Shopping on line can be easy, simple and save you lots of money. It can also take a lot of your time, frustrate you, and result in unwanted purchases. Now the same can be said for regular high street shopping, but with the vast opportunity presented by the Internet it will pay you to spend a few minutes reading this and understanding how to better optimize your Anaerobic Digestion shopping experience:

1. Compare - without doubt the biggest advantage that the Anaerobic Digestion offers shoppers today is the ability to compare thousands of Anaerobic Digestion at a time. This is a great thing, but not necessarily all the time! Too much can be daunting at times so take advantage of the great comparison sites and where possible let them do the hard work for you.

2. Research - if it has been said it will be on the internet. Ignorance is no longer a justifiable reason for buying the wrong thing. Take the time to research in detail everything that you could possible want to know about

3. Testimonials - don't know anybody that has bought a Anaerobic Digestion? Wrong! If the Anaerobic Digestion is good the internet will let you know. Use the Internet as a friend and get testimonials before you buy.

4. Questions - Got a question about Anaerobic Digestion then search the Forums, FAQ's, Blogs etc. Don't be afraid to ask .....

5. Reputation - Never heard of the company selling Anaerobic Digestion? Don't worry, no reason why you should know every company in the world, but you know someone that does! Use the internet to find out what people are saying about Anaerobic Digestion and build up a picture of their reputation for sales, returns, customer service, delivery etc.

6. Returns - still worried that even after all of the above your Anaerobic Digestion wont be what you want? Check out the returns policy. There is so much competition now that someone, somewhere is bound to offer the terms that you are comfortable with.

7. Feedback - happy with your Anaerobic Digestion then let people know, after all you are depending on others people input in your buying decision, so why not give a little back.

8. Security - check for the yellow padlock on the Anaerobic Digestion site before you buy, and the s after http:/ /i.e. https:// = a secure site

9. Contact - got a question about Anaerobic Digestion, or want to leave a comment then check out the sites contact page. Reputable companies have them and respond.

10. Payment - ready to pay for your Anaerobic Digestion, then use your credit card or PayPal! Be aware of companies that don't accept them, there may be genuine reasons but given the huge amount of choice you have when buying online there is no reason at all not to buy via credit card or PayPal.

silage digester

Anaerobic digestion (AD) is the natural process of biological degradation of organic material in the absence of air. An anaerobic digester is a man-made system that harnesses this process to treat waste and produce biogas and anaerobic digestate, a soil-improving material. The biogas can then be converted into heat and electricity. An introduction to anaerobic digestion, www.anaerobic digestion.com, retrieved 17.08.07 Anaerobic digestion, www.energy.ca.gov, retrieved 17.08.07 Anaerobic digestion is widely used as a stabilisation process for the treatment of wastewater sludges and organic wastes. The process provides volume and mass reduction of the input material, whilst delivering valuable renewable energy. Anaerobic digestion, http://www.monsal.com, retrieved 18.09.07 A biogas plant is an anaerobic digestion system that is designed and operated specifically for the purpose of producing biogas. Application of biogas installation to produce electrical power in Szczecin, barzyk.pl, retrieved 17.08.07

Anaerobic digestion has a long history dating back to the 10th Century BC. It is used to treat many biodegradable wastes including sewage, industrial effluents, farm wastes and the organic component of municipal solid waste.Svoboda, I (2003) Anaerobic digestion, storage, olygolysis, lime, heat and aerobic treatment of livestock manures, www.scotland.gov.uk, retrieved 17.08.07

The four Anaerobic digestion#Stages of anaerobic digestion are hydrolysis, acidogenesis, acetogenesis and methanogenesis. These stages result from the biological treatment of organic material 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 configurations of anaerobic digestion systems including:

Anaerobic digestion is considered to be a sustainable technology due to its ability to recover energy from biodegradable waste and prevent the emission of methane into the atmosphere. These factors enable anaerobic digestion to be a method of generating renewable energy and helping to reduce the emission of greenhouse gases into the atmosphere.

Background History The first recorded use of biogas was for heating bath water in Assyria during the 10th century BC. In the 17th century, Jan Baptita Van Helmont found that decaying organic material produced flammable gases. In 1776, Count Alessandro Volta found 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.Cruazon, B. (2007) History of anaerobic digestion, web.pdx.edu, retrieved 17.08.07 Anaerobic digestion, www.waste.nl, retrieved 19.08.07

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. Research was done that led to the discovery of anaerobic bacteria which in turn led to more research into the required growth conditions for methanogenic bacteria.Humanik, F. et al (2007) Anaerobic digestion of animal manure, www.epa.gov, retrieved 17.08.07 This work was further developed during World War II where in Germany and France there was an increase in the application of anaerobic digestion for the treatment of manure.

Biochemistry In an aerobic system, the end products are primarily CO2 and H2O which are the stable, oxidised forms of carbon and hydrogen. If the biodegradable material contains nitrogen, phosphorus and sulphur, then the end products may also include NO3−, PO43− and SO42−.

In an anaerobic system, there is an absence of gaseous oxygen. In the case of anaerobic digestion, oxygen is prevented from entering the system through physical containment in sealed digestion tanks. The oxygen source may be the organic material itself or it may be supplied by inorganic oxides (in 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 CO2. In the presence of specialised methanogens, the intermediates are converted to the 'final' end products of CH4, CO2 with trace levels of H2S.Adapted from User:Mbeychok (1967) Aqueous Wastes from Petroleum and Petrochemical Plants, First edition, John Wiley & Sons, Library of Congress Control Number What is the anaerobic digestion process?, www.anaerobic-digestion.com, retrieved 17.08.07

Applications anaerobic digesters as part of a mechanical biological treatment systemAnaerobic 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. Anaerobic Digestion Page, www.remade.org.uk, retrieved 17.08.07 Friends of the Earth (2004) Anaerobic digestion Briefing Paper, www.foe.co.uk, retrieved 17.08.07Cardiff University (2005) Anaerobic Digestion Page, www.wasteresearch.co.uk, retrieved 17.08.07 An anaerobic digester configured to produce energy in this way is also be termed a biogas plant.

Environmental pressure on solid waste disposal methods in developed countries has increased the applicaiton of anaerobic digestion as a process for reducing waste volumes and generating useful byproducts. 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.Juniper (2005) MBT: A Guide for Decision Makers – Processes, Policies & Markets, www.juniper.co.uk, (Project funding supplied by Sita Environmental Trust), Accessed 22.11.06

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 or silage for dedicated biogas production. After sorting or screening to remove physical contaminants, such as metals and plastics, from the feedstock the material is often shredded, minced, or hydrocrushed Finstein, M. S., Zadik, Y., Marshall, A. T. & Brody, D. (2004) The ArrowBio Process for Mixed Municipal Solid Waste – Responses to “Requests for Information”, Proceedings for Biodegradable and Residual Waste Management, Proceedings. (Eds. E. K. Papadimitriou & E. I. Stentiford), Technology and Service Providers Forum, p. 407-413 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.

Process stages There are four key biological and chemical stages of anaerobic digestion: Anaerobic digestion, www.waste.nl, retrieved 19.08.07Ciborowski, P (2004) Anaerobic Digestion in the Dairy Industry, Minnesota Pollution Control Agency Air Innovations Conference, www.epa.gov, retrieved 19.08.07

  • 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.
  • 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.
  • 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.
  • 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 There are three principal products of anaerobic digestion: biogas, digestate and water. Abstract from Operation of Municipal Wastewater Treatment Plants Manual of Practice-MOP 11 Fifth Edition, www.e-wef.org, retrieved 19.08.07 Feasibility study concerning anaerobic digestion in Northern Ireland, www.eunomia.co.uk,retrieved 19.08.07

    Biogas Biogas is a gaseous mixture comprising mostly methane and carbon dioxide, Beginners guide to biogas, www.adelaide.edu.au, retrieved 19.08.07 How Anaerobic Digestion (Methane Recovery) Works, www.eere.energy.gov, retrieved 19.08.07 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. GE Energy - Jenbacher Gas Engines for Power Generation, www.power-technology.com, retrieved 19.08.07 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. UK Biomass Strategy 2007, www.defra.gov.uk, retrieved 19.08.07

    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. Fact sheet on anaerobic digestion, www.waste.nl, retrieved 19.08.07 Biomass and biogas, www.globalwarming101.com, retrieved 19.08.07

    Fibrous; acidogenic digestateThe 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. Oaktech Environmental Consultation Response, www.alexmarshall.me.uk, retrieved 19.08.07

    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 in its successful application. The more putrescible the material the higher the gas yields possible from the system. 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 it will be to handling with standard pumps instead of concrete pumps 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 moisture content of the target feedstock will also affect what type of system is applied to its treatment. In order to use a high solids anaerobic digester for dilute feedstocks, bulking agents such as compost should be applied to increase the solid content of the input material.

    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.

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



    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 suffer from odour issues which can be a severe problem when they are emptied. Typically 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. Bioreactor landfills and anaerobic lagoons can take this batch form.

    In the the more common continuous process organic matter is constantly 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, continuously stirred tank reactors, UASB, EGSB and IC reactors. BIOPAQ® IC, www.paques.nl, retrieved 19.08.07 Biological processes with Biomar® technology, www.envirochemie.com, retrieved 19.08.07

    Temperature There are two conventional operational temperature levels for anaerobic digesters, which are determined by the species of methanogens in the digesters:Song, Y.C., Kwon, S.J., Woo, J.H. (2004) Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge, Water Res. 2004 Apr;38(7):1653-62



    Methanogens come from the primitive group of bacteria called the archaea. This bacterial 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, 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:



    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%. Feasibility study concerning anaerobic digestion in Northern Ireland, www.eunomia.co.uk,retrieved 19.08.07

    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: Feasibility study concerning anaerobic digestion in Northern Ireland, www.eunomia.co.uk,retrieved 19.08.07



    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. HIMET—A Two-Stage Anaerobic Digestion Process for Converting Waste to Energy, www.gastechnology.org, retrieved 19.08.07

    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.Finstein, M. S. (2006) ArrowBio process integrates preprocessing and advanced anaerobic digestion to recover recyclables and generate electricity, www.oaktech-environmental.com, retrieved 19.08.07

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



    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 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. Removal of hydrogen sulfide from anaerobic digester gas, U.S. Patent, www.patentstorm.us, retrieved 17.08.07

    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. Siloxane quantification, removal and impact upon landfill gas generation facilities, www.p2pays.org, retrieved 17.08.07Wheles, E. & Pierece, E. (2004) Siloxanes in landfill and digester gas, www.scsengineers.com, retrieved 17.08.07

    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. Haase biogas energy centre, www.haase-energietechnik.de, retrieved 19.08.07

    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 a sequencing batch reactors or reverse osmosis unit.

    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.Doelle, H. W. (2001) Biotechnology and Human Development in Developing Countries, www.ejbiotechnology.info, retrieved 19.08.07

    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. Introduction and spanish organic waste situation, www.compostnetwork.info, retrieved 19.08.07

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

    Climate change Methane produce in anaerobic digestion facilities can be utilised to replace methane derived from fossil fuels. Digestion systems website, www.anaerobic-digestion.com, retrieved 19.08.07 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. Benefits of anaerobic digestion, www.afbini.gov.uk, retrieved 17.08.07 Questions about biomass energy, www.dti.gov.uk, retrieved 17.08.07 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. Global warming methane could be far more potent than carbon dioxide www.newmediaexplorer.org, retrieved 17.08.07 In this way correctly engineered and utilised anaerobic digestion can be considered to be sustainable and biogas considered to be a Renewable energy.

    See also

    References External links


    silage digester

    Anaerobic digestion (AD) is the natural process of biological degradation of organic material in the absence of air. An anaerobic digester is a man-made system that harnesses this process to treat waste and produce biogas and anaerobic digestate, a soil-improving material. The biogas can then be converted into heat and electricity. An introduction to anaerobic digestion, www.anaerobic digestion.com, retrieved 17.08.07 Anaerobic digestion, www.energy.ca.gov, retrieved 17.08.07 Anaerobic digestion is widely used as a stabilisation process for the treatment of wastewater sludges and organic wastes. The process provides volume and mass reduction of the input material, whilst delivering valuable renewable energy. Anaerobic digestion, http://www.monsal.com, retrieved 18.09.07 A biogas plant is an anaerobic digestion system that is designed and operated specifically for the purpose of producing biogas. Application of biogas installation to produce electrical power in Szczecin, barzyk.pl, retrieved 17.08.07

    Anaerobic digestion has a long history dating back to the 10th Century BC. It is used to treat many biodegradable wastes including sewage, industrial effluents, farm wastes and the organic component of municipal solid waste.Svoboda, I (2003) Anaerobic digestion, storage, olygolysis, lime, heat and aerobic treatment of livestock manures, www.scotland.gov.uk, retrieved 17.08.07

    The four Anaerobic digestion#Stages of anaerobic digestion are hydrolysis, acidogenesis, acetogenesis and methanogenesis. These stages result from the biological treatment of organic material 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 configurations of anaerobic digestion systems including:

    Anaerobic digestion is considered to be a sustainable technology due to its ability to recover energy from biodegradable waste and prevent the emission of methane into the atmosphere. These factors enable anaerobic digestion to be a method of generating renewable energy and helping to reduce the emission of greenhouse gases into the atmosphere.

    Background History The first recorded use of biogas was for heating bath water in Assyria during the 10th century BC. In the 17th century, Jan Baptita Van Helmont found that decaying organic material produced flammable gases. In 1776, Count Alessandro Volta found 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.Cruazon, B. (2007) History of anaerobic digestion, web.pdx.edu, retrieved 17.08.07 Anaerobic digestion, www.waste.nl, retrieved 19.08.07

    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. Research was done that led to the discovery of anaerobic bacteria which in turn led to more research into the required growth conditions for methanogenic bacteria.Humanik, F. et al (2007) Anaerobic digestion of animal manure, www.epa.gov, retrieved 17.08.07 This work was further developed during World War II where in Germany and France there was an increase in the application of anaerobic digestion for the treatment of manure.

    Biochemistry In an aerobic system, the end products are primarily CO2 and H2O which are the stable, oxidised forms of carbon and hydrogen. If the biodegradable material contains nitrogen, phosphorus and sulphur, then the end products may also include NO3−, PO43− and SO42−.

    In an anaerobic system, there is an absence of gaseous oxygen. In the case of anaerobic digestion, oxygen is prevented from entering the system through physical containment in sealed digestion tanks. The oxygen source may be the organic material itself or it may be supplied by inorganic oxides (in 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 CO2. In the presence of specialised methanogens, the intermediates are converted to the 'final' end products of CH4, CO2 with trace levels of H2S.Adapted from User:Mbeychok (1967) Aqueous Wastes from Petroleum and Petrochemical Plants, First edition, John Wiley & Sons, Library of Congress Control Number What is the anaerobic digestion process?, www.anaerobic-digestion.com, retrieved 17.08.07

    Applications anaerobic digesters as part of a mechanical biological treatment systemAnaerobic 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. Anaerobic Digestion Page, www.remade.org.uk, retrieved 17.08.07 Friends of the Earth (2004) Anaerobic digestion Briefing Paper, www.foe.co.uk, retrieved 17.08.07Cardiff University (2005) Anaerobic Digestion Page, www.wasteresearch.co.uk, retrieved 17.08.07 An anaerobic digester configured to produce energy in this way is also be termed a biogas plant.

    Environmental pressure on solid waste disposal methods in developed countries has increased the applicaiton of anaerobic digestion as a process for reducing waste volumes and generating useful byproducts. 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.Juniper (2005) MBT: A Guide for Decision Makers – Processes, Policies & Markets, www.juniper.co.uk, (Project funding supplied by Sita Environmental Trust), Accessed 22.11.06

    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 or silage for dedicated biogas production. After sorting or screening to remove physical contaminants, such as metals and plastics, from the feedstock the material is often shredded, minced, or hydrocrushed Finstein, M. S., Zadik, Y., Marshall, A. T. & Brody, D. (2004) The ArrowBio Process for Mixed Municipal Solid Waste – Responses to “Requests for Information”, Proceedings for Biodegradable and Residual Waste Management, Proceedings. (Eds. E. K. Papadimitriou & E. I. Stentiford), Technology and Service Providers Forum, p. 407-413 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.

    Process stages There are four key biological and chemical stages of anaerobic digestion: Anaerobic digestion, www.waste.nl, retrieved 19.08.07Ciborowski, P (2004) Anaerobic Digestion in the Dairy Industry, Minnesota Pollution Control Agency Air Innovations Conference, www.epa.gov, retrieved 19.08.07

  • 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.
  • 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.
  • 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.
  • 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 There are three principal products of anaerobic digestion: biogas, digestate and water. Abstract from Operation of Municipal Wastewater Treatment Plants Manual of Practice-MOP 11 Fifth Edition, www.e-wef.org, retrieved 19.08.07 Feasibility study concerning anaerobic digestion in Northern Ireland, www.eunomia.co.uk,retrieved 19.08.07

    Biogas Biogas is a gaseous mixture comprising mostly methane and carbon dioxide, Beginners guide to biogas, www.adelaide.edu.au, retrieved 19.08.07 How Anaerobic Digestion (Methane Recovery) Works, www.eere.energy.gov, retrieved 19.08.07 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. GE Energy - Jenbacher Gas Engines for Power Generation, www.power-technology.com, retrieved 19.08.07 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. UK Biomass Strategy 2007, www.defra.gov.uk, retrieved 19.08.07

    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. Fact sheet on anaerobic digestion, www.waste.nl, retrieved 19.08.07 Biomass and biogas, www.globalwarming101.com, retrieved 19.08.07

    Fibrous; acidogenic digestateThe 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. Oaktech Environmental Consultation Response, www.alexmarshall.me.uk, retrieved 19.08.07

    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 in its successful application. The more putrescible the material the higher the gas yields possible from the system. 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 it will be to handling with standard pumps instead of concrete pumps 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 moisture content of the target feedstock will also affect what type of system is applied to its treatment. In order to use a high solids anaerobic digester for dilute feedstocks, bulking agents such as compost should be applied to increase the solid content of the input material.

    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.

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



    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 suffer from odour issues which can be a severe problem when they are emptied. Typically 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. Bioreactor landfills and anaerobic lagoons can take this batch form.

    In the the more common continuous process organic matter is constantly 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, continuously stirred tank reactors, UASB, EGSB and IC reactors. BIOPAQ® IC, www.paques.nl, retrieved 19.08.07 Biological processes with Biomar® technology, www.envirochemie.com, retrieved 19.08.07

    Temperature There are two conventional operational temperature levels for anaerobic digesters, which are determined by the species of methanogens in the digesters:Song, Y.C., Kwon, S.J., Woo, J.H. (2004) Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge, Water Res. 2004 Apr;38(7):1653-62



    Methanogens come from the primitive group of bacteria called the archaea. This bacterial 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, 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:



    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%. Feasibility study concerning anaerobic digestion in Northern Ireland, www.eunomia.co.uk,retrieved 19.08.07

    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: Feasibility study concerning anaerobic digestion in Northern Ireland, www.eunomia.co.uk,retrieved 19.08.07



    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. HIMET—A Two-Stage Anaerobic Digestion Process for Converting Waste to Energy, www.gastechnology.org, retrieved 19.08.07

    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.Finstein, M. S. (2006) ArrowBio process integrates preprocessing and advanced anaerobic digestion to recover recyclables and generate electricity, www.oaktech-environmental.com, retrieved 19.08.07

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



    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 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. Removal of hydrogen sulfide from anaerobic digester gas, U.S. Patent, www.patentstorm.us, retrieved 17.08.07

    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. Siloxane quantification, removal and impact upon landfill gas generation facilities, www.p2pays.org, retrieved 17.08.07Wheles, E. & Pierece, E. (2004) Siloxanes in landfill and digester gas, www.scsengineers.com, retrieved 17.08.07

    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. Haase biogas energy centre, www.haase-energietechnik.de, retrieved 19.08.07

    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 a sequencing batch reactors or reverse osmosis unit.

    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.Doelle, H. W. (2001) Biotechnology and Human Development in Developing Countries, www.ejbiotechnology.info, retrieved 19.08.07

    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. Introduction and spanish organic waste situation, www.compostnetwork.info, retrieved 19.08.07

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

    Climate change Methane produce in anaerobic digestion facilities can be utilised to replace methane derived from fossil fuels. Digestion systems website, www.anaerobic-digestion.com, retrieved 19.08.07 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. Benefits of anaerobic digestion, www.afbini.gov.uk, retrieved 17.08.07 Questions about biomass energy, www.dti.gov.uk, retrieved 17.08.07 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. Global warming methane could be far more potent than carbon dioxide www.newmediaexplorer.org, retrieved 17.08.07 In this way correctly engineered and utilised anaerobic digestion can be considered to be sustainable and biogas considered to be a Renewable energy.

    See also

    References External links




    anaerobic digestion
    Waste treatment in this fashion uses the same process as that which naturally occurs in landfills. In an oxygen free environment it reduces organic ...

    Monsal - anaerobic digestion technology - biogas technology - enzymic ...
    Monsal are a specialist environmental technology business offering technology, products, processes and services for the water and waste industries. Monsal is the UK market leader ...

    Anaerobic Digestion Technology – Enzymic Hydrolysis – Monsal
    Monsal are a specialist environmental technology business offering technology, products, processes and services for the water and waste industries. Monsal is the UK market leader ...

    Anaerobic Digestion (AD) Technical Pages. Anaerobic treatment and ...
    Source for supplier independent information on the Anaerobic Digestion (AD) process and related subjects.

    Anaerobic digestion
    Anaerobic digestion Briefing September 2007 Currently, much of our biodegradable waste such as food, garden waste, card and paper is sent to landfill, where it breaks down to ...

    Anaerobic Digestion
    Anaerobic Digestion breaks down biodegradable waste such as food, animal manures and cardboard in the absence of oxygen in an enclosed chamber.

    Anaerobic digestion - Wikipedia, the free encyclopedia
    Anaerobic digestion is a process in which microorganisms break down biodegradable material in the absence of oxygen. The process is widely used to treat wastewater sludges and ...

    070820 Anerobic Digestion - Environmental Services Association
    Events. Managing Organic Waste – Does Anaerobic Digestion hold the key? 16 October 2007 One Great George Street, Westminster, London. AD and the changing economics of new waste ...

    Understanding Anaerobic Digestion
    The Environment and Sustainable Technology Understanding Anaerobic Digestion Alastair D. Martin BSc PhD CEng MIChemE Director: Environment and Sustainable Technology Programmes ...

    Environment Agency - Anaerobic processes
    Information on and cases studies of anaerobic biological waste treatment methods. ... Anaerobic. Certain bacteria in the absence of oxygen convert organic matter into a gas ...

     

    Anaerobic Digestion



     
    Copyright © 2008 Hintcenter.com - All rights reserved.
    Home | Terms of Use | Privacy Policy
    All Trademarks belong to their repective owners. Many aspects of this page are used under
    commercial commons license from Yahoo!