Anaerobic Digestion
...rbon dioxide. This is because methane can be burnt as a fuel to produce energy, giving-off carbon dioxide, but displacing the need to burn carbon-emitting fossil fuels. This option also reduces the amount of organic waste for final disposal – whilst composting reduces the volume of waste by 30%, anaerobic digestion can reduce it by 50%. There are a few anaerobic digestion plants working in the UK, mainly at sewage works. Many organic wastes, such as sewage sludge, animal slurry, food waste, and separated household organic wastes, can be digested. They are stored in sealed vessels at the site. Then they are processed to produce an organic 'soup' that is mixed with bacteria to activate it. Then activated soup is then fed into the digester. The digester is a large sealed vessel that is heated to the optimum temperature for the bacteria to work. Most systems are mesophilic – they use bacteria, like the gut bacteria of ruminant animals, that live at around 37oC. Some systems use specialised bacteria that work at higher temperatures – called thermophilic systems – but these are not common because the energy balance isn't so good for power production. The content of the digester is stirred to prevent settlement. As the bacteria work, bio-gas bubbles off. The gas ...