|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Biogas generation: developments. Problems, and tasks - an overview|
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
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
solar radiation, prevailing climatic conditions, and
strength of village population need to be
contraints: Digester design; high
Transportation costs of digester materials;
installation and maintenance costs;
increasing labour costs in distribution of
biogas products for domestic purposes
of cheap construction materials, emphasizing
low capital and maintenance costs and simplicity of
operation; provision of subsidies and loans that are
own or have access to relatively
large number of cattle
community development, ownership and biogas distribution schemes
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.
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
|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,
|Loading rate||Dependent upon digester size, dilution
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,
|Establishment of "no
leak" conditions, use of
external protective coating materials (PVC,
|Occurrence of CO2
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
|Risks to health
and plant crops resulting
from residual accumulation of toxic materials
and encysted pathogens
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
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
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-