Biogas

- A mixture of flammable gases formed by the break down of human and animal waste in anaerobic conditions. 

- Up to 80% is methane, but this can vary. 

- A 'cleaner' and renewable alternative to the fossil fuels. 

- Waste contains carbohydrates, which can be utilised as an energy source, it is put into the generator. 

- Also added is bacteria, the optimum temperature for this is 30C (hence the more efficient systems being in hotter climates). 

- Anaerobic respiration takes place to form products of biogas (methane) and slurry (can be used as fertiliser). 

- Overall the reaction is exothermic, so the generators can be modified to insulate this heat reducing the energy required. 

- Used mainly on small/medium farms. 

- Gas is collected above the digester but released to the side. 

- Advantages: Easy to construct and operate, reliable technology and steady gas pressure.

- Disadvantages: Metal gas holder can be expensive and liable to rusting. 

- Gas is collected and released from above digester. 

- Advantages: Low initial cost and running costs (well insulated hence less energy required), long life (no metal parts). 

-Disadvantages: Gas pressure likely to fluctuate and sealing of gas not secure.