|Guide to the Development of On-Site Sanitation (WHO, 1992)|
Human excrete should be regarded as a natural resource to be conserved and reused under careful control rather than being discarded. Excreta for reuse are derived from:
- nightsoil, including that collected by municipal systems or private contractors, and the nightsoil of individual households or groups of households and used on their own gardens or farms;
- solids from full pit latrines;
- sludge, scum and liquor from septic tanks, aqua-privies, vaults and cesspits; and
- raw and treated sewage and sludge from sewage treatment works (which are outside the scope of this book).
Solids from pit latrines are innocuous if the latrines have not been used for two years or so, as in alternating double pits. Raw excrete from all other sources are likely to include recently excreted faeces and may therefore contain active pathogens.
There are three basic methods of using this resource: agriculture, aquaculture, and biogas production.
Use in agriculture
Human excrete are a rich source of nitrogen and other nutrients necessary for plant growth. The most common method of reuse is direct application to the soil as a fertilizer. Nightsoil contains about 0.6% nitrogen, 0.2% phosphorus and 0.3% potassium, all of which are valuable plant nutrients. The humus formed by decomposed faeces also contains trace elements which reduce the susceptibility of plants to parasites and diseases. Humus improves the soil structure, enhancing its water-retaining qualities and encouraging better root structure of plants. Soil containing humus is less subject to erosion by wind and water and is easier to cultivate.
For centuries, untreated nightsoil has been widely used as a fertilizer in east and south Asia, although there is an increasing awareness of the public health dangers involved. Pathogens of all kinds can remain viable in the soil and on crops (see Table 2.4, page 14). Death of pathogens on crops is usually caused by desiccation and direct sunlight, so pathogens are generally more persistent in humid cloudy climates than in arid areas.
It has been suggested that the use of raw excrete and the effluent from septic tanks is acceptable only if confined to industrial crops and foodstuffs that are cooked before being eaten. However, even with these crops, there is considerable risk of pathogen transmission to agricultural workers, to people involved in transporting crops, and to those processing industrial crops or preparing food for cooking. Therefore such use must be carefully planned with strict surveillance by the health authorities.
The risks arising from pathogen transmission from the use of untreated excrete or sludge on food crops may be greater for populations with high levels of hygiene and health (for example people in towns) than for agricultural workers living in areas where excretaderived diseases are endemic (Feachem et al., 1983).
Excreta on paddy fields
Fields with crops standing in water during part or all of the vegetation period are potential transmission sites for schistosomiasis if fresh excrete are used as fertilizer (Cross & Strauss, 1985).
Fertilization of trees
Treated or untreated sewage is sometimes used to irrigate trees. This practice is most common in arid climates, where trees are watered to control desertification, to provide shade and windbreaks, or to cultivate coconuts and some other food crops. The main health risk is to workers and members of the public who have access to the plantation.
Excreta on pasture
When excrete are applied to land on which cattle graze there is a danger of the spread of beef tapeworm, whose eggs may survive on soil or pasture for more than six months.
Excreta may be treated in various ways to eliminate the possibilities of disease transmission. Apart from storage in double-pit latrines, the most appropriate treatment for on-site sanitation is composting.
Composting consists of the biological breakdown of solid organic matter to produce a humic substance (compost) which is valuable as a fertilizer and soil conditioner. It has been practiced by farmers and gardeners throughout the world for many centuries. In China, the practice of composting human wastes with crop residues has enabled the soil to support high population densities without loss of fertility for more than 4000 years (McGarry & Stainforth, 1978).
Nightsoil or sludge may be composted with straw and other vegetable waste, or with mixed refuse from domestic, commercial or institutional premises. The process may be aerobic or anaerobic.
Aerobic bacteria combine some of the carbon in organic matter with oxygen in the air to produce carbon dioxide, releasing energy. Some energy is used by the bacteria to reproduce. The rest is converted to heat, often raising the temperature to more than 70°C. At high temperatures there is rapid destruction of pathogenic bacteria and protozoa, worm eggs and weed seeds. All faecal microorganisms, including enteric viruses and roundworm eggs, will die if the temperature exceeds 46 °C for one week. Fly eggs, larvae and pupae are also killed at these temperature. No objectionable odour is given off if the material remains aerobic.
In the absence of oxygen, nitrogen in organic matter is converted to acids and then to ammonia; carbon is reduced to methane and sulfur to hydrogen sulfide. There is severe odour nuisance. Complete elimination of pathogens is slow, taking up to twelve months for roundworm eggs, for example.
The traditional method of composting is to pile vegetable waste with animal manure and nightsoil or sludge on open ground. Aerobic conditions may be maintained by regular turning of the material, which also has the advantage of making the moisture content more uniform throughout the tip. Under aerobic conditions, rapid decomposition of organic matter takes place in the first 2 4 weeks. The process is considerably shorter than under anaerobic conditions. Controlled composting in mechanized composting equipment shortens the process even more.
According to Flintoff (1984) there are five preconditions for successful composting:
- suitability of the wastes;
- marketability of the product;
- support of authorities, particularly those in agriculture;
- a price for the product that is acceptable to farmers; and
- a net cost (i.e., process costs less income from sale) that can be sustained by the operating authority.
In developing countries most domestic refuse is vegetable matter, and there may be little paper, glass or metal. Where these materials are more common, paper can be composted and some glass is acceptable in compost if it is ground up at some stage of the composting process. Metals need to be removed. Textiles, plastics, leather and the like may be removed or they may be shredded and included in the compost. Dust and ash may also be included but, if they form too large a proportion of the refuse, the value of the compost is reduced.
Working over refuse heaps with forks to break down large lumps helps the composting process. Broken-down refuse has a greater surface area for air to enter and for bacteria to attack. It allows less penetration of rain and fly control is easier.
Control of composting
Too much moisture in a heap of composting material fills the spaces between particles, preventing air from getting in. On the other hand, bacteria do not flourish if the material is too dry. The optimum moisture content is 40-60%. Moisture content can be increased by spraying a compost heap with water, and can be decreased by adding dry straw or sawdust. Frequent turning allows a heap to dry naturally by evaporation.
For optimum value to plants, the ratio of available carbon to nitrogen in compost should be about 20. In the composting process carbon is used by the bacteria, so the best raw material for composting has a higher carbon: nitrogen ratio, say about 30. The carbon: nitrogen ratio of nightsoil is about 6, of fresh vegetable waste around 20, and of dry straw over 100. The ratio of mixed household refuse is often in the range 30 50, but it may be higher if there is a high paper content. The desirable ratio of 30 can sometimes be obtained by judicious mixing of incoming waste, for example by adjusting the proportions of nightsoil or sludge and vegetable waste. It is rarely practical to determine the carbon: nitrogen ratio by chemical analysis; a good operator learns to judge what mix of materials will produce the best compost.
During composting the volume is reduced by 40-80% and the weight by 20-50%.
Windrows and pits
Unless expensive mechanical plant is used, aerobic composting of municipal refuse is usually carried out in long heaps called windrows. The best height for windrows is about 1.5 m. In heaps more than 1.8 m high, the material at the bottom becomes too compressed; in heaps less than I m high, too much of the heat generated by the bacteria is lost.
The width and length of windrows should be planned for the most efficient handling of materials and the best utilization of the area available. The initial width is often 2.5 3.5 m at the bottom. In dry weather the cross-section should be trapezoidal, as shown in Fig. A1.1, but during the rainy season a more rounded shape prevents the material getting too wet.
For composting small quantities (for example, from a single village), refuse should be stored until there is enough to make a pile about 3 m in diameter and 1.5 m deep.
For composting nightsoil, a common method is to place alternate layers of nightsoil (about 50 mm thick) and vegetable waste (about 200 mm thick) in pits or windrows. Fig. A1.2(a) shows how a windrow can be formed to ensure destruction of faecal pathogens by high temperature. Vegetable matter below and at the edges provides some insulation. Fig. A1.2(b) shows an alternative method: after a windrow has been in use for two or three days and the temperature has risen, a trench or pocket is formed in the centre and nightsoil is poured in.
Temperature, aeration and turning
Providing that the material being composted remains aerobic the temperature may rise to 45-50 °C during the first 24 hours. A few days later it will reach 60-70-C, well above the lethal temperature for all pathogenic organisms. Fig. A1.3 shows the variation in temperature during aerobic composting of mixed municipal refuse; the points marked T indicate when the material was turned for aeration.
Various methods of aeration have been tried. For a small refuse heap (as in a village), refuse can be tipped over bamboo or timber poles which are removed when the heap is complete, leaving holes through which air reaches the refuse (see Fig. A1.4). Other approaches, including forced aeration (using compressed air blowers or suction) and use of porous floors, have not been very successful in keeping large masses of material aerated.
Material in windrows can be turned by labourers using forks or by adapted earth-moving equipment. Turning should keep the heap aerobic. In addition, material at the outside should be moved to the centre, because the outer layers may:
- be too wet because of rain;
- be too dry owing to evaporation (especially on the side facing the wind);
- be unaffected by the temperature rise in the centre of the windrow;
- contain large numbers of flies, fly eggs and larvae.
Some operators turn their windrows every two or three days. However, aerobic conditions can be maintained with less frequent turning after the first week or so. One suggested pattern is to turn the windrow after one day and then on the 3rd, 7th, 14th and 21st days. On the 28th day the material is put into a storage area to await removal.
Generally compost is only required at certain times of the year. If there is only one harvesting and sowing season a year, an area sufficient to store most of the year's production of compost may be required. During storage, compost continues to "mature", but high temperatures cannot be maintained. The time taken for stabilization depends on the initial carbon: nitrogen ratio, the moisture content, maintenance of aerobic conditions and the particle size. Unless precautions are taken, fly breeding may be a problem when compost is stored.
Condition and quality of compost
Tests of compost during and after stabilization show whether the process is going well and whether the finished product is suitable for agricultural use. Except in a large mechanical composting plant, the condition of the compost is gauged by simple methods. It is reasonable to assume that pathogenic organisms will be killed if the temperature rises above 65°C. This can be confirmed by poking an iron bar or wooden stick into the heap and pulling it out after about ten minutes. It should then be too hot to hold. The temperature falls when stabilization is complete. Absence of an unpleasant smell and absence of flies also indicate satisfactory aerobic composting (Flintoff, 1984). An experienced operator can check that all is well from the appearance of the composting material. It should look moist, but not so wet that liquid seeps out. While aerobic stabilization is progressing the appearance will change from day to day. Anaerobic conditions are shown by a pale green, slightly luminous appearance of material inside the heap.
Farmers and market gardeners may want to know the chemical composition of compost derived from nightsoil or sludge. The major plant nutrients (nitrogen, phosphorus pentoxide and potassium oxide) are likely to be about 3% by weight, three times the concentration in compost from municipal refuse.
Use in aquaculture
The practice of depositing excrete into fish ponds or tanks is
common in many Asian countries. In some places, latrines are placed immediately
over or alongside ponds; elsewhere nightsoil is tipped from carts, tankers or
buckets. Nutrients in excrete result in a rich algal growth, which encourages
aerobic conditions and provides food for certain fish.
Carp and tilapia are especially suitable for such ponds, but a variety of fish species may coexist, some feeding on large algae, some on small algae, some on zooplankton; some prefer the bottom layer, some the top. Fish are usually netted for human consumption, but in some places they are dried and ground up for feed for poultry or animals. Ducks may also be kept on the ponds.
There are three health risks associated with fish farming in ponds that receive excrete.
(1) Pathogens may be transmitted on the body surfaces or in the intestines of the fish without causing overt disease in the fish; the pathogens may then be passed to people handling the fish.
(2) Helminths, particularly flukes, may be transmitted to people who eat infected fish that has not been properly cooked.
(3) Helminths with intermediate hosts (such as Schistosoma with water snails) may continue their life cycle in ponds.
The WHO publication, Guidelines for the safe use of wastewater and excreta in agriculture and aquaculture (Mare & Cairncross, 1989), gives further useful information.
The search for alternative sources of energy has led to widespread use of organic waste to produce a combustible fuel which can be used for domestic cooking. Basically, a biogas plant consists of a chamber in which excrete are fermented, producing gas which contains about 60% methane. The biogas is collected at the top of the chamber, from which a pipe leads to domestic appliances or to flexible storage containers.
A few biogas plants operate entirely on human excrete. For example, in Patna, India a 24-seat pour-flush latrine serves several thousand people and generates sufficient energy to light a 4-km length of road. However, most plants, of which there are more than 7 million in China (Li, 1984), are dependent on animal excrete with which human excrete are processed. A medium-sized buffalo or cow provides about twenty times as much gas as a person. The minimum feed is that from one cow and a family of people, although it is more usual to add excrete from at least four cows. In China it is customary to produce biogas from the excrete of pigs.
Although there are many variations, the most common types of domestic plant have a floating or fixed dome under which the gas collects. The floating dome type, shown in Fig. A1.5, is widely used in India. In China, masonry or concrete fixed domes are usual, as shown in Fig. A 1.6. They are generally cheaper than those with a floating roof. The daily gas output is approximately equal to one-third the volume of the digester.
Excreta are often mixed with straw or other vegetable waste, such as that used for animal bedding, and equal quantities of water added to make a slurry. This is fed to the inlet side of the chamber. Effluent slurry is removed after a retention time of 30 50 days. Biogas production is greater at higher temperatures. At 30°C the rate of generation of gas is about twice that at 25°C, and little gas is produced if the temperature is below 15 C.
The effluent slurry is usually dried in the open and used as a fertilizer. On a dry solids basis, the nitrogen content is greater than in untreated excrete because of the loss of carbon in the gas. The nutrients in effluent slurry, whether dried or applied directly to land, are more readily taken up by plants.
Retention of excrete in biogas tanks results in the death of many pathogens, including Schistosoma eggs. A few hookworm eggs survive, and there is high survival of roundworm eggs.
CROSS, P. & STRAUSS, M. (1985) Health aspects of nightsoil and sludge use in agriculture and aquaculture. Dubendorf, International Reference Centre for Waste Disposal (IRCWD Report No. 04185).
FEACHEM, R. G. ET AL. (1983) Sanitation and disease: health aspects of excreta and wastewater management. Chichester, Wiley.
FLINTOFF, F. (1984) Management of solid wastes in developing countries, 2nd ed. New Delhi, WHO Regional Office for South-East Asia (South-East Asia Series No. 1).
McGARRY, M. G. & STAINFORTH, J. (1978) Compost, fertilizer, and biogas production from human and animal wastes in the People's Republic of China. Ottawa, International Development Research Centre.
MARA, D. & CAIRNCROSS, S. ( 1989) Guidelines for the safe use of wastewater and excreta in agriculture and aquaculture. Geneva, World Health Organization.