Anaerobic digestion facts for kids

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Schematic of the Biogas Reactor
Schematic of an anaerobic digester as part of a sanitation system. It produces a digested slurry (digestate) that can be used as a fertilizer, and biogas that can be used for energy.

Anaerobic digestion is the way microorganisms break down organic matter without oxygen. This process can happen naturally, but it is called anaerobic digestion only if it is supported and contained. An anaerobic digester is an industrial system that supports these natural process to treat waste, produce biogas that can be used to power electricity generators, provide heat and produce soil improving material.


Anaerobic Lagoon at Cal Poly
Anaerobic lagoon and generators at the Cal Poly Dairy, United States

Anaerobic digestion is particularly suited to wet organic material and is commonly used for effluent and sewage treatment. Anaerobic digestion is a simple process that can greatly reduce the amount of organic matter which might otherwise be destined to be dumped at sea, landfilled or burnt in an incinerator.

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 such as silage for dedicated biogas production. In Germany and continental Europe these facilities are referred to as biogas plants. A co-digestion or co-fermentation plant is typically an agricultural anaerobic digester that accepts two or more input materials for simultaneous digestion.

In developing countries simple home and farm-based anaerobic digestion systems offer the potential for cheap, low-cost energy for cooking and lighting. Anaerobic digestion facilities have been recognized by the United Nations Development Programme as one of the most useful decentralized sources of energy supply. From 1975, China and India have both had large government-backed schemes for adaptation of small biogas plants for use in the household for cooking and lighting. Presently, projects for anaerobic digestion in the developing world can gain financial support through the United Nations Clean Development Mechanism if they are able to show they provide reduced carbon emissions.

Utilising anaerobic digestion technologies can help to reduce the emission of greenhouse gasses in a number of key ways:

  • Replacement of fossil fuels
  • Reducing or eliminating the energy footprint of waste treatment plants
  • Reducing methane emission from landfills
  • Displacing industrially-produced chemical fertilizers
  • Reducing vehicle movements
  • Reducing electrical grid transportation losses

Methane and power produced in anaerobic digestion facilities can be utilized to replace energy derived from fossil fuels, and hence reduce emissions of greenhouse gasses. This is due to the fact that the carbon in biodegradable material is part of a carbon cycle. The carbon released into the atmosphere from the combustion of biogas has been removed by plants in order for them to grow in the recent past. This can have occurred within the last decade, but more typically within the last growing season. If the plants are re-grown, taking the carbon out of the atmosphere once more, the system will be carbon neutral. This contrasts to carbon in fossil fuels that has been sequestered in the earth for many millions of years, the combustion of which increases the overall levels of carbon dioxide in the atmosphere.

If the putrescible waste processed in anaerobic digesters was disposed of in a landfill, it would break down naturally and often anaerobically. In this case the gas will eventually escape into the atmosphere. As methane is about twenty times more potent as a greenhouse gas as carbon dioxide this has significant negative environmental effects.

Digester liquor can be used as a fertiliser supplying vital nutrients to soils. The solid, fibrous component of the digested material can be used as a soil conditioner to increase the organic content of soils. The liquor can be used instead of chemical fertilisers which require large amounts of energy to produce and transport. The use of manufactured fertilisers is therefore more carbon intensive than the use of anaerobic digester liquor fertiliser. In countries, such as Spain where there are many organically depleted soils the markets for the digested solids can be equally as important as the biogas.

In countries that collect household waste, the utilization of local anaerobic digestion facilities can help to reduce the amount of waste that requires transportation to centralized landfill sites or incineration facilities. This reduced burden on transportation reduces carbon emissions from the collection vehicles. If localized anaerobic digestion facilities are embedded within an electrical distribution network, they can help reduce the electrical losses that are associated with transporting electricity over a national grid.

Power generation

Gas street lamp

Biogas from sewage works is sometimes used to run a gas engine to produce electrical power; some or all of which can be used to run the sewage works. Some waste heat from the engine is then used to heat the digester. It turns out that the waste heat is generally enough to heat the digester to the required temperatures. The power potential from sewage works is limited. The scope for biogas generation from non-sewage waste biological matter – energy crops, food waste, abattoir waste etc is much higher, estimated to be capable of about 3,000 MW. Farm biogas plants using animal waste and energy crops are expected to contribute to reducing CO2 emissions and strengthen the grid while providing UK farmers with additional revenues.

Some countries offer incentives in the form of, for example, Feed-in Tariffs for feeding electricity onto the power grid in order to subsidize green energy production.

Grid injection

Biogas grid-injection is the injection of biogas into the natural gas grid. Until the breakthrough of micro combined heat and power, two-thirds of all the energy (the heat) produced by biogas power plants was lost, as a result of using the grid to transport the gas to customers. As an alternative, the electricity and the heat can be used for on-site generation, resulting in a reduction of losses in the transportation of energy.

Anaerobic digestion Facts for Kids. Kiddle Encyclopedia.