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Biogas


Table of Contents

Biogas is a methane rich gas produced through the anaerobic digestion (ie. without air) of organic wastes. It can be generated from cattle dung and animal wastes, and with substantially more difficulty, from some crop residues. Although these feedstocks are frequently used directly as cooking fuel, in most areas they are not preferred fuels and are used only when wood is not available.

Biogas systems offer multiple benefits. The digester-effluent is usually a good fertiliser, and, if connected to latrines, biogas plants can provide valuable sanitation services. For cooking and other thermal household tasks, it is simple and reasonably efficient to use the gas directly in conventional low-pressure gas burners. Biogas can also provide lighting when used in mantle lamps. In societies where suitable feedstocks are readily available, small family-sized biogas digesters were thought to have considerable potential.

[top] [end]International experience

A number of countries have initiated biogas programmes - China and India have on a large scale, and there is significant experience of commercial biogas in Nepal. Results have been mixed, especially in the early stages. Quality control and management problems have resulted in a large number of failures.

Biogas experience in Africa has been on a far smaller scale and has been often disappointing at the household level. The capital cost, maintenance, and management support required have been higher than expected. Under subsistence agriculture, access to cattle dung and to water that must be mixed with slurry has been more of an obstacle than expected. Possibilities are better where farming is done with more actively managed livestock and where dung supply is abundant - as in rearing feedlot-based livestock. The initial enthusiasm for biogas has thus been somewhat dampened by experience. Because of its requirement for relatively large amounts of animal dung, the niche for household biogas plants is likely to remain small. Poor families often do not have access to the necessary quantity of dung, and better-off families with sufficient animals often prefer to purchase fuel and fertiliser rather than spend time gathering dung and managing the often-temperamental digesters. Even so, in the right social and institutional context, and with appropriate technical expertise, the potential for biogas remains significant. A new initiative on Biogas was launched in May 2007 called the Biogas Africa Initiative.

[top] [end]Lessons learnt

[top] [end]I. Benefits of biogas technology

What makes biogas an attractive option is the fact that this technology can provide solutions to a variety of problems simultaneously: In general it has been proven that the energy aspect alone does not justify the cost for biogas technology. But the essential benefits of biogas plants are not manifested in individual cost-efficiency calculation. The overall objective, to which biogas technology contributes, is environmental protection which includes energy-related objectives (decrease of greenhouse gas emissions as well as deforestation) and the improvement of livelihoods of biogas users.

Well-functioning biogas systems can yield a whole range of benefits for their users, the society and the environment in general:
  • production of energy (heat, light, electricity)
  • transformation of organic waste into high quality fertilizer
  • improvement of hygienic conditions through reduction of pathogens, worm eggs and flies
  • reduction of unpleasant odors
  • reduction of workload, mainly for women, in firewood collection and cooking
  • environmental advantages through protection of soil, water, air and woody vegetation
  • micro-economical benefits through energy and fertilizer substitution, additional income sources and increasing yields of animal husbandry and agriculture
  • macro-economical benefits through decentralized energy generation, import substitution and environmental protection
  • Biogas technology can substantially contribute to conservation and development, if the concrete conditions are favorable

[top] [end]II. Limitations of Biogas Technology

Biogas systems are functioning under a variety of climatic conditions. However, a widespread acceptance and dissemination of biogas technology has not yet materialized in many countries. One main reason is the required high investment capital. Often the reasons for failure were also the unrealistically high expectations of potential users. Biogas technology cannot solve every problem of a farm, a village or a big animal production unit. If disappointment is to be avoided, the limitations of biogas technology should be clearly spelt out.
  • An obvious obstacle to the large-scale introduction of biogas technology is the fact that the majority of rural populations often cannot afford the cost of investment for a biogas plant. The installation of a few biogas plants often can only be afforded by better-off farmers. High up-front investment costs for even small biogas units are still not affordable for poor households.
  • The technical viability of biogas technology has been generally proven in field test and projects; the economic viability of biogas digesters is under discussion and did not prove to be viable for some contexts. The establishing of an efficient and sustainable dissemination structure continues to remain the key problem of numerous biogas projects. Numerous problems have arisen when mass dissemination of biogas units/digesters is attempted: in particular the dung collection has proved more problematic than anticipated, particularly for farmers who do not keep their livestock in one location. The viability and reliability of biogas projects usually depend on a number of factors, such as:
    • Quantity of available biomass/animal waste: Sufficient biomass/manure on a continuous basis should be available to maintain installed biogas units. Project experiences show that if more biogas units were installed than biomass manure has been available, unreliable and disrupted energy services were a consequence.
    • Location of biogas project: if a project combines the provision of energy services with income-generation, such as the production and selling of manure as fertilizer, the local market situation plays a role, as it is critical to have a sustainable local demand for fertilizers and a critical mass of users. Users, such as farmers, will loose interest in using biogas units if there is no financial benefit associated with producing and marketing manure.
    • Ownership issue: Users of biogas units should, if possible, make a financial contribution to the installation of biogas units, to develop an ownership perception of the energy provider.
    • Combined biogas units: General consensus emerged from practice is that larger combined septic tanks/biogas units run by institutions such as schools or hospitals are more viable than small-scale biogas digesters.

[top] [end]III. Dissemination & promotion strategies for biogas

The implementation of biogas projects and programmes, even on a small-scale level, must take into account the underlying socio-cultural, political, economic and ecological conditions. As an appropriate technology, mainly for rural areas, the realization of economically viable and sociologically and ecologically beneficial biogas projects heavily relies on social and political acceptance. The basic prerequisite for successful, comprehensive introduction and popularization of biogas technology is the effective motivation and mobilization of potential target groups.

A successful dissemination strategy will require steps within the following fields of activity: information and public relation campaigns; educational and training programs; financial promotion; politico-administrative and organizational aspects; social acceptance.

Checklist for introduction & promotion of biogas technology:
  • region with favorable climatic conditions
  • existence of a potential target group
  • private sector involvement
  • informal sector involvement
  • government involvement
  • organizations/networks to cooperate with
  • economic viability on micro- and macro level
  • financing program and the cost of programme
  • material requirements
  • technological standards
  • available know-how on planning, management, technician and artisan level
  • the role of subsidies
  • kinds of information, propagation, awareness creation
  • assessment of sustainability

[top] [end]IV. The Technology

There are various types of plants. Concerning the feed method, three different forms can be distinguished: (1) Batch plants; (2) Continuous plants; (3) Semi-batch plants. Batch plants are filled and then emptied completely after a fixed retention time. Each design and each fermentation material is suitable for batch filling, but batch plants require high labor input. As a major disadvantage, their gas-output is not steady. Continuous plants are fed and emptied continuously. They empty automatically through the overflow whenever new material is filled in. Therefore, the substrate must be fluid and homogeneous. Continuous plants are suitable for rural households as the necessary work fits well into the daily routine. Gas production is constant, and higher than in batch plants. Today, nearly all biogas plants are operating on a continuous mode. If straw and dung are to be digested together, a biogas plant can be operated on a semi-batch basis. The slowly digested straw-type material is fed in about twice a year as a batch load. The dung is added and removed regularly.

[top] [end]Resources


A number of useful publications are available from the meeting "How to create a market for domestic biogas plants?" of the "Network of Experts on Domestic Biogas" held on 5 and 6 April 2006 in Hanoi and focused on the promotion of biogas.

[top] [end]External links and references


Categories: Fuels| Biogas


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Page created: 27 September 2003; Last edited: 14 September 2007; Version: 10
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