Around 100 million tonnes of solid waste is sent each year to landfill sites in England and Wales. This is 25 per cent of all waste and 60 per cent of MSW. The long-term aim of landfill management is to reach a point where management and monitoring is no longer necessary and the land can be reused. Landfill sites may take over a century to reach 'completion' or stabilisation, which is when their undisturbed state poses no further threat to health or the environment.
Waste in traditional UK landfill sites goes through several stages, including anaerobic stages where microbial activity - such as fermentation - degrades most of the waste. Disadvantages of this method are that there is a long period after the majority of degradation has taken place, but gases, such as methane, and leachate (percolating liquids) are still being produced. These sites must be managed and monitored for years, if not decades. Advantages of anaerobic methods are that the methane can be sold as a source of energy for up to 10-15 years and some pesticides degrade better.
As an alternative to anaerobic decomposition, remediation of old landfill sites by aerating the waste has been undertaken in Germany, the United States, Italy and The Netherlands, with considerable success. At a pilot scale, aeration has also been used in new waste to accelerate stabilisation. Advantages of aerobic landfill techniques are that stabilisation is reached more rapidly; there is a reduced after care period by several decades; and less leachate produced due to evaporation. The amount of methane produced is also reduced, although emissions such as carbon monoxide may increase, instead. Odours at aerobic sites are less pungent and are of an organic rather than chemical nature: hydrogen sulphide and ammonia odours are typical of anaerobic sites. Disadvantages of aerobic techniques are that toxic metals are not immobilised to the same extent as they are in anaerobic sites, and it is a more expensive method to carry out.
The authors point out that the current EU Landfill Directive1 requests Member States to divert bio-degradable waste from landfills at the rate of 25 per cent (of 1995 values) by 2006, 50 per cent by 2009 and 65 per cent by 2013. The best option may be a hybrid system that uses aerobic techniques to accelerate the stabilisation of a site after it has been anaerobic for about 20 years. Late aeration allows energy generation from methane to continue and would not involve aerating the site throughout its life. It would also mean certain pesticides had already been degraded and toxic metals immobilised.