Terminology for Anaerobic Digesters

Anaerobic Digester Terminology

Looking for more information on the terms & acronyms related to anaerobic digesters and their usage? We provide a glossary of terms related to these tools and processes for ease of understanding.

∙  MI Department of Agriculture & Rural Development
∙  American Biogas Council 

Acidogenic – acid producing

Activated Carbon – Charcoal that has been heated or otherwise treated to increase its adsorptive power 

Ambient – outside air temperature

Amine Scrubber – Amine gas treating, also known as gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various aklylamines to remove hydrogen sulfide and carbon dioxide from gases

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 (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

Anaerobic Lagoon – (or Manure Lagoon) is a man-made outdoor earthen basin filled with animal waste that undergoes anaerobic respiration as part of a system designed to manage and treat refuse created by animal feeding operations

Anaerobic Sequencing Batch Reactor –a suspended growth reactor treating waste in four distinct phases over a 12 hour cycle, including digester feeding, digester mixing and gas production, biomass and solids settling, and liquid effluent discharge

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

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

Biofiltration – A pollution control technique using living material to capture and biologically degrade process pollutants. Common uses include processing waste water, capturing harmful chemicals or silt from surface runoff, and microbiotic oxidation of contaminants in air

Biomethane— biogas-derived, high-BTU gas that is predominately methane after the biogas is upgraded to remove most of the contaminants and a majority of the carbon dioxide (CO2) and nitrogen (N2) found in biogas

Biochemical Methane Potential (BMP)—Maximal potential production of biogas by a substrate (m3 biogas/tons of VS). Also, a common test done to a feedstock to estimate the amount of biogas a certain feedstock or mix of feedstocks might produce

Biosolid – Sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. It can also refer to the settled suspension obtained from conventional drinking water treatment, and numerous other industrial processes

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 and used as a measure of the degree of pollution in waterways

BTU—Unit of energy. British Thermal Unit 

Certified Digestate: Digestate that has been certified to meet the health and safety criteria of the American Biogas Council’s Digestate Standard Testing and Certification Program at www.Digestate.org 

Complete mix digester – a tank designed above or below ground as part of a manure management system to handle manure containing 2 to 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

Conditioned Biogas—medium-BTU biogas that is stripped of some trace contaminants and water, but maintains the relative mix of carbon dioxide (CO2) and methane (CH4)

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

Digestate – The material remaining after the anaerobic digestion of a biodegradable feedstock. Anaerobic digestion produces two main products: digestate and biogas. It is produced both by acidogenesis and methanogenesis and each has different characteristics

Digester – a sealed container or tank, where the biological digestion can occur of animal manure and biogas formed

Dry Digester – The anaerobic digestion system in which most bio-solid organic material is placed for anaerobic digestion 

Dryer – A machine or device used to remove the humidity from a raw biogas substance to assist in the transformation of biogas to biomethane 

Effluent – organic liquid and solid material (slurry) leaving a digester.

Feedstock – liquid and solid material fed to the digester, usually manure, 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

High Strength—a term that is usually applied to industrial wastewater to indicate that it contains a higher than normal percentage of solids or other soluble or suspended material 

Holding Tank – A large container in which liquids are temporarily held 

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 – (or asynchronous generator) is a type of AC electrical generator that uses the principles of induction motors to produce power. Induction generators operate by mechanically turning their rotor faster than a synchronous speed, giving negative slip. This type of generator operates in parallel with the utility for its phase, frequency and voltage and cannot operate in isolation (stand alone) – i.e., it cannot operate without the power company

Industrial Wastewater – Wastewater not otherwise defined as domestic wastewater, including the runoff and leachate from areas that receive pollutants associated with industrial or commercial storage, handling or processing facilities

Inorganic Material – Compounds derived from other than vegetable or animal sources, generally do not contain carbon atoms 

Iron Sponge – A machine that removes sulfides during the anaerobic digestion process 

Loading rate – the total amount of solids and liquids fed to the digester daily

Manure – consists of animal urine and feces, wasted feed and bedding collected to put into the digester as influent

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

Methanogenic – methane producing microbes

Microturbine – a small-scale gas turbine generation system to combust gas and generate electricity

Municipal Solid Waste (MSW) – (or trash/garbage) refuse or rubbish is a waste type consisting or everyday items that are discarded by the public 

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

Pathogen – a bacterium, virus, or other microorganism that can cause disease

Psychrophilic – less than 68°F

Plug-flow digester – a tank designed for a manure management system which handles manure 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

Renewable Compressed Natural Gas (R-CNG): RNG that is compressed to a high pressure, often for use as a transportation fuel

Renewable Liquefied Natural Gas (R-LNG): RNG that is converted to liquid form, often for use as a transportation fuel

Renewable Natural Gas (RNG): biomethane that is upgraded to natural gas pipeline quality standards such that it may blend with, or substitute for, geologic natural gas, including odorizing

Residence Time – The average length of time during which a substance, a portion of material, or an object is in a given location or condition, such as adsorption or suspension 

Return Activated Sludge (RAS) – The settled activated sludge (biomass) that is collected in a secondary clarifier and returned to the secondary treatment process to mix with incoming wastewater. This returns a concentrated population of microorganisms back into the aeration basin

Settled solids – the separated manure solids which settle to the bottom of the digester

Silica Gel – hydrated silica in a hard granular hygroscopic form used as a desiccant 

Siloxanes – A compound having a molecular structure based on a chain of alternate silicon and oxygen atoms, esp. (as in silicone) with organic groups attached to the silicon atoms 

Sludge – Thick, soft, wet waste or a similar viscous mixture of liquid and solid components, esp. the product of an industrial or refining process

Slurry – the mixture of manure 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

Sulfide – A binary compound of sulfur with another element or group 

Syngas: a gas mixture composed primarily of hydrogen (H2) and carbon monoxide (CO), along with hydrocarbons from the thermochemical decomposition of organic or inorganic materials 

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 manure 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. A measure of the combined content of all inorganic and organic substances contained in a liquid in molecular, ionized or micro-granular suspended form 

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. E.g., a pesticide

Volatile acids – these are produced in the digester by acid-forming bacteria and then used by the methane-forming bacteria to produce methane

Volatile solids – the organic matter in manure which can be converted to gas

Volatile solids loading rate – the total amount of volatile solids fed to the digester daily (note: volatile solids are what the microbes use to make methane gas)

AD – Anaerobic Digester
ASBR – Anaerobic Sequencing Batch Reactor
BMP – Biomethane Potential
BOD – Biological Oxygen Demand
HRT – Hydraulic Retention Time
RAS – Return Activated Sludge
RNG- Renewable Natural Gas
SRT – Solids Retention Time
TDS – Total Dissolved Solids
TPAD – Temperature-phased anaerobic digester
TSS – Total Suspended Solids
VS-Volatile Solids
VA – Volatile Acids

AU – Animal unit
Btu – British thermal units
cfd (ft3/day) – cubic feet per day
gpd – gallons per day
kW – kilowatt
kWh – kilowatt hours

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