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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
close this folderMicro-organisms as tools for rural processing of organic residues
View the document(introductory text...)
View the documentIntroduction
View the documentMicrobial utilization of mono- and di-saccharide residues
View the documentMicrobial conversion of starchy residues
View the documentMicrobial conversion of complex mixtures of compounds (Polysaccharides, Proteins, Lipids, etc.)
View the documentMicrobial utilization of cellulose and ligno-cellulose residues
View the documentAlgal culture as a source of biomass
View the documentMicrobial utilization of silviculture biomass
View the documentMicro-organisms and marine and freshwater biomass
View the documentInternational studies on processing organic residues
View the documentReferences


Micro-organisms have been closely associated with transforming or cycling organic matter in nature for as long as such material has existed. But it has only been within the past 100 years that certain of these associations have become known, or that advantage has been taken of helpful microbes in rural agricultural practices. Predictions are, however, that greater use will have to be made of beneficial micro-organisms.

I need not discuss the important part micro-organisms play in the production of humus, nor how they help cycle all elements or substances in the soil and thereby provide the nutrients necessary for healthy crops. These topics are beyond the scope of this paper. Neither will I discuss in depth how microbes fix an estimated 150 to 175 million tons of atmospheric nitrogen per year, which is several times more than the total commercial production of nitrogen fertilizer in 1977; nor how micro-organisms may be degrading over 1,500 million tons of pesticides and large quantities of other complex synthetic substances that find their way into the environment each year.

The concept of utilizing excess biomass or waste from agricultural and agro-industrial residues to produce energy, feeds or foods, and other useful products is not necessarily new. For centuries agricultural residues and wood have been used as sources of fuel, food, construction materials, and paper-making, as well as for other purposes. Recently, fermentation of biomass has gained considerable attention because of the forthcoming scarcity of fossil fuels, and because it is necessary to increase the world food and feed supplies - especially those high in protein.

Most attention today is being given to the possible use of micro-organisms to convert relatively high-quality biomass (corn and grains, sugar-cane juice, etc.) to fuel. Although this topic will be discussed later, certain technical and economic restrictions exist that must be removed if significant fuel production is to result from fermentation of such high-quality biomass, because these substrates have other important possible uses. This does not mean, however, that residues from farm crops, livestock feedlots, agro-industries, forest operations, and other similar practices should be excluded. This is especially so in circumstances where their removal does not eventually reduce the quality of the land, permit soil erosion, or produce other harmful effects on crops.

A logical classification of agricultural and agro-industrial materials has recently been published by Rolz (1) His data (Table 1) illustrate the variation in the structure of the substances, and the nature of the by-products that may be available for utilization by micro-organisms

TABLE 1. Classification of Agricultural and Agro-Industrial By-products

Group Predominant compound Activity By-product
I High proportion of di- and mono-saccharides Sugar-cane growing and processing Molasses
    Pulp elaboration Sulphite liquors
    Cheese-making Whey
II Di- and mono-saccharides with some structural polysaccharides Fresh fruit collection centres Rejected or damaged fruit
    Rum and liquor making Wash waters
III Mixture of soluble organic compounds, including starch,

sugars, proteins, pectin,

acids, etc.

Fruit and vegetable processing Wastes from washing, peeling, and blanching
    Tuber and grain processing Wastes from sorting and washing
    Coffee processing Washing and pulping waters
    Meat processing (beef, pork, poultry) Washing and scalding waters
IV Complex mixtures of structural

polysaccharides and other

compounds such as proteins,

lipids, etc.

Fruit and vegetable processing Peels, insoluble solids from pulp and seeds
    Animal and poultry production Manure
    Animal slaughtering and meat processing Suspended solids
    Coffee processing Pulp
    Sugar-cane and oil palm processing Residual solids
    Alcohol and alcoholic beverage processing Residual solids
V Structural cellulose and lignin in high proportion Cereal, sugar-cane, and rice growing and processing Straw, husks, bagasse
    Corn growing and processing Stocks and cobs
    Citronella and lemon grass processing Bagasse
    Coffee and cacao processing Husks
    Cotton seed processing Hulls, linters
    Forest processing Bark, sawdust, wastes

Source: Rolz (1).

Several calculations have been made of the quantities of biomass produced annually in the world by photosynthesis, and the resulting agro-industrial wastes One estimate is that 1.7 x 10(11) tons of biomass are produced annually, and that 98 per cent of this amount is not used in an economically sound manner DaSilva, Olembo, and Burgers (2) present data on some agricultural residues in six European countries (Table 2); these constitute about 98 million tons each year. For other countries, the three authors estimate the following: Malaysian oil palm and rice mill wastes in 1974 were 3 million and 250,000 tons, respectively. In Egypt, 600,000 tons of maize cobs, 1.5 million tons of dry rice straw, and 40,000 tons of sugar pith residues accumulate annually. About 100,000 tons of sugar-cane bagasse are burned in Bangladesh each year. In Western Australia, 10 million tons of wheat and barley straw and chaff are produced annually. Recent similar estimates for the United States by Pimentel and associates (3) are presented in Table 3.

TABLE 2. Agricultural Wastes in Certain European Countries

  Amounts in tons    
Country Cereal straw* Corn stover Beet pulp by-product
Belgium 1,760,000 - 120,000
Federal Republic      
of Germany 8,600,000 - 700,000
France 26,000,000 9,000,000** 1,100,000
Italy 8,850,000 4,000,000  
Netherlands 1,350,000 - 110,000
United Kingdom 26,000,000 - 375,000

Data from Battelle Document 75712, Courtesy DaSilva, Olembo, Burgers (2).

* Amount varied in different countries from 1.5 to 5.0 tons/ hectare.

** Yield 1.8 tons/hectare.

TABLE 3. Sources of Biomass Available Annually in the United States

Source Available


Mt, dry


Mt, dry



kcal x 1012

Livestock manure 255 128 27
Crop remains 430 0 0
Food-processing wastes(20 - 70% moisture) 4 4 18
Food-processing wastes(70 - 90% moisture) 14 14 10
Forestry remains 340 44 50
Forestry-processing wastes 81 20 85
Fuel-wood production 10 10 40
Fuel-wood plantations 60 60 52
Hydrocarbon plantations 16 16 13
Aquatic plants 17 3 0.6
Urban refuse 123 66 41
Industrial wastes 40 5 4
Municipal sewage 13 2 1 3
TOTAL 1,403 372 341,9

Source: Pimentel et al. (3).

Before any organic residue or high-quality biomass material is considered for microbial conversion to other substances, a number of factors must be taken into consideration. For instance: (i) Is there a ready and continuous supply of the raw product to be converted? (ii) If the material is removed from cropland or forests, will this contribute to soil erosion and depletion of plant nutrients? (iii) Are expensive equipment and large amounts of capital necessary for the processing? and (iv) Are such things as an external energy supply and large amounts of water necessary?

After considering the above factors, the following question may be asked: Which microorganism or microorganisms possess potentials for the bioconversion of the organic material under consideration? First, we must keep in mind that in natural conditions the indigenous microbial flora is only one component of a complex, dynamic biomass undergoing interaction in the transformation of organic matter. Only in a few cases can any one species or genus be given sole credit for natural bioconversions. For example, in the transformation of green fodder or forage crops into silage, the complex fermentation process involves plant enzymes as well as several groups of microorganisms present in the fodder and in the environment. Likewise, in the production of biogas from organic wastes, methane bacteria may be responsible for the gases produced, but this is not the only biological process taking place in the digester.

Even though mixed cultures of micro-organisms are usually involved in the transformation of organic residues, there are cases where pure cultures of bacteria, yeasts, moulds, or enzyme preparations can be used for processing such materials; these will be discussed later

Because other papers in these proceedings are also devoted to topics that can be included under the broad title of this paper, the following discussion will be restricted to a few possible microbial processes involved in the transformation of by-products or residues listed in Tables 1-3. Some of these processes have been, or can be, adapted on a small scale to rural regions, but others currently require fairly sophisticated knowledge or equipment for operation.

One of the major problems facing us today is how to adapt technical skills to various regions where people differ in their cultural or social customs, where natural resources vary, where the economy is dissimilar, or where environmental conditions may limit certain processes. More careful thought must be given in future developments as to whether the so-called "high technology" will be the best choice for people in every nation, or whether more attention needs to be given to what Norman refers to as "soft technologies" (4); Hedén speaks of as "self-reliance in an equilibrium society" (5); or what DaSilva, Olembo, and Burgers consider "low capital" vs. "high capital" technologies (21. The results presented in this Symposium can help point the way for leaders in various countries to make certain important decisions for future development.

An extensive discussion of even the major organic residues that can be utilized by microorganisms in a rural environment cannot be covered in one article. So I have selected only a few substances from the groups used for classification in Table 1.