When the landfill is complete, a final cover is placed over the entire surface. This minimises the passage of water into the landfill, reducing subsequent problems with leachate, and helps control the migration of gases from the landfill. The final cover also provides a further barrier between the waste and the environment, while allowing plant growth and landscaping. There has been a general move towards containing landfilled wastes through increased engineering. The principle compo- nents are usually the containment liner, its protection layer, leachate drainage layer and a top cover.
Many complex reactions can occur between the extremely heterogeneous components of landfilled waste.
Biological processes Generally, more than half of household waste is organic. Finally, conditions may become aerobic again as the landfill becomes more stabilised. Chemical processes Two general types of chemical reactions take place within landfilled waste. Firstly, oxidation, using trapped oxygen, which soon becomes depleted. Secondly, acid-metal reactions, due to the presence of organic acids and CO2.