A glossary of terms commonly used in connection with the anaerobic digestion of organic waste materials
ACIDOGENESIS: The second stage of the conversion of large organic molecules to volatile fatty acids.
AERATION: The process of exposing bulk material, such as digestate or solid wastes, to intimate contact with air.
AEROBE, facultative: An organism that can respire in either the presence or the absence of atmospheric oxygen.
AEROBE, obligate: An organism that requires free (atmospheric) oxygen for respiration.
AEROBIC BACTERIA: Bacteria which live and reproduce usually in an environment containing oxygen, such as atmospheric oxygen, which is available for their respiration.
ANAEROBE, obligate: An organism that cannot tolerate the presence of atmospheric oxygen.
ALCOHOL: The family name of a group of organic chemical compounds composed of carbon, hydrogen, and oxygen (C2H6O) which occurs widely in nature and is used in solvents, antifreezes, chemical manufacture, and as a fuel. Alcohol commonly is obtained by fermentation.
ALDEHYDE: An aldehyde is a highly-reactive chemical compound created by oxidizing different alcohols to make resins and organic acids.
ANAEROBIC: Without the presence of free oxygen.
ANAEROBIC DIGESTER: A reactor that is constructed to effect the degradation of organic matter by anaerobic bacteria.
ANAEROBIC DIGESTION The degradation and stabilisation of organic materials brought about by the action of anaerobic bacteria with the production of biogas (bio-methanisation).
BATCH FEED: A process by which the reactor is filled with feedstock in discrete amounts, rather than continuously.
BIODEGRADABLE: Capable of being broken down biochemically by the action of micro-organisms.
BIOGAS: A mixture of gases, predominantly methane and carbon dioxide, produced by anaerobic digestion.
BIOGAS UPGRADE: Refers to the process of purifying and compressing biogas to make it suitable for use in CNG, and for injection into natural gas mains.
CALORIFIC VALUE: The amount of heat that can be obtained from a fuel, usually expressed in terms of kilojoules per unit weight of the fuel.
CARBON DIOXIDE: A colourless, odourless, non flammable gas and one of the two main constituents of biogas; chemical formula CO2.
COMPOST: Relatively stable decomposed organic material that has undergone aerobic action to transform the material to a humus.
COMPOSTING: The aerobic stabilisation of organic material (including digestate) for example by mechanical turning (windrowing) or by static pile aeration.
CONTINUOUS-FEED DIGESTER: A digester which is regularly charged with small quantities of fresh feedstock, which automatically displaces an equal volume of spent material, the process continuing without interruption.
CONTRARIES: Material unsuitable for anaerobic digestion which is ideally removed from the feedstock before it enters the process.
DEGRADATION: A particular type of gradual decomposition that usually proceeds in well-defined stages to give products with fewer carbon atoms than the original compound. The term is often applied to decomposition resulting from the action of micro-organisms.
DENITRIFICATION: Anaerobic reduction of nitrogen compounds, such as nitrates, to elemental nitrogen.
DEWATERING: The process of removing water from the effluent slurry of a digester by, for example, centrifuge or filtration.
DIGESTATE: The sludge or spent slurry discharged from a digester. May also refer to a crystalized residue left after evaporation of the water.
DIGESTER: A unit such as an enclosed tank, cylinder or silo in which anaerobic digestion of organic wastes takes place.
EFFLUENT: The liquid produced following dewatering of the digestate from a digester.
ENZYME: A complex organic substance (protein) produced by living cells and having the property of accelerating biochemical reactions.
FACULTATIVE: The ability of micro-organisms to live under either aerobic or anaerobic conditions.
FEEDSTOCK: Largely organic material fed into a digester, including solid and liquid wastes.
FERMENTATION: Changes in organic matter or organic wastes brought about by micro-organisms, part of the fermentation taking place in a closed system, i.e. a digester or drum.
GASHOLDER: A separate structure that receives and stores biogas produced in a digester.
GRIT: Heavy mineral matter often present in digester feedstock, such as sand, gravel and cinders, which may accumulate in the bottom of the digester.
HALOPHILIC: Primitive bacteria tolerant of high levels of salts.
HUMUS: The end product of a composting or digestion process.
HYDROGEN SULPHIDE: A colourless, odorous and corrosive gas which is found as a minor constituent of biogas; chemical formula H2S.
HYDROLYSIS: The first stage of the breakdown of organic matter by anaerobic bacteria.
INOCULANT/ INOCULUM Any material, such as previously digested feedstock, that is added to incoming feedstock, to hasten the degradation of organic matter and the production of biogas through the introduction of appropriate micro-organisms.
INORGANIC MATTER: Material, such as grit, inorganic salts, metals, glass etc., which is not degraded by micro-organisms.
MESOPHILIC: Moderate temperature range, 20-45OC, usually 35OC is considered optimum.
METABOLISM: The biochemical changes in living cells by which energy is provided for vital processes and activities, and new material is synthesised.
METHANE: A colourless, odourless, flammable gas and a main constituent of biogas; chemical formula CH4.
METHANOGENS: Methane-forming bacteria which use acetate and hydrogen as energy sources.
METHANOGENESIS: The final conversion stage of volatile fatty acids and hydrogen into biogas.
MOISTURE CONTENT: The quantity of water present in sludge and screenings which is usually expressed as a percentage of the wet waste.
ORGANIC MATTER: Material from animal and vegetable sources which can be degraded by micro-organisms.
PATHOGEN: An organism which causes disease.
pH: A measurement of the concentration of hydrogen ions in a solution used as an indicator of acidity (pH<7) or alkalinity (pH>7).
PUTRESCIBLE: Liable to rot or decompose, the best example being food waste.
RETENTION TIME: The period of residence in a given volume or unit. It is normally calculated by dividing the active volume of the unit by the rate of flow of the feedstock through it. Commonly, the number of days that organic matter or bacteria remain in a digester.
SLURRY: A mixture of solids and liquid in a fluid state.
STABILISATION: The degradation of putrescible organic substances by aerobic and/or anaerobic microbic populations to yield a biochemically stable product.
THERMOPHILIC: Adapted to a relatively high temperature, in the range of 50-65OC.
TOXICITY: A condition which inhibits or destroys the growth or formation of organisms.
VOLATILE FATTY ACIDS (VFAs): The first degradation product of anaerobic digestion prior to methane creation.
VOLATILE SOLIDS: The organic matter in a sample, usually expressed as a percentage of the total solids.