Anaerobic digestion is the natural decomposition of organic matter in an anaerobic environment (without air). This degradation is performed by anaerobic micro-organisms under controlled conditions, including temperature, and leads to the production of the so-called biogas. This technology constitutes an interesting solution for the treatment of the biodegradable fraction of household waste (kitchen and garden waste and paper cardboards) as far as this fraction represents 60 to 65 % of the waste input to be treated. Processing organic solid waste by anaerobic digestion leads to:     

 Energy production in the form of biogas containing around 55 % methane,
Recycling of residual organic material in the form of a stabilised soil conditioner.

New regulations, ecological requirements and available technologies lead the municipalities to consider more and more the waste treatment according to a multi-process approach. If possible and considering economic requirements, this multi-process approach takes into account different techniques adapted to recovery and/or treatment of the different waste materials.

That is how simultaneous combinations can take place including the recycling of certain materials which are reintroduced in the industrial network (glass, metals, plastic, some types of paper), the biological treatment of the organic material (kitchen waste, garden waste, dirty paper), the incineration or the RDF (combustible) material (non recycled paper and plastics) and the dumping of non-valorizable materials or final residues from the previous treatments.

The biological treatment of the organic material is then naturally important in this multi-process approach in that non-stabilised organic material cannot be directly dumped. Furthermore, the incineration of the wet organic material cannot be considered as a recycling nor a valorisation process because of the destruction of the organic material and the low energy recovery by the combustion.

When a selective collection can be organised in order to obtain a source separated organic material almost free of refuses, a very high quality stabilised soil conditioner is available after treatment.

In the case of a bulk waste collection, an automatic sorting unit, upstream from the treatment, permits the separation of the organic material from the other waste.

Downstream from selective collection or downstream from bulk collection followed by mechanical sorting, the biological treatment of the organic material can be made by anaerobic digestion (oxygen-free fermentation) also called methanisation, or by aerobic composting or else by a combination of anaerobic digestion and aerobic composting.

Compared to usual composting, the anaerobic digestion (with or without composting) has the following advantages:       

the anaerobic digestion leads to the production of a high methane content gas: the biogas. This technique allows a net energy production with several ways of valorisation (steam, methane gas, electricity, fuel),

 the oxygen-free fermentation takes place in closed fermentation tanks called digesters, avoiding any smell nuisances. It is noticeable that during the fermentation process, the volatile organic smelly acids which are intermediate components of anaerobic digestion, are naturally transformed in biogas inside the digesters. In aerobic composting volatile fatty acids can be emitted into the air and must be recovered and treated in a biofilter.

this technique does not require a large land area.

The anaerobic digestion of the organic material is usually followed by a short aerobic phase (about two weeks) which completes the stabilisation of the digested matter. After this phase, the compost is fully stabilised (degree 5 of the German norm LAGA M10) and can be stored and commercialised. The possible excess process water, related to the incoming waste moisture, can be discharged into the sewage network, after an additional purification step if necessary, depending on local regulations.

Fields of application of anaerobic digestion processs