Waste treatment in this fashion uses the same process as that which naturally occurs in landfills. In an oxygen free environment it reduces organic waste to a relatively stable solid residue (digestate) similar to compost. Anaerobic digestion is particularly suited to wet, organic material and as such has been used for the treatment of sewage sludge for over a century.

The organic portion is separated from the waste to remove plastic, glass and metals and then placed in a sealed reactor. The conditions necessary for biological degradation are created which allows for the production of biogas, a mixture of methane and carbon dioxide. This can then be used as a fuel. The methane is generally burnt on site for heating or to produce electricity on a small scale. The digester will require an energy input to retain the material at elevated temperatures (between 20 and 40°C - the mesophilic range, or up to 60°C - the thermophilic range) in order to speed digestion. Some of the energy available from the methane generated can be recycled for this purpose, typically 20-40% of the total energy produced. Alternatively the process can produce solid fuel, a fibre product for board manufacture, or more commonly the residue is used as a soil conditioner. (Warmer Information Sheet: Anaerobic Digestion).

If no other market is available the product material is sometimes sent direct to landfill as a biologically inactive waste or as daily cover. But if it is to be used as a soil conditioner then it needs to be ensured of being free of contaminants. There is the risk that the digestate will contain high proportions of heavy metals. Anticipating this, it is necessary to carefully oversee the feedstock to the reactor. Also to be effective it will need to contain sufficient quantities of nitrate and phosphorous. For use as a fertiliser the digestate may still need to undergo a final brief composting stage to ensue complete breakdown of the material. It can then be used on golf courses, roadsides, forestry etc.

This treatment option is done on a broad variety of scales. Always done in batches which using a thermophilic process can take between two and three weeks (or a standard single-stage mesophilic digester which to complete the process is quoted -Warmer Information Sheet - to take between 12 and 25 days). The digesters can contain between only a matter of tens of tonnes and 3,000 tonnes. More than one digester would likely be used alongside each other to boost the output of the operation. The process achieves a 5% reduction in weight.

An analysis can be conducted for anaerobic digestion in a similar manner to the previously used technique to estimate the potential of generating electricity by using all the UK's available waste. There is about 90Mt of waste produced in the UK each year. 62% of this waste stream is said to be biodegradable (DETR, Limiting Landfill, 1999). This biodegradable waste will produce about 150m3/tonne of biogas at 60% methane concentration (Warmer Information Sheet, 1998). Using a 70% process efficiency, 70% load factor, and the known 37GJ/tonne energy content for methane, an estimate can be produced. After accounting for the 20-40% of energy needed to maintain the digestion is accounted for, anaerobic digestion could provide the UK with about 1.4GW. This represents about 1.9% of the UK's installed capacity.

Comparing to the other processes anaerobic digestion solely for the purpose of electricity generation is about twice as productive as total landfill gas power, but only a third as efficient as mass burn, and only a fifth as fruitful as gasification.

From an ecological point of view it is quite a sound process. Impacts related to visual intrusion, pests and noise will be similar to other waste management options and with proper planning can be minimised to acceptable levels. The input of waste, seen as a liability, can be reduced to a saleable soil conditioner. All the greenhouse gas generated is burnt for energy recovery rather than letting some of it escape to the atmosphere as would occur in landfill. CO2 is emitted but as it comes from organic material this has a short carbon cycle and so has no overall environmental impact. However, as this waste management option only deals with part of the waste stream, it is not on it's own going to solve the sustainable waste management dilemma.

Anaerobic digestion has not taken off as a waste treatment and disposal option in the UK mainly due to the lack of market for the produced soil conditioner.