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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 transportation. |
| Site selection | Nature of
subsoil, water table, and availability of solar radiation, prevailing climatic conditions, and strength of village population need to be considered. |
|
| 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 necessary. |
|
| 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, creosotes |
|
| Occurrence of
CO2 reducing calorific value of biogas |
Reduction in
CO2 content through passage in lime-water |
|
| Occurrence of
water condensate in gas supply system (blockage, rusting) |
Appropriate
drainage system using condensate traps |
|
| 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 pressure |
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 |
|
| Residue utilization |
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 aesthetics |
| 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- sufficiency |
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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.