### ZIMSEC O Level Combined Science Notes: Biogas as a source of energy

• Biogas is the gas that is obtained from fermenting manure and other organic waste
• It is a mixture of methane, carbon dioxide and hydrogen sulphide
• Cattle manure is commonly used as a waste to feed biogas plants
• Small plants have been built in some parts of Zimbabwe
• Manure (from both cattle, other animals and humans) as well as organic waste is put into a sealed pit
• Bacteria digests the manure and the gas is given off, collected and then piped to burners
• The sludge in the pit has to be agitated by rotating the floating gasometer tank on top several times a day
• Eventually the sludge collected has to be  removed and the process started again
• The sludge can be used as a fertilizer for crops
• The bacteria responsible for the fermentation of waste requires an environment lacking oxygen
• $\text{organic waste}\xrightarrow{fermentation}\text{methane + carbon dioxide}$
• Biogas is not easy to store so it is used as it is produced

Biogas plant design

Factors affecting the production of biogas

• Several factors affect the amount of gas that is produced by a biogas plant
• These factors include:
• The type of waste
• The temperature-optimum temperatures for the production of biogas are between 35°C and 55°C
• The pH of the solution which should not be too acidic or too alkaline
• Metals such as copper and zinc will stop the fermentation of the waste by bacteria
• Time-the length of time that the waste is in the digestor will depend on the type of waste and factors mentioned above

Uses of biogas

• Biogas has several uses including:
• cooking
• refrigeration
• in gas lights

To access more topics go to the Combined Science Notes page.

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