Introduction to Biogas

When animal species ingest (eat food), when it gets to the stomach the bacteria and enzymes in it. This produces gas in the stomach called biogas which contains methane (CH4) as a major component (approx. 70%) and carbon dioxide (CO2-25%) and other trace elements.  Therefore biogas is a technology where organic waste viz kitchen waste, human waste and vegetable waste are made to biodegrade/ferment anaerobically (in the absence of oxygen) to produce a flammable gas and the residue becoming organic fertilizer. With a vast population present on earth it becomes imminent that we start thinking ahead on ways we ought to use to sustain future generation. The first recorded biogas on the continent of Africa was in the beginning of the 1980’s in Guinea. For close to 16 years they were only 60 digesters installed all with an average capacity of 30m3. This whole process occurs in a concrete dome as shown below:


Before feeding the digester, the excreta, especially fresh cattle dung, has to be mixed with water at the ratio of 1:1 on a unit volume basis (i.e. same volume of water for a given volume of dung) and a temperature range of 35˚C or lower. However, if the dung is in dry form, the quantity of water has to be increased accordingly to arrive at the desired consistency of the substrate (e.g. ratio could vary from 1:1.25 to even 1:2). The dilution should be made to maintain a total solid content from 7 to 10%. If the dung is too diluted, the solid particles will settle down into the digester and if it is too thick, the particles impede the flow of gas formed at the lower part of digester. Furthermore, most biogas plants are designed for a total solids content of about 8%. A change of this ratio will have an impact on the HRT (hydraulic retention time) and the hydraulic functioning of the plant. The gas is collected for further use in heating, cooking and lighting. The paste that remains is called slurry and is very rich in nitrogen and hence it becomes a preferred fertilizer/manure.



Although the uptake of this technology is still at a slow pace, there are certain organizations, schools and farms that have led the pack and have greatly lowered their electricity bills using a locally generated product, biogas. A school in Zimbabwe called Monte Casino generates gas from human waste and their toilets are constructed in a way that students’s waste is collected in brick containers acting as digesters (KAWI Series, 1999). It further states that the school generates gas from pigs and cattle. Malawi also has a school called Phwezi Girls Secondary School which makes use of this human waste but the biggest digester measuring 4 000m3 is in Dwangwa, Malawi and the gas is used to distill ethanol. 


With all these benefits, some of you might be wondering then why this technology has not made it in the market with a bang. In the next issue this week I will be looking at some of the economic, social and financial challenges faced by this technology.

The author of these documents is a qualified Fuels and Energy Engineer and can be contacted on for your views and comments. 


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