Thursday, August 18, 2011
Biogas is an inflammable gas produced by bacteria in the process of fermentation of organic matter such as animal manure under anaerobic conditions. The process takes place in what is called a digester. The end results of the fermentation are:
i. Biogas – mainly composed of Methane (60 – 65%) and carbon dioxide (30 – 35%). The remaining is composed of hydrogen, Nitrogen, Sulphide, Carbon dioxide and other gases.
ii. Slurry – this is a mixture of liquid and solid effluents of the anaerobic digestion, a good fertilizer.
The process of decomposition can take place only if:
I. Oxygen is excluded from the process (air – tight conditions)
II. The raw material contains Nitrogen
III. The temperature is favourable – the optimum is about 35 degrees Celsius (a range of 20 – 40) as a temperature much below or above this retards or arrests the process
IV. The reaction is slightly alkaline, with pH of about 7.5
Application/use of Biogas digester
i. A bio digester makes an important contribution and improvement of natural resources and environment through provision of energy for cooking and lighting
ii. It can be used as a tool for clearing and digesting animal manure making it better and ready as fertilizer for use in gardens and fish ponds
iii. It improves the sanitary conditions in homesteads.
Among other advantages are: - being smokeless, reduce amount of petroleum products needed by a household, saves household time and money as well as firewood. The gas is non – poisonous.
Biogas can be produced from and animal (cattle, pigs and chicken) waste.
Preparation of the materials
Organic waste and water – the essential source of biogas are collected in one place (collection chamber). Ensure that the organic matter does not contain stones, straw, unconsumed feeds and other non fermenting solids. Mix the waste with water in adequate proportions. The ratios of mixing are:
Cow dung to water is 1:2
Pig dung to water is 5:4
Chicken dung to water is 1:1
Feeding /releasing the mixture into the digester through the inlet opening
The digestion process involves the fermentation of organic waste under anaerobic condition.
The bacteria secrete enzymes that ferment hydrolyzed compounds into hydrogen. A small amount of carbon gets converted to volatile fatty acids.
The higher fatty acids are caxxxx to hydrogen + acetic acid. The methane producing bacteria utilize acetic acid, hydrogen forlic acid and Carbon dioxide to form Methane plus Carbon dioxide, the main products of biogas.
The digestion process requires aerobic conditions and warmth.
Storage and use of digested products – the biogas can be stored in the digester itself for some Plants, and in others a separate gas holder is provided.
The slurry is stored in a well – covered outlet chamber from where it is removed for use as fertilizer in crop fields and fish ponds.
The initial investment is the major constraint in acquiring a biogas plant and careful plant management can help to maximize its effectiveness. The farmer can reduce costs and increase the output of the biogas plant by:
I. Plant selection: it is important to build the size of biogas plant according to the quantity of dung available and the needs of a household. A 2 – 3 cubic meter digester will be adequate for a family of 5 – 8 members, requiring 3 – 4 cattle.
II. Plant placement: the digester should be placed near cattle shed/manure source and also near the kitchen for the best efficiency. There should be enough water around and should be built in the open where heat from the sun will assist more rapid fermentation.
III. Plant construction/installation – the animal shed should be constructed so as to allow easy collection of dung and urine to be mixed with water. The design should help to exclude straw, stones, and other non – fermenting materials. The digester foundation should be properly built and follow the plan of construction especially with the use of materials. Do not skimp on the use of cement or necessary fittings. If possible, use plastic or stainless steel gas delivery systems to avoid corrosion.
Use good quality appliances for which the flames can be adjusted to blue colour and white colour for lighting.
The outlet tank should be adequately covered to prevent accidents and the slurry should flow freely to the gardens or compost tanks.
IV. Plant utilization: the manure added should be free of straw, soil and should be properly diluted with water and urine. Feed adequate manure periodically although necessarily daily. Prior preparations should be made before lighting the gas to avoid wastages. Open gas top after lighting the match stick and adjust the top of the blue flames or white light.