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Anaerobic Digester Terms & Definitions
Ambient - outside air temperature.
Anaerobic –in the absence of oxygen microbes breakdown organic material (i.e., animal manure).
Anaerobic Bacteria – microbes whose metabolisms require the absence of oxygen to survive
Anaerobic Digestion – the breakdown of animal manure and other organic material in the absence of oxygen, (methane producing bacteria are most active in two temperature ranges, 95 to 105°F and 130 to 135°F. This is a living system and must be treated as such. The organic material is decomposed by acid formers into fatty acids and then into biogas by methane formers or methanogens.
Biofibers – the solid material separated from the effluent stream after treatment by an anaerobic digester. This is the solid material that could not be volatilized into biogas.
Biogas – the gas produced from decomposition of livestock manure in an anaerobic digester consisting of 60-80 percent methane, 30-40 percent carbon dioxide, and other trace gases such as hydrogen sulfide, ammonia and hydrogen.
BOD (Biochemical Oxygen Demand) – a qualitative measurement indicating how fast biological organisms use up oxygen in a water body. It is an indication of the availability of nutrients and food in the water.
Complete Mix Digester - a tank designed above or below ground as part of a manure management system to handle manure containing 2-10 percent solids. The digester is heated and mixed mechanically or with gas-mixing systems to keep the solids suspended. This maximizes biological activity for destruction of volatile solids, methane production, and odor reduction.
Covered Lagoon Digester – an anaerobic lagoon is commonly used when manure has less than 2 percent solids. Decomposition of the manure occurs, methane is produced, and effluent odor is reduced. The lagoon is covered with a gas-tight cover to capture the biogas.
Digester – a sealed container or tank, where the biological digestion can occur of animal manure and biogas formed.
Effluent – organic liquid and solid material (slurry) leaving a digester.
Feedstock – liquid and solid material fed to the digester, usually manure and other organic material, also known as influent.
Hydraulic Retention Time (HRT) – the average length of time the liquid influent remains in the digester for treatment.
Influent – liquid and solid material fed to the digester, usually manure.
Induction Generator – this type of generator operates in parallel with the utility for its phase, frequency and voltage and cannot operate in isolation (stand alone) -- in other words, it cannot operate without the power company.
Loading Rate – the total amount of solids and liquids fed to the digester daily.
Mesophilic – the temperature range of 95 to 105°F in which methanogenic microbes thrive.
Methane – a combustible gas produced by anaerobic digestion; also the principal component of natural gas.
Methanogens – methane producing microbes.
Microturbine – a small-scale gas turbine generation system to combust gas and generate electricity.
Net Metering – an agreement with the utility company to purchase the electricity produced by the digester system at a rate equal to the farm electricity purchase rate.
Psychrophilic – less than 68°F.
Settled Solids – the separated manure solids which settle to the bottom of the digester.
Slurry – the mixture of solids and water processed in the digester.
Thermophilic – temperature range of 125 to 135°F where certain methanogenic bacteria are most active, the greatest pathogen destruction occurs in this temperature range.
Total Dissolved Solids (TDS) – the volume of solid material that cannot be filtered out.
Total Suspended Solids (TSS) - the volume of solid material that can be filtered out.
Toxicant – a component in manure or some other feedstock causing an adverse effect on bacterial metabolism.
Volatile Acids – these are present in the feedstocks and also produced in the digester by acid-forming bacteria and then used by the methane-forming bacteria to produce methane.
Volatile Solids – the organic matter which can be converted to gas.
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