Biogas Glossary

Ammoniacal Nitrogen (N-NH4 ou N-NH3)

A physico-chemical parameter expressing the concentration of a sample in ammoniacal nitrogen in its aqueous form (N-NH4) or its gaseous form (N-NH3). The analysis of ammoniacal nitrogen is essential in process monitoring and to know the value of the input. Ammonia being toxic exceeded a certain level in digesters, it is important to monitor it closely. Ammonia originates primarily from the digestion of proteins and its initial presence in the input. It is however essential to distinguish the toxicity rate applicable to free ammonia and not necessarily to ammoniacal nitrogen. It is therefore necessary to calculate the free ammonia content as a function of pH and temperature.


This value allows us to standardize the ratio of ammonia to total nitrogen of the input.


Process of the treatment of organic matter by fermentation in the absence of oxygen. The process of biological degradation is carried out in one or more anaerobic digesters. The output is a digestate, in the form of a more or less liquid fraction, as well as biogas.


Maximal potential production of biogas by a substrate (m³ biogas/tons of VS).


A gas produced by the fermentation of organic matter in the absence of oxygen. Crude biogas refers to the gaseous effluent discharged from an anaerobic digester or biomethanizer. Biogas consists of 60 to 80% methane (CH4), 30 to 40% carbon dioxide (CO2) and other trace gases, such as hydrogen sulphide (H2S), ammonia (NH3), and hydrogen (H).


A process that reduces the concentration of contaminants in biogas, such as water, carbon dioxide, hydrogen sulphide, ammonia, etc. Primary purification refers to the process that reduces the concentration of hydrogen sulphide and water in the biogas. Secondary purification refers to the process that reduces the concentration of carbon dioxide in biogas resulting in biomethane (conditioning, treatment, purification).


Gas obtained by the purification of biogas that can be injected into a gas network or used as a replacement for natural gas.


A sealed container or tank, where the biological anaerobic digestion of animal manure or organic matter occurs and from which results the production of biogas.

Buffer Capacity

Indicates the ability of the environment to be influenced by bases or acids. Used to characterize inputs and to monitor digesters’ health. A too low buffer capacity means that the organic matter is poor and poorly buffered, resulting in faster digestibility. A buffer capacity that is too high can indicate the presence of organic acids or buffered compounds, such as proteins.


The C/N ratio represents the carbon portion of the organic material on the total nitrogen portion. It is usually calculated for inputs and digestate. It will be closely related to the other Nprot/Ntot and N-NH3/Ntot values.

Carbon & Degradability Profile

The degradability is calculated by a carbon balance: Carbon input (Feed MO) – Carbon present in the digestate = Degraded carbon. This balance will be broken down by a calculation of the carbon balance produced by the carbon of CH4, the carbon of the CO2 and the metabolites produced.


A controlled temperature, constant volume, mechanically mixed vessel designed to maximize biological treatment, methane production, and odor control as part of a manure management facility with methane recovery.


The biological decomposition and stabilization of organic matter under conditions which allow the development of elevated temperatures as the result of biologically produced heat. When complete, the final product is sufficiently stable for storage and application to land without adverse environmental effects.


Non-biodegradable material present in SSO and which does not contribute to its methanogenic potential.


DGE is a way to measure the required volume of an alternative source of energy in order to be comparable to the energy potential of diesel. Thus, DGE is a way to evaluate the CNG vehicle storage required.


Liquid, pasty or solid residue derived from the biomethanization of organic materials. The raw digestate denotes the effluent at the exit of the biomethanizers. The dehydrated digestate is the solid fraction produced at the dehydration step (solid-liquid separation of the raw digestate). The dried digestate refers to the digestate which has undergone the dehydration and drying processes.


Dry materials (DM) is what is obtained when water is removed from a product.


Liquid and solid material fed to the digester.


Organic residual materials used as fertilizers in agricultural, horticultural and forestry applications or for the rehabilitation of degraded sites.


An atmospheric gas, which is transparent to incoming solar radiation but absorbs the infrared radiation emitted by the Earth’s surface. The main greenhouse gases are carbon dioxide, methane, and CFCs.


The average length of time the liquid influent remains in the digester for treatment. HRT may go up to 50 days.


Stage during which the macromolecules (proteins, lipids, carbohydrates) are hydrolyzed to monomers.


Stage of conditioning the inputs or the digestat which consists in heating them during a given time, to reduce their content in pathogens (pasteurization).


Refers to organic matter from ICI.


Organic or non-organic chemical compounds essential for plant growth.


Psychrophilic : 15 to 25 °C (opt. : 20°C)

Mesophilic : 30 to 40 °C (opt. : 37°C)

Thermophilic : 50 to 60 °C (opt. : 55°C)


Amount of organic matter arriving at the anaerobic digestion system every day, expressed in kg of volatile solids per day per cubic meter of digester (kg VS/d/m³). This feeding rate is calculated based on system performance and the hydraulic retention time (HRT). This dictates the nutritional pressure of VS applied to the bacteria. The higher the OLR, the less the digestate will be degraded and the more likely it is to reduce the burden of methanogenic microorganisms. A low OLR with high HRT may create metabolites lethal to methanogens. An OLR of between 2.5 and 4 kg VS/J/m³ in mesophile and between 4 and 6.5 kg VS/J/m³ in thermophile complies with the sound operation of a digester.

Physico Parameters

Parameters allowing to control the quantity and quality of inputs. These parameters can also predict trends during atypical feeding (pH, Buffer capacity, Redox, FOS-TAC, C/N, Ntot, Nprot, N-NH3, Nprot/Ntot, N-NH3/Ntot, buffer capacity/Nprot).


A constant volume, flow-through, controlled temperature biological treatment unit designed to maximize biological treatment, methane production, and odor control as part of a manure management facility with methane recovery.

Potential of Hydrogen (PH)

The pH indicates, by a regular follow-up, the good health of each stage of the anaerobic digestion process.

Production and %CH4

System performance measures. These should be as stable as possible. They reflect performance and accuracy in the stability of other parameters, so they are a consequence, a symptom.


Protein nitrogen is used to characterize the protein content of inputs and digestate. The difference between the two measures informs us about the proportion of degraded proteins.


Term used to describe the use of organic matter in agricultural, horticultural or forestry applications or for the rehabilitation of degraded sites.


Reduction-Oxidation potential.


Organic vegetable and animal materials derived primarily from the preparation, consumption and distribution of food and beverages and sorted at the place where these residual materials are produced, generally sorted by municipalities and ICIs.

Status Parameters

Allow to track and control the process in a stable and secure manner (HRT, ORL, CH4, pH, T°C, Buffer capacity, Redox, FOS-TAC, Carbon balance, N-NH3, N-NH3/Ntot, N-NH3/CT).


Total nitrogen is a measure used to characterize inputs, but it remains a vague measure because it will have to be broken down by a calculation of protein nitrogen (Nprot) and ammonia nitrogen (N-NH4). The latter will, however, be a parameter for monitoring the process.


Physico-chemical parameter expressing the rate of solids in a liquid sample.


Use of a product in a value-added application.


These are produced in the digester by acid-forming bacteria and then used by the methane-forming bacteria to produce methane.

Volatile Fatty Acids (VFA)

An analysis of the Volatile Fatty Acids (VFA) profile allows to identify an unstable or even toxic biochemical state. Because short chain fatty acids are lethal to some bacteria this can impair digestion and production. Such imbalance could also create, under certain conditions, a problem of foaming. Analysis of the VFA profile is not done on a regular basis but rather in case of problems, quality control or when using a new input.


Physico-chemical parameter expressing the rate of volatile solids in a liquid sample.