Chapter 6

Product processing and waste management

INTRODUCTION

Animals are kept - including those in urban conditions - because they serve some purpose: pleasure; emotional attachment; direct production of milk, eggs or power; or indirect production such as savings with which, eventually, to start an enterprise that is not necessarily related to animal production. Indeed, it is quite normal to observe a gradual change from the keeping of a few chickens to the operation of a small restaurant, or from the keeping of a few goats to the keeping of a calf and, eventually, the running of a small dairy enterprise (however small that may be). The evolution from a bullock cart to a small transport company is a rather logical process, and this example stresses how important urban livestock can be: not only as a source of direct production but also in providing "triggers" for the internal dynamics of a developing society.

In cities, perceptions about the processing and use of produce and waste vary considerably among social categories. While poor people may tend to emphasize access to quantity, this does not imply that they do not value quality. For example, they may strongly prefer local chicken over meat from commercial broilers. Rich sectors are often thought to stress access to quality, such as hygiene and ease of preparation; they may also be implying - often unknowingly - a need for quantity. A case of the latter occurs when policy- makers encourage increased production of food from animal origin as incomes increase.

Subsistence and semi-commercial farmers can be said to produce in the low-input mode, while large-scale commercial farmers operate in the high-input mode. These modes can roughly be described as low external input agriculture (LEIA) and high external input agriculture (HEIA), and a few major differences between these two approaches are illustrated in Table 16. This chapter discusses product processing and waste management, particularly for subsistence and semi-commercial livestock keepers, but the distinctions shown in Table 16 should be borne in mind. Because of its access to external resources, HEIA may tend more towards the use of accelerating technologies and management, whereas small-scale LEIA tends to adjust continuously to local conditions. More research into the application of these concepts would be highly relevant for policy setting in (urban) livestock development.

TABLE 16

Effects of HEIA and LEIA modes of farming on aspects of production, processing and waste handling

	HEIA	LEIA
Major drive for production	Demand for produce that is good enough to purchase such inputs as feed and veterinary services	Availability of cheap resources (such as vegetable waste) that are put to use in spite of their low commercial value
Capacity to absorb such local waste as food residues	Limited; emphasis on resource quality concerns	High; emphasis on access to quantity
Quality of produce	Critical consumers are prepared to pay, but are not interested in by-products and willing to waste unwanted dead animals, dung, etc.	Critical consumers are capable of turning every bit of waste (guts, dung, carcasses) into something of use
Major consumer concerns	Gaining access to regular supply of good-quality meat, eggs, milk, etc. without running into health problems of toxins, obesity, coronary disease, etc.	Gaining occasional access to valued luxuries (including even the intestines or head of an animal)
Reasons for waste management	Complaints from neighbours	Waste is seen as something useful
Importance of regular market	High, if imports are necessary	Low; consumption tends to be adjusted to availability
Resources for investment, orientation to scale/degree of standardization	Relatively abundant, large-scale and standard processing and marketing	Small-scale innovative local solutions and processing

PRODUCT PROCESSING

Appropriate preservation and processing techniques offer the opportunity to eliminate some limitations of the urban livestock products marketing system and increase the income of livestock producers and those who process livestock products such as meat and milk. Unfortunately, meat, fish and dairy products are all (low acid) foods that support bacterial growth. This means that the foods spoil rapidly and can cause severe food poisoning. In many developing countries, producers and consumers in small-scale, low-input systems are forced to complete the cycle from procurement to consumption in a very short period (mostly within one day) to avoid hygiene, health and environmental problems. There will always be a demand for fresh products ("hot" meat, raw milk), but appropriate preservation and processing can decrease the need for facilities in urban, densely populated areas, resulting in a reduced burden on the environment. Furthermore, such techniques enable livestock keepers and their surrounding communities to produce products that can be stored hygienically under ambient temperatures. Processing and preservation can also increase the revenues from some meat and milk products by value adding. Prices per unit of weight for processed products such as sausages, dried meats, cooked ham, cheeses and yoghurt are higher than those for the raw material. Parts that are graded as "less valuable" because of poor sensory properties such as taste and texture (meat from internal organs and the head, blood) may be especially valuable from the nutritional and processing point of view.

Meat products

Many different animals are used as sources of meat in developing countries. These range from domesticated cows, pigs and chickens to camels, horses, hamsters and wild animals such as deer, snails, boars and monkeys. The amount and type of meat eaten in a particular country are determined by factors such as cost, availability and cultural or religious acceptability. In developing countries, meat is usually eaten fresh and the meat products that are found in industrialized countries are not commonly available or in significant demand. However, meat is dried, smoked or salted to preserve it for a longer storage life and to change its flavour and texture in order to increase variety in the diet.

In low-input countries, slaughtering conditions are often unhygienic and animals may be slaughtered and displayed for sale in the open air without any covering. This enables bacteria to be transmitted to meat by flies, animals and birds. Harmful micro-organisms may grow on meat and cause food poisoning when eaten. These, together with such infectious organisms as parasites which grow in the meat, make the careful selection of raw materials, proper handling, hygiene and preparation of meat products essential. However, it should be stressed that local processing can be, and often is, done hygienically. Photo 21, in Chapter 2, shows how animals can be slaughtered in a clean way, while milk for direct consumption can be obtained at the farmer's doorstep. Wealthy urbanites like to come to dairy farms directly adjoining their quarters to be guaranteed a good product. The following section discusses some meat processing techniques.

Dried, salted and smoked meat. Dried, salted and smoked meats are found widely in Africa, where they are important traditional foods. One of the best known examples is biltong, which is used as a snack food in southern Africa, but similar products are made in other parts of the continent. Biltong is made from strips of dried, salted meat which are dark-brown and have a salty taste and a flexible, rubbery texture. The principles of preservation are to inhibit enzyme and microbial action by adding salt to the surface of the meat and to remove moisture by drying (Table 17).

In Asia, chunks of pig meat (or other meats such as beef or chicken) are salted to preserve them. They are later cooked after the salt has been thoroughly washed out. This product can be kept for about a month, but rancidity and spoilage develop gradually. The slaughtered animal is turned over a fire, so that the entire body is exposed to the flames. The viscera are removed and the animal is then cut into pieces. These are dipped in a saturated salt solution and the pieces of meat are then pressed, rubbed with salt crystals, wrapped tightly in cloth, tied closely with a rope and hung up in bags to drain.

Smoked pork is a common product in West Africa, where it is made from the shoulders, feet, head or offal, hot-smoked at temperatures of about 100 to 120°C and eaten in soups or stews. It has a well-developed smoky flavour but is only partially cooked. It can be slightly salted or unsalted, and usually has mixed spices applied. Smoking is done on a grill mounted on a tripod. The meat is turned regularly for six to ten hours to ensure uniform heating and smoking.

TABLE 17

Preparation of biltong

Ground and sliced meat. In urban areas, ground and sliced meat snack foods are common. The process of grinding meat into small pieces (e.g. minced meat) or pastes (e.g. sausage meat) is aimed at changing the texture of the meat and allowing it to be formed into different shapes. Grinding meat does not help to preserve it and can even cause more rapid spoilage because bacteria from hands or dirty equipment have more chance of becoming mixed with the meat. Grinding also releases enzymes from the meat which cause changes to its flavour and texture. When ground meat is being produced the operators must be thoroughly trained in hygienic processing, which should be done quickly and preferably at a low temperature to slow down bacterial growth. In developing countries, these products are not preserved for later consumption, but are made as street foods that are consumed immediately.

Table 18 gives an overview of different meat preservation techniques suitable for small-scale systems in urban areas in developing countries. In many cases a combination of treatments may be used.

BOX 11 MEAT PRODUCTS IN WEST AFRICA In West Africa, minced beef is mixed with chilli pepper and moulded around disposable wooden skewers. The skewers are then made to stand upright in a bed of sand around a low charcoal fire and are turned at regular intervals to roast the meat evenly. Pork kebab is a grilled product that is prepared from cut, deboned pieces of pork that have been skewered on a long thin sharpened stick. The grill is a 0.5 to 1 m2 metal mesh, placed on a metal bowl half filled with sand. A charcoal fire is made on the sand in the metal bowl. The product is commonly sold as a snack along the roadside where the cooking takes place.

BOX 12 CHICKEN SATAY IN INDONESIA In Indonesia, the sounds of suburban streets and alleys in shanty towns include the calls of vendors shouting "ayam". Ayam is the Malay word for chicken, and satay ayam are tiny pieces of chicken (including intestines) that are strung on bamboo sticks to be roasted over a small charcoal fire on the little cart or the pikul. Roadside stalls and restaurants also offer various forms of this roasted meat, which is often produced from local waste in the immediate neighbourhood, as snacks and products with much value added.

If simple hygienic standards are maintained, processing methods including carcass dressing, cutting, meat grinding and subsequent treatments (heat, fermentation) can take place at the household level, thus providing work and increasing family incomes.

TABLE 18

Methods, technologies and examples of meat preservation

Method	Technology	Example
Lowering the moisture content to control/inhibit microbial growth	Evaporation of water by drying (sun, open air) or replacement by food additives such as sugar. Sometimes spices are added which also have an antimicrobial effect. Only simple equipment needed	Dried meat (biltong - southern Africa; kilishi), Chinese sausage
Increasing the acidity to inhibit microbial growth	Fermentation or addition of organic acids	(Dried) ham (Western countries), naem (South Asia)
Treatment with or without additives to inhibit microbial growth	Salting, curing, smoking	Biltong (southern Africa)
Thermal treatment to reduce microbes	Boiling, cooking, roasting	Boiled meat (Nigeria, Ethiopia), smoked pork (West Africa), pork kebab

Sources: Heinz (FAO, 1995c).

Milk products

Cow milk is popular in many countries, but milk from goats, water buffaloes, horses and sheep is also very important in many developing countries. The degree to which milk consumption and processing occur differs from region to region. It is dependent on many factors, including geographic and climatic conditions, availability and cost of milk, food taboos and religious restrictions. Where processing exists, many traditional techniques can be found for producing indigenous milk products. The basic objective of milk processing is to improve the quality of raw milk by its transformation to products with prolonged shelf-life and increased hygiene. Fermented milk products such as yoghurt and soured milk contain bacteria from the Lactobacilli group. These bacteria aid digestion and help prevent illness caused by other bacteria. In addition, fermentation removes milk sugar (lactose) from milk and people who suffer from lactose intolerance1 can therefore consume fermented dairy products.

Milk is a perishable commodity that spoils very easily. Its low acidity and high nutrient content make it the perfect breeding ground for bacteria including those that cause food poisoning (pathogens). Bacteria from the animal, utensils, hands and insects may contaminate the milk and their destruction to preserve the milk and make it safe is the main reason for processing. Infections in the animal that cause illness, such as tuberculosis, may be passed directly to the consumer through milk. It is therefore extremely important to ensure that the bacterial activity in raw milk is of an acceptable level and that no harmful bacteria remain in the processed products.

PHOTO 46

Satay sellers in Indonesia sell tiny pieces of chicken that are strung on bamboo sticks to be roasted over a small charcoal fire on the little cart

PHOTO 47

A meat shop in Bolivia selling different kinds of meat products of local origin

Technology levels differ according to local conditions. Milk is either marketed raw or converted by such technologies as boiling, natural fermentation, souring, mechanical separation and heating. In India, local tea shops are an important outlet for milk produced in the alleys of urban neighbourhoods; but in many cities around the world, milk is purchased almost directly from the cow and is consumed locally, often in small quantities, within an hour of purchase. In many temperate countries, a major reason for pasteurizing milk was to reduce infection of tuberculosis. In many tropical countries, fresh milk is not consumed, milk is always boiled and there are so many other sources of infection that pasteurization is seen as an unnecessarily expensive step; instead it denies access to a valuable food resource to the people (especially children) who would benefit most from it.

Soured milk. Traditional soured milk is a thick clotted product similar to yoghurt but with a stronger flavour and a more acidic taste. It has a similar shelf-life of three to eight days and is used as a drink or as an accompaniment to meals in some countries. Preservation is due to the production of lactic acid by naturally occurring lactic acid bacteria in the untreated milk. The high levels of acid inhibit the growth of any spoilage bacteria and pathogens that may be present in the raw milk.

Butter and ghee. To make butter or ghee, fresh milk is pasteurized and then cooled to 37°C before the addition of a starter culture, which may be a small amount left over from a previous batch. The milk is then incubated for several hours in a warm place (30 to 37°C) until it sets. The resulting yoghurt is cooled for several hours, and then salted and churned to produce lactic butter. Alternatively, cow milk can also be put into well- cleaned and smoked clay containers and left to solidify for three to four days depending on the ambient temperature. It is then agitated by shaking the tightly closed container (which has a small vent for releasing the air that builds up during the first few minutes of shaking). The butter granules float to the surface of the buttermilk, growing in size as the agitation nears completion. The butter is collected by hand and washed with clean water twice or three times before being packed in aluminium, clay or plastic containers.

Ghee is a clear, golden brown or yellow fat (at temperatures below 30°C) or oil (at temperatures above 30°C) with the characteristic flavour of milk fat. It is made from cow or buffalo milk and is in high demand in Asia and parts of Africa as a cooking oil for domestic use and as an ingredient for local food products (e.g. baked products, confectionery). The principles of preservation are: 1) to destroy enzymes and contaminating micro-organisms by heat; and 2) to remove water from the oil to prevent rancidity and microbial growth during storage. Ghee has a long shelf-life owing to its low moisture content if stored in an airtight, light-proof and moisture-proof container in a cool storage room to slow down rancidity.

Yoghurt and curd. Yoghurt and curd are produced mainly in urban areas. Yoghurt is made from a variety of milks but the term "curd" usually applies to fermented and thickened buffalo milk. Both products are white with a creamy consistency. Set yoghurt is a smooth, firm, white gel with a characteristic acidic taste made by fermenting cow milk. Other similar products can be made from goat or mare milk. Yoghurt is widely used in Africa, Asia and Latin America to accompany a main meal, as a dessert or as a dressing for vegetable salads. It has a shelf-life of three to eight days, depending on the storage conditions. Stirred yoghurt is fermented in bulk, stirred and then dispensed into pots or sold into customers' containers. Set yoghurt is made by pouring the inoculated milk into pots and fermenting it in the pot.

TABLE 19

Production of yoghurt and curd

Milk	Collect the milk in carefully cleaned covered vessels.
Pasteurize	Milk is heated to 80 to 85°C for 15 to 20 minutes or to 70°C for 20 to 30 minutes. This is especially important if local animals are thought to be infected with tuberculosis bacteria.
Cool	To 44 to 45°C, as quickly as possible, by dipping the container into a tub of cold water.
Inoculate	With actively growing starter cultures of Lactobacillus bulgaricus and Streptococcus thermophilus (either bought as a dry powder or saved from a previous batch). Maintain a temperature of 42 to 44°C for approximately five hours until desired degree of acidity is achieved to form the correct consistency.
Cool	To room temperature or in a refrigerator.
Store	In a cool place
Ambient temperatures are not adequate in most climates and a heated incubator is needed. Simple and inexpensive ways of incubating yoghurt include keeping the jars/pots surrounded by warm water or putting it into thermos flasks. Alternatively, an insulated box can be made from a block of 10-cm thick polystyrene with indentations into which to fit small containers. The containers are filled with the warm yoghurt mixture and a polystyrene lid placed on top. The insulation of the box will prevent loss of heat and maintain the required temperature of the product. A similar idea uses a polystyrene box of approximately 0.75 m3 fitted with a 40 W electric light bulb. The heat from the bulb maintains the temperature within the required range. To make curd, buffalo milk is filtered and boiled, the scum is removed and it is cooled to room temperature. A few spoonfuls of a previous batch of curd are added and it is then mixed well and poured into clay pots. These are sealed by wrapping a piece of paper over the pot and allowing it to stand for 12 hours. Filtering removes any particles and boiling kills micro-organisms as well as causing separation of some cream. The starter culture from a previous batch inoculates the milk, and the resulting lactic acid retards the growth of other micro-organisms and causes the curd to set. Fermentation also develops the characteristic flavour of the product. Curd is eaten or distributed and sold immediately after manufacture because storing it for more than a few days at room temperature makes it too sour.

Unfermented cheese. Unfermented cheese is made by coagulating the milk solids in buttermilk and it is used as a component of traditional dishes. It is white in colour and similar to European cottage cheese in appearance, but is more sour in taste and has a pronounced cheese flavour. Fermented milk is heated gently in a clean pot until coagulation is complete, after about 30 minutes. The cheese is collected by draining the whey and is then put on a straw screen to drain. The product must be sold or eaten within 24 hours if refrigeration is not available.

Table 20 shows some milk processing methods and products that are suitable for small-scale systems in urban areas.

The distribution of dairy products to urban populations has always been shared between the informal and the formal sectors, often in competition. Formal channels are catering increasingly to supermarket chains, often basing distribution on production from elsewhere, from rural areas or even from abroad. Informal market channels based on local supply may turn out to be a lifeline for the poorest categories of consumers, and special care should be taken to avoid the outbreak of diseases that are transferred by, especially raw, milk and include tuberculosis and brucellosis. Shifting production and quality control to the outside would be an accelerating technology: a more defusing approach would be to focus on education through, for example, non-governmental organizations at the community and household levels.

TABLE 20

Methods, technologies and examples of local (household-level) milk processing

Method	Technology	Example
Increasing acidity to inhibit microbial growth and change the taste	Fermentation with lactic acid or addition of organic acids Equipment needed includes a cooler	Soured milk, yoghurt, curd
Mechanical separation to extend shelf-life	Curdling, addition of solids, drying	Ghee, cheese, butter
Heat treatment to reduce microbes and enzyme activities	Pasteurization	Soft cheese, pasteurized milk

Sources: Heinz (FAO, 1995c).

Conclusion

Meat and milk processing takes place at the community and household levels and contributes, often significantly, to the household income and the community food requirements. Large central facilities are not always appropriate to local demand and are often expensive, polluting and unhygienic. Efforts can also be made to improve existing facilities and family-level processing techniques to ensure a hygienically and economically sound product without excessive pollution of the environment. Community-based organizations could play a major role in this.

PHOTO 48

"Milk intermediaries" transport milk daily from peri-urban settlements to be marketed and processed in Cap Haitien (Haiti)

PHOTO 49

In Peru milk delivery in peri-urban areas by pick-up truck

WASTE PROCESSING2

Every production process generates waste. One essential (defusing) characteristic of small-scale urban livestock is that little waste is generated. Among other factors, this is the result of utilizing waste from other resources which, in LEIA systems, tends to be recycled and used to a higher extent than it is in the more commercial HEIA mode. Furthermore, the waste that is generated tends to be less concentrated, i.e. LEIA systems are better able to internalize their own waste than HEIA systems. Nevertheless, inadequate animal waste disposal systems and unhygienic processing of animal products such as milk and meat can cause health and environmental problems, even in small-scale enterprises.

Waste from meat processing

Many slaughterhouses are situated in residential areas because of the demand for fresh "hot" (non-refrigerated) meat. Most of these lack functioning waste treatment facilities, and thus cause pollution of water, soil and air. A recent development, especially in Asia and Latin America, is the emergence of a great number of small, hygienically unsatisfactory slaughtering facilities while, at the same time, reasonably good central abattoirs have been abandoned because of their higher costs. Essentially, this is a "struggle for size". The advantages of scale also imply disadvantages: centralized processing implies advantages in terms of standardization and qual ity control, but it also concentrates waste, thus causing health and environmental problems.

The waste generated in meat processing units is composed basically of three categories: water, organic matter and minerals. Water, which itself does not cause pollution, is the vehicle for the other two categories. Slaughtering requires large amounts of hot water for sterilization and cleaning (Steinfeld, De Haan and Blackburn, 1996). A major accelerating aspect of pollution lies, therefore, in the use of water that is dispersed while it is carrying unwanted waste products. Animal waste that is handled without the addition of extra water causes fewer problems. Once the waste is diluted with water, the whole treatment process becomes more complicated because, among other causes, water makes it more difficult for a basically defusing technology such as anaerobic fermentation to take place.

The most effective way of reducing waste production is to prevent unnecessary mixing. As much blood as possible should be collected and processed at source; the use of water should be minimal; meat and water should be separated as soon as possible; rumen and intestines can be rendered directly, or processed for future use; and large intestines can be used as sausage casings. The next step is to develop clean processing methods involving specific technical and economic expertise. The treatment of waste prior to its discharge into the environment (end-of-pipe treatment) is the third step. This step includes ensuring the optimal use of edible and non- edible by-products (Figure 5) so as to minimize the amount of waste to be treated in wastewater treatment plants, which is generally a very expensive process. The sludge from these plants, provided it does not contain toxic compounds, can be used as fertilizer.

Waste from leather production. The most toxic component in leather production is chromium which is used to make hides resistant to bacteria and high temperature (Photo 51). Tannery wastewater containing high concentrations of (chromium) salt and hydrogen sulphide greatly affects water quality and fish and other aquatic life. When it is applied to the land, soil production is affected, the land may become infertile and groundwater quality is also affected. As in meat production, a reduction of water use in leather production decreases the polluting value of the wastewater. Furthermore, the use of chemicals, especially chromium but also lime, salt and sulphide (which is used mainly for preservation purposes), should be reduced. Sludge from wastewater treatment plants is too toxic to be usable and has to be processed or temporarily dumped at special dumping grounds.

Waste from milk processing

In industrialized countries, the bulk of milk is factory-processed, but home or village processing is still common in many developing countries, accounting for 80 to 90 percent of the total in Africa and about 50 percent in Latin America (De Haan, Steinfeld and Blackburn, 1996). Furthermore, there is a return to home processing in developed countries because of the possibilities that it offers for value addition, for instance in the case of biological products.

Hardly any solid waste is produced during the processing of milk. The major contribution to the waste load comes from wastewater from cleaning operations throughout the production process. It is not possible to identify particular waste producing practices; the style of management determines how the water consumption and operation processes are carried out. In general, waste from milk processing, being usually small-scale, does not cause severe environmental and health problems because, among other factors, such liquid by-products as whey or buttermilk can be fed to calves or other animals.

Technologies. Table 21 indicates waste management problems, possible solutions and ways of implementing measures at different levels. The processes to be used to treat wastewater produced by the industries mentioned in this chapter do not much differ from one another. Treatment technologies should be cheap, efficient and easy to operate. As mentioned before, efficient water management is to be the primary action; there are no generally valid solutions. Repressive methods that merely outlaw the processing of animal products at the household or community level will not work, and will only lead to fraud and corruption unless they are reinforced by strong government intervention such as that in Singapore, where pig production was banned. One way of addressing the issue is to sensitize and educate producers at the community and household levels. This is where most local meat production and processing takes place, and where all stakeholders (producers, processors, consumers, local institutions) are involved. Action at the level of social organization can be backed up by technical measures, such as recycling and reusing liquid and solid livestock wastes, and should be encouraged (FAO, 1991; 1995c).

PHOTO 50

Hygiene conditions are maintained at this slaughterhouse in Peru

PHOTO 51

Leather production takes place on rooftops and is a major business in Fez (Morocco), but causes water pollution and bad odours

Manure

Poultry manure is normally a valuable fertilizer, or even a feed ingredient, and is not considered a problem unless it contains a lot of chemicals and medicine/hormone residues. Cattle manure, too, is rarely an environmental risk, unless large numbers of animals are concentrated in restricted areas far from agricultural fields. Waste gener ated from industrial swine and poultry units, especially in intensive systems in Western and some Asian countries, is often seen as the main polluter of sources of surface and groundwater (Photo 52). Many technological solutions are now being implemented in industrialized countries, but none of these is likely to be useful for small semi-commercial livestock systems in cities. For these systems, the use of manure in urban gardens has far more potential. Several manure-processing techniques are available, and these decrease the burden on the environment and improve the characteristics of the manure as a fertilizer. They include mixing manure with sawdust or crop residues, composting and producing humus (with worms - vermiculture).

Biogas

In many countries, dried manure is used as a cheap fuel in rural and also urban areas. Substantial numbers of trees can be saved because of this (Photo 53), and the ash from manure can be used as a fertilizer. However, this practice contributes significantly to increased air pollution, and much nitrogen goes up in smoke. In zero- grazing and other systems where animals are kept in small spaces, and where water and temperature are not limiting factors, dung can be used far more efficiently when it is processed through biodigesters, the effluent of which can be used as fertilizer. All types of organic material, including animal manure, can be processed through anaerobic digestion in a biodigester, and the technology requires only one large initial investment, after which it runs by itself. Gas, liquid and solid phases are produced. The gas can be used as fuel, and the liquid and solid parts as fertilizer. There are two main types of small-scale biogas digester: batch fed, which are filled, then digest and then are emptied; and continuous load digesters, which are filled and produce gas and fertilizer continuously.

Despite the merits of biogas technology, biodigesters are widespread only in India (more than 5 million installations) and China (nearly 3 million installations). Recently, an ambitious programme in Nepal has been accelerating the market development of small bio digesters and, in many other countries, some practical experience of disseminating biodigester technology has been gained, often with the support of researchers (Photo 13). However, in most cases, larger-scale dissemination has not taken place, despite the economic, environmental and social benefits of the technology. The main reasons for this slow take-up seem to be that the multiple benefits of biodigesters have to be demonstrated to the end users in order to convince them to invest in the technology, and a market-driven institutional infrastructure needs to be in place to facilitate the large-scale dissemination of the technology.

Types of digesters. Among the small-scale and low-cost biodigesters, three basic designs can be distinguished: the floating-drum, also known as the "Indian design"; the fixed-dome, known as the "Chinese design"; and the flexible-bag digester. In general, a well-constructed fixed-dome biodigester has the longest life span, at 20 years or more. The floating-drum digester can have a comparable lifetime, but the recurrent costs are higher as the steel drums have to be replaced every five to ten years as a result of corrosion. The lifetime of the flexible-bag digester is difficult to indicate, as some do not last more than a couple of months, while others function satisfactorily for several years.

The gas pressure in the different designs also varies. Fixed-dome types are characterized by a varying but high pressure, with a maximum of about 1 000 mm of water pressure. The floating-drum design has a stable but lower pressure, at about 100 mm of water pressure; and the gas pressure in flexible-bag digesters is even lower. Users frequently prefer a higher gas pressure, as it is more comfortable to work with owing to the better control of the gas flame that it allows.

Feedstock. Many different organic materials can be fed into a biodigester. To allow the bacteria to do their work it is important that the organic matter be accessible to them. This means that pretreatment is sometimes necessary, e.g. through chopping and/or composting of crop wastes. The feedstock that is easiest to use is cattle dung, as it already contains the right bacteria and the vegetable matter has been broken down during its passage through the guts (and teeth) of the cow. Human excrement and manure from chickens and pigs are also useful, but they do not contain the right bacteria and they need a starter in the form of, for example, slurry from a working biodigester.

Neither the feedstock nor the water used to dilute it before it goes into the digester should contain toxins that would kill bacteria (e.g. antibiotics, detergents and disinfectants). These stop the digestion process completely, a phenomenon that is known as "the digester going sour" because it gives off an unpleasant acidic smell. For example, the dung of cows that have received antibiotics should not be fed into the digester.

Cultural aspects might influence the selection of feedstock. For example, Hindus in India usually accept cow dung for use in biodigesters, while pig dung and human excrement are often not accepted.

TABLE 21

Waste management at different levels

Level	Problem	Solution	How to cope
Government	Water, soil, air pollution Obstructed sewage systems Public health	Purification plants (expensive) Reallocation of processing units	Taxes and environment levies (on pigs in Singapore; Chark [FAO, 1998b]) Facilitate education and sensitization activities
Producer, processor	Health problems Decreased (farm) output and revenues	Improved processing methods (blood recuperation, separation of solid and liquid parts, disinfecting) Efficient utilization of by-products	Education Awareness raising Support from NGOs and other organizations at the community level
Consumer	Decreased quality of agricultural products Smell Decreasing recreation facilities	Protest to neighbours Complain at municipality	Move to other areas Ignore the problems Community action

Biogas. The principal uses of biogas are for household energy, such as cooking and lighting (Photo 54), but larger installations can produce sufficient gas to fuel engines, enabling the productive use of energy, e.g. for powering mills and water pumps. The efficiency of biogas production varies enormously. Different sources show ranges between 0.01 and 0.98 m3/kg manure. Besides environmental temperature and type of biodigester, efficiency is influenced mainly by the quality of the manure used.

BOX 14 MANURE UTILIZATION IN MEXICO CITY In Mexico City, manure poses very few problems. Organic waste from stables is used as fertilizer for the production of nopal leaves (Opuntia ficus indica. ). This plant has minimal water requirements and is cultivated on about 6 000 ha of small terraced plots on eroded and poor soils inside the city. About 75 percent of the leaves produced are consumed by city dwellers (UAM, 1999).

PHOTO 52

Disposal of dung from large-scale poultry systems in China often causes environmental problems

PHOTO 53

In Toliara (Madagascar), as in many developing country cities, charcoal is the main source of fuel for cooking and heating

PHOTO 54

Preparing chapatis (traditional bread) on biogas in Uttar Pradesh (India)

Slurry. In nearly all cases, biodigesters have been promoted as producers of energy in the form of biogas. A shortcoming of many biogas programmes is that too little attention has been paid to the excellent properties of the slurry as an organic fertilizer for crops and trees. In addition, the digestion process substantially reduces the amounts of pathogenic germs and weed seeds that were present in the original feedstock. Another advantage is that it makes the nutrients that are already present in the feedstock (e.g. nitrogen, potassium and phosphorus) more readily available to crops. Furthermore, the biogas process reduces the odours typically given off by manure and compost.

The combination of these properties helps to explain the renewed interest in biodigesters in Europe. The use of slurry in agriculture improves the recycling of nutrients, reduces the risks of diseases and plagues and substitutes chemical fertilizers to a large extent. In addition, it reduces emissions of greenhouse gases because biodigesters produce renewable energy (biogas) and because chemical fertilizers can be replaced by organic fertilizers (the slurry). Chemical fertilizers, on the other hand, require substantial quantities of fossil fuels for their production.

The main disadvantage of the slurry is that it is liquid, with a dry matter content of less than 10 percent. Therefore, when the slurry is used as an organic fertilizer, transport to the fields is complicated or expensive because it contains more than 90 percent water. The use of the slurry is thus usually limited to fields in the neighbourhood of the digester. Larger installations in industrialized countries separate the slurry into a liquid and a solid fraction; the solid fraction can be applied on fields at a distance from the digester.

Additional benefits and drawbacks of biogas. Biogas usually replaces fuelwood for cooking and also provides gas for lighting. Users confirm that cooking with biogas is more comfortable than cooking with fuelwood and provides a better energy service. For example, a biogas burner requires very little attention, it allows for better control over the heat source and, in practice, it reduces the time spent on cooking. The financial benefits of a biodigester for household cooking and lighting are probably less obvious. Only when fuelwood or other fuels are scarce do the economics of a digester become convincing.

BOX 15 FAMILY-LEVEL FIXED-DOME BIODIGESTER One cow produces an average of 10 kg of wet dung a day, equivalent to approximately 2 kg of dry matter. If cow dung is to be the main feedstock, the dung of approximately six cows (diluted with water) will be sufficient for a small biodigester of 9 m3. This biodigester would produce 2 m3 of biogas a day at 25°C, sufficient for the cooking needs of a family of around six people. At 30°C, the same digester would yield 3 m3 of biogas a day, sufficient for the cooking and lighting needs of the same family. In this case, the biogas would replace an average of 10 kg of fuelwood and 0.5 litres of kerosene per day, or roughly 4 000 kg of fuelwood and 200 litres of kerosene a year. In Cambodia, it has been demonstrated that gas produced from the manure of at least three cows or eight pigs is sufficient to replace about 75 percent of the fuelwood normally used by a family of six people (Dalibard, 1995).

The health benefits of biogas are mainly related to substantial reductions of smoke and indoor air pollution compared with those from traditional wood fires. This has a positive impact on the health situation of (mainly) women and children: it reduces the incidence of respiratory infections and eye ailments associated with smoke-filled environments. Another advantage is improved sanitation when toilets are connected to the biodigester. However, there are sometimes cultural impediments to the use of biogas and slurry from human excrement inputs.

Space for the biogas pit is not a major problem because it can be constructed directly underneath the place where animals are kept.

¹ Many people of non-European origin suffer from lactose intolerance, i.e. they lack a certain enzyme (lactase) that transforms lactose (milk sugar) into glucose. They are genetically not adapted to consuming cows' milk because they have only been exposed to cattle during recent centuries.

² This section is based on Verheijen et al. (1996).