1. Anaerobic Digestion

[GLOSS]Anaerobic digestion[/GLOSS] (AD) is the bacterial [GLOSS]fermentation[/GLOSS] of organic material. Anaerobic digesters produce conditions that encourage the natural breakdown of organic matter by bacteria in the absence of air. AD provides an effective method for turning residues from livestock farming and food processing industries into:

• Biogas, which can be used to generate heat and/or electricity;
• Fibre, which can be used as a nutrient-rich soil conditioner;
• Liquor, which can be used as liquid fertiliser.

The rate of breakdown depends on the nature of the incoming material and the operating temperature.

The [GLOSS]biogas[/GLOSS] is typically made up of 65% methane and 35% carbon dioxide with traces of nitrogen, sulphur compounds, volatile organic compounds and ammonia. This biogas can be combusted directly in modified gas boilers or can be used to run an internal combustion engine. The calorific value of this biogas is typically 17 to 25 MJ/m3, approx. between 50% and 70% that of natural gas and can be combusted directly in modified natural gas boilers or used to run internal combustion engines. Typical composition of biogas is given in Table 1.

Biogas can also be generated in landfill ([GLOSS]landfill gas[/GLOSS]) through the similar digestion process. However, the composition of the landfill gas is different because of the wide mixture of organic waste.

The remainder, fibre, consists of an odour free residue with appearance similar to peat (although
it is not strictly comparable as peat is nutrient-free), and which has some value as a soil conditioner in some instances as an alternative to peat.

The biogas produced through the AD process usually needs to be cleaned to varying degrees as soon as possible after generation for two main reasons:

• The gas is corrosive and may damage engines;
• For health and safety reasons.
• To bring it up to pipeline quality for remote (from the AD) use.

The other digestate, the liquid has a low but diverse level of nutrients. It can be used as a liquid fertiliser in a planned fertiliser regime. As it has high water content, the liquor also has irrigation benefits, so it can be used for ‘[GLOSS]fertigation[/GLOSS]’ on agricultural land. However, as it contains particles, it should not be used for fertigation in greenhouses because it can block feeder pipes if not separated effectively.

2. How does AD Work?

AD equipment consists, in simple terms, of a heated digester tank, a gasholder to store the biogas, and a gas-burning engine/generator set, if electricity is to be produced. The digestion process takes place in a warmed, sealed airless container (the digester), which creates the ideal conditions for the bacteria to ferment the organic material in oxygen-free conditions. The digestion tank needs to be warmed and mixed thoroughly to create the ideal conditions for the bacteria to convert organic matter into biogas (a mixture of carbon dioxide, methane and small amounts of other gases).

3. What are the Main AD Processes?

An overview of the main AD processes is shown in Figure 1. These processes are described in detail in the following paragraphs.

[GLOSS]Mesophilic digestion[/GLOSS]. The digester is heated to 30 –
35ºC and the feedstock remains in the digester typically for 15 – 30 days. Mesophilic digestion tends to be more robust and tolerant than the thermophilic process, but gas production is less, larger digestion tanks are required and sanitisation, if required, is a separate process stage.

[GLOSS]Thermophilic digestion[/GLOSS]. The digester is heated to
55ºC and the residence time is typically 12 – 14 days. Thermophilic digestion systems offer higher methane production, faster throughput, better pathogen and virus ‘kill’, but require more expensive technology, greater energy input and a higher degree of operation and monitoring.

Digestion refers to various reactions and interactions that take place among the methanogens, non-methanogens and substrates fed into the digester as inputs. This is a complex physiochemical and biological process involving different factors and stages of change. This process of digestion (methanisation) is summarized below in its simple form. The breaking down of inputs, that are complex organic materials, is achieved through three stages as described below [3]:

1. Hydrolysis: The waste materials of plant and animal origins consist mainly of carbohydrates, lipids, proteins and inorganic materials. Large molecular complex substances are solubilised into simpler ones with the help of extra cellular enzyme released by the bacteria. This stage is also known as polymer breakdown stage. For example, the cellulose consisting of polymerised glucose is broken down to dimeric, and then to monomer sugar molecules (glucose) by cellulolytic bacteria.
2. Acidification: The monomer such as glucose which is produced in Stage 1 is fermented under anaerobic condition into various acids with the help of enzymes produced by the acid forming bacteria. At this stage, the acid-forming bacteria break down molecules of six atoms of carbon (glucose) into molecules of less atoms of carbon (acids), which are in a more reduced state than glucose. The principal acids produced in this process are acetic acid, propionic acid, butyric acid and ethanol.
3. Methanisation: The principle acids produced in Stage 2 are processed by methanogenic bacteria to produce methane. The reactions that takes place in the process of methane production is called methanisation and is expressed by the following equations

The above equations show that many products, by-products and intermediate products are produced in the process of digestion of inputs in an anaerobic condition before the final product (methane) is produced. Obviously, there are many facilitating and inhibiting factors that play their role in the process, such as material fed, pH value, temperature, loading rate, retention time, toxicity, etc.

During the digestion process 30 – 60% of the digestible solids are converted into biogas. This gas is a relatively rich fuel gas and may be fired to generate heat or electricity or both. It can be burned in a conventional gas boiler and used as heat for nearby buildings including farmhouses, and to heat the digester. It can be used to power associated machinery or vehicles. Alternatively, it can be burned in a gas engine to generate electricity. If generating electricity, it is usual to use a more efficient combined heat and power (CHP) system, where heat can be removed in the first instance to maintain the digester temperature, and any surplus energy can be used for other purposes. A larger scale CHP plant can supply larger housing or industrial developments, or supply electricity to the grid.

Figure 1: Overview of the AD process. Source [1]

As fresh feedstock is added to the system, digestate is pumped from the digester to a storage tank. Biogas continues to be produced in the storage tank; collection and combustion may be an economic and safety requirement. The residual digestate can be stored and then applied to the land at an appropriate time without further treatment, or it can be separated to produce fibre and liquor. The fibre can be used as a soil conditioner or composted prior to use or sale. The liquor contains a range of nutrients and can be used as a liquid fertiliser, which can be sold or used on-site as part of a crop nutrient management plan.

Nutrient analysis data for typical liquids and fibres are given in Table 2.

Table 1. Typical data on composition of biogas. Source: [2]

Table 2. Nutrient analysis of the fibre and liquor from the AD of farm slurry/manure [1]

1 Figures are not for elemental mass but mass of compounds of nitrogen, phosphate and potash.
2 The fibre and liquor also contain trace elements including magnesium, manganese, sulphur, calcium, zinc, copper, boron and sodium

4. Benefits and Problems of AD Processes

AD offers a great potential for using a renewable energy source for electricity, heat and combined heat and power generation. It is also carbon neutral, i.e. it does not generate extra carbon dioxide, and can therefore reduce overall quantities of carbon dioxide in the atmosphere. The digestate, if correctly used, can reduce demand for synthetic fertilisers and other soil conditioners, which may be manufactured using less sustainable methods.

The environmental benefits of the AD processes can be briefly summarised as follows [1]: better energy balance than in another energy production, reducing greenhouse gases, displacing use of finite fossil fuels, recycling nutrients, reducing land and water pollution, reducing demand for peat, supporting organic farming, reducing odour, efficient electricity distribution.

The following problems may occur in AD farms: potential emissions, traffic movements, noise, health and safety hazards, animal disease, and visual impact.

Keywords:

Anaerobic digestion, biofuel, biogas, fermentation, fuel, mesophilic, thermophilic.

Source:

[1] British BioGen, Trade Association to the UK Bioenergy Industry

http://www.britishbiogen.co.uk/gpg/adgpg/adgpgintro.htm#whatisad
[2] Basic information on biogas. The Finnish Biogas Association.

http://www.kolumbus.fi/suomen.biokaasukeskus/en/enindex.htm
[3] Biogas technology: A training manual for extension. Sustainable Development Department, Food and Agriculture Organisation of the United Nations
(FAO). http://www.fao.org/sd/EGdirect/EGre0021.htm