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Anaerobic digestion is a process in which microorganisms break down biodegradable material in the absence of oxygen. The products of this are biogas (a mixture of carbon dioxide (CO2) and methane) and digestate (a nitrogen-rich fertiliser). The biogas can be burned to produce heat, or can be cleaned and used in the same way as natural gas or as an automobile fuel. The waste digestate is rich in minerals and can be used as an agricultural fertiliser or soil conditioner.
The wet anaerobic process is widely used to treat wastewater sludges and organic wastes because it provides volume and mass reduction of the input material. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere.
Dry anaerobic digestion has been used widely too. For example, there is the Axpo Kompogas AG system. This is a fully developed system and has produced 27 million Kwh of electricity and Biogas in 2009. The oldest of the companies own lorries has achieved 1.000.000 kilometers driven with biogas from household waste in the last 15 years.
Anaerobic digestion is a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production helping replace fossil fuels. Also, the nutrient-rich solids left after digestion can be used as fertilizer
2 types of processes exist: the process for wet anaerobic digestion' and for dry anaerobic digestion.
In both types of process, there are a number of bacteria that are involved in the process of anaerobic digestion including acetic acid-forming bacteria (acetogens) and methane-forming bacteria (methanogens). These bacteria feed upon the initial feedstock, which undergoes a number of different processes converting it to intermediate molecules including sugars, hydrogen & acetic acid before finally being converted to biogas.
Different species of bacteria are able to survive at different temperature ranges. Ones living optimally at temperatures between 35-40°C are called mesophiles or mesophilic bacteria. Some of the bacteria can survive at the hotter and more hostile conditions of 55-60°C, these are called thermophiles or thermophilic bacteria. Methanogens come from the primitive group of archaea. This family includes species that can grow in the hostile conditions of hydrothermal vents. These species are more resistant to heat and can therefore operate at thermophilic temperatures, a property that is unique to bacterial families.
As with aerobic systems the bacteria in anaerobic systems the growing and reproducing microorganisms within them require a source of elemental oxygen to survive.
In an anaerobic system there is an absence of gaseous oxygen. In an anaerobic digester, gaseous oxygen is prevented from entering the system through physical containment in sealed tanks. Anaerobes access oxygen from sources other than the surrounding air. The oxygen source for these microorganisms can be the organic material itself or alternatively 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, then 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 with trace levels of hydrogen sulfide. In an anaerobic system the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane.
Populations of anaerobic bacteria typically take a significant period of time to establish themselves to be fully effective. It is therefore common practice to introduce anaerobic microorganisms from materials with existing populations. This process is called 'seeding' the digesters and typically takes place with the addition of sewage sludge or cattle slurry.
There are four key biological and chemical stages of anaerobic digestion:
In most cases biomass is made up of large organic polymers. In order 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 by 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 (VFA’s) with a chain length that is greater than acetate must first be catabolised into compounds that can be directly utilised by methanogens.
The biological process of acidogenesis is where there is 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 by-products. The process of acidogenesis is similar to the way that milk sours.
The third stage of anaerobic digestion is acetogenesis. Simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid as well as carbon dioxide and hydrogen.
The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here methanogens utilise the intermediate products of the preceding stages and convert them into methane, carbon dioxide and water. It is these components that makes 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, non-digestable material which the microbes cannot feed upon, along with any dead bacterial remains constitutes the digestate.
A simplified generic chemical equation for the overall processes outlined above is as follows:
C6H12O6 → 3CO2 + 3CH4
Configuration of anaerobic digester
Anaerobic digesters can be designed and engineered to operate using a number of different process configurations:
- Batch or continuous
- Temperature: Mesophilic or thermophilic
- Solids content: High solids or low solids
- Complexity: Single stage or multistage
Batch or continuous
A batch system is the simplest form of digestion. Biomass is added to the reactor at the start of the process in a batch and is sealed for the duration of the process. Biogas production will be formed with a normal distribution pattern over time. The operator can use this fact to determine when they believe the process of digestion of the organic matter has completed.
There are two conventional operational temperature levels for anaerobic digesters, which are determined by the species of methanogens in the digesters:
- Mesophilic which takes place optimally around 37°-41°C or at ambient temperatures between 20°-45°C where mesophiles are the primary microorganism present
- Thermophilic which takes place optimally around 50°-52° at elevated temperatures up to 70°C where thermophiles are the primary microorganisms present.
There are a greater number of species of mesophiles than thermophiles. These bacteria are also more tolerant to changes environmental conditions than thermophiles. Mesophilic systems are therefore considered to be more stable than thermophilic digestion systems. Thermophilic digestion systems are considered to be less stable, however the increased temperatures facilitate faster reaction rates and hence faster gas yields. Operation at higher temperatures facilitates greater sterilisation of the end digestate.
A drawback of operating at thermophilic temperatures is that more heat energy input is required to achieve the correct operational temperatures. This increase in energy is not be outweighed by the increase in the outputs of biogas from the systems. It is therefore important to consider an energy balance for these systems
Typically there are two different operational parameters associated with the solids content of the feedstock to the digesters:
Digesters can either be designed to operate in a high solids content, with a total suspended solids (TSS) concentration greater than 20%, or a low solids concentration less than 15%.
High-solids digesters process a thick slurry that requires more energy input to move and process the feedstock. The thickness of the material may also lead to associated problems with abrasion. High-solids digesters will typically have a lower land requirement due to the lower volumes associated with the moisture.
Low-solids digesters can transport material through the system using standard pumps that require significantly lower energy input. Low-solids digesters require a larger amount of land than high-solids due to the increase volumes associated with the increased liquid: feedstock ratio of the digesters. There are benefits associated with operation in a liquid environment as it enables more thorough circulation of materials and contact between the bacteria and their food. This enables the bacteria to more readily access the substances they are feeding off and increases the speed of gas yields.
Number of stages
Digestion systems can be configured with different levels of complexity:
- One-stage or single-stage
- Two-stage or multistage
A single-stage digestion system is one in which all of the biological reactions occur within a single sealed reactor or holding tank. Utilising a single stage reduces construction costs, however facilitates less control of the reactions occurring within the system. Acidogenic bacteria, through the production of acids, reduce the pH of the tank. Methanogenic bacteria operate in a strictly defined pH range. Therefore the biological reactions of the different species in a single stage reactor can be in direct competition with each other. Another one-stage reaction system is an anaerobic lagoon. These lagoons are pond-like earthen basins used for the treatment and long-term storage of manures. Here the anaerobic reactions are contained within the natural anaerobic sludge contained in the pool.
In a two-stage or multi-stage digestion system different digestion vessels are optimised to bring maximum control over the bacterial communities living within the digesters. Acidogenic bacteria produce organic acids and more quickly grow and reproduce than methanogenic bacteria. Methanogenic bacteria require stable pH and temperature in order to optimise their performance.
Typically hydrolysis, acetogenesis and acidogenesis occur within the first reaction vessel. The organic material is then heated to the required operational temperature (either mesophilic or thermophilic) prior to being pumped into a methanogenic reactor. The initial hydrolysis or acidogenesis tanks prior to the methanogenic reactor can provide a buffer to the rate at which feedstock is added
The residence time in a digester varies with the amount and type of feed material, the configuration of the digestion system and whether it be one-stage or two-stage.
In the case of single-stage thermophilic digestion residence times may be in the region of 14 days, which comparatively to mesophilic digestion is relatively fast. The plug-flow nature of some of these systems will mean that the full degradation of the material may not have been realised in this timescale. In this event digestate exiting the system will be darker in colour and will have more odour.
In two-stage mesophilic digestion, residence time may vary between 15 and 40 days.
In the case of mesophilic UASB digestion hydraulic residence times can be (1hour-1day) and solid retention times can be up to 90 days. In this manner the UASB system is able to separate solid an hydraulic retention times with the utilisation of a sludge blanket.
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. They also allow excess material to be continuously extracted to maintain a reasonably constant volume within the digestion tanks.
Dry anaerobic digestion
This process uses no manure at all and is thus more suitable for certain applications in which no manure needs to be processed. This process can be done at several ways. For example, there is the Wiessmann-Bioferm "Kompoferm" process. There is also the Axpo Kompogas AG system, the Dranco process as designed by OWS, as well as a system by Jan Klein Hesselink.
There are three principal products of anaerobic digestion: biogas, digestate and water.
Biogas is the ultimate waste product of the bacteria feeding off the input biodegradable feedstock, and is mostly methane and carbon dioxide, with a small amount hydrogen and trace hydrogen sulfide. Most of the biogas is produced during the middle of the digestion, after the bacterial population has grown, and tapers off as the putrescible material is exhausted. The gas is normally stored on top of the digester in an inflatable gas bubble or extracted and stored next to the facility in a gas holder.
The methane in biogas can be burned to produce both heat and electricity, usually with a reciprocating engine or microturbine often in a cogeneration arrangement where the electricity and waste heat generated are used to warm the digesters or to heat buildings. Excess electricity can be sold to suppliers or put into the local grid. Electricity produced by anaerobic digesters is considered to be renewable energy and may attract subsidies. Biogas does not contribute to increasing atmospheric carbon dioxide concentrations because the gas is not released directly into the atmosphere and the carbon dioxide comes from an organic source with a short carbon cycle.
Biogas may require treatment or 'scrubbing' to refine it for use as a fuel. Hydrogen sulfide is a toxic product formed from sulfates in the feedstock and is released as a trace component of the biogas. If the levels of hydrogen sulfide in the gas are high, gas scrubbing and cleaning equipment (such as amine gas treating) will be needed to process the biogas to within regionally accepted levels( determined by the US EPA or the English and Welsh Environment Agency). An alternative method to this is by the addition of ferric chloride FeCl3 to the digestion tanks in order to inhibit hydrogen sulfide production.
Volatile siloxanes can also contaminate the biogas; such compounds are frequently found in household waste and wastewater. In digestion facilities accepting these materials as a component of the feedstock, low molecular weight siloxanes volatilise into biogas. When this gas is combusted in a gas engine, turbine or boiler, siloxanes are converted into silicon dioxide (SiO2) which deposits internally in the machine, increasing wear and tear can also contaminate the biogas; such compounds are frequently found in household waste and wastewater.
In digestion facilities accepting these materials as a component of the feedstock, low molecular weight siloxanes volatilise into biogas. When this gas is combusted in a gas engine, turbine or boiler, siloxanes are converted into silicon dioxide (Si02) which deposits internally in the machine, increasing wear and tear.
Advantages of Anaerobic Digestion
When organic waste is deposited in landfill sites, it produces an enormous amount of greenhouse gases, including Carbon Dioxide and Methane. AD can play an important role as a means of dealing with organic waste and eliminating, by more efficient capture and treatment, these greenhouse gas emissions; converting them to useful products. AD recovers energy which would otherwise be lost to the atmosphere and produces valuable biofertilisers. The biogas can be used to generate electricity, heat, biofuels or cleaned and injected into the gas grid.
- Bokashi anaerobic digestion: a domestic anaerobic digestion process, ie for kitchen scraps
- Original:Recycling agricultural wastes to produce hot water
- Lorry has driven 25 times around the earth using gas from Biodegradable waste
- See brochure Bioferm_Trockenfermentation.pdf at www.graskracht.be
- See graskracht_25-11-2011_Isabella Wierinck_OWS.pdf at www.graskracht.be
- See 25092012 Droogvergisten Jan klein Hesselink Ekwadraat.pdf at www.graskracht.be
- Note that although it appears similar to composting it isn't the same, as dry digestion uses anaerobic digestion, composting uses a aerobic digestion. The air is kept out of the dry digestion process using airtight seals (still allowing to let the biogas out)