Appendix 2: Glossary

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Acid-forming Bacteria - The group of bacteria in a digester that produce volatile acids as one of the by-products of their metabolism.

Aerobic - In the presence of free oxygen.

Aerobic Bacteria - Bacteria which live and reproduce only in an environment containing oxygen which is available for their respiration, such as atmospheric oxygen or oxygen dissolved in water.

Alkaline - The condition in which there is present a sufficient amount of alkali substances to result in a pH above 7.0.

Anaerobic - Without the presence of free oxygen.

Anaerobic Bacteria - Bacteria that live and reproduce in an environment containing no free or dissolved oxygen.

Anaerobic Contact Process - An anaerobic digestion process in which the microorganisms are separated from the effluent slurry by sedimentation or other means and returned to the digester to increase the rate of stabilization.

Anaerobic Digester - A reactor which is constructed to degrade organic matter by anaerobic bacteria.

Anaerobic Digestion - The process of degradation and stabilization of organic materials by the action of anaerobic bacteria with the production of biogas (biomethanation). The process is very slightly exothermic (heat- producing).

Active Volume - The actual volume available in a digester for bacterial action. It is calculated by subtracting the volume occupied by grit and scum from the volume of the digester occupied by sludge.

Batch-Feed Digester - A digester which retains all the feedstock added in a single charge. Discharge of the entire batch occurs at the end of the retention time.

Benefits - Tangible benefits of a biogas system are those that are easily quantifiable and have a monetary value. Such benefits include the value of the gas and the digested "furry produced, both as fertilizer and as peat-moss substitute. Intangible benefits are those that are not so easily quantified or related to a monetary value. Examples include the value of an improvement in environmental sanitation, odour prevention et Biofeed - Solids recovered from digested sludge and used as animal fodder.

Biogas - A mixture of gases, predominantly methane and carbon dioxide, produced by anaerobic digestion. Traces of hydrogen sulphide and ammonia are present in varying quantities.

Biogas Plant - A plant used to process organic matter to produce biogas and sludge.

Buffer Capacity - A measure of the resistance to changes in pH caused by the compounds in the sludge.

Cabutz - The solid fraction that remains after the separation of the digested slurry from the thermophilic anaerobic digestion of cow manure.

Carbon dioxide - CO2, The gas resulting from burning or complete oxidizing of any carbon source.

Carbon monoxide - CO, The gas resulting from incomplete burning or incomplete oxidizing of any carbon source.

Calorific Value - The amount of heat that can be obtained from a fuel, usually expressed in terms of calories per unit weight (or volume) of the fuel.

Catabolism - Destructive metabolism involving the production of energy and resulting in the breakdown of complex materials within the organism.

Carbon/Nitrogen Ratio (C/N Ratio) - The ratio of organic carbon to that of total nitrogen.

Coliform - A rod-shaped bacterium found in intestinal tracts of most animals, which is often used as an indicator to detect faecal contamination.

Composting - Controlled decomposition of organic matter under aerobic conditions by which material is transformed to humic material. The process is exothermic resulting in a rise in temperature. The process is used to improve the quality of manure as organic fertilizer.

Continuous-Feed Digester - A digester which is regularly charged continuously or with small amounts of fresh slurry at short intervals; the freshly charged slurry automatically displaces an equal volume of effluent and the process continues without interruption.

Degradation - The breakdown of (organic) substances by chemical, physical, and/or biological action.

Denitrification - Anaerobic reduction of nitrogen compounds, such as nitrates, to elemental nitrogen.

Detention Time - The theoretical 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 liquid through it.

Dewatering - The process of removing water from the effluent slurry of a digester by evaporation or filtration.

Digester - The vessel in which anaerobic digestion takes place, which may be constructed also to store the biogas produced by anaerobic digestion.

Digester Slurry - Mixture of fermented organic matter and water.

Digestion - The controlled decomposition of organic substances, normally under anaerobic conditions.

Effluent - The sludge or spent slurry emerging from a digester.

Endothermic reaction - The chemical reaction in which energy is needed (or absorbed from the surrounding) in order for it to take place.

Exothermic reaction - The chemical reaction in which energy is liberated to the surrounding) when it takes place.

Enzyme - A complex organic substance (mostly a protein) produced by living cells and having the property of accelerating transformations such as digestion processes.

Facultative - The ability of microorganisms to live under either aerobic or anaerobic conditions.

Floating Gasholder - A biogas container consisting of an inverted open top tank floating over a liquid such as digester slurry or water; it rises when it fills with biogas and sinks as the gas is depleted. The weight of the floating cover controls the pressure of the gas which is discharged from the gasholder.

Gasholder - A separate system that receives and stores the gas produced in a digester.

Grit - Heavy mineral matter such as sand, gravel, and cinders, often present in digester feedstock which accumulates in the bottom of the digester.

Humus - The end product of a comporting process consisting mainly of humic acids, lignin and cellulose.

Hydraulic Retention Time - The average time that a liquid stays in a reactor before it is discharged. It is equal to the active volume of the reactor divided by the flow rate of the liquid entering it. It is usually expressed in days but may be as short as hours.

Inactivation - The process by which parasite eggs, wild fungi, pathogenic bacteria and viruses are rendered inactive and hence unable to propagate.

Influent - The feeding materials or slurry incoming to a digester.

Inoculant, Inoculum - Any material, such as digested feedstock, that is added to a newly started digester to start the degradation of organic matter and the production of methane.

Inorganic Matter - Material in solution or suspension, such as sand, salt, iron, calcium, and other minerals, which are not degraded by microorganisms.

Manure - Animal excrete, normally faecal matter from livestock.

Manure Slurry - The mixture of manure and water coming from livestock pens.

Mesophyllic - Within a moderate temperature range, normally 30-40C.

Metabolism - The biochemical changes in living cells by which energy is provided for vital processes and activities, and new material is synthesized (catabolism + anabolism).

Methane (CH4) - A colourless, odourless, flammable gas and the main constituent of natural gas, coal gas and biogas.

Methane Forming Bacteria - The group of bacteria in a digester that uses acetate and H2 as energy sources and produces methane.

Night Soil - Human faeces and urine collected by buckets or vacuum trucks.

Organic Matter - Materials which come from animal or vegetable sources. Organic matter generally can be degraded by microorganisms.

Pathogen - Disease-causing organism.

Peatrum - The sieved digested slurry after thermophilic digestion of cattle rumen content and manure.

Plug Flow - Movement without mixing in the axial (longitudinal) direction in a digester. The opposite of complete-mixing in digesters.

Retention Time - The number of days that organic matter or bacteria remain in the digester. See also detention time.

Sludge - The slurry of settled particles resulting from the process of sedimentation.

Sludge Digestion - A process by which organic matter in sludge is gasified, liquefied, mineralized, or converted to a more stable form, usually by anaerobic organisms.

Specific gas volume - Daily volume of biogas produced per unit volume of digester.

Supernatant - Liquid removed from settled sludge. Supernatant commonly refers to the liquid between the sludge in the lower portion and the scum on the surface of an anaerobic digester or the liquid material left after separation of the slurry.

Suspended Solids - Solids that are in suspension in water or other liquids.

Thermophilic - Digestion at a relatively high temperature, normally in the range of 50-70C.

Toxicity - A condition that will inhibit or destroy the growth or function of a living organism.

Total Solids - The sum of dissolved and suspended constituents in a sample, usually stated in milligrams per litre or percent.

Volatile Acids - Short chain (Cl - C2) fatty acids which are produced by acid forming bacteria. They are soluble in water, can be steamdistilled at atmospheric pressure, and are commonly reported as equivalent acetic acydlatile Solids - The solids that volatilize and therefore are lost on ignition of a sample of dry solids at 55C - Representing the organic matter in the sample. The volatile solids are expressed as a percentage of the total solids.


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