Autogenerative high-pressure fermentation (AHPD) is a

biogas Biogas is a gaseous renewable energy source produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, Wastewater treatment, wastewater, and food waste. Biogas is produced by anaerobic ...

production technique that operates under elevated gas pressure. This pressure is naturally generated by the

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and

archaea Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...

through the gases they release. First described by R. Lindeboom of University of Wageningen (WUR) in 2011, a

batch reactor A batch reactor is a chemical reactor in which a non-continuous reaction is conducted, i.e., one where the reactants, products and solvent do not flow in or out of the vessel during the reaction until the target reaction conversion is achieved. By e ...

was pressurized to 58 bar, yielding a

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

concentration of 96% in the resulting

. This method is also commonly referred to as High Pressure Anaerobic Digestion (HPAD) in scientific literature. AHPD leverages the higher solubility of carbon dioxide (CO2) at 0.031 mol/L/bar compared to methane (CH4) at 0.0016 mol/L/bar. This difference allows more CO2 to dissolve in the digestate, while

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

(H2S) also dissolves more efficiently under pressure. The result is biogas with a higher methane content, which requires less upgrading to meet

natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...

standards, ultimately reducing processing costs.

Microbial composition

Individual species of

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

have different optimal conditions in which they grow and replicate most rapidly.There is a specific

range Range may refer to: Geography * Range (geographic), a chain of hills or mountains; a somewhat linear, complex mountainous or hilly area (cordillera, sierra) ** Mountain range, a group of mountains bordered by lowlands * Range, a term used to i ...

around that optimum in which a specie is able to survive. Factors such as the pH, temperature,

osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a Solution (chemistry), solution to prevent the inward flow of its pure solvent across a semipermeable membrane. It is also defined as the measure of the tendency of a soluti ...

(often caused by

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

) all contribute to the optimal condition of all microorganisms. For example, in terms of pressure, some species are able to survive in

extremophile An extremophile () is an organism that is able to live (or in some cases thrive) in extreme environments, i.e., environments with conditions approaching or stretching the limits of what known life can adapt to, such as extreme temperature, press ...

conditions such as extreme radiation, temperature, salinity or pressure.

Piezophile A piezophile (from Greek "piezo-" for pressure and "-phile" for loving) is an organism with optimal growth under high hydrostatic pressure, i.e., an organism that has its maximum rate of growth at a hydrostatic pressure equal to or above , when tes ...

microorganisms have their optimal growth condition at a pressure equal to or above 10 megapascals (99 atm; 1,500

psi Psi, PSI or Ψ may refer to: Alphabetic letters * Psi (Greek) (Ψ or ψ), the twenty-third letter of the Greek alphabet * Psi (Cyrillic), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviat ...

). Some bacteria and archaea have adapted to life in the deep oceans, where the pressure (

Hydrostatic pressure Hydrostatics is the branch of fluid mechanics that studies fluids at hydrostatic equilibrium and "the pressure in a fluid or exerted by a fluid on an immersed body". The word "hydrostatics" is sometimes used to refer specifically to water and o ...

) is much higher than at sea level. For example, the methane-producing archaea species

Methanocaldococcus ''Methanocaldococcus'' formerly known as ''Methanococcus'' is a genus of coccoid methanogen archaea. They are all mesophiles, except the thermophilic ''M. thermolithotrophicus'' and the hyperthermophilic ''M. jannaschii''. The latter was dis ...

, Methanothermococcus,

Methanopyrus ''Methanopyrus'' is a genus of methanogen, with a single described species, ''Methanopyrus kandleri''. It is a rod-shaped hyperthermophile, discovered on the wall of a black smoker from the Gulf of California at a depth of 2,000 m, at temper ...

and Methanotorris have been found in

hydrothermal vents Hydrothermal vents are fissures on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hots ...

in the ocean floor. Research at the

University of Groningen The University of Groningen (abbreviated as UG; , abbreviated as RUG) is a Public university#Continental Europe, public research university of more than 30,000 students in the city of Groningen (city), Groningen, Netherlands. Founded in 1614, th ...

(RUG) has shown that the bacterial community is affected by pressure from composition changes. This makes it possible to influence the

anaerobic digestion Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to Waste management, manage waste or to produce fuels. Mu ...

process.A further development of this technique is the addition of

hydrogen gas Hydrogen is a chemical element; it has symbol H and atomic number 1. It is the lightest and most abundant chemical element in the universe, constituting about 75% of all normal matter. Under standard conditions, hydrogen is a gas of diatomi ...

to the reactor. According to

Henry's law In physical chemistry, Henry's law is a gas law that states that the amount of dissolved gas in a liquid is directly proportional at equilibrium to its partial pressure above the liquid. The proportionality factor is called Henry's law constant ...

, this gas also dissolves more at increased pressure. The result is that it can be better absorbed by bacteria and archea. In turn, it converts the hydrogen gas with the already dissolved carbon dioxide into additional methane. This combination of techniques was described in detail by Kim et all in 2021, known to be a process called

biological methanation Biological methanation (also: biological hydrogen methanation (BHM) or microbiological methanation) is a conversion process to generate methane by means of highly specialized microorganisms (Archaea) within a technical system. This process can be ...

. On Michael Liebreich's hydrogen ladder 5.0, this form of biogas upgrading is at step C. This is considerably higher than applications as fuel in vehicles. These are spread over steps D to G. Although the technique is usually used as a fermentation process for thick liquid flows and solid biomass, it can also be applied as anaerobic

Wastewater treatment Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on ...

. In South Korea, they have succeeded in operating a UASB reactor (a form of anaerobic wastewater treatment) at 8 Bar. A biogas was then created with a methane content of 96.7%. A remarkable finding was that the grains in the sludge that are so similar in characteristic of the UASB technique were well preserved. This was because more

Extracellular polymeric substance Extracellular polymeric substances (EPS) are biopolymer, natural polymers of molecular mass, high molecular weight secreted by microorganisms into their environment. EPS establish the functional and structural integrity of biofilms, and are consid ...

(EPS) was formed in the

biofilm A biofilm is a Syntrophy, syntrophic Microbial consortium, community of microorganisms in which cell (biology), cells cell adhesion, stick to each other and often also to a surface. These adherent cells become embedded within a slimy ext ...

. Microorganisms make these to protect themselves against difficult conditions, in this case the extreme pressure.

References

Sustainable energy Biogas technology Anaerobic digestion {{Improve categories, date=August 2024

Microbial composition

See also

References