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 manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.
Anaerobic digestion reduces sludge volume and pathogen content while generating methane-rich biogas that can be used for renewable energy production. The liquid remainder, digestate, can be further treated and, in some cases, beneficially reused.
Anaerobic digestion occurs naturally in some soils and in lake and oceanic basin sediments, where it is usually referred to as "anaerobic activity". This is the source of marsh-gas methane, as discovered by Alessandro Volta in 1776.
Anaerobic digestion comprises four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis.
The digestion process begins with bacterial hydrolysis of the input materials. Insoluble organic polymers, such as carbohydrates, are broken down to soluble derivatives that become available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. In acetogenesis, bacteria convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide, amongst other compounds. Finally, methanogens convert these products to methane and carbon dioxide. The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.
Anaerobic digestion is used as part of the process to treat biodegradable waste and sewage sludge. As part of an integrated waste-management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize.
Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide, and traces of other "contaminant" gases. This biogas can be used directly as fuel, in cogeneration plants or upgraded to natural-gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.
Process
Many microorganisms affect anaerobic digestion, including acetic-acid-forming bacteria (acetogens) and methane-forming archaea (methanogens). These organisms promote a number of chemical processes in converting the biomass to biogas.
Gaseous oxygen is excluded from the reactions by physical containment. Anaerobes use electron acceptors from sources other than oxygen gas. These acceptors can be the organic material itself or may be supplied by inorganic oxides from within the input material. When the oxygen source in an anaerobic system is derived from the organic material itself, the "intermediate" end products are primarily alcohols, aldehydes, and organic acids, plus carbon dioxide. 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.Adapted from In an anaerobic system, the majority of the chemical energy contained within the starting material is released by methanogenic archaea as methane.
Populations of anaerobic microorganisms typically take a significant period of time to establish themselves to be fully effective. 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.
Process stages
The four key stages of anaerobic digestion involve hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The overall process can be described by the chemical reaction, where organic material such as glucose is biochemically digested into carbon dioxide () and methane () by the anaerobic microorganisms: Hydrolysis In most cases, biomass is made up of large organic polymers. For the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts. These constituent parts, or monomers, such as sugars, are readily available to other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore, hydrolysis of these high-molecular-weight polymeric components is the necessary first step in anaerobic digestion. Through hydrolysis, the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids. Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules, such as volatile fatty acids (VFAs) with a chain length greater than that of acetate must first be catabolised into compounds that can be directly used by methanogens. Acidogenesis The biological process of acidogenesis results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here, VFAs are created, along with ammonia, carbon dioxide, and hydrogen sulfide, as well as other byproducts. The process of acidogenesis is similar to the way milk sours. Acetogenesis The third stage of anaerobic digestion is acetogenesis. 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. Methanogenesis The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here, methanogens use the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. These components make up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8. The remaining, indigestible material the microbes cannot use and any dead bacterial remains constitute the digestate.
Main Parameters that affect performance Process performance depends on several operational parameters, including temperature (mesophilic conditions at approximately 35–37 °C or thermophilic conditions at 50–55 °C), organic loading rate (OLR), pH (typically maintained between 6.8 and 7.2), and solids retention time (SRT). Proper control of these parameters is essential to prevent process instability and volatile fatty acid accumulation.
Configuration
Anaerobic digesters can be designed and engineered to operate using a number of different configurations and can be categorized into batch vs. continuous process mode, mesophilic vs. thermophilic temperature conditions, high vs. low portion of solids, and single-stage vs. multistage processes. Continuous processes require more complex design, but still may be more economical than batch processes, because batch processes require more initial building money and a larger volume of the digesters (spread across several batches) to handle the same amount of waste as continuous-process digesters. Higher heat energy is required in a thermophilic system compared to a mesophilic system, but the thermophilic system requires much less time and has a larger gas output capacity and higher methane gas content. For solids content, up to 15% solid content is considered low. Above this level is considered high solids content and can also be known as dry digestion. In a single-stage process, one reactor houses the four anaerobic digestion steps. A multistage process uses two or more reactors for digestion to separate the methanogenesis and hydrolysis phases.
Batch or continuous Anaerobic digestion can be performed as a batch process or a continuous process. In a batch system, biomass is added to the reactor at the start of the process. The reactor is then sealed for the duration of the process. In its simplest form, batch processing needs inoculation with already-processed material to start the anaerobic digestion. In a typical scenario, biogas production will be formed with a normal distribution pattern over time. Operators can use this fact to determine when they believe the process of digestion of the organic matter has completed. There can be severe odour issues if a batch reactor is opened and emptied before the process is well completed. A more advanced type of batch approach has limited the odour issues by integrating anaerobic digestion with in-vessel composting. In this approach, inoculation takes place through the use of recirculated degasified percolate. After anaerobic digestion has completed, the biomass is kept in the reactor, which is then used for in-vessel composting before it is openedaikantechnology.com Retrieved 10. Feb. 2012. As the batch digestion is simple and requires less equipment and lower levels of design work, it is typically a cheaper form of digestion.Anaerobic digestion , energy.ca.gov. Retrieved 18.06.09. Using more than one batch reactor at a plant can ensure constant production of biogas.
In continuous digestion processes, organic matter is constantly added (continuous complete mixed) or added in stages to the reactor (continuous plug flow; first in – first out). Here, the end products are constantly or periodically removed, resulting in constant production of biogas. A single digester or multiple digesters in sequence may be used. Examples of this form of anaerobic digestion include continuous stirred-tank reactors, upflow anaerobic sludge blankets, expanded granular sludge beds, and internal circulation reactors.BIOPAQ IC, paques.nl. Retrieved 19.08.07.Biological processes with Biomar technology envirochemie.com. Retrieved 24.10.2012.
Temperature The two conventional operational temperature levels for anaerobic digesters determine the species of methanogens in the digesters: Mesophilic digestion takes place optimally around 30–38 °C, or at ambient temperatures of 20–45 °C, where mesophiles are the primary microorganisms present. Thermophilic digestion takes place optimally around 49–57 °C, or at elevated temperatures up to 70 °C, where thermophiles are the primary microorganisms present.
A limit case has been reached in Bolivia, with anaerobic digestion in temperature working conditions of less than 10 °C. The anaerobic process is very slow, taking more than three times the normal mesophilic time process. In experimental work at University of Alaska Fairbanks, a 1,000-litre digester using psychrophiles harvested from "mud from a frozen lake in Alaska" has produced 200–300 litres of methane per day, about 20 to 30% of the output from digesters in warmer climates. Mesophilic species outnumber thermophiles, and they are also more tolerant to changes in environmental conditions than thermophiles. Mesophilic systems are, therefore, considered to be more stable than thermophilic digestion systems. 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. The increased temperatures facilitate faster reaction rates, and thus faster gas yields. Operation at higher temperatures facilitates greater pathogen reduction of the digestate. 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.Animal by-products introduction, ec.europa.eu. Retrieved 24.10.07.