| Bioconversion of Organic Residues for Rural Communities (1979) |
|Strategies for developing small-scale fermentation processes in developing countries|
c. rolz., j.f. menchú, s. de cabrera, r. de leon, and f. calzada
central american research institute for industry (icaiti), guatemala city, guatemala, central america
tropical areas around the world are considered unparalleled green factories where, under mild and appropriate environmental conditions, agro-industrial operations strive to produce cash export crops like coffee, cocoa, sugar-cane, and fruits. the global participation by the fermentation industries is negligible in this region despite much renewable raw material potential in the agricultural wastes from these crops. the reason? simple enough: lack of scientific, technical, and managerial skills.
in tropical countries, the majority of the population lives outside the main cities in what has been loosely termed rural communities. international economic and scientific organizations have recently asked: can small-scale, simple fermentation processes be developed for these groups? if so, what kind of processes should be considered? what type of technology is required?
the first question is what kind of fermentation developments is appropriate for less developed countries? there are several alternatives, among which are the following.
1. traditional units. for example, submerged tanks, using soluble carbohydrate as raw materials to produce antibiotics, enzymes, amino acids, and microbial biomass. however, this kind of operation would compete with existing industries in the developed world. it would, therefore, be unlikely to obtain support from international industrial development institutions.
2. simple, small-scale processes for rural communities biogas and soil conditioners are the best examples. some in the developed world suggest that these are the most feasible for developing countries, but they must be proved to be commercially successful.
3 . a new breed of processes, a new set of rules: fermentation-plantation units established in rural areas. these should be self-sufficient in energy and utilize all the wastes. in this new technology, solid substrate fermentations and multiple-product units would be favoured. complexity of the unit and products would depend on local conditions. this alternative will probably get some sympathy but very little support from the developed countries, unless dramatic events, such as those which initiated the "new economic order" discussions develop in future.
deciding which alternative is best for all rural communities is unreasonable, as natural circumstances vary from country to country, unless the alternative chosen is regarded as a general policy framework. on this basis, many governments have already endorsed the second alternative. the industrial sector would, however, prefer the last one if it is eventually developed by engineers and scientists in third world countries.
in asian countries, anaerobic fermentation of organic matter, especially animal wastes and night soil, to produce energy (biogas) and to act as a soil conditioner (sludge residue) has been recommended as one of the simplest methods for treating these wastes in order to minimize public health hazards and, at the same time, obtain valuable products two recent reports from international development institutions give an in-depth technological state-of-the-art review (1,2). it is from these reports that the following points are drawn.
practical experience in biogas generation in rural communities overwhelmingly resides in asian countries, e.g., india, the people's republic of china, taiwan, korea, and japan. animal wastes and night soil have been the main substrates. as barnett et al. state, "the technical and economic evaluation of these technologies has often been rudimentary..." and "the data that currently exist on the viability of biogas plants are not only very unreliable, but are obtained from a narrow range of possible plant designs and socio-economic and agroindustrial environments" (2). of the tens of thousands of biogas plants that have been constructed-the majority built not more than ten years ago - it is not known whether they are still functioning (after an initial test period sponsored by governmental agencies), or at what level of efficiency. more information is required before this approach can be recommended for large-scale adoption with any assurance of economic success and cultural acceptance (1).
it is interesting to summarize some of the tasks for research and development recommended in a technical assessment of biogas plants made in india, one of the leading countries in this field (3):
a. more basic knowledge is needed on the metabolic path ways for methane formation.
b. a search should be made for new fermentation materials.
c. more treatment processes should be explored.
d. fermentation components (ch4 evolution?) as a function of temperature, pressure, agitation, solids particle size, viscosity of suspension need to be determined.
e. operation techniques need to be tested, e.g., continuous, one-day feeding, fed-batch, batch.
f. more must be learned about equipment design: building materials, gas holders, heating, gas purification.
g. how will biogas be best utilized (cooking, lighting, power conversion) ?
h. which is the best use of sludge: handling (drying, composting, algae growth, compacting), and will it have value as fertilizer?
i. socio-economic aspects must be considered: cultural acceptance, optimum number and capacity of plants, and infrastructures.
the point is that there is quite a bit of technical experience in small-scale methane generation in rural conditions, especially in asian countries, employing animal wastes and night soil as substrates. but whether this technology could be directly transferred to africa and latin america and be accepted and economically successful is an open question. it is essential that research of a fundamental and applied nature be done with the more abundant ligno-cellulosic agro-industrial wastes.
large-scale methane generation is also being considered in developed countries for treating organic municipal and feed lot wastes (4 - 8). although not necessarily related to rural operation, some of the more complex, recently developed engineering techniques could eventually be scaled down. future technical advances need careful evaluation.
the role that methanogens play in mixed bacterial populations, and how methane is biosynthesized from either acetate or co2 and h2 in natural habitats, under mesophilic conditions, as in lake and marine soil sediments, and in the animal rumen have been the subjects of recent reviews (9 - 12). methanogenic bacteria have been shown to alter the metabolic pattern of chemo-organotrophic bacteria. hydrogen produced by them is oxidized to methane by methanogens coupled with the reduction of co2. this co-operation among bacteria has been called "inter-species hydrogen transfer." it is not an obligatory interaction, but occurs with positive effects for each participating species. it is not exclusive for methanogens; it might be considered a general reaction involving all terminal micro-organisms. a recent example has been described by weimer and zeikus (13) and is illustrated diagrammatically in figure 1. while with clostridium thermocellum growing on cellulose or cellobiose, ethanol and hydrogen are produced, in co-culture of cl. thermocellum and methanobacterium thermosutotrophicum there is complete conversion of h2 to chin, a shift in the conversion of acetyl co a from ethanol to acetic acid, which results in more electrons being available for h2 production and hence, methane. no methanogenesis was observed from acetate. the cellobiose was fermented more rapidly than was cellulose.
Figure. 1. Interaction between Clostridium thermocellum and Methanobacterium thermoautotrophicum
These comments stress the point that the biogenesis of methane is still a subject for research. More basic knowledge of how mixed bacterial populations are able to attack lignocellulosic biopolymers and eventually produce CH4 and CO2 is needed.
In any event, the degradation of the plant's structural biopolymers like cellulose and hemicellulose will be the rate-limiting sept. This biological reaction occurs in nature either under complete anaerobiosis or full aerobiosis, in mesophilic (20 to 45 C) or thermophilic (above 45°C) conditions (14). There is recent experimental evidence that there is a positive correlation between temperature and rate of cellulose hydrolysis until denaturation of enzymes becomes limiting, either under anaerobic (15, 16) or aerobic conditions (17). Biogas production was also enhanced when produced at high temperatures from wastes with a high cellulose content, in this case urban refuse (18,19). Still, the characterization of the cellulose enzyme complex among micro-organisms is only beginning. Cellulolytic enzymes are found widely scattered among the major taxonomic groups, but only in the higher fungi are they a feature of the group as a whole (20) This is one of the reasons why it has been studied in detail in Sporotrichum pulverulentum (21), Trichoderma viride (22), Trichoderma koningii (23,24), Fusarium solani (25), and in a culture of a Thermoactinomyces sp. (26).
Figure 2 illustrates a summarized form of a proposed mechanism for ligno-cellulosic polymers under aerobic conditions. Note that cellulose is degraded to glucose through the concerted action of multiple enzymes controlled by an induced-catabolite repressed system. Lignin is assumed to enter the cycle through redox reactions (dashed lines in the figure) The reader is referred to recent review articles dealing with this very important subject (27 29), whose thorough understanding will give guidelines for the anaerobic degradation of agro-industrial by-products in substrate selection, treatment, operating temperature, number of fermenters and mode of operation, and rate models for biogas production.
Because cellulose is the most abundant renewable resource on earth, it has been considered one of the possible future raw materials for fuels, chemicals, feed, and food The Gaden Humphrey diagram shown in Figure 3 illustrates this point (30). This means that many scientists around the world will be working with cellulose as their raw material, and needless to say, a great many technical advances are forecast for the near, mid-, and long-term future.
Anaerobic digestion of cellulosic by-products for biogas and soil conditioner production should be placed among competing alternatives, and also as a complementary step in an integrated scheme for by-product utilization. It would be a mistake to consider it the only solution to fermentation schemes for small-scale processing in rural areas. This would be frustrating and a serious limitation for further development.
Every raw material available in a region must go through at least a preliminary cost-benefit analysis, as well as local socio-economic review as the first step in strategy selection and process synthesis. In this exercise, all possible products, their cost and demand, should be established. It is not possible to consider all available alternatives in this paper, especially when one considers the heterogeneity and diversity of agro-industrial products (31). As an epilogue, therefore, a qualitative analysis will be made on the utilization of an important byproduct of coffee-producing countries: coffee pulp.
Coffee pulp is the skin of the coffee fruit. It represents 40 per cent by weight (fresh) of the fruit, and is separated from the fruit by mechanical action with the help of water. Another coffee by-product is mucilage, which can be removed from the depulped fruit by natural solid fermentation. It represents 20 per cent by weight (fresh) of the fruit. Both can be obtained together without introducing drastic and costly changes in coffee processing itself. As a matter of fact, it would be better to depulp the fruit with a minimum amount of water. The same water could be used to separate most of the mucilage from the fruit, leaving natural fermentation to finish the process. The water could be circulated and treated later on.
During the last few years, the total quantity of coffee pulp produced in Central America has varied between 800,000 and 1 million metric tons. There is a wide variation in the number and capacity of coffee processing units, at least three orders of magnitude in daily rate. Some of them are easily accessible by road or train; others are quite isolated in mountainous regions.
The pulp is now disposed of in an inefficient manner. It is usually dumped in open piles and left there for several months. After that storage period, it is mainly used as an organic fertilizer or soil conditioner in the coffee fields. Several things are wrong with this system For instance, in most of the pile, anaerobic conditions are rapidly established and putrefactive fermentation sets in. This causes very bad odours, and the pile becomes a haven for insects. Moreover, constant flow of pulp liquor leaches out of the pile and is usually channeled directly to the rivers or into open oxidation ponds, contributing to serious water pollution and, again, bad odour. Pulp has occasionally been used as ruminant feed, but because of improper handling and storage, results have been contradictory and not reproducible.
There are alternatives for coffee pulp processing that should be explored. Pulp is a potential fermentation substrate. It should be obtained as fresh as possible from the coffee processing units, and depulping should take place with a minimum of water or none at all. The alternatives are:
a. to use the fresh pulp in one of the following ways:
(i) composting it by controlled aerobic solid state fermentation;
(ii) mixing it directly with other feed ingredients and using it as feed for cattle;
(iii) mixing it with feed ingredients and then ensiling;
(iv) drying it and use it as feed;
(v) using it to produce biogas and organic fertilizer through controlled anaerobic fermentation;
b. to treat the pulp to remove excess water and soluble compounds from the solid matrix, use the liquid portion as a substrate for microbial biomass production, and use the solid matrix as in alternative 1.
Local conditions will dictate which method is most appropriate Research is needed on all of them. At ICAITI, we are developing the technology for the following systems:
a. Small-scale composing of fresh pulp - demonstration unit.
b. Large-scale composting of solid matrix with previous liquid separation - pilot plant.
c. Biomass production from the liquid (submerged production of filamentous fungi) - pilot plant.
d. Biogas generation from the liquid portion - rate and and modeling studies.
Previous work on fungal biomass production has already been published (31 - 38).
From the above examination of processing alternatives for coffee pulp, it can readily be seen that organic fertilizer and feed are the preferred products. The former can be produced directly under aerobic conditions (compost), or by producing biogas as a byproduct through anaerobiosis.
In order to give sound answers and not biased approximations to questions on which product is better and what alternative should be chosen, two conditions are required. (i) adequate experimental data, and (ii) knowledge of local conditions. Research activities carried out at the site will provide the answers.