Anaerobic digestion
In the presence of specialised methanogens, the intermediates are converted to the 'final' end products of methane, carbon dioxide, and trace levels of hydrogen sulfide.
Therefore, common practice is to introduce anaerobic microorganisms from materials with existing populations, a process known as "seeding" the digesters, typically accomplished with the addition of sewage sludge or cattle slurry.
[17] The overall process can be described by the chemical reaction, where organic material such as glucose is biochemically digested into carbon dioxide (CO2) and methane (CH4) by the anaerobic microorganisms.
Here, simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen.
In contrast, while thermophilic digestion systems are considered less stable, their energy input is higher, with more biogas being removed from the organic matter in an equal amount of time.
In countries where legislation, such as the Animal By-Products Regulations in the European Union, requires digestate to meet certain levels of pathogen reduction there may be a benefit to using thermophilic temperatures instead of mesophilic.
For example, certain processes shred the substrates to increase the surface area or use a thermal pretreatment stage (such as pasteurisation) to significantly enhance the biogas output.
[38] High solids digesters also require correction of conventional performance calculations (e.g. gas production, retention time, kinetics, etc.)
[39] Low solids (wet) digesters can transport material through the system using standard pumps that require significantly lower energy input.
[47] Continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mix the contents, enabling the bacteria and the food to be in contact.
The anaerobic digestion process can be inhibited by several compounds, affecting one or more of the bacterial groups responsible for the different organic matter degradation steps.
[55] Almost any organic material can be processed with anaerobic digestion;[56] however, if biogas production is the aim, the level of putrescibility is the key factor in its successful application.
To use a high-solids anaerobic digester for dilute feedstocks, bulking agents, such as compost, should be applied to increase the solids content of the input material.
The maceration of solids can be achieved by using a chopper pump to transfer the feedstock material into the airtight digester, where anaerobic treatment takes place.
Techniques to determine the compositional characteristics of the feedstock are available, while parameters such as solids, elemental, and organic analyses are important for digester design and operation.
A 2024 study has shown that anaerobic digestion, combined with adsorption in activated carbon and voltage application can remove up to 61% of PFAS from sewage sludge.
[85] In developing countries, simple home and farm-based anaerobic digestion systems offer the potential for low-cost energy for cooking and lighting.
[94] Farm biogas plants using animal waste and energy crops are expected to contribute to reducing CO2 emissions and strengthen the grid, while providing UK farmers with additional revenues.
[95] Some countries offer incentives in the form of, for example, feed-in tariffs for feeding electricity onto the power grid to subsidize green energy production.
Several technologies are available for this purpose, the most widely implemented being pressure swing adsorption (PSA), water or amine scrubbing (absorption processes) and, in recent years, membrane separation.
[102] In October 2010, Didcot Sewage Works became the first in the UK to produce biomethane gas supplied to the national grid, for use in up to 200 homes in Oxfordshire.
[123] In certain applications, in situ treatment can be used to increase the methane purity by reducing the offgas carbon dioxide content, purging the majority of it in a secondary reactor.
[124] In countries such as Switzerland, Germany, and Sweden, the methane in the biogas may be compressed for it to be used as a vehicle transportation fuel or input directly into the gas mains.
[125] In countries where the driver for the use of anaerobic digestion are renewable electricity subsidies, this route of treatment is less likely, as energy is required in this processing stage and reduces the overall levels available to sell.
[127][128] The second byproduct (acidogenic digestate) is a stable, organic material consisting largely of lignin and cellulose, but also of a variety of mineral components in a matrix of dead bacterial cells; some plastic may be present.
[142][143] After 1920, closed tank systems began to replace the previously common use of anaerobic lagoons – covered earthen basins used to treat volatile solids.
[140] In addition to high energy prices, factors affecting the adoption of anaerobic digestion systems include receptivity to innovation, pollution penalties, policy incentives, and the availability of subsidies and funding opportunities.
Feed-in tariffs in Germany were enacted in 1991, also known as FIT, providing long-term contracts compensating investments in renewable energy generation.
The German government responded by amending FIT four times between 2000 and 2011, increasing tariffs and improving the profitability of anaerobic digestion, and resulting in reliable returns for biogas production and continued high adoption rates across the country.
[148][149] Anaerobic digesters have caused Fish kills (e.g. River Mole, Devon,[151] River Teifi,[152] Afon Llynfi,[153] and loss of human life (e.g. Avonmouth explosion) There have been explosions of Anaerobic Digesters in the US[154] (Jay, Maine Pixelle Specialty Solutions' Androscoggin Mill;[155] Pensacola (Cantonment) 22 January 2017 (Kamyr digester explosion);[156] EPDM failure March 2013 Aumsville, Oregon;[157] February 6, 1987, Pennsylvania two workers at a wastewater treatment plant were re-draining a sewage digester when an explosion lifted the 30-ton floating cover, killing both workers instantly;[158] Southwest Wastewater Treatment Plant in Springfield, Missouri),[159] in the UK (for example at Avonmouth and at Harper Adams College, Newport, Shropshire[160][161]), plus In Europe, there were about 800 accidents on biogas plants between 2005 and 2015, e.g. in France (Saint-Fargeau)[162][163] (though few of them were 'serious' with direct consequences for the human population).
