Landfill. Under the hierarchy, landfill is seen as the least desirable option. Biological processes within the landfill ensure that, over a period of time, any biodegradable waste is degraded, neutralised and stabilised to form an essentially inert material. However, methane and carbon dioxide which are ‘greenhouse gases’ are generated throughout the degradation period. The European Union, through the Waste Landfill Directive (Council Directive 1999/31/EC 1999) has set targets for the reduction of biodegradable waste going to landfill, to encourage more recycling and to reduce emissions of the greenhouse gases. Where disposal to landfill occurs, the process is controlled, ensuring that human health is not endangered or harm to the environment does not occur. Landfill sites are often used mineral workings, which are required to be infilled after use and consequently, the disposal of certain types of waste such as treated and inert wastes into landfill, can be beneficial and eventually result in recovered land. A further major consideration for landfill disposal is the leachate, the potentially toxic liquid residue from the site, which may enter the water course.