Anaerobic Digestion

What is anaerobic digestion (AD)?

Anaerobic DiagramAnaerobic digestion is the bacterial process by which organic material decomposes to produce a 'digestate' or residue and a very interesting byproduct or 'biogas'. The biogas is typically 60% methane and 40% carbon dioxide and has a medium BTU energy value. This gas can be burnt in boilers or used to fuel engines to produce heat and power.

The process of anaerobic digestion

The process of anaerobic digestion involves a variety of different groups of bacteria. The methane producing bacteria are known as 'archaebacteria' and are some of the oldest known life forms. They evolved at a time when the atmosphere contained no oxygen and the earth was 'anaerobic'. Today, these organisms have retreated to specialist environments such as marshes, deep underground or at the bottom of the sea where conditions remain with very low levels of oxygen. They digest and respire using different chemical reactions to aerobic bacteria, producing methane rather than carbon dioxide alone.

Sealed prefabricated vessels or 'Digesters' are mostly used, however, landfills also act as very large 'digesters' and the biogas produced here is known as 'Landfill Gas'. It has a similar energy value but also contains nitrogen and a wide spectrum of often quite exotic chemicals derived from the heterogenous mixture of materials that have been landfilled.

Gas Storage
Gas Stacks

Gas storage

Landfill gas stacks for
safe disposal of excess landfill gas

Landfill gas is carefully controlled and the huge volumes of gas produced are used to generate heat and power. Where excess gas is produced, this needs to be controlled by flaring.

Is AD carbon neutral?

The gases produced by anaerobic digestion are carbon neutral as they only contain carbon taken up during the growth of any organisms which contributed to the organic matter in the first place. By comparison, ‘Natural gas’ is formed from the decomposition of organisms from millions of years ago, therefore is considered to be a fossil fuel. Biogas has the advantage that it is produced from the decomposition of recently formed organic waste; therefore biogas supply is renewable as it can be produced continuously from a variety of sources.

What feedstocks can be used?

Gas Stacks
On farm (piggery waste) anaerobic digestion facility

Almost any form of organic matter can be digested anaerobically to produce biogas. Organic wastes are most commonly used and include: farm/ animal waste, human sewage, food waste and domestic/ commercial wastes (often referred to as ‘Municipal Solid Waste’ or MSW). Woody materials are the least suitable organic material as it contains a high proportion of lignin, which is very difficult to break down anaerobically.

Food waste can be especially high in energy, making it valuable for biogas production. Oxfordshire council have introduced food waste bins, separate to garden waste bins specifically for directing to anaerobic digestion. Waitrose is the leading food retailer employing anaerobic digestion, sending approximately 50% of its food waste to anaerobic digestion plants and aims to send 95% by this route by 2013.

Increasingly, there is greater consideration given to producing crops specifically for AD. Crops rich in sugar, oils or even starch can be very successfully digested with the digestate often having value as an animal feed. There are ethical issues and land use issues associated with these feedstocks and lively debates continue.

Flow Diagram
Flow diagram showing the scheme for crop digestion
used at the Strem biogas plant

The Process

The first step of anaerobic digestion is the collection of the waste or organic material and suitable preparation. This may involve masceration or chopping and stirring to keep the solid contents of the liquor reasonably high. The liquor is then pumped into a vessel or other sealed container. By law, the waste must be liquefied first, to allow a more uniform and efficient digestion due to the formation of a higher surface area for subsequent bacterial attack. Pasteurisation may be necessary, and is performed before digestion of organic wastes. Anaerobic conditions form naturally once the container is airtight.

On an industrial level the concentrations of different gases formed during decomposition may be monitored to maximise decomposition and methane formation.

There are several optimal temperature ranges where specific bacteria flourish. The most usual operation is ‘Mesophilic’ or around 35 degrees Centigrade. Thermophilic operation is at 55 degrees Centigrade and is faster but of course requires more energy input to sustain this level. The main trade off is the size of vessel required for a given throughput – Thermophilc having a smaller vessel.

Different populations of bacteria break down different components of organic wastes. The first stage is hydrolysis. Complex insoluble carbohydrates, cellulose, proteins and fats are broken down by hydrolytic bacteria. This initial break down increases availability of material for digestion by other bacteria. The second stage is acidogenesis. Methanol is produced to be later broken down into methane. The final stage of digestion where methane is produced is methanogenesis. This stage is carried out by methanogenic bacteria, producing the final product: biogas.

Gas Stacks
Austria's first biogas feeding-plant injects purified biogas into the existing gas grid

Carbon dioxide can be removed after production of biogas, prior to combustion using Pressure Swing Adsorption (PSA) or Pressurised Water Scrubbing (PWS). PSA involves removal of carbon dioxide from the biogas by attachment to a solid surface. PWS involves pumping biogas through a tank of water. A proportion of gases dissolve into the water. This allows some hydrogen sulphide and ammonia to be removed as well, overall purifying the gas. Purified gas can be injected into the natural gas network or used as a vehicle fuel.

The water industry has been using anaerobic digestion effectively for many years to dispose of large volumes of organic wastes, mostly sewage. Gas is used to heat the digesters and also to produce electricity at the larger sites in CHP (combined heat and power) schemes.

Biogen has recently developed an anaerobic digester which can process packaged food (e.g. out of date foods from supermarkets). This is important as it minimises time needed to unpackage waste foods, broadening food waste availability.

What are the advantages?

Biogas has much lower carbon emissions than natural gas, allowing carbon reduction targets to be met more easily. Biogas is a renewable source of energy, unlike natural gas, which is considered to be a fossil fuel.

Anaerobic digestion can be performed economically on a small or large scale. The volume of biogas produced can be matched to predicted future demand.

Anaerobic digestion captures methane produced during decomposition. Methane is a significant greenhouse gas, approximately 40-60 times more powerful than carbon dioxide. Methane concentrations in the atmosphere are the highest they have ever been for millions of years due to human activity.

Another advantage of anaerobic digestion is processing of bulky organic wastes under enclosed conditions, minimising odour and other problems. Anaerobic digestion reduces the volume of organic waste sent to landfill, where it produces methane which is released into the atmosphere unless captured.

Unlike standard aerobic composting, anaerobic digestion kills most pests contained in the waste, including unwanted weed seeds and insect pests. This reduces the need for harmful pesticides and improves fertiliser quality for organic farming.

Remaining waste contains highly concentrated nutrients and can be composted to reduce it further. The waste product can be used effectively as a fertiliser or a soil conditioner.

What are the disadvantages?

Smell is one of the main disadvantages. Although the process is mostly contained, the leakage of even small volumes of gas can lead to significant odour of the typical 'bad egg' kind due to the presence of mercaptans and hydrogen sulphide.

Waste water and dust production add to processing costs. Dust can be reduced by using filters and careful handling of the organic wastes. Water is more of an issue as removing nitrates from water used in the system can be expensive but essential to prevent eutrophication.

Digestate from certain wastes, including sewage has to be carefully monitored if used as a soil conditioner. This is due to the possible contamination by heavy metals.

Costs

The larger and more advanced the system is, the more expensive it will be; however, there are significant economies of scale. On farm digester systems are particularly difficult to make economic. Centralised AD systems or ‘CAD’ systems work on mainland Europe and could be a solution in the UK. Such projects involve a number of farmers using one centralised plant.

BV Dairy, Dorset, invested in a £2.3m anaerobic digestion plant. They are now supplying 60% of their own energy (3.8 GWh) from decomposition of their waste. Any unused electricity is sold to the grid.

See also the information on the RHI (Renewable Heat Incentive) which allows for a premium payment to be made relative to the heat energy produced from AD systems.

Safety

Accumulation of hydrogen sulphide during decomposition of organic matter can be an issue as it is a poisonous gas. However, there is a wide range of safety measures available, and hydrogen sulphide is generally only a problem in the biogas digestion/ storage chambers, where it tends to sink and accumulate.

Planning

Anaerobic digestion has major potential for expansion as a carbon friendly renewable energy resource. Friends of the Earth estimate that only 0.4% of food waste in the UK is currently anaerobically digested for energy. In 2010 there were approximately 30 anaerobic digesters on farms in the UK.

Digestion plants are generally more acceptable to the public than other forms of waste processing providing the odour problem can be contained. This makes planning permission easier to obtain and often allows digestion plants to be sited nearer to highly populated areas that give a greater supply of food waste over a shorter distance.

The Government is currently reforming the UK planning permission system in favour of renewable energy projects. This should reduce planning times further in the future.

Keys links

  • International Energy Agency - www.iea-biogas.net - Involved in IEA Task 37, which aims to increase use of AD plants.
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