per day in eastern Nigeria. Generally, active adults eating a high-fibre diet and living in a rural area produce more faeces than children or elderly people living in urban areas eating a low-fibre diet. Both Shaw (1962) and Pradt (1971) suggested that the total amount of excreta is about one litre per person per day.

The amount of urine is greatly dependent on temperature and humidity, commonly ranging from 0.6 to 1.1 litres per person per day.

In the absence of local information the following figures are suggested as reasonable averages:

 

- high-protein diet in a temperate climate: faeces 120 g, urine 1.2 l, per person per day.
- vegetarian diet in a tropical climate: faeces 400 g, urine 1.0 l, per person per day.

Table 5.1. Quantity of wet faeces excreted by adults (in grams per person per day)

Place

Quantity

Reference

China (men)

209

Scott (1952)

India

255

Macdonald (1952)

India

311

Tandon & Tandon (1975)

Peru (rural Indians)

325

Crofts (1975)

Uganda (villagers)

470

Burkitt et al. (1974)

Malaysia (rural)

477

Balasegaram & Burkitt (1976)

Kenya

520

Cranston & Burkitt (1975)

Decomposition of faeces and urine

As soon as excreta are deposited they start to decompose, eventually becoming a stable material with no unpleasant smell and containing valuable plant nutrients. During decomposition the following processes take place.

 

• Complex organic compounds, such as proteins and urea, are broken down into simpler and more stable forms.

• Gases such as ammonia, methane, carbon dioxide and nitrogen are produced and released into the atmosphere.

• Soluble material is produced which may leach into the underlying or surrounding soil or be washed away by flushing water or ground-water.

• Pathogens are destroyed because they are unable to survive in the environment of the decomposing material.

The decomposition is mainly carried out by bacteria although fungi and other organisms may assist. The bacterial activity may be either aerobic, i.e., taking place in the presence of air or free oxygen (for example, following defecation and urination on to the ground), or anaerobic, i.e., in an environment containing no air or free oxygen (for example, in a septic tank or at the bottom of a pit). In some situations both aerobic and anaerobic conditions may apply in turn. When all available oxygen has been used by aerobic bacteria, facultative bacteria capable of either aerobic or anaerobic activity take over, and finally anaerobic organisms commence activity.

Pathogens may be destroyed because the temperature and moisture content of the decomposing material create hostile conditions. For example, during composting of a mixture of faeces and vegetable waste under fully aerobic conditions, the temperature may rise to 70°C, which is too hot for the survival of intestinal organisms. Pathogens may also be attacked by predatory bacteria and protozoa, or may lose a contest for limited nutrients.

Volumes of decomposed human wastes

As excreta become decomposed they are reduced in volume and mass owing to:

 

- evaporation of moisture;

- production of gases which usually escape to the atmosphere;

- leaching of soluble substances;

- transport of insoluble material by the surrounding liquids;

- consolidation at the bottom of pits and tanks under the weight of superimposed solids and liquids.

Little information is available regarding the rate at which the reduction takes place although there are indications that temperature is an important factor (Mara & Sinnatamby, 1986). Weibel et al. (1949) measured the sludge accumulation rate in 205 septic tanks in the United States of America, and obtained the results shown in Fig. 5.1; other authors have reported the accumulation rates listed in Table 5.2.


Fig. 5.1. Rate of accumulation of sludge and scum in 205 septic tanks in the United States of America (from Weibel et al., 1949)

 

WHO 91416

Table 5.2. Excreta accumulation rates (litres per person per year)

Location

Accumulated excreta

Remarks

Reference

Zimbabwe

20

Latrine regularly washed down; degradable cleaning material

Morgan & Mara (1982)

West Bengal

25

Wet pit - ablution water used

Wagner & Lanoix (1958)

West Bengal

34

Wet pit

Baskaran (1962)

Philippines

40

Wet pit; degradable cleaning material

Wagner & Lanoix (1958)

USA

42

Faeces (adult); half amount for children

Geyer et al. (1968)

Brazil

47

Dry pit

Sanches & Wagner (1954)

Philippines

60

Dry pit; degradable cleaning material

Wagner & Lanoix (1958)

The factors with the biggest effect on the sludge accumulation rate are whether decomposition takes place above or below the water table and the type of anal cleaning material used. Decomposition under water produces a much greater reduction in volume than decomposition in air. This is due to better consolidation, more rapid decomposition and removal of the finer material in the water flow. Anal cleaning materials vary widely around the world, from those requiring little or no storage space; such as water, to those having a greater volume than the excreta, such as corn cobs, cement bags or stones.

Table 5.3. Suggested maximum sludge accumulation rates (litres per person per year)

 

Sludge accumulation rate

Wastes retained in water where degradable anal cleaning materials are used

40

Wastes retained in water where non-degradable anal cleaning materials are used

60

Waste retained in dry conditions where degradable anal cleaning materials are used

60

Wastes retained in dry conditions where non-degradable anal cleaning materials are used

90

When designing a latrine it is strongly recommended that local sludge accumulation rates should be measured. In the absence of local data, the volumes given in Table 5.3 are suggested as a maximum. There is some evidence to indicate that these figures are on the high side. However, if refuse is added to excreta, the accumulation rate may be much greater.

Where excreta are stored for short periods only, such as in double pit latrines or composting toilets, the reduction process may not be complete before the sludge is removed. In such cases it will be necessary to use higher sludge accumulation rates than indicated above. A 50% increase is tentatively suggested.

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