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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
close this folderEnvironmental goals for microbial bioconversion in rural communities
View the document(introduction...)
View the documentIntroduction
View the documentHealth and water economy
View the documentFertilizer and energy economy
View the documentConcluding remarks
View the documentReferences
View the documentDiscussion summary: Papers by Porter, Berk and La Rivière


Environmental Goals

The meaning of the word "environmental" has rapidly broadened over the past decade under the influence of the United Nations Conference on the Human Environment held in Stockholm in 1972. The 21 goals the UN Environment Programme has set for itself for 1982 include not only preservation of air, soil, and water quality and of genetic resources but also improvement of the human environment in its widest sense - that is, with respect to water, food, energy, shelter, and the ecologically sound use of natural resources (1). Thus, we now have dovetailing and blurring of demarcation lines between, say, environmental and agricultural programmes and goals. Under these circumstances, I will not attempt to draw up a sharp definition of environmental goals, but, for the sake of expediency, interpret this term, for the purposes of this paper, as pertaining to the protection of human health and of the quality and productive capacity of water and land resources, which will include recycling of minerals and energy recovery from wastes.

Waste may be defined as any material which technology does not yet know how to use at a given time and place. Hence, this term has a specific meaning only in a specific context.

Microbial Bioconversions

Since time immemorial microbial conversions have had an impact on the global environment. Microbes have helped to shape the environment as it is today, and, through their massive action in the bio-geo-chemical cycles of carbon, nitrogen, and sulphur, they help maintain a steady state in the biosphere, for instance, by balancing CO2 fixation by CO2 production in the mineralization process, and by counteracting denitrification by nitrogen fixation. Because these conversions can be influenced by man to only a small extent, we cannot say that they serve environmental goals. Instead, their results have important environmental consequences that we are obliged to accept from nature. Thus, a discussion of these conversions falls outside my mandate; I mention them briefly because the two most important types of man-mediated microbial conversions are, to some extent, intensified versions of natural conversions (Table 1).

TABLE 1. Types of Microbial Conversions

A. Natural conversion processes in bio-geo-chemical cycles, having important environmental consequences.
B. Man-mediated conversions: (i) Product-oriented: fermentation industry. Sterilization and use of selected

organisms possible. Usually produces waste.

  (ii) Raw material-oriented: waste treatment. Sterilization and use of selected

organisms impossible. Serves environmental goals only.

This is least true with respect to the modern bioconversions performed in the fermentation industry, but still, we should not forget that many of its processes also find their early origin in the microbial mineralization of freshly harvested foods like milk, grapes, and cabbage that happened to yield spoilage products that were palatable, more durable, and hygienically safe, in the form of yogurt and cheese, wine, and sauerkraut. The modern processes merely provide rigorous guidance for these desired spoilage processes in order to guarantee a reproducible product. In general, however, the modern fermentation industry is based on product-oriented processes rather than on processes designed to utilize a given raw material. Good examples are the production of antibiotics, pharmaceuticals, enzymes, polymers, and other chemicals, where in all cases the growth medium is selected for optimum formation of high-priced products, at the same time making possible the use of aseptic conditions and selected, pure cultures.

In strong contrast to this are the man-mediated bioconversions of the second type, the microbiological waste treatment processes. Initiated by civil engineers on an empirical basis, these processes quietly developed into perhaps the largest existing microbiological industry without attracting much interest or support from microbiologists and biotechnologists. This is probably explained by the fact that the processes in question are oriented on whatever raw material is given, instead of on a product that is chosen because it can be sold. As there are no profits in the ordinary sense, the installations for waste treatment have to be cheap, and sterilization, which would allow use of selected strains, is out of the question. Furthermore, these processes must be designed so as to cope with waste flows that change in size and composition beyond the control of the plant operator. In fact, they are no more and no less than intensified versions of the natural mineralization process.

These two types of processes have been developed over the past 100 years or so along parallel but separate lines, virtually without interaction. During the past few years, however, we have reached an exciting point where these two lines of development are converging. The environmental crisis, coupled with imminent food and energy shortages, is bringing about co-operation between the two, suddenly creating wider scope for innovation through interaction. On the one hand, the fermentation industry is considering the use of raw materials hitherto designated as waste, and thus tends to serve environmental goals. The industry is also moving from the production of fine chemicals into producing bulk chemicals like ethanol and single-cell protein; the acetone-butanol fermentation, which has survived in Egypt, may well make a come-back.

On the other hand, the waste treatment industry is forced to go beyond its original restricted environmental mandate and seeks ways and means to turn wastes into useful materials rather than merely eliminate their health hazards and nuisance value (Figure 1). This opens the way for new processes that serve multiple goals simultaneously, i.e., pollution abatement and production of energy, food, fodder, fuel, and fertilizer. This includes the interesting possibility of designing processes which would be considered unrealistic when judged from only one point of view, but feasible if judged from several directions.

Figure. 1. Evolution of the Aims of Waste Treatment and of the Fermentation Industry

The fact that one-sided judgements still too often prevail should not discourage the scientist. It is better to have a process that is economically unfeasible but environmentally sound than to have no process at all. Economic considerations and yardsticks, like soy prices, are man-made and reflect the political will of the day; unlike the laws of nature, they should not be taken too seriously.

In taking a closer look at what conventional waste treatment achieves, I am strongly tempted to use the subtitle "From sense to nonsense, and back to sense again." In Figure 2, agricultural products are considered to consist of minerals and of energy that the sun has slung around them in the form of organic matter. We then see that only a fraction of the harvested plant or slaughtered animal is actually used for the energy metabolism of, and assimilation by, the consumer; the rest becomes human or animal waste, a spoiled product, or waste consisting of unused parts of the original product.

Figure. 2. Fate of Agricultural Products; "lost" minerals have to be replaced in order to keep production capacity constant.

If all these forms of waste remain untreated or unused, they pose a threat to the environment; human and animal wastes carry well known health hazards, while organic matter, apart from its odour nuisance, threatens the oxygen balance of surface waters and discharge minerals leading to eutrophication. These threats are not significant as long as population density is low and the degree of dilution high - conditions that permit natural mineralization to cope effectively with the disturbance. When waste concentrations rise to the point where natural purification is overtaxed, it makes sense to provide for waste treatment.

Most of the processes used are oxidative microbiological conversions that dissipate the energy and the minerals contained in the waste and even require, for aeration, an amount of energy roughly the same in magnitude as that present in the original waste. This does make sense as long as energy and minerals are cheap and plentiful and as long as the discharged minerals do not cause eutrophication. However, when discharged phosphorus and nitrogen compounds reach levels in the receiving waters that do cause eutrophication, further treatment, the so-called tertiary treatment, which "unfixes" the bound nitrogen in the waste (at great expense) by transforming it through denitrification to nitrogen gas, becomes necessary.

The treatment also precipitates the phosphates as insoluble aluminium or ferric salts that have no fertilizer value. It is at this stage where the borderline between sense and nonsense is being approached, especially when prices of energy and fertilizer are mounting, thus calling for a re-orientation of the process which brings recovery of energy and minerals within reach. Once thinking along these lines begins, another striking element of inefficiency comes to mind: high-quality water is often used for the transport of wastes, especially of the domestic type. This is not only wasteful but also impedes treatment and recovery by over-diluting the waste.

Thus, the question we have to discuss is: how can the arsenal of available bioconversions outlined above be made to serve environmental goals in rural communities, including not only health and water quality preservation, but also promotion of mineral and energy economy?

Rural Communities

To conclude this introduction, I shall say a few words about the important boundaries the frameworks of rural communities impose: a village is not a spaceship. This means that, in creating some degree of self-sufficiency, we are not only bound by the prevailing availability of energy and material, including water and minerals, but also by the constraints of the information economy. Rural self-reliance cannot be assured by mere import of information in the form of prototypes; we also need a "lock and key" complementary process brought about by training the population if the imported processes are to perpetuate themselves. Because training requires free time and other resources, the capacity of a village to incorporate a training component is limited. Hence, if we want to avoid irresponsible use of the "black box" principle the methods introduced have to be simple, rational, and transparent. Most of all, the goals they are to serve must be perceived from within and not merely imposed from outside. Microbiological methods create a special problem here because microbes must be recognized as both friends and foes, even though they are invisible to the naked eye Also, acceptance of environmental goals, especially those with long-term effects, will not be easy

The matrix of types of wastes, conversions, and goals in Table 2 will serve as the skeleton for a more detailed discussion. The many inter-relationships obviously create such complexity that only an arbitrary approach can provide a semblance of order In the following discussion, health will be combined with water economy, and energy economy with that of minerals.

TABLE 2. Matrix of Waste Types, Conversions, and Their Goals

Waste types




Photosynthetic Goals


Animal - Activated sludge - Oxidation ponds Economy of
Agricultural - Trickling filter - Fish ponds - water
Industrial - Oxidation ditch   - minerals
  - Composting Anaerobic - energy

-Mushroom production - Digestion - fodder  
  - Fermentative    
  upgrading   - food