Biomethanisation is a process of fermentation similar to what takes place in a cow’s first stomach. Materials entering a digester (fermentation tank) undergo biological degradation by microorganisms. This fermentation has the particularity of occurring in the absence of oxygen (anaerobic digestion). The decomposition of materials by biomethanisation produces two types of outputs: biogas and digestate.
Biomethanisation requires large quantities of inputs mostly produced in sectors generating high waste volumes. This is why main users are farmers, cooperatives, industrialists or municipalities.
Different biomethanisation materials
Almost all organic materials can undergo the biomethanisation process, each with a different yield. Only highly woody materials should be avoided. Input feedstocks determine the types of biomethanisation:
- Treatment of agricultural materials and organic waste from the food industry: materials can be liquid (slurries, whey, etc.), liquefiable solids within a few days of digestion (fruit and vegetable waste, corn silage, grass or some straw manure), or solid (straw or certain types of manure);
- Liquid effluent treatment: industrial wastewater, sewage sludge, etc.;
- Household waste treatment: household waste has a high organic content, but requires prior sorting;
- Technical landfill: Landfills that have previously received organic biodegradable waste are now covered, allowing the recovery of the biogas that escapes. However as biodegradable waste should be collected more and more precisely, the following process will disappear in the coming years.
The degradation of raw materials by biomethanisation leads to the creation of two types of products:
- Biogas, which is a gas composed mainly of methane (CH4) and carbon dioxide (CO2). It has many energy uses: it can be used in cogeneration installations, injected into a natural gas network or even be used as biofuel (biomethane).
- The digestate, which is the remaining matter organic left after digestion, is mainly used as a fertiliser for soils.