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.
Fixed Film Digester – a tank designed as part of a manure management system to handle manure up to 3 percent solids.  The digester is temperature controlled and a media is placed inside the digester.  This design allows the microbial populations to attach to the media and grow as a biofilm (fixed film), thus preventing the microbes from being removed with the effluent.

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.

Plug-Flow Digester – a tank designed for a manure management system which handles organic material containing 11-14 percent solids.  The digester is given daily influent plugs that flow-through the digester.  The digester is heated.  This helps with the destruction of volatile solids, methane production and odor reduction.

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.

Synchronous Generator – this type of generator can operate in parallel with the utility or operate in isolation from the power company (stand alone).  This generator does not need the utility voltage to create electricity; the machine is self excited.  Generally, more expensive utility breaker controls are required.
Temperature-Phased Anaerobic Digester (TPAD) – two tanks designed as part of a residuals management system.  The digesters are heated, the first digester in the thermophilic temperature range and the second digester in the mesophilic temperature range.  This will maximize biological activity for the destruction of volatile solids, methane production and odor reduction.

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.