Technical biogas terms

Biogas technology can be a challenging subject, especially for newcomers. You will quickly come across many technical terms you are usually not confronted with in everyday life.

We want to help you navigate the deep jungle of specific terms related to this interesting technology and have listed the most commonly used expressions in alphabetical order below.


Biogas Gaseous product of anaerobic digestion containing usually 50-60 % methane (CH4) but also CO2, ammonia, hydrogen sulphide, water vapour and other components
Biomethane Biogas purified to a degree that the resulting gas contains up to 99% of CH4, 0-5% of CO2 and other components (i.e. trace gases, impurities, water vapour). Its properties are similar to those of natural gas, making it suitable for use as vehicle fuel and in heating applications.


Energy crops Biomass utilised for energy-related purposes. Agricultural raw material such as: maize, beet, grass, sorghum or green rye. Energy crops are ensiled before being utilised.
Manure Animal excrements usually with straw or other bedding material.
Silage Bulk storage of raw agricultural material by lactic acid fermentation. It destroys the wax layer and increases the digestibility and methane yield.
Sludge Muddy or slushy mass, deposit, or sediment such as precipitated solid matter produced by water and sewage treatment processes.
Slurry Liquid mixture of something such as mud, animal manure or dust.
Substrate/feedstock Raw material for digestion.

Biogas plant parts

Anaerobic lagoon Earthen basin used as digester or as digestate storage volume.
Biogas plant Plant designed specifically for the production, storage and utilisation of biogas.
CHP – combined heat and power unit Internal combustion engine coupled to a generator to convert chemically bound energy (biogas) into electrical and thermal energy.
Desulphurisation Chemical, biological, physical or combined method of reducing the concentration of hydrogen sulphide in the raw biogas before being burnt in the CHP or upgraded to biomethane.
Digestate storage tank (liquid-manure pond) Tank or pond in which liquid manure, slurry or digestate substrate is stored prior to subsequent use.
Digester Container where a substrate is microbiologically degraded and biogas is produced.
Dry fermentation Anaerobic digestion of substrate with DM > 25% (see below under “Biology process” for further info) that remains stackable througout the whole process.
Feeder Device that loads the feedstock into the digester.
Gas flare Safety device, used for burning the surplus of biogas when it can’t be used in the CHP unit.
Gas storage tank Gas-tight vessel or plastic sheeting sack in which biogas is held in temporary storage.
Hygienisation Additional treatment or disinfection that reduces/eliminates pathogens/phytopathogens in the substrate or the digestate.
Mixing pit Container used to pretreat the feedstock and mix it before its fermentation into the digester.
Post-digester / Secondary digester Container where the digestate treated in the first digester is stored for further fermentation.
Silage clamp Concrete structure with walls and a sealed base used for ensiling and storing feedstock (usually energy crops).
Solid infeed Method of loading non-pumpable structures or substrate mixtures directly into the digester.
Weighbridge Large scale used to weigh the truck carrying the incoming feedstock which is transported to the biogas plant area.
Wet fermentation Anaerobic digestion of wet substrate with DM < 25%. During fermentation the substrate becomes pumpable and mixable.

Biology process

Acetogenesis 3rd stage of the anaerobic digestion: degradation of short-chain fatty acids, acetic acid, CO2 and hydrogen with formation of CO2.
Acidogenesis 2nd stage of the anaerobic digestion: degradation of dissolved polymers like sugar, amino and fatty acids with formation of H2.
Ammonia NH3 – nitrogeous gas arising from the degradation of nitrogen-containing compounds such as protein, urea and uric acid.
Anaerobic digestion Biotechnological process taking place in the absence of air (atmospheric oxygen) with the aim of degrading organic matter.
Biodegradation Breaking down of organic matter, e.g. plant and animal residues, into simpler compounds by microorganisms.
Biogas yield [m³/t oDM] or [m³/t FM] Biogas volume produced by the specific substrate.
Buffer capacity Amount of acid or bases which can be absorbed without any significant pH-value changes.
C:N ratio Mass ratio of total carbon to total nitrogen in organic matter.
Co-fermentation Fermentation of feedstocks which are not the main feedstock of the biogas plant; usually used for co-fermenting organic wastes with manure or with sewage sludge
Co-substrate Feedstock for digestion which does not account for the largest percentage of the feedstock.
Digestate Residue (liquid or solid) from biogas production.
Dry matter (DM) [%] Moisture-free content of a mixture of substances after drying at 105 °C.
FOS [g CaCO2/l] (=Freie organische Säuren) – free organic acids.
Hydraulic retention time (HRT) [d] The average number of days the feedstock remains in the digester.
Hydrolysis 1st stage of the anaerobic digestion: degradation of proteins, polysaccharides, carbohydrates and fats with formation of CO2 and H2.
Inhibitor Substances/compounds that slow down or stop the biological process.
Mesophilic digestion Anaerobic digestion taking place at temperatures between 37 and 42 °C.
Methane content [%] Percentage of methane in the raw biogas.
Methanogenesis 4th stage of the anaerobic digestion: degradation of the products from the previous stages with formation of methane, CO2 and water.
Organic loading rate [kg VS/(m³*d)] Amount of volatile solids (VS) fed into a digester per day in relation to the volume of the digester.
Percolation liquid (leachate) Circulated liquid in garage type dry digestion which runs through the feedstock continuously to keep the fermentation process going.
pH-value Numeric scale that specifies the acidity/basicity of a solution that goes from 0 to 14. Solutions with a pH value of 7 are neutral, <7 are acid and >7 are basic.
TAC [g HACeq /l] Total inorganic carbon; indicator for the alkaline buffer capacity.
Thermophilic digestion Anaerobic digestion taking place at temperatures between 50 and 57 °C.
Total solids (TS) [%] See „Dry matter“.
Volatile solids (VS) [% of fresh matter] [% of dry matter] Also: Organic dry matter (oDM or oTS); the volatile solids is what remains after the water content and inorganic matter have been removed from the substrate.

Financial terms

Feed-in tariff Payment made to households or businesses generating their own electricity through the use of methods that do not contribute to the depletion of natural resources. It is proportional to the amount of power generated.
Flexible production Generation of electrical and thermal energy with the CHP unit when the income from their selling is higher. This kind of production needs additional biogas storage.
Gate fee Charge levied upon a given quantity of waste received at a waste processing facility.


Gate fee