Bio 3 - Microbiology - Biogas

Revision cards for Biogas! Hope this helps! :]

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Everyone needs fuel of some sort, but there is a finite amount of fossil fuels like coal, oil and gas to use. Even wood and peat are getting scarcer. Around the world, we all need other, renewable forms of fuel. The generation of Biogas from human and animal waste is becoming increasingly important in both the developing and the developed world.

Biogas is a flammable mixture of gases formed when bacteria break down plant material or the waste products of animals in anaerobic conditions. It is mainly methane, but the composition of the mixture varies. It depends on what is put into the generator and which bacteria are present

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Around the world, millions of tonnes of faeces and urine are made by animals like cows, sheep, pigs and chickens. We produce out fair share of waste materials as well. Also, in many places, plant material grows very rapidly. Both the plant material and animal waste contain carbohydrates. They make up a potentially enormous energy resource - but how can it be used?

The bacteria involved in biogas production work best at a temperature of around 30 degrees C. So biogas generators tend to work best in hot countries. However, the process generates heat (exothermic reaction). This means that if you put some heat energy in at the begining to start things off, and have your generator well insulated to prevent heat loss, biogas generators will work anywhere

Under ideal conditions, 10kg of dry dung can produce 3m^3 of biogas. That will give you 3 hours of cooking, 3 hours of lighting or 24 hours of running a fridge. 

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At the moment biogas generators around the world operate on a relatively small scale. They supply the energy needs of one family, a farm or at most a whole village.

What you put into your generator has a big effect on what comes out. Biogas units are widely used in China. There are well over 7 million biogas units, which produce as much energy as 22 million tonnes of coal. Waste vegetables, animal dung and human waste are the main raw materials. These Chinese digesters produce excellent fertiliser but relatively low quality biogas.

In India, there are religious and social taboos against using human waste in biodigesters. As a result only cattle and buffalo dung is put into the biogas generators. This produces very high quality gas, but much less fertiliser.

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There are also different sizes and designs of biogas generators. The type chosen will depend on local conditions, for example many fermenters are sunk into the ground, which provides very good insulation. Others are built above ground, which may be easier and cheaper but offers less insulation. If night time temperatures fall a long way it could cause problems.

Many countries are now looking at biogas generators and experimenting with using them on a larger scale. The waste material we produce from sugar factories, sewage farms and rubbish tips could be used to produce biogas. We have some problems to overcome with scaling the process up, but early progress is promising.

Biogas could well be an important fuel for the future for all of us. It would help us to get rid of much waste as well as providing a renewable energy source.

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