PowerPedia:Anaerobic digestion

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Anaerobic digestion (AD) is the harnessed and contained, naturally occurring process of anaerobic decomposition. An anaerobic digester is an industrial system that harnesses these natural process to treat waste, produce biogas that can be used to power electricity generators, provide heat and produce soil improving material.

Introduction

Anaerobic digesters have been around for a long time and they are commonly used for sewage treatment or for managing animal waste. Increasing environmental pressures on waste disposal has increased the use of AD as a process for reducing waste volumes and generating useful byproducts. It is a fairly simple process that can greatly reduce the amount of organic matter which might otherwise end up in landfills or waste incinerators .

Almost any organic material can be processed in this manner. 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 a airtight digester often with extra water added depending on the digestion process and feedstock.

Stages of anaerobic digestion

There are two conventional operational temperature levels:

• Mesophilic which takes place optimally at 37°C or at ambient temperatures between 20°-40°C with mesophile bacteria
• Thermophilic which takes place at elevated temperatures up to 70°C with thermophile bacteria

The residence time in a digester varies with the amount of feed material, type of material and the temperature. In the case of mesophilic digestion, residence time may be between 15 and 30 days. In the case of mesophilic UASB digestion hydraulic residence times (1hour-1day) and solid retention times (<90 days) are separated. In the thermophillic phase the process can be faster, requiring only about two weeks to complete. Thermophilic digestion is more expensive, requires more energy and is less stable than the mesophillic process. Therefore, the mesophillic process is still widely in use.

Many continuous digesters have mechanical or hydraulic devices to mix the contents and to allow excess material to be continuously extracted to maintain a reasonably constant volume.

The digestion of the organic material is done by a range of many different species of naturally occurring bacteria, all doing a different job at a different step in the digestion process. Maintaining suitable conditions in the digester is essential in maintaining a healthy bacterial population.

Four stages of anaerobic digestion have been recognised.

1. The first is 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 acidogenesis where a further breakdown into simpler molecules occurs, producing ammonia, carbon dioxide and hydrogen sulfide as byproducts.
3. The third stage is acetogenesis where the simple molecules from acidogenesis are further digested to produce carbon dioxide, hydrogen and mainly acetic acid, although higher-molecular-weight organic acids (e.g., propionic, butyric, valeric) are also produced.
4. The fourth stage is methanogenesis where methane, carbon dioxide and water are produced.

By-products of anaerobic digestion

There are three principal by-products of anaerobic digestion. Biogas, a gaseous mixture comprising mostly of methane and carbon dioxide, but also containing a small amount hydrogen and occasionally trace levels of hydrogen sulphide. Biogas can be burned to produce electricity, usually with a reciprocating engine or microturbine. The gas is often used in a cogeneration arrangement, to generate electricity and use waste heat to warm the digesters or to heat buildings. Excess electricity can be sold to electricity suppliers. Electricity produced by anaerobic digesters is considered to be green energy and may attract subsidies such as Renewables Obligation Certificates.

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 in size. Toward the end of the reaction, only the hardest to digest materials remain, leading to a decrease in the amount of biogas produced.

The second by-product (acidogenic digestate) is a stable organic material comprised largely of lignin and chitin. There is 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 fiberboard.

The third by-product is a liquid (methanogenic digestate) that is rich in nutrients and can be an excellent fertilizer dependent on the quality of the material being digested. If the digested materials include low levels of toxic heavy metals or synthetic organic materials such as pesticides or PCBs, the effect of digestion is to significantly concentrate such materials in the digester liquor. In such cases further treatment will be required in order to dispose of this liquid properly. In extreme cases, the disposal costs and the environmental risks posed by such materials can offset any environmental gains provided by the use of biogas. This is a significant risk when treating sewage from industrialized catchments.

Nearly all digestion plants have ancillary processes to treat and manage all of the by-products. The gas stream is dried and sometimes sweetened before storage and use. The sludge liquor mixture has to be separated by one of a variety of ways, the most common of which is filtration. Excess water is also sometimes treated in sequencing batch reactors (SBR) for discharge into sewers or for irrigation. Digestion can be either wet or dry. Dry digestion refers to mixtures which have a solid content of 30% or greater, whereas wet digestion refers to mixtures of 15% or less.

Reactor types

The two main types of reactors are continuous and batch. Batch is the simplest, with the biomass added to the reactor at the beginning and sealed for the duration of the process. In the continuous process, which is the more common type, organic matter is constantly added to reactor and the end products constantly removed, resulting in a much more constant production of biogas.

Although there will always be a net loss in energy in the whole system (the energy to grow the biomass is more than the output of the reactor), for the processing of waste organic material, anaerobic digestion is the preferable choice because it is environmentally friendly. The biggest impacts on the environment include the energy and materials used to build the plant, transport costs and fuel use in transporting material to site and visual and audible impacts of the site operation. Odor can be a severe problem during emptying cycles. This is a particularly difficult issue for batch reactors.

Considerations

To be economically viable, there must be a market for the end products. Biogas can be sold or used in almost all parts of the world, where it will offset demand on fossil fuel stocks. The digester liquor is suitable for use as a fertilizer, although frequently supplemental nutrients need to be added.

The sludge component, even when dried and available as a soil conditioner, is not easily disposed of. However, it has its uses in non-agricultural areas, such as golf courses, and as cover for landfills. In some localities, the sludge itself is used as a fuel in heating systems, and the residual ash is disposed of in a landfill.

Contribution to prevention of climate change

Production of Renewable Fuel

Processing biodegradable waste using anaerobic digestion helps to reduce global warming. If this waste was landfilled it would break down naturally however the biogas would escape directly into the atmosphere. In this way anaerobic digestion is considered to be a sustainable technology and biogas is considered to be a renewable fuel.

Associated technologies

Mechanical biological treatment

Main article: mechanical biological treatment

New developments in anaerobic digestion have led to systems being integrated with sorting units. Mixed waste streams such as unsorted household waste can undergo a mechanical pretreatment stage. These systems come under the category of mechanical biological treatment. They enable the recovery of the organic fraction of the waste in a form that can be processed in anaerobic digesters.

Inhibition of methanogenesis & production of alcohols

Main article: bioconversion of biomass to mixed alcohol fuels

Anaerobic digestion can be inhibited from reaching the methanogenic stage. The organic acids (i.e., carboxylic acids) from the acidogenic and acetogenic stages of the digestion can be recovered. The acids can then undergo further chemical transformations into useful chemicals or fuels.

Potential in the Hydrogen Economy

Main article: steam methane reforming

As anaerobic digestion is a renewable source of methane it offers the potential to contribute to the hydrogen economy:

Steam methane reforming (SMR) is the most common method of producing commercial bulk hydrogen. It is also the least expensive method. At high temperatures (700 – 1100 °C) and in the presence of a metal-based catalyst, steam reacts with methane to yield carbon monoxide and hydrogen.

CH₄ + H₂O → CO + 3 H₂

The United States produces nine million tons of hydrogen per year, mostly with steam reforming of natural gas. This process is different from catalytic reforming, an oil refinery process that also produces significant amounts of hydrogen along with high octane rating gasoline.

Anaerobic decomposition

Anaerobic decomposition is the reduction of the net energy level and change in chemical composition of organic matter caused by microorganisms in an oxygen-free environment. A detailed description of the process can be found tamu.edu, compost, chapter 1 (http://aggie-horticulture.tamu.edu/extension/compost/chapter1.html).

Anaerobic digester types

Below are a list of different anaerobic digester types:

• Anaerobic activated sludge process
• Anaerobic clarigester
• Anaerobic contact process
• Anaerobic expanded-bed reactor
• Anaerobic filter
• Anaerobic fluidised bed
• Anaerobic lagoon
• Anaerobic migrating blanket reactor AMBR
• Batch system anaerobic digester
• Expanded granular sludge bed digestion EGSB
• Hybrid reactor
• Imhoff tank
• One-stage anaerobic digester
• Submerged media anaerobic reactor SMARs
• Two-stage anerobic digester
• Upflow anaerobic sludge blanket digestion UASB
• Upflow and down-flow anaerobic attached growth

Anaerobic lagoon

Anaerobic lagoons are used to dispose of animal waste, particularly that of cows and pigs. The waste is washed into the lagoon by flushing the animal pens with water. Solid waste, particularly the fibrous type of cows, is sometimes separated before the wastewater enters the lagoon to prevent the buildup of solid material. Anaerobic organisms naturally present in the manure and the environment decompose the waste in the anaerobic conditions of the lagoon.

Areas with cold winters are inappropriate for anaerobic lagoons because the activity of the microorganisms is highly dependent on temperature. It is critical to have the proper size for the lagoon, with volume being more important than surface area. A minimum of two meters is necessary for anaerobic conditions, but the depth should not exceed 6 meters. Sometimes a secondary lagoon is used to accept wastes while the primary lagoon is undergoing maintenance or for other purposes.

If the anaerobic lagoon system is being used for energy production, the primary lagoon has a cover floating on the surface of the water. The cover captures the biogas produced by anaerobic bacteria. The biogas produced by anaerobic lagoons is 50 to 75% methane, with carbon dioxide making up most of the rest. The gas is usually used to produce electricity using a microturbine or reciprocating engine, but it can also be used for water or space heating. The gas usually undergoes pretreatment, particularly dehydration, prior to combustion. Sometimes the carbon dioxide, which is incombustible, is also removed.

Related

• Biogas powerplant
• List of solid waste treatment technologies
• List of waste water treatment technologies
• Mechanical biological treatment
• Sewage treatment
• Agricultural wastewater treatment
• List of waste water treatment technologies
• Biogas
• Methane
• Landfill

References and external articles

• European Anaerobic Digestion Network (http://www.adnet.org)
• Independent Anaerobic Digestion Website (http://www.anaerobic-digestion.com)
• Methane (Biogas) from Anaerobic Digesters (http://web.archive.org/web/20041124201613/www.eere.energy.gov/consumerinfo/factsheets/ab5.html?print)
• Application of anaerobic digesters (http://www.arrowbio.co.uk)
• Anaerobic Digestion (http://www.remade.org.uk/Organics/anaerobicdigestion.htm) Remade Scotland Anaerobic Digestion Page.
• Friends of the Earth (http://www.foe.co.uk/resource/briefings/anaerobic_digestion.pdf) FOE (2004 Anaerobic digestion Briefing of the breakdown of organic matter by anaerobic organisms in environments lacking oxygen
• Anaerobic Digestion (http://www.wasteresearch.co.uk/ade/efw/anaerobic.htm) Cardiff University Anaerobic Digestion Page
• ArrowBio Reference (http://www.alexmarshall.me.uk/index_files/documents/HarrogateProceedings.pdf) Finstein, M. S., Zadik, Y., Marshall, A. T. & Brody, D. (2004) The ArrowBio Process for Mixed Municipal Solid Waste – Responses to “Requests for Informationâ€?, Proceedings for Biodegradable and Residual Waste Management, Proceedings. (Eds. E. K. Papadimitriou & E. I. Stentiford), Technology and Service Providers Forum, p. 407-413

See also

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