The methanisation , or anaerobic digestion , is the biological natural process of degradation of the Organic matter in absence of oxygen. It is found in the sediments, the marshes, the rice plantations, like in the digestive tract of certain animals (termites, ruminants,…). The degraded organic matter is found mainly in the form of Biogaz (with more than 90%). The remainder is used for the growth and the maintenance of the micro-organisms.

Methanisation plays a big role in the Cycle of carbon and could contribute to the climatic Modifications. The great quantities of methane present in the form of Hydrate of methane in the Pergélisol S and the Sédiment S sailors, could if they were brutally salted out to accelerate climate warming.

“Methanisation”, natural process, should not be confused with the “Méthanation”, industrial Procédé of catalytic conversion of the Hydrogène and the Carbon monoxide into Méthane.

Biological process

Methanisation is ensured thanks to the joint action of Microorganismes pertaining to different microbial Populations in interaction constituting a network Trophique. Three successive phases classically are distinguished:

• the Hydrolysis and the Acidogénèse
• the Acétogénèse
• the Méthanogénèse

The hydrolysis and the acidogénèse

The complex organic matter is first of all hydrolized in simple substances. The decomposition is carried out by enzymes exocellulaires and can become the limiting stage in the case of made up with difficulty hydrolysables such as cellulose, the starch or greases. Then, these substrates are used at the time of the stage of acidogénèse by the species known as acidogenes, which will produce alcohols and organic acids, as well as Hydrogène and Carbon dioxide.

The acétogénèse

The stage of acétogénèse allows the transformation of the various compounds resulting from the preceding phase into direct precursors of methane: the Acetate, carbon dioxide and the Hydrogen. One distinguishes two groups from bacteria acetogenes:

• the producing bacteria obliged of hydrogen , anaerobes strict, also called OHPA (“Obligate Hydrogen Producing Acetogens”). They are able to produce acetate and of H₂ starting from the metabolites reduced resulting from the acidogenèse such as propionate and the butyrate. The accumulation of hydrogen leads to the stop of the acétogenèse by bacteria OHPA. This implies the need for a constant elimination of produced hydrogen. This elimination can be realized thanks to the syntrophic association of these bacteria with micro-organisms hydrogénotrophes.
• the bacteria acetogenes not syntrophes whose metabolism is mainly directed towards the production of acetate. They develop in the mediums rich in carbon dioxide. The bacteria “homo-acetogenes” belong to this group, they use hydrogen and carbon dioxide to produce acetate. They do not seem to enter in competition for hydrogen with the Archaea methanogenes hydrogénotrophes and are present in quantity much smaller in the anaerobic biotopes.

The méthanogénèse

See also: Archéobactérie méthanogène

The méthanogénèse is ensured by strict anaerobic micro-organisms which belong to the field of the Archaea . This last stage leads to the production of Méthane. It is carried out by two possible ways: one starting from hydrogen and of the Carbon dioxide by the species known as hydrogénotrophes , and the other starting from acetate by the species acétotrophes . Their growth rate is weaker than that of the Bactéries acidogenes.

CO₂ + 4:00 ₂ → CH₄ + 2:00 ₂O

CH₃COOH → CH₄ + CO₂

Physicochemical conditions

Methanisation is a biological process complexes which requires the installation of certain physicochemical conditions for which the biological reaction is optimized. The Archaea methanogenes are strict organizations Anaérobie S. They develop in a satisfactory way when the Potentiel of oxydoreduction compared to the normal electrode with the hydrogen (Eh) of the medium is very low (- 300 mV). Methanisation generally takes place in mode mésophile (30-40°C) or thermophilous (45-60°C), in a range of pH ranging between 6 and 8 with an optimum between 6,5 and 7,2. The Archaea methanogenes have requirements out of particular trace elements like the Fer, the Molybdène, the Nickel, the Magnésium, the Cobalt, the Cuivre, the Tungstène and the Sélénium. The Pression partial hydrogen must remain in lower part of 10^-4 bar in gas phase.

Methanisation and depollution

Methanisation, as a Bioprocédé, can be implemented in a Digesteur, to cleanse rejections charged in Organic matter while producing energy in the form of methane. Methanisation makes it possible to treat rejections as various as waste water, muds of purification plants, the animal manure, waste of the food industry, the agricultural household refuse, waste, etc Methanisation with valorization of the Biogaz produced (thermal and/or electric energy production by combustion of methane) has all its place among the whole of the various solutions of production of renewable energy while making it possible to achieve two complementary goals: to produce energy while reducing the polluting load of waste and the organic effluents and also, according to the nature of the starting product, to produce a Digestat stabilized usable like Fertilizer or organic amendment.

Today the principal industrial applications profiting from the contributions of methanisation for the treatment of their rejections are identified by the Government agency Of the Environment and the Control of Energy (ADEME) as being: agricultural digestion (animal manure), the digestion of domestic and comparable solid waste (biodéchets), the digestion of the Clarification sludge urban and the digestion of the industrial effluents. Concerning this last scope of application, methanisation is a very competitive treatment compared to the aerobic purification. It is applied mainly to treat the effluents of industries Agroalimentaire S strongly charged and the effluents with the Fermentation (75% of the digesters with strong load in operation in 2006).

The use of the methane, produced starting from the methanisation of muds of purification plant, for the operation of the urban buses makes important great strides in certain town of France like Lille. The improvement and the reduction of the costs of the membrane techniques of separation of gases should make it possible to consider the possibility of a purification of biogas on the production site.

Methanisation of liquid mixtures

Soluble methanisation of organic matter or in suspension in a liquid phase is adapted to several products which one has in this form, which can be supplemented by solid waste. The basic commodities are then frequently the effluents of breeding (Lisier S) and the Clarification sludge (often of mixed muds which gather primary muds and biological muds). These primary products one has in general systematically can be supplemented various organic waste, and in particular of greases whose capacity methanogene is strong (exits for example of slaughter-houses, or the pretreatment of the purification plants). The liquid state of the mixture allows a mixing to obtain a good homogeneity of the matter and temperature.

Methanisation of solid waste

The methanisation of solid waste applies to the majority of organic waste:

• Municipal: food waste, newspapers, packing, textiles, green waste, by-products of the urban cleansing;

• Industrial: muds of agribusiness industries, waste of transformation of vegetable and animal industries, fermentable fraction of the industrial waste banals (DIB);
• Agricultural: dejections of animals, vegetable substrates solid.

One generally speaks about solid methanisation (comparable with digestion in dry medium) when waste to be digested has a dry matter rate to the top of 15-20%. The advantages of such a process compared with that, traditional, treatment in liquid medium are for example the space saver, the facility of treatment of the produced digestat, the saving in water and the economy in term of heating (if need be) thanks to the reduction of volumes of the digesters.

Since about thirty years, the industrial facilities treating this type of waste showed their economic profitability. According to their source, waste can undergo a pretreatment such as: separation, sorting, reduction of the size by crushing, sifting by the size or pasteurization.

At the end of 2002,78 production facilities of methanisation of household waste and comparable were in service in Europe for a capacity for treatment of 2,3 million tons of waste per annum. The new capacities installed into 2002 rise with 813.000 tonnes/an.

France was the first country to be launched out in the methanisation of household waste in 1988 to Amiens, and counts since 2002 an additional unit in Varennes-Jarcy (the Essonne). One to two projects should concretize itself in the Seine Saint Denis.

The “outgoing Digestat” of the digester or engine is a treated and stabilized waste. The organic soil conditioner of this residue resulting from the methanisation of fermentable waste is of good quality and can be used for or not food food cultures like the green areas according to the regulation, the nature of the treated products and them analyzes of this digestat.

See too

Related articles

• Purification of water
• W: Sewage treatment

External bonds

• the Laboratory of Biotechnology of the Environment: Laboratory of INRA
• an application of the methanisation of Volta to create a company of depollution (video educational on the site of the Educational Channel to the Request)

• Methanisation.info Site of information on methanisation

Random links:

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