Anaerobic digestion

Anaerobic digestion (AD) is the process of the breakdown of organic matter by anaerobic organisms in environments lacking oxygen. Anaerobic digesters use the natural process of anaerobic digestion to treat waste, produce renewable energy or both.

Anaerobic digesters have been around for a long time, and they are commonly used for sewage treatment and for managing animal waste. Increasing environmental pressures on waste disposal has increased the use of digestion 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 materials such as waste paper, grass clippings, leftover food, sewage, and animal waste. After sorting or screening to remove inorganic or hazardous materials such as metals and plastics, the material to be processed is often shredded or minced to achieve a better reaction (ultrasound has even been used in the process to aid in the break up of solids). Breaking the material into smaller pieces provides the bacteria with more surface area, allowing them to complete the process quicker. The material is then fed into a sealed digester. In the case of dry materials, water is added.

Stages of anaerobic digestion

There are two conventional processes—mesophilic, which takes place at ambient temperatures typically between 20° and 40°C, and thermophilic, which takes place at elevated temperatures, typically up to 70°C. The residence time in a digester varies with the type and amount of feed material and the temperature. In the case of mesophilic digestion, residence time may be between 15 and 30 days; the thermophillic process is usually faster, requiring only about two weeks to complete. However, thermophilic digestion is more expensive, requires more energy, and is less stable than the mesophillic process. Therefore, the mesophillic process is still in use.

Many continuous digesters have mechanical devices to slowly mix the contents and to allow excess material to be continuously bled off to maintain a reasonably constant volume.

The digestion of the organic material is done by a range of many different species of different 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 digestion have been recognized.

The first is hydrolysis, in which complex organic molecules are broken down into simple sugars, amino acids, and fatty acids with the addition of hydroxyl groups.
The second stage is acidogenesis, where a further breakdown occurs producing ammonia, carbon dioxide and hydrogen sulfide.
The third stage is acetogenesis where the products of acidogenesis are further digested to produce products such as carbon dioxide, hydrogen and mainly acetates, although higher-molecular-weight organic salts (e.g., propionate, butyrate, valerate) are also produced.
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 for thermophilic digesters or to heat buildings. Excess electricity can be sold to electricity suppliers. 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 is a liquid 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 industrialised catchments.
The third by-product is a stable organic material comprised largely of lignin and chitin, but also of a variety of plastics and mineral components in a matrix of dead bacterial cells. This resembles domestic compost and can be used as compost or to make low grade building products such as fiberboard.

A less common by-product of anaerobic digestions are the organic acids (i.e., carboxylic acids) produced as in the MixAlco Process.

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.

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.

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Environmental engineering

Anaerobní digesce

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Stages of anaerobic digestion

By-products of anaerobic digestion

Reactor types

Considerations

See also

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