Anaerobic digestion

Anaerobic digestion (AD) is the biological degradation of organic material in the absence of air. An anaerobic digester is a man-made system that harnesses this natural process to treat waste, produce biogas that can be converted to heat and electricity and anaerobic digestate, a soil improving material. Anaerobic Digestion (AD) is the preferred stabilisation process for the treatment of wastewater sludges and organic wastes. The process provides volume and mass reduction and delivers valuable renewable energy with biogas production.

A biogas powerplant is an anaerobic digestion system that is designed and operated specifically for the purpose of generating energy.

Anaerobic digestion has a long history dating back to the 10th Century BC. It is presently used to treat many biodegradable wastes including sewage, industrial effluents, farm waste and the organic component of municipal solid waste.

The four stages key of anaerobic digestion are hydrolysis, acidogenesis, acetogenesis and methanogenesis. These stages result from the biological treatment of organic waste by two key bacterial groups- acetogens and methanogens.

A simplified overall chemical reaction of the process can be summarised as: C6H12O6 → 3CO2 + 3CH4

There are a number of different configurations of anaerobic digestion systems that will include either:
 * Batch or continuous loading
 * One-stage or multi-stage digestion tanks
 * Mesophilic or thermophilic operational temperatures
 * High-solids or low-solids feedstock content

Anaerobic digestion can be considered to be a sustainable technology and has many environmental benefits that lead to it contributing to the reduction of emissions of greenhouse gases to atmosphere.

History of anaerobic digestion
Biogas was first recorded to have been used for heating bath water in Assyria during the 10th century BC and then in Persia during the 16th century. In the 17th century, Jan Baptita Van Helmont found that decaying organic material produced flammable gases. In 1776, Count Alessandro Volta resolved that there was a direct connection between how much organic material was used and how much gas the material produced. In 1808, Sir Humphry Davy determined that methane was present in the gases produced by cattle manure.

The first anaerobic digester was built by a leper colony in Bombay, India in 1859. In 1895 the technology was developed in Exeter, England, where a septic tank was used to generate gas for street lighting. Also in England, In 1904 the first dual purpose tank for both sedimentation and sludge treatment was installed in Hampton. In 1907, in Germany, a patent was issued for the Imhoff tank

In the 1930’s, people began to recognise anaerobic digestion as a science, and research was done that led to the discovery of anaerobic bacteria and that led to more research into the required conditions to grow methane bacteria. This work was further developed during World War II where in Germany and France there was increased digestion of manure.

Anaerobic biochemistry
In an aerobic system using free gaseous oxygen (or air), the end products are primarily CO2 and H2O which are the stable or oxidised forms of carbon and hydrogen. If the organic waste contains nitrogen, phosphorus and sulphur, then the end products may also include NO3−, PO43− and SO42−.

In contrast to this, in an anaerobic system, there is an absence of free gaseous oxygen. In the case of anaerobic digestion, oxygen is prevented from entering the system through physical containment and isolation from the atmosphere in sealed digestion tanks. The oxygen source may be the organic waste itself or it may be supplied by inorganic oxides (in the waste). When the oxygen source in an anaerobic system is derived from the organic waste itself, then the 'intermediate' end products are (primarily) alcohols, aldehydes, and organic acids plus CO2. In the presence of specialised methanogens, the intermediates are converted to the 'final' end products of CH4, CO2 with trace levels of H2S.

Uses of anaerobic digestion
Anaerobic digesters are commonly used for effluent and sewage treatment or for managing animal waste. Anaerobic digestion is a simple process that can greatly reduce the amount of organic matter which might otherwise end up in landfills or waste incinerators. In developing countries simple home and farm-based anaerobic digestion systems offer the potential for cheap, low cost energy from biogas.

Increasing environmental pressures on solid waste disposal in developed countries have increased the use of anaerobic digestion as a process for reducing waste volumes and generating useful byproducts. Here anaerobic digestion may either be used to process the source separated fraction of biodegradable waste or alternatively combined with mechanical sorting systems to process mixed municipal waste. These facilities fall under the category of mechanical biological treatment.

Almost any organic material can be processed with anaerobic digestion. This includes biodegradable waste materials such as waste paper, grass clippings, leftover food, sewage and animal waste. Anaerobic digesters can also be fed with specially grown energy crops to boost biodegradable content and hence increase biogas production. After sorting or screening to remove inorganic or hazardous materials such as metals and plastics, the material to be processed is often shredded, minced, or hydrocrushed to increase the surface area available to microbes in the digesters and hence increase the speed of digestion. The material is then fed into an airtight digester where the anaerobic treatment takes place.

Stages of anaerobic digestion
There are four key biological and chemical stages of anaerobic digestion:


 * 1) The first is the chemical reaction of hydrolysis, where complex organic molecules are broken down into simple sugars, amino acids, and fatty acids with the addition of hydroxyl groups.
 * 2) The second stage is the biological process of acidogenesis where a further breakdown by acidogens into simpler molecules, volatile fatty acids (VFAs) occurs, producing ammonia, carbon dioxide and hydrogen sulfide as byproducts.
 * 3) The third stage is the biological process of acetogenesis where the simple molecules from acidogenesis are further digested by acetogens to produce carbon dioxide, hydrogen and mainly acetic acid.
 * 4) The fourth stage is the biological process of methanogenesis where methane, carbon dioxide and water are produced by methanogens.

A simplified generic chemical equation of the overall process is as follows: C6H12O6 →  3CO2 + 3CH4

Products of anaerobic digestion
There are three principal products of anaerobic digestion: biogas, digestate and water.

Biogas
Biogas is a gaseous mixture comprising mostly methane and carbon dioxide, but also containing a small amount hydrogen and trace levels of hydrogen sulfide. The methane in biogas can be burned to produce electricity, usually with a reciprocating engine or microturbine. The gas is often used in a cogeneration arrangement where both electricity is generated and waste heat 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 green energy and may attract subsidies.

Since the gas is not released directly into the atmosphere and the carbon dioxide comes from an organic source with a short carbon cycle biogas does not contribute to increasing atmospheric carbon dioxide concentrations; because of this, it is considered to be an environmentally friendly energy source. The production of biogas is not a steady stream; it is highest during the middle of the reaction. In the early stages of the reaction, little gas is produced because the number of bacteria is still small. Toward the end of the reaction, only the hardest to digest materials remain, leading to a decrease in the amount of biogas produced.

Digestate
Digestate can come in three forms; fibrous, liquor or a sludge-based combination of the two fractions. In two-stage systems the different forms of digestate come from different digestion tanks. In single stage digestion systems the two fractions will be combined and if desired separated by further processing.

Fibrous; acidogenic digestate
The second by-product (acidogenic digestate) is a stable organic material comprised largely of lignin and chitin, but also of a variety of  mineral components in a matrix of dead bacterial cells; some plastic may be present. This resembles domestic compost and can be used as compost or to make low grade building products such as fibreboard.

Liquor; methanogenic digestate
The third by-product is a liquid (methanogenic digestate) that is rich in nutrients and can be used as a fertiliser dependent on the quality of the material being digested. Levels of potentially toxic elements (PTEs) should be chemically assessed. This will be dependent upon the quality of the original feedstock. In the case of most clean and source-separated biodegradable waste streams the levels of PTEs will be low. In the case of wastes originating from industry the levels of PTEs may be higher and will need to be taken into consideration when determining a suitable end use for the material.

Wastewater
The final output from anaerobic digestion systems is water. This water originates both from the moisture content of the original waste that was treated but also includes water produced during the microbial reactions in the digestion systems. This water may be released from the dewatering of the digestate or may be implicitly separate from the digestate. It will typically contain high BOD and COD that will require further treatment prior to being released into water courses or sewers. This can be achieved by oxygenation of the end effluent in tanks associated with the digesters.

Feedstock considerations
The first and foremost issue when consideration of the implementation of anaerobic digestion systems is the feedstock. Digesters typically can accept any biodegradable material, however the level of putrescibility is the key factor. The more putrescible the material the higher the gas yields possible from the system. The anaerobes can breakdown material to varying degrees of success from readily in the case of short chain hydrocarbons such as sugars, to over longer periods of time in the case of cellulose and hemicellulose. Anaerobic microorganisms are unable to break down long chain woody molecules such as lignin. Anaerobic digesters were typically designed for operation using sewage sludge and manures. Sewage and manure are not however the material with the most potential for anaerobic digestion as the biodegradable material has already had the energy content taken out by the animal which produced it.

A second consideration related to the feedstock will be moisture content. The wetter the material the more suitable the material will be to handling with pumps instead of screw presses and physical means of movement. Also the wetter the material, the more volume and area it takes up relative to the levels of gas that are produced.

The level of contamination of the feedstock material is a key consideration. If the feedstock to the digesters has significant levels of physical contaminants such as plastic, glass or metals then pre-processing will be required in order for the material to be used. If it is not removed then the digesters can be blocked and will not function efficiently. It is with this logic in mind that mechanical biological treatment plants based on anaerobic digestion are designed.

Process configuration
Anaerobic digestion systems can be designed to operate in a number of different configurations:


 * Batch or continuous
 * Temperature: Mesophilic or thermophilic
 * Solids content: High solids or low solids
 * Complexity: Single stage vs multistage

Batch or continuous
A batch system is the simplest form, where the biomass added to the reactor at the beginning and sealed for the duration of the process. Batch reactors can suffer from odour issues which can be a severe problem during emptying cycles. Typically biogas production will form in a normal distribution pattern. The operator can use this fact to determine when they believe the process of digestion of the organic matter has completed. Bioreactor landfills and anaerobic lagoons can take form.

In the continuous process, which is the more common type, organic matter is constantly added, or added in stages to the reactor. Here the end products are constantly or periodically removed, resulting in constant production of biogas. Examples of this form of anaerobic digestion include UASB, EGSB and IC reactors.

Temperature
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 with mesophiles - mesophilic archaea - are the primary microorganism
 * Thermophilic which takes place optimally around 50°-52° at elevated temperatures up to 70°C where thermophiles - thermophilic archaea - are the primary microorganisms

Methanogens come from the primitive group of bacteria called the archaea. This bacterial family includes species that grow at the hostile conditions hydrothermal vents. These species are more resistant to heat and can therefore operate at thermophilic temperatures, which is unique to bacterial families.

Typically there are a greater number of species of mesophiles that are present in mesophilic digestion systems. These bacteria are more tolerant to changes environmental conditions than thermophiles. Mesophilic systems are therefore considered to be more stable than thermophilic digestion systems.

As mentioned above, 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. In countries where legislation, such as the Animal By-Products Regulations in the European Union, requires end products to meet certain levels of bacteria in the output material this may be a benefit.

A draw back of operating at thermophilic temperatures is that more heat energy input is required to achieve the correct operational temperatures. This increase in energy may not be outweighed by the increase in the outputs of biogas from the systems. Hence it is important to consider an energy balance for these systems.

Solids content
Typically there are two different operational parameters associated with the solids content of the feedstock to the digesters:


 * High-solids
 * Low-solids

Digesters can either be designed to operate in a high solids content, with a total suspended solids (TSS) concentration greater than c20%, or a low solids concentration less than 15%.

High solids digesters process a thick slurry that will require more energy input to move and process the feedstock. They 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 pumps that require significantly lower energy input. Low solids digesters will require a larger amount of land than high solids due to the increase volumes. There are benefits associated with operation in a liquid environment enabling more thorough circulation of materials and contact between the bacteria and their food.

Complexity
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. This gives benefits associated with lower construction costs, however there is less control of the reactions occurring within the system.

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. 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. 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.

Residence time
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 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 typically 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.

Further treatment
Many digestion plants have ancillary processes to treat and manage the by-products. These systems can include:


 * Biogas refinement
 * Digestate maturation
 * Effluent treatment

Biogas refinement
Biogas may require further treatment and cleaning or 'scrubbing' to further refine it for other uses.

Hydrogen sulphide

Hydrogen sulphide is a toxic product of the anaerobic decomposition of sulphates contained within the input feedstock. This hydrogen sulphide is released as a trace component of the biogas.

National environmental inforcement agencies such as the US EPA or the English and Welsh Environment Agency put strict limits on the levels of gases containing hydrogen sulphide. The US EPA has mandated that industrial facilities may not burn any fuel gas that contains more than 160 ppm by volume (0.016 percent by volume) of hydrogen sulfide.

Therefore, if the levels of hydrogen sulphide in the 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.

Siloxanes

If siloxanes are present in the gas, they will adversely affect gas engines. The siloxane forms mineralised deposits on the physical elements of the engine, which will increase wear and tear. Therefore, increased levels of siloxane will render greater attention to the maintenance of the gas engine. Over certain threshold levels the gas will not be suitable for processing in the gas engine.

Methane concentration

In countries such as Switzerland, Germany and Sweden the methane in the biogas may be concentrated in order for it to be used as a vehicle transportation fuel or alternatively input directly into the gas mains. In countries where the driver for the utilisation 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 over all levels available to sell.

Digestate maturation
Digestate typically contains elements such as lignin that cannot be broken down by the anaerobic microorganisms. Also the digestate may contain ammonia that is phytotoxic and will hamper the growth of plants if it is used as a soil improving material. For these two reasons a maturation or composting stage may be employed after digestion. Lignin and other materials are available for degradation by aerobic microorganisms such as fungi helping reduce the overall volume of the material for transport. During this maturation the ammonia will be broken down into nitrates, improving the fertility of the material and making it more suitable as a soil improver.

Effluent treatment
The wastewater exiting the anaerobic digestion facility will typically have elevated levels of BOD and COD. Some of this material is termed 'hard COD' meaning it cannot be accessed by the anaerobic bacteria for conversion into biogas. If this effluent was put directly into watercourses it would negatively affect them by causing eutrophication. As such further treatment of the wastewater is often required. This treatment will typically be an oxidation stage where air is passed through the water in sequencing batch reactors or similar aeration tanks.

Consideration of suitability
As with all industrial systems, to be economically viable, there must be a use, market or acceptable disposal point for the outputs of anaerobic digestion. Biogas can be sold or used in almost all parts of the world, where it can offset demand on fossil fuel stocks. Alternatively biogas can be used to provide cheap sources of energy in the developing world and help reduce methane emissions to atmosphere.

Digestate liquor can be used as a fertiliser supplying vital nutrients to soils. The solid, fibrous component of digestate can be used as a soil conditioner. This material can help boost the organic content of soils. There are some countries, such as in Spain where there are many organically depleted soils, the markets for the digestate can be just as important as the biogas.

When considering alternatives to anaerobic digestion such as composting, anaerobic digestion performs excellently with higher renewable energy production and lower carbon emissions.

Contribution to prevention of climate change
Methane produce in anaerobic digestion facilities can be utilised to replace methane derived from fossil fuels. The carbon in biodegradable waste is part of a carbon cycle, as such the carbon released from the combustion of biogas can be thought of as having been removed by plants in the recent past, for instance within the last decade, but typically within the last growing season. If these plants are re-grown, as is the case with crops, it can be argued that the systems can be considered to be carbon neutral. This contrasts to carbon in fossil fuels that has been sequestered in the earth for many thousands of years.

Furthermore, if the putrescible waste feedstock to the digesters was landfilled, it would break down naturally and often anaerobicly, in this case the gas may escape into the atmosphere; As methane is about twenty times more potent as a greenhouse gas than carbon dioxide this would be considered more harmful. In this way correctly engineered and utilised anaerobic digestion can be considered to be sustainable and biogas considered to be a renewable fuel.