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

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.