This is the old United Nations University website. Visit the new site at http://unu.edu
Contents - Previous - Next
Agricultural Services Division, Food and Agriculture Organization Headquarters, Rome, Italy
A residue is a substance resulting from the processing of a product. A residue becomes a coproduct or a byproduct when profitable use is made of it. If this is not the case, the residue becomes a waste, which is defined as a material with no apparent market, social, or environmental value - at times even a negative one - that the producer no longer wants in a given place at a given time. According to these definitions, our task will then be to identify, classify, and quantify residues and to turn wastes into by-products.
The distinction between main product, by-product, and waste is not always clear. Molasses has established itself as a product of world commerce in its own right, yet it is still a waste in certain parts of the world. The meal of soya bean, initially a by-product of oil production, has become a main product, i.e., soy flour. Another sign of independency of a byproduct is when a specific name comes into use for its identification, e.g., middlings, polish, tankage, distillers' grains, and gluten meal.
In addition to the well established by-products, there are the poorer branches of the residue family. They are under-utilized or are completely wasted for a variety of reasons. An FAO project manager has defined these wastes as "resources out of place." This may be too ambitious a statement, because it is exactly the place of production that often creates a major constraint for profitable utilization. However, it shows a positive attitude of looking realistically at wastes as potential resources
Organic residues and their potential end-uses can be classified in a number of ways. This will enable us to pre-select areas on which to focus our activities and assist us in determining our first options for their potential use. In Tables 1 and 2, an attempt has been made at such a classification. The entries marked with an asterisk are of particular relevance to the subject of residue conversion by the action of micro-organisms.
TABLE 1 Classification of Organic Residues
|Agriculture||- crop and animal wastes|
|Fisheries||- shrimp heads, fish trimmings, trash fish, etc.|
|Forestry||- bark, shavings, sawdust, logging wastes, etc.|
|Related industries||- bagasse, hulls, cakes, pulps, bran, etc.|
|Home/community/municipality||- garbage, sewage|
|By commodity or commodity groups||- animals, beverage industries, cereals, fibres, fruits|
|and vegetables, milk and dairy products, oilseeds and|
|nuts, rubber, spices and essential oils, starchy roots|
|and tubers, sugars|
|By geographical location||- national, regional, rural, etc.|
|By physical state||- solid, slurry, liquid, gaseous|
|Common properties||- meals and press cakes, straws, fruit pulps, etc.|
|Common main component||- sugary, starchy, cellulosic residues, etc.|
TABLE 2. Potential End-Uses of Organic Residues
|Food||- fermented foods.|
|Feed||- direct use|
|- upgrading (physical, chemical, microbial *)|
|- microbial biomass*|
|Fertilizer||- direct use|
|- residue of biogas production*|
|Energy||- biogas *|
|- producer gas|
|- direct use (combustion)|
|Construction materials||- boards, panels, bricks|
|Paper pulp||- Paper, paperboard, packaging materials|
|- alcohol *|
|- organic acids'|
Before dealing with the quantitative aspects of residues, some of the constraints that can hamper effective residue utilization should be mentioned. Remoteness of the centres of production becomes a major constraint when local conditions do not allow for absorption of the residue, and costs of transport to other locations are prohibitive.
This is the case with molasses in certain African countries. Scattered production of the residues, which is a common feature in agricultural production, raises collecting costs. Straws and other field-crop residues tend to have this handicap. Residues with a strong seasonal character, unless they can be stored, put a heavy burden on investment costs. Fruit and vegetable canning residues are an example. Dilution of the residue by either processing or washing water quite often makes the residue economically inaccessible. Lack of know-how and/or technologies adjusted to local conditions in scale and simplicity can be a factor obstructing the way to residue utilization. On occasion, reservations and restrictions of a social nature have to be overcome, which, for instance, has been experienced in the application of small biogas units, because of the type and handling of the raw materials involved.
All these possible constraints make it necessary to tread very carefully when considering residue utilization schemes. Apart from the availability of the raw material and the appropriate technologies, the local infrastructure should be carefully looked into. It should not be forgotten either that the incentive for profitable residue utilization is frequently linked with, or dependent on, political decisions This is clearly shown in some industrialized countries where penalties on pollution have shifted the economic feasibility of residue utilization practices. Lack of credit facilities, especially in rural areas, may block a whole utilization scheme even if the other requirements for successful implementation have been fulfilled.
It is, therefore, extremely difficult to prepare a general set of guidelines for the selection and application of residue technologies, because social, economic, and environmental conditions vary with time, and from place to place.
After this general description of some of the main characteristics of residue utilization, the second part of this paper will concentrate on the quantitative aspects of residues.
Availability is defined by the Oxford Dictionary as "capable of being used." This is a state of affairs that one would like to reach after all impeding constraints relevant to supply of raw material have either been demonstrated not to exist or have been removed. This also means that the commodity production statistics as published by FAO have little practical value apart from obtaining a general view of the overall magnitude of quantities of residues that are produced. A typical example is rice bran oil. In Japan, practically all oil is extracted from rice bran and two-thirds of that is used as edible oil. In India, only one-sixth of the oil present in bran is recovered as crude oil, of which only a small fraction is used in food for humans. Some planners have jumped to the conclusion that there would be an enormous scope for supplementing the vegetable oil supply for human consumption in Southeast Asia, forgetting that, in contrast to Japan, most of the rice milling is done in small-scale mills, making solvent extraction a much more difficult and costly technology to apply.
The above considerations motivated FAO in 1977 to initiate a survey with the aim of obtaining more information on the actual availability of residues, in particular those that are now being wasted. Some 450 institutions (including government departments) and persons, mostly in developing countries, known to be engaged in residue utilization, were contacted by questionnaire. The emphasis was put on collecting information on quantities within constricted areas, present utilization or disposal practices, existing constraints towards increased utilization, and proposals for utilization schemes. The survey involved 128 countries; responses were received from 57 with a total of 115 completed questionnaires. For 45 countries sufficient information was received to make possible the preparation of a first country profile. In total, 72 project ideas for research, development, and demonstration projects on a national basis were received.
It must be borne in mind that, although this survey has been a first useful step towards quantitative appraisal of residues, it remains a collection of contributions mostly from individuals. The survey is, therefore, far from complete, but FAO hopes to make this a continuing effort.
In addition, it should be mentioned that, in spite of the emphasis on constricted areas, the figures on quantities are again mostly of a general nature without reference to the area in which the residues are produced.
Nevertheless, several conclusions can be drawn from the survey. The use of cereal residues was reported most often (22 countries!, followed by residues of the sugar industry (20 countries). The beverage industry (19 countries) and animal by-products (18 countries) followed closely. In a middle group of 12 to 15 countries, fruit and vegetable residues, forestry residues, oilseed processing waste, and residues of fishery industries were featured. Little information was received on starchy roots, municipal wastes, rubber, or dairy by-product residues.
The actual utilization and disposal for residues showed a rather uniform pattern. Burning in the field, ploughing under, and use as roughage and litter for animals is most frequently applied to straw, stalks, and other bulky agricultural residues. Some wet agricultural residues are used as feed, but the bulk seems to be wasted. There is considerable wastage in the effluents of certain processing industries, such as brewing and palm oil, olive, coffee, and fish processing. A number of residues are used as an energy source, e.g., bagasse, husks, and wood processing residues.
The constraints impeding the increased or improved utilization of residues as reported by the survey are very much the same as mentioned earlier in this paper. From the replies, lack of appropriate technologies and qualified personnel rank as the biggest constraints, followed by difficulties of collection due to poor road networks and transport facilities. Lack of financial means is often mentioned, as well as the uncertainty of marketing outlets.
The type of assistance requested centres mainly on requests for technical advice, implementation of research, demonstration projects, and financing.
For the 72 project proposals received through this survey, the following sub-division applies:
|animal by-products and residues||8|
|beverage industry residues||8|
|forestry and cellulosic residues||10|
|fruits and vegetables||6|
|marine and fresh-water products||8|
|oilseeds and nuts||5|
Milk and dairy, municipal and domestic wastes, and starchy roots and tubers are incorporated with other residues in a group of 14 under the heading "various."
There are 20 proposals that make a direct reference to microbiological conversion: nine for biogas (Burma, Cyprus, Jamaica, Korea, Mauritius, Senegal, Spain, Thailand, Regional Central America); five for biomass protein (Algeria, Peru, the Philippines, Somalia, Thailand); one for composting (the Philippines), and four unspecified (Colombia, Egypt, Malaysia, and the Sudan). However, it is likely that more proposals would qualify for microbiological activities.
The figures and proposals from this survey now at hand can be seen and used only as an indication. Any activity in a selected priority area should, however, be preceded by an on-the-spot, in-depth feasibility study. It is FAO's intention to increase and improve knowledge on residue availabilities by periodical updating of data. This will be done concurrently with the revision of the Directory of Institutions, the Compendium of Technologies, and the Bibliography, which were published in 1978. All relevant documents can be obtained free of charge from FAO Headquarters in Rome.
With the first phase of this systematic effort to establish an information data base on residue utilization concluded. FAO will now put increased emphasis on field project development A number of such projects are already being implemented for fertilizer, feed, and energy in particular
An intensified search for the exploitation of the microbe in residue utilization, especially when directed to technologies that can be transferred and applied successfully at the village level, should have full support. FAO will be happy to co-operate within the limits of its programme scope and means.
At issue was the implication, drawn from the preceding papers, that it is less desirable to return agricultural residues to the soil than to process them for feed or other purposes. It was pointed out that ploughing back the residues helps to prevent erosion, thus conserving soil and water and maintaining the soil structure. An estimated 3,000 million tons of topsoil are said to be lost annually in the United States from the cultivation of corn, cotton, and other crops, in addition to the loss of 123 kg per hectare of nitrogen by the removal of residues.
Ploughing back residues significantly improved subsequent crop yields in the Guatemalan highlands where the soil had an initial humus content of less than one per cent.
While examples of the benefits of returning residues to the soil could be given in some instances, in other places there would be no such advantage. In these cases, the chemical treatment of straws before their use for feed could be a better method of residue management. It is simple, it has been demonstrated to be practical, and there is a demand for the feed, although the economics and magnitude of various methods of treatment in any particular set of circumstances remain to be determined.
Contents - Previous - Next