|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Analysis of bioconversion systems at the village level|
Barnett's and companion articles in Biogas Technology in the Third World: A Multidisciplinary Review (1) provide an admirable framework for evaluation of alternative decisions in biogas systems. These references should be required reading for decisionmakers in the rural energy area. The present study may be considered complementary because it looks at bioconversion in general. The analysis is restricted to bioconversion for energy, feed, and possibly food.
Table 2 demonstrates the alternative possibilities for using a common residue, i.e., straw. What criteria should be used by rural people to derive the optimum benefit from the straw? As can be seen in the table, the farmer has a number of choices: how does he decide whether to sell that straw in exchange for other goods, digest it for energy, or use it as fuel or feed directly? It is thus necessary to evolve some simple rules to enable him to make a decision. If self-reliance in energy and feed is the goal considered politically desirable, then the man who owns the straw should clearly be steered away from marketing it for alternate uses not leading to energy and feed.
TABLE 2. Various Present and Future Uses of Straw
Straw and similar residues
1. Building material/composites
3. Feed for animals
4. Ensilage and/or storage
6. Any alternate bioconversion (For example, termites, mushrooms)
7. Any alternate marketability (For example, packing materials)
This is, in many ways, the crux of the problem. What are the socio-political and socioeconomic targets for the rural areas? It seems that unless a determined effort is made to propagate self-reliance, the increasing demands of population, industries, and cities will continue to denude the rural areas even of residues, and rural people will be forced to substitute high technology products, such as kerosene, for their basic needs. However, it is true that even for strictly bioconversion processes, the choice is not obvious, nor are the criteria for making the choice. Figure 1 shows various possible ways to use cattle dung and urine. Such schemes can be visualized for other residues. However, even for the same residue the scheme might vary from place to place.
In looking at alternate possibilities for residue bioconversion, two considerations are of the utmost importance: (a) time, and (b) tradition and acceptability. Figure 1 accounts for these by using the width of the arrows for popularity or tradition: the broader the arrow, the greater the usage of the method. In typical marginal communities, anaerobic manure piles are the rule (process B), perhaps because of ignorance about other methods. This method of bioconversion is also the most time-consuming, as shown by the length of the arrow. For other residues, the most acceptable mode might be the most rapid one because of the need for capital generation. Process A is very efficiently practiced in China, but not to a great extent in India. However, it could be made popular in India.
Process C is for feeding the manure directly to algae/fish ponds, a process that is common in Southeast Asia, e.g., the Philippines. It is not widely practiced in India, though it happens in many stagnant bodies of water naturally. Process D is the biogas process, which could become widely acceptable provided that the capital and technology inputs are there. Process E is almost unknown in the rural areas, though it has the great advantage of being quick. This involves drying the dung, pulverizing it, and feeding it in admixture with bran, etc., directly to poultry. The present practice is for village chickens to grub around in dung heaps, because farmers provide very little feed to poultry.
In the case of dung utilization, time is considered a zero-value entity; if the option for the residue were biogas generation (D), followed by A or C on the slurry, the time needed would still be very much shorter than that required for process B. Thus, there is a need to evolve a system, perhaps a judicious mix of all the processes, that will optimize the output to the farmer. This figure is presented to emphasize the need to build in the rate of biomass production or use along with energy usage rate. Quite often developmental efforts do not include the time component. An example of this is the large effort spent on upgrading cellulosic waste. Unless an alternative is available simultaneously, a waste such as bagasse will continue to be burned in huge quantities, regardless of its potential for conversion to fodder and food.
Table 3 presents all the variables that have to be considered in making a decision on a bioconversion process. If the residue is seasonal, then the process should be designed with minimum idle time for any equipment. This is the kind of information that has to be included as an input under item 1. Similarly, collection efficiency and analysis of residue have to be determined. For items 2 and 3, consideration should be given to whether the residue is individually owned and used or is used by the community and also to whether the desired benefit is short-term capital generation so that the village can then have longer-term benefits or whether planning should be for the long run. Usually, in the poorer Indian villages, it seems advisable to sacrifice even efficiency (if necessary) for quick capital generation, because this is what is desperately needed. In any case, it appears that technology, to stay useful in the poorest surroundings, should be highly adaptive, evolutionary, and integrated.
TABLE 3. Considerations in Choosing a Bioconversion Process
|1. Nature of residue||Seasonal/perennial? Collection Efficiency?|
|2. Ownership of residue||Individual/community?|
|3. Desired benefit||Capital/fodder, food/energy? Short or long-term? For one
man or community?
|4. Technology||Inputs - advanced or low availability?|
|5. Capital||Inputs - high or low availability?|
|6. Labour||Skill level? Availability/seasonality?|
|7. Energy||Inputs - high or low availability?|
|9. Delay loop||Time for finished steps of bioconversion|
|10. Other factors||Possibility of employment
Health and environment
Alternate marketability (other than bioconversion)
Items 4 to 8 list the main requirements for capital, energy, land, etc. in terms of input and availability. The inputs that have to be listed in such an analysis are based on proces-specific considerations and availability on region-specific considerations. For example, a process such as yeast culture might involve high energy input in a low energy availability environment and, therefore, be undesirable. Items 9 and 10 are equally important. A process might have to be rejected because the residue is needed urgently for an alternate use; therefore, the time rate of utilization and availability of the end-product become paramount considerations. Also, environmental factors and employment possibilities might dictate the total choice of a bioconversion process.
Thus, a proper analysis of the bioconversion of organic residues needs to encompass all possible factors and combinations. Usually the choice is not as difficult as it appears, because very few residues are available for bioconversion, except in the more prosperous villages. These, however, are a minority. In some areas where MCRC is active, the choice seems to favour a process that will lead to initial capital generation. Once capital is generated, time rate of utilization becomes less important and more elaborate, but more efficient, designs can be put into use.
To show how the factors in Table 3 are applied, the use of a residue, biogas effluent, for three bioconversion processes leading to poultry feed or fodder is discussed next. All three processes have potential application in the rural areas, though perhaps not in the immediate future. In the next section, processes leading to different end-uses, energy and fodder, are considered and compared.
Consider the mass culture of algae, yeast, and photosynthetic bacteria on biogas effluent. Spirulina has been grown by MCRC on such effluent, unfiltered (2 - 5 per cent of the total culture), with an initial boost of 50 per cent by weight in Zarrouk's medium (2). The open-air culture is harvested every other day and dried in solar driers. The skill level needed is high-school-trained workers; other than that, there are no special requirements. The methods, culture ponds, etc., are described in MCRC's Technical Notes (3). The average yield during the months of June to September 1978 was 10 g/m²/day in ponds with a maximum depth of 30 cm. This was a period of unusually heavy monsoon.
Compared to the yeasts, algae cultures, especially the blue-green algae, are very slow yielding. Irgens and Clarke (4) have reported the possibility of yeast culture in anaerobic digester supernatant supplemented with 1 - 2 per cent carbohydrate. The energy and skill requirements are very high, especially in the harvesting cycle. Unless the yeast is used for feeding as a slurry, this process might not, in spite of its higher yields, be adaptable. However, the potential for single-cell protein production at the rural level is high because of the low land requirement (see, for example, Slesser ), high nutrient value, even accounting for nucleic acid content, and availability of substrate.
The situation with photosynthetic bacteria (6) is similar, except that the yield is even higher. At MCRC, work is continuing on cultural photosynthetic bacteria on gobar gas effluent in inexpensive, sealed PVC bags. Harvesting the biomass is difficult, so the slurry must be fed directly. The land requirement for this procedure is low, but the skill, energy, and monitoring demands are very high. The comparison is given as an example of the choices available. The best choice would obviously be for algal cultures. with yeast as a future possibility.
Deciding among widely differing end-products is even more difficult. For example, if the choice is between making compost out of a residue or converting it to energy or to edible biomass for animals or humans, comparison must be made of the benefit of something that is expressed in megajoules versus something that may have an additional value (other than energy) as a protein source. The mere caloric value of a foodstuff is inadequate as a basis for comparison with other materials as far as its benefits are concerned. There is a need for a common yardstick that combines the various uses of organic residues and presents one basis for comparison. MCRC is working on this problem.