This is the old United Nations University website. Visit the new site at

Contents - Previous - Next

Bottlenecks, considerations, and research and development

Bioconversion of organic domestic and farm residues has become attractive as its technology has been successfully tested through experience on both small- and large-scale projects. Feeding upon renewable resources and non-polluting in process technology, biogas generation serves a triple function: waste removal, management of the environment, and energy production. Nevertheless, there are still several problems (14, 19, 20) that impede the efficient working of biogas generating systems (Table 5).

TABLE 5. Considerations Relating to Bottlenecks in Biogas Generation


Aspect Bottlenecks Remarks
Planning Availability and ease of transportation of raw

materials and processed residual products

Use of algae and hydroponic plants offsets high

transportation costs of materials not readily at

hand. Easily dried residual products facilitate


Site selection Nature of subsoil, water table, and availability of

solar radiation, prevailing climatic conditions, and

strength of village population need to be


Financial contraints: Digester design; high

Transportation costs of digester materials;

installation and maintenance costs;

increasing labour costs in distribution of

biogas products for domestic purposes

Use of cheap construction materials, emphasizing

low capital and maintenance costs and simplicity of

operation; provision of subsidies and loans that are

not burdensome.

Necessity to own or have access to relatively

large number of cattle

Well-planned rural community development, ownership and biogas distribution schemes


Social contraints and psychological

prejudice against the use of raw materials

Development of publicity programmes to

counteract contraints compounded by illiteracy;

provision of incentives for development of small-

scale integrated biogas systems.

Technical Improper preparation of influent solids

leading to blockage and scum formation

Proper milling and other treatment measures (pre-

soaking, adjustment of C/N ratio); removal of inert

particles: sand and rocks.

Temperature fluctuations Careful regulation of temperature through use of

low-cost insulating materials (sawdust, bagasse,

grass, cotton waste, wheat straw); incorporation of

auxiliary solar heating system.

Maintenance of pH for optimal growth of

Methanogenic bacteria

C/N ratio

Appropriate choice of raw material, regulation of

C/N ratio and dilution rate.

Appropriate mixing of N-rich and N-poor

substrates with cellulosic substrates.

Dilution ratio of influent solids content Appropriate treatment of raw materials to avoid

stratification and scum formation.

Retention time of slurry Dependent upon dilution ratio, loading rate,

digestion temperature.

Loading rate Dependent upon digester size, dilution ratio,

digestion temperature.

Seeding of an appropriate bacterial

Population for biogas generation

Development of specific and potent cultures.
Corrosion of gas holder Construction from cheap materials (glass fibre,

clay, jute-fibre reinforced plastic) and/or regular

cleaning and layering with protective materials

(e.g., lubricating oil).

Pin-hole leakages (digester tank, holder,

inlet, outlet)

Establishment of "no leak" conditions, use of

external protective coating materials (PVC,


Occurrence of CO2 reducing calorific

value of biogas

Reduction in CO2 content through passage in


Occurrence of water condensate in gas

supply system (blockage, rusting)

Appropriate drainage system using condensate


Occurrence of H2S leading to corrosion On a village scale, H2S removed by passing over

ferric oxide or iron filings

Improper combustion Designing of air-gas mixing appliances necessary
Maintenance of gas supply at constant


Regulation of uniform distribution and use of gas;

removal of water condensate from piping systems;

appropriate choice of gas holder in terms of weight

and capacity



Risks to health and plant crops resulting

from residual accumulation of toxic materials

and encysted pathogens

Avoid use of chemical industry effluents; more

research on type, nature, and die-off rates of

persisting organisms; minimize long transportation

period of un-dried effluent

Health Hazards to human health in transporting

night soil and other wastes (gray-water)

Linkage of latrine run-offs into biogas reactors

promotes non-manual operations and general


Safety Improper handling and storage of methane Appropriate measures necessary for plant

operation, handling, and storage of biogas through

provision of extension and servicing facilities

Rural communities using the integrated system are appropriate examples of recycled societies that benefit from low-capital investments on a decentralized basis and such communities are attuned to the environment. The technology thus seeded and spawned is, in essence, a populist technology based on "Nature's income and not on Nature's capital."

Biogas generated from locally available waste material seems to be one of the answers to the energy problem in most rural areas of developing countries. Gas generation consumes about one-fourth of the dung, but the available heat of the gas is about 20 per cent more than that obtained by burning the entire amount of dung directly. This is mainly due to the very high efficiency (60 per cent) of utilization compared to the poor efficiency (11 per cent) of burning dung cakes directly.

Several thousand biogas plants have been constructed in developing countries. A screening of the literature indicates that the experience of pioneering individuals and organizations has been the guiding principle rather than a defined scientific approach. Several basic chemical, microbiological, engineering, and social problems have to be tackled to ensure the large-scale adoption of biogas plants, with the concomitant assurances of economic success and cultural acceptance. Various experiences suggest that efficiency in operation needs to be developed, and some important factors are: reduction in the use of steel in current gas plant designs; optimum design of plants, efficient burners, heating of digesters with solar radiation, coupling of biogas systems with other non-conventional energy sources, design of large-scale community plants, optimum utilization of digested slurry, microbiological conversion of CO2 to CH4, improvement of the efficiency of digestion of dung and other cellulosic material through enzyme action and other pre-digestion methods, and anaerobic di gestion of urban wastes

We may summarize some of the research and development tasks that need to be undertaken as follows.

In basic research:

a. Studies on the choice, culture, and management of the micro-organisms involved in the generation of methane.

b. Studies on bacterial behaviour and growth in the simulated environment of a digester (fermentation components: rate, yield of gas, composition of gas as a function of variables - pH, temperature, agitation - with relation to substrates - manure, algae, water hyacinths).

In applied research:

a. Studies on improving biogas reactor design and economics focusing on: alternative construction materials in stead of steel and cement; seeding devices; gas purification methods; auxiliary heating systems; insulator materials; development of appropriate appliances for efficient biogas utilization (e.g. burners, lamps, mini tractors, etc.).

b. Studies for determining and increasing the traditionally acknowledged fertilizer value of sludge.

c. Studies on quicker de-watering of sludge.

d. Studies on deployment of methane to strengthening small-scale industries, e.g., brick-making, welding, etc.

In social research:

a. Effective deployment of the written, spoken, and printed word in overcoming the social constraints to the use of biogas by rural populations.

b. Programmes designed to illustrate the benefits accruing to rural household and community hygiene and health.

c. Programmes designed to illustrate the need for proper management of rural natural resources and for boosting rural crop yields in counteracting food and feed unavailability and insufficiency.

d. On-site training of extension and technical personnel for field-work geared to the construction, operation, maintenance, and servicing of biogas generating systems.

e. Involvement and training of rural administrative and technical personnel in regional, national, and international activities focusing on the potentials and benefits of integrated biogas systems.

Table 6 shows a number of the benefits of biogas utilization, set against the related drawbacks of presently used alternatives.

Present problems Benefits of Biogas
Depletion of forests for firewood and causation of

ecological imbalance and climatic changes

Positive impact on deforestation; relieves a portion of the

labour force from having to collect wood and transport coal;

helps conserve local energy resources

Burning of dung cakes: source of environmental

pollution; decreases inorganic nutrients; night soil

transportation a hazard to health

Inexpensive solution to problem of rural fuel shortage;

improvements in the living and health standards of rural

and village communities; provides employment

opportunities in spin-off small-scale industries

Untreated manure, organic wastes, and residues lost as

valuable fertilizer

Residual sludge is applied as top-dressing; good soil

conditioner; inorganic residue useful for land reclamation

Untreated refuse and organic wastes a direct threat to health Effective destruction of intestinal pathogens and parasites;

end-products non-polluting, cheap; odours non-offensive

Initial high cost resulting from installation, maintenance, storage, and distribution costs of end-products System pays for itself
Social constraints and psychological prejudice to use

of human waste materials

Income-generator and apt example of self-reliance and self-



1. J.W.M. LaRivière and E.J. DaSilva, "Farming Microbes for Food, Fuel and Fibre," Unesco Courier, June 1978.

2. J.R. Porter, "Microbiology and the Food and Energy Crises," Amer. Soc. Microbiol. News 40: 813 11974).

3. J.R. Porter, "Microbiology and the Disposal of Solid Wastes," Amer. Soc. Microbial. News 40:826 (1974).

4. J.R. Porter, "Micro-organisms as Natural Resources for Food and Energy," presented to the Asian Regional Seminar on Contributions of Science and Technology to National Development, New Delhi, 4 - 6 October 1978.

5. E.J. DaSilva, R. Olembo, and A. Burgers, "Integrated Microbial Technology for Developing Countries: Springboard for Economic Progress," Impact Sci. Soc. 28: 159 (1978).

6. M.P. Bryant, in H.G. Schlegel and J. Barnea (eds.) Microbial Energy Conversion, pp. 399 412, Erich Gottze KG, Gottingen, W. Germany, 1976.

7. J.W.M LaRiviere, "Microbial Ecology of Liquid Waste Treatment," Adv. Microbial Ecol. 1: 215 (1977).

8. J.W.M. LaRiviere, "Microbiological Production of Methane from Waste Materials," J. Sci. Soc. Thailand 3: 5 11977).

9. D,F. Torien, W.H.J. Hattingh, J.P. Kotze, P.G. Thiel, W.A. Pretorius, G.G. Cillie, M.R, Henzen, G.J. Stander, and R.D. Baillie, "Anaerobic Digestion - Review Paper," Water Res. 3: 385 (1);459111);5451111); 623 (IV) 11969).

10. D.F. Torien and J.P. Kotzé, "Population Description of the Non-Methanogenic Phase of Anaerobic Digestion," Water Res. 4:129 (i); 285 111); 305 (III); 315 (IV) 11970).

11. R.S. Wolfe, "Microbial Formation of Methane," Adv. Microbiol. Physiol. 6:107 (1971).

12. M.P. Bryant, E.A. Wolin, M,J. Wolin, and R.S. Wolfe, "Methanobacillus omelianskii, a Symbiotic Association of Two Species of Bacteria," Arch. Mikrobiol. 59: 20 (1967).

13. C. Bell, S. Boulter, D, Dunlop, and P. Keiller, Methane: Fuel of the Future, Andrew Singer, Cambridgeshire, U.K., 1973.

14. ESCAP Document RAS/74/041/A/01/01, "Biogas Technology and Utilization," ESCAP, Bangkok, 1975.

15. T.D, Biswas, "Biogas Plants: Prospects and Limitations," Invention Intelligence 12: 71 (1977).

16. M.J. McGarry and J. Stainforth, Compost, Fertilizer and Biogas Production in the People's Republic of China, p, 94, International Development Research Centre, Ottawa, IDRC-TS8e, 1978.

17. L.G. Fry, Practical Building of Methane Power Plants for Rural Energy Independence, L.G. Fry, Santa Barbara, California, 1 974.

18. U. Loll, in H.G. Schiegel and J. Barnea (eds.), Microbial Conversion, pp. 361 - 378, Erich Gottze KG, Gottingen, W. Germany, 1976.

19. National Academy of Sciences, Methane Generation from Human, Animal and Agricultural Wastes, NTIS Accession No. PB-276-469, NAS, Washington, D.C., 1977.

20. C. Prasad, N. Prasad, and A. Reddy, "Big-gas Plants: Prospects, Problems and Tasks," Economic and Political Weekly 9: 1347 (1974).

21. R.B. Singh, Bio-gas Plant: Generating Methane from Organic Wastes, Gobar-Gas Research Station, Ajitmal, Etawah, India, 1971.

22. E.J. DaSilva, A. Burgers, and R. Olembo, "Health and Wealth from Waste; an Economic Incentive for Developing Countries," Impact Sci. Soc. 26: 323 (1976).

23. J. Tinbergen, Reshaping the International Order: A Report to the Club of Rome, E.P. Dutton, New York, 1976.

24. J.R. Benemann, J.C. Weissman, B.L. Koopman, and W.J. Oswald, "Energy Production by Microbial Photosynthesis," Nature 268: 19 (1977).

25. A. Fernandez, Gobar-Gas Plant - How and Why, Seva Vani, May - June 1976.

26. G. Shelef, R. Moraine, A. Meydan, and E. Sandbank, in H.G. Schlegel and J. Barnea (eds.), Microbial Energy Conversion, pp. 427 - 442. Erich Gottze KG, Gottingen, W. Germany, 1976.

27. E.P. Eckholm, "The Energy Crisis: Firewood," World Watch Paper 1, Worldwatch Institute, Washington, D.C., 1975.

28. J. Parikh and K. Parikh, in H. G. Schlegel and J. Barnea (eds.), Microbial Energy Conversion. pp. 555 - 591, Erich Gottze KG, Gottingen, W. Germany, 1976.

Discussion summary

The question arose as to whether retention time in the biogas fermentation could be reduced by mixing. There seems to be very little in the literature on the subject, and, although information is now becoming available from the United States National Academy of Sciences and the Economic and Social Commission for Asia and the Pacific (ESCAPI, much more is needed. There is a great deal of information on domestic sludge, and it is now possible to treat dissolved residues, e.g., potato, in continuous anaerobic processes.

Contents - Previous - Next