|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Environmental goals for microbial bioconversion in rural communities|
Besides potential toxic effects of industrial wastes, human and animal wastes pose the gravest health hazards because they carry the pathogens of the water-borne diseases. In addition to hygienic quality of water, the quantity available is also important. Some so-called water-borne diseases (2) can be prevented if sufficient washing takes place, even with water of less than drinking quality (Table 3). Also, vicious circles of water-borne infections occur in farm animals, e.g., Salmonella in pigs (3).
TABLE 3. Classification of Infective Diseases in Relation to Water Supply
|Category||Examples||Relevant water improvements|
|1. Water-borne infections:||Improve quality|
|(a) Classical||Typhoid*||Aim for maximum microbiological quality of water|
|(b) Non-classical||Infective hepatitis*||Improve microbiological quality of water|
|2. Water-washed infections:||Improve quantity|
|(a) Skin and eyes||Skin sepsis and ulcers|
|Yaws||Provide a greater volume of water, facilitate access, and encourage its use|
|(b) Diarrhoeal diseases||Bacillary dysentery*|
|3. Water-based infections:||Specific measures:|
|(a) Penetrating skin||Schistosomiasis||Reduce contact with infested water|
|(b) Ingested||Guinea worm||Protect water source|
|4. Infections with water-related insect|
|(a) Biting near water||Sleeping sickness||Clear vegetation|
|(b) Breeding in water||Onchocerciasis||Avoid need to visit source|
|Yellow fever||Provide reliable supply|
|5. Infections primarily of defective Sanitation:||Hookworm||Provide sanitary fecal disposal|
|(To some extent, most diseases in|
|previous categories also)|
Source: Pacey (2).
* These diseases may be spread by any process that allows material from human faeces to be ingested; i.e., they may spread either as water-borne or as water-washed infections.
Microbial methods for destruction of pathogens are, in addition to self-purification after dilution in surface waters, found to coincide with those of conventional waste treatment that have built-in mechanisms for destruction of pathogens. One example is parasitism by Bdellovibrio through the action of bacteriophages and photooxidation. This may lead to destruction of 90 - 99 per cent of pathogens and a reduction in biological oxygen demand (BOD).
In screening the array of existing biological treatment methods (Figure 3) for suitability in energy-poor rural areas, oxidative methods are disqualified because they are costly, consume energy, and deplete the land of minerals. In addition, conventional waste collection and transport systems must be discounted in many cases because they require a sewerage system which uses and contaminates large quantities of water. Also, dilution precludes methods like drying followed by incineration, and, even more important, dilution prevents composting.
Thus, the anaerobic and photosynthetic methods appear to be the best candidates for rural application, and from the health and water-quality point of view, no objections arise. This is not the case with waste collection and transport in the undiluted state, which has definitely higher health hazards than are occasioned by the flush toilet sewerage method. Because the latter is generally too expensive, night soil collection coupled with special sanitary practices will ultimately be the best choice.
1. Emphasis should be placed on hygienic collection of undiluted human and animal wastes at the source, to be followed either by immediate anaerobic digestion, or by hygienically safe transport to composting sites. Study of the equipment required has been neglected for a long time, but recently the World Bank has started a comprehensive research project on this subject (4), which has already shown that safe night soil collection in villages is feasible for about one-eighth of the price of conventional sewerage systems. At the 1977 GIAM V Conference held in Bangkok, Nimpuno presented a comparative study of sanitary privies, latrines, and composting toilets (5). Recent reports from the People's Republic of China indicate that utilization of human and animal waste in the undiluted state is widely and successfully practiced and appears to enjoy firm popular support (6).
2. Where space, solar radiation, and water availability permit, photosynthetic waste treatment in oxidation ponds after appropriate dilution is a suitable and cheap method that attains hygienic safety and water quality protection, The method essentially converts organic waste material into algal cell material (Figure 4), which, in conventional ponds, is discharged with the effluent. Harvesting of the algae offers a wide scope of rural applications, as the material may serve as fertilizer, fodder, or as a source of energy through anaerobic conversion to methane.
3. In cases where the population is traditionally and firmly committed to thorough cooking of the fish they consume, direct discharge of wastes into fish ponds appears to be a valuable alternative (7). To what extent the fish feed directly on the waste and indirectly on the algae grown from it, remains to be elucidated.
4. Animal wastes lend themselves, in some cases, to fractionation followed by partial refeeding of the protein fraction (bacterial cells), while undigested cellulose can be upgraded by fermentation to serve as a fodder supplement. However, simple technologies for this process have not yet emerged, and special care must be taken to avoid recycling animal pathogens.
5. Conversion of animal waste, like horse dung mixed with straw, into human food in the form of mushrooms represents the most ambitious example of upgrading the lowest-quality waste to a high-quality product in one simple step, skipping intermediate stages in the food chain. This practice is now widely adopted in Southeast Asia (8).
6. In all cases where fresh human and animal wastes are being processed, a minimum sanitary discipline should be taught to the population, not only with respect to personal hygiene, but also with regard to ground-water quality protection in areas where wells are used for drinking water.
7. Providing an adequate water supply in rural areas is best done by protecting the water source against pollution by waste treatment methods 1 - 3 indicated above. If these are carefully followed, a minimum of final treatment often suffices to produce safe water. Simple treatments include: 48 hours of sedimentation to kill off the cercariae of Schistosomes (2), and slow sand filtration (9) which, through oxidative action by the microbial film, removes both pathogens and the last traces of organic matter. Oil drums can serve as small-scale filters.
8. In situations where water quality is threatened by large amounts of agro-industrial wastes (e.g., from rubber processing), microbial conversions of waste to useful products should receive a higher priority than conventional oxidative treatment.
9. While the dimensions of the problems of the water-health complex are staggering - more than 80 per cent of the world's rural population has "no reasonable access to safe water" (2) - it should be recognized that the problems present themselves in great variety, each demanding its own special solution Moreover, they cannot be divorced from seemingly unrelated factors, for instance, the level of food energy intake, which determines the distance from which water can be fetched (10).