BIOGAS

Biogas

Biogas is a technology in which organic waste such as human excreta, animal dung, vegetable waste or kitchen residue is made to ferment in anaerobic conditions to produce an inflammable gas (flatulence) and organic manure as a by-product. The gas produced is called biogas; it contains a large amount of methane gas and can be used directly as a gas for heating, cooking and lighting.

Biogas is a product of the fermentation at a relatively high rate of humidity of organic matter from human and animal waste, and from crop residues. The digester is a concrete dome. The fermentation of animal waste is produced in a fermentation vat, and stored in a tank. It is then piped to households. Slurry is the residue that remains after gas is collected. This residue is high in nitrogen content, which is the basic ingredient for fertilizer. Most digesters are constructed near an orchard or vegetable garden so that the slurry can be used as manure.

Nowadays villagers or communities centrally manage the use of dung. The dung is collected as soon as possible while fresh. If the material is collected late, the amount of microbiological action that will have taken place may lower the efficiency of the dung, particularly in the production of biogas. The fresh cow dung and water are fed in through an inlet into the airtight digester; which has an outlet of gas leading to the appliances. In the digester a chemical reaction takes place. Anaerobic respiration by bacteria results in biogas production, the gas moves through pipes. These must be made of a non-corrosive metal, as the gas is highly corrosive. When digestion is complete, the waste matter called slurry comes out through the slurry outlet, and this can then be used as manure. The digester has to be fed regularly to have a constant supply of gas. The digester requires a lot of water to function efficiently.

In addition, biogas plants must be properly constructed and managed. There should be no blockage of pipes which could lead to gas leakage. If there is a sludge build up, overflowing of slurry occurs resulting in unpleasant smell. The plant manager must ensure that corrosion of gasholders does not exist by planting the tanks with an oil base paint. Where dung is collected and used as fertilizer, the quality of nitrogen content in animal waste depends on the quality of the food the animal receives.

Plant sizes and number of cows required and gas storage capacities

PLANT SIZE

NUMBER OF COWS

GAS STORAGE CAPACITY

8m³

1-2

0.75m³

12m³

2-3

1.0m³

16m³

3-4

2.5m³

30m³

8-10

4.5m³

50m

15-20

7.5m³

The above five sizes are available as designed by CAMARTEC1, (Center for Agriculture Mechanization, and Rural Technology)

Technical sub-systems for biogas production and storage for utilization, the silayo (1992) comprises the following:

  • Procurement of organic materials: Organic materials and water, which are essential sources of biogas, should be gathered in one place.
  • Preparation of digestive substrate: The organic materials (substrate) are mixed with water in adequate proportions.
  • Feeding the digester: The prepared substrate is fed to the digester through the inlet opening.
  • The digestion process: This is decomposition and fermentation of substrate by bacteria in the digester. The result is the formation of biogas and slurry.
  • Storage of digestion products: The primary product is biogas, the chemical energy rich substance which can be stored in the digester itself, Or in a separate gas-holder, depending on the design of the digester. Slurry as a secondary product can be directly or stored in a pit or converted into compost for future use.
  • Use of digestion products: Biogas is a rich source of energy; therefore it can be used for cooking and lighting. Slurry is mainly used as a fertilizer on agricultural crops. It can also be used to enrich animal feed with nitrogen.

Advantages

  • It is clean and efficient fuel which burns without smoke or smell.
  • It is environmentally - friendly, does not pollute the environment, the burning of the methane gas saves the environment from greenhouse effect, since the destructive methane gas is burnt out.
  • It is good for rural communities.
  • The slurry can also be used for raising fish.
  • Helps to reduce deforestation and replace fuel wood.
  • Mosquitoes and flies are expelled from the surroundings of a biogas plant and therefore cannot breed in the slurry.
  • Biogas technology serves as an effective control of parasitic diseases. The pathogens, gems and other disease-causing organisms are destroyed in the digestion process.
  • It saves money and time and releases people from the task of fetching and buying firewood.
  • It leaves cooking utensils clean since the flame does not produce soot.
  • The bi-product of the digested organic matter (sludge) is a rich organic fertilizer.
  • Biogas can be used to run generator for producing electricity.
  • Animals are a source of protein and can also feed the biogas digester with their dung to increase energy supplies.

Disadvantages

  • Cultural, traditional, and governmental factors undermine biogas technology and are not yet committed to the technology
  • Manufacturers making biogas equipment and accessories are few.
  • Scattered settlement in rural areas.
  • There is limited capacity for trained people who can install and maintain biogas digesters.
  • The cost of transporting waste can also make biogas expensive.
  • Societies consider cow dung and human fecal waste as traditionally unacceptable and would not like to use biogas energy.

In terms of the economic benefits, the fact that biogas production substitutes for oil products, thus protecting the user from price changes, is probably a more important reason for encouraging its adoption than any consideration of climate change. The Kyoto Agreement advocates the setting up of a trading system in carbon emissions.

As Zimbabwe takes climate change seriously, a good biogas awareness raising programme is needed for quick uptake by rural communities.  The mass adoption of biogas will certainly be one of the strategies to be used to minimize the country’s emissions. Individual farmers would then have an incentive for installing biogas technology.

To increase viability of the technology, the following avenues must be explored:

  • Resources should be directed to promoting the technology in high potential areas.
  • Private sector involvement in promotion and dissemination is necessary for successful adoption of the technology.
  • There is need to counter the already negative image of the technology that exist by promoting proven designs and providing post installation support.
  • Cheaper low costs designs need to be developed to lower the cost entry barrier
  • Access to credit for end-users and small enterprises promoting biogas

Promoting entities should develop sufficient capacity to advice potential users on available options Institutions and organizations must work together to convince communities that human waste can be used as a resource. There should be equitable distributions of energy sources in African countries, governments should ensure that all rural areas located in the passage of power lines should automatically be electrifies, A fair principle should also be applied to both urban and rural areas. Energy sources should not only be harnessed for use in urban areas, it must also go to the rural areas. Energy should be affordable to the poor; it must be priced in such a way that the poor can also afford it.