Figure 1 is a picture of a biogas plant. Briefly explain how biogas is generated.

Biogas is produced by the anaerobic decomposition of animal and plant waste in the presence of water. In this oxygen-free environment, bacteria break down the waste and release methane-rich gas. The mixture is fed into an underground digester tank through an inlet pipe. As bacteria ferment the waste, biogas is collected through a gas outlet, while the leftover slurry is removed via an outlet pipe.