Introduction to Anaerobic Digestion

 

The process of Anaerobic Digestion (AD) involves the breakdown of organic material by bacteria in an oxygen-free environment. This can occur in bogs, landfills, on the bottom of lakes or in stomachs of animals such as cattle. The end product is biogas.

 

Biogas
 

Biogas is a mixture of methane (50-75%), carbon dioxide (25-45%) and small amounts of water (2-7%), as well as trace gases such as sulphur hydrogen, oxygen, nitrogen, ammonia and hydrogen.

Farm, municipal or industrial-based AD plants convert waste material into biogas. Waste/feedstock is pumped into a closed vessel (digester) which has been inoculated with suitable bacteria. Anaerobic (0% oxygen) conditions are then maintained in the vessel and the temperature is held at a constant value (typically 40oC).

The biogas produced can be upgraded to fossil ('natural') gas quality, but is normally used on site to generate heat and power electricity.

The AD process residue or digestate can be separated into a liquid fraction and a fibrous fraction.The liquid fraction can be returned to the land as a fertiliser and the solid fibre used as a soil conditioner.

Average composition of biogas

ComponentChemical symbolConcentration
MethaneCH450 - 75 volume - %
Carbon dioxideCO225 - 45 volume - %
Water vapourH2O2 - 7 volume - %
OxygenO2< 2 volume - %
NitrogenN2< 2 volume - %
AmmoniaNH3< 1 volume - %
HydrogenH2< 1 volume - %
Hydrogen sulphideH2S20 – 20.000 ppm
[ppm: Parts per million; Volume. - %: volumetric percentage]
 
Methane
 

Methane in biogas corresponds chemically to natural gas and is the most energetic component of the gas. One cubic metre (m3) of methane has an energy content of approximately 10 kilowatt hours (9.97 kWh) or 36 MJ/m3. The energetic use of a cubic metre of biogas with a methane content of 55% is approx. 5.5 kWh (app. 21 MJ/m3).

Thus the average heat value of a cubic metre of biogas is approximately 0.55 litres fuel oil. The residues make an ideal fertilizer in arable farming/crop production.

 
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