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.

Environmental science / Environmental technology:

* Air pollution control
* Air pollution dispersion modeling
* Alternative energy
* Biofuel
* Composting
* Ecoforestry
* Energy conservation
* Environmental design
* Future energy development
* Green building
* Hydrogen technologies
* Recycling
* Renewable energy
* Renewable energy development
* Remediation
* Solid waste treatment
* Sustainable architecture
* Sustainable energy
* Sustainable development
* Waste water treatment
* Water purification
* Waste management

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 have 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 hydro-crushed 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 two conventional operational temperature levels:

* Mesophilic which takes place optimally around 37°-41°C or at ambient temperatures between 20°-45°C with mesophile bacteria
* Thermophilic which takes place optimally around 50°-52° 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 thermophilic 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 mesophilic process. Therefore, the mesophilic 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 involves 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 recognized.
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 sulfide. 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, 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 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:
There is a range of types of anaerobic digesters, however the two main types of operations 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. Batch reactors can suffer from odour issues which can be a severe problem during emptying cycles.

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. The carbon in biodegradable waste is part of a complete carbon-cycle: the carbon released from the combustion of biogas was removed by plants in the recent past, and does not contribute to the global accumulation of carbon in the same manner that fossil fuels do. Furthermore, if this waste was landfilled it would break down naturally and the biogas would escape directly into the atmosphere. Using the biogas for energy is an intermediate use that does not affect the overall cycle. 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:
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.


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 / 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.