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'''Anaerobic Digestion''' (AD) is a process where biodegradable material is broken down by micro-organisms in the absence of oxygen in an enclosed vessel. The process produces a [[Biogas]] which can be used for generating renewable power and a nutrient-rich solid/liquor known as [[digestate]] which can be used as fertiliser and compost. Anaerobic Digestion uses biodegradable waste materials from [[Agricultural Waste]] such as manure and slurry, [[Food Waste]] and [[Sewage Sludge]]. Where the process utilises [[Animal By-product Material]], it must comply with the [[Animal By-Products (Enforcement) (England) Regulations 2011]] requirements.  
'''Anaerobic Digestion''' (AD) is a process where biodegradable material is broken down by micro-organisms in the absence of oxygen in an enclosed vessel. The process produces a [[Biogas]] which can be used for generating renewable power and a nutrient-rich solid/liquor known as [[digestate]] which can be used as fertiliser and compost. Anaerobic Digestion uses biodegradable waste materials from [[Agricultural Waste]] such as manure and slurry, [[Food Waste]] and [[Sewage Sludge]]. Where the process utilises [[Animal By-product Material]], it must comply with the [[Animal By-Products (Enforcement) (England) Regulations 2011]] requirements.  


[[File:Biogen AD plant.jpg|350px|left|Biogen Ad Plant in Gwynedd 11,500 tonnes of food waste per year - source Biogen.co.uk]]__TOC__
[[File:Biogen AD plant.jpg|400px|left|Biogen Ad Plant in Gwynedd 11,500 tonnes of food waste per year - source Biogen.co.uk]]__TOC__
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==Overview==
==Overview==
'''Anaerobic Digestion''' (AD) is a process where biodegradable material is broken down by micro-organisms in the absence of oxygen in an enclosed vessel. [[Biogas]] is produced that can be used for the generation of power (electic and heat), or refined and used as a 'biomethane', and [[Digestate]] is produced which can be used as a 'biofertiliser'.  
'''Anaerobic Digestion''' (AD) is a process where biodegradable material is broken down by micro-organisms in the absence of oxygen in an enclosed vessel. [[Biogas]] is produced that can be used for the generation of power (electic and heat), or refined and used as a 'biomethane', and [[Digestate]] is produced which can be used as a 'biofertiliser'.  
AD plants can be categorised in a number of different ways with different commentators using slightly different approaches and slightly different datasets. This combined with the ongoing development of plants over time (with plants moving from development into operation each year) means that it is difficult to build a definitive list of the number and types of plants in operation. As a result this page summarises data from different sources without attempting to correlate them to one another where the links are unclear. The broad classification by feedstock is as follows:
[[File:NEW AD diagram LR.jpg|600px|left]]<ref>[http://www.abdabioresources.org/about-ad/what-is-it/ The AD Cycle - Reproduced from the [[ABDA]] website]</ref>
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===Biogas===
The process produces a [[Biogas]] which is around 60% methane, 40% carbon dioxide and traces of other contaminant gas (depending on the feedstock being digested). [[Biogas]] can be combusted to provide heat, electricity, or both. Alternatively the [[Biogas]] can be 'upgraded' to pure methane, often called biomethane, by removing other gases. This pure stream of biomethane can then be injected it into the mains gas grid or used as a road fuel. One cubic metre of [[biogas]] at 60% methane content converts to 6.7 kWh energy<ref>http://www.biogas-info.co.uk/about/biogas/</ref>.
 
===Digestate===
[[Digestate]] is a nutrient-rich substance produced by AD that can be used as a fertiliser. It consists of left over indigestible material and dead micro-organisms - the volume of digestate will be around 90-95% of what was fed into the digester. Digestate is not compost, although it has some similar characteristics. In order for the material to no longer be considered as waste and be used as a fertiliser, the digestate must meet the standards set out in the Quality protocol and [[PAS110]]<ref>[http://www.wrap.org.uk/content/bsi-pas-110-specification-digestate [[WRAP]] Website With Link to Download]</ref> ([[SEPA]] position statement in Scotland<ref>http://www.biofertiliser.org.uk/pdf/SEPA-Position-Statement.pdf</ref>)
 
===Definition by Input===
AD plants can be categorised in a number of different ways with different commentators using slightly different approaches and slightly different datasets. This combined with the ongoing development of plants over time (with plants moving from development into operation each year) means that it is difficult to build a definitive list of the number and types of plants in operation. As a result this page summarises data from different sources without attempting to correlate them to one another where the links are unclear.  


[[File:Summary Table of AD Descriptions.png|800px|left|Summary Table of AD Descriptions Primarily by Feedstock ]]
The broad classifications by feedstock that can be used in AD are summarised in the table below including both waste and non-waste materials (with crops including maize, grass silage, energy beet and whole crop cereals) :
[[File:Summary Table of AD Descriptions.png|800px|left|Summary Table of AD Descriptions Primarily by Feedstock]]
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This classification can be further divided by the nature of the [[Food Waste]] (i.e. [[Animal By-product Material|ABPR]] and non-[[Animal By-product Material|ABPR]] material) - where the process utilises [[Animal By-product Material|ABPR]] materials, it must comply with the [[Animal By-Products (Enforcement) (England) Regulations 2011]] requirements and some Agricultural plants, for example, only utilise non-[[Animal By-product Material|ABPR]] material so as to avoid the need to comply with these requirements. Additionally, the three different feedstocks in the 'Agricultural' classification can be co-digested in different combinations to optimise plant productivity and yield of [[biogas]] and can be talked about in those terms.[[Sewage Sludge]] doesn't tend to be blended with other materials and tends to be a dedicated AD solution in its own right - primarily as it becomes impossible to to deliver [[End of Waste Criteria|End of Waste]] [[digestate]] with the introduction of [[Sewage Sludge]] as an input feedstock.
===Definition by Output===
The broad classifications by output are summarised in the table below:




reports commentators numbers without trying to cross-reference
The process of AD occurs in several stages and requires a community of micro-organisms:
# Hydrolysis - large, complex polymers like carbohydrates, cellulose, proteins and fats are broken down by hydrolytic enzymes into simple sugars, amino acids and fatty acids.
# Acidogenesis - simple monomers are broken down into volatile fatty acids
# Acetogenesis - the products of acidogenesis are broken down into acetic acid, releasing hydrogen and carbon dioxide.
# Methanogenesis - bacteria called methane formers produce methane either by cleaving two acetic acid molecules to form carbon dioxide and methane, or by reduction of carbon hydroxide with hydrogen <ref>http://www.biogas-info.co.uk/about/ad/</ref>
[[File:NEW AD diagram LR.jpg|600px|left]]<ref>[http://www.abdabioresources.org/about-ad/what-is-it/ The AD Cycle - Reproduced from the [[ABDA]] website]</ref>
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===Feedstock===
Anaerobic Digestion uses biodegradable waste materials from a range of different sources:
* [[Food Waste]]
* [[Agricultural Waste]] such as manure and slurry
* [[Sewage Sludge]]
* Crops (including maize, grass silage, energy beet and whole crop cereals).


Where the process utilises [[Animal By-product Material]], it must comply with the [[Animal By-Products (Enforcement) (England) Regulations 2011]] requirements.


In addition, AD projects are required to report on their '''[[Biomass Sustainability]]''' under the [[ROC]], [[FIT]] and [[SEG]] schemes, which demonstrates sustainability of the [[biogas]] produced based upon:
* meeting greenhouse gas criteria and is made from solid biomass (i.e. Farm AD based on a crop such as maize) that also meets the land criteria
* is made from feedstock which is waste (i.e. Food AD) or
* consists of both of the above (i.e. Farm AD that is multi-streamed).


===Feedstock Sectors===
AD plants tend to be broadly categorised into the following groups:
* Agricultural (often grouped simply into 'Farm AD')
* Mixed Agricultural/Food Waste (also often grouped simply into 'Farm AD')
* On-site Industrial (for example specialised applications for waste milk at a diary)
* Food Waste AD (often simply termed 'Food AD')
* Sewage (Sludge) AD
* Other




'''Food AD''' and '''Farm AD''' projects (the former being 'pure' [[Food Waste]] and the latter being either single stream (i.e. based on manures alone) or multi-streamed (i.e. based on a mix of crop and/or manure and/or [[Food Waste]]). [[Sewage Sludge]] doesn't tend to be blended with other materials and tends to be a dedicated AD solution in its own right - primarily as it becomes impossible to to deliver [[End of Waste Criteria|End of Waste]] compost with the introduction of [[Sewage Sludge]] as an input feedstock.


===Biogas===
AD projects are required to report on their '''[[Biomass Sustainability]]''' according to the size of the plant (as shown in the last column of the table above) under the [[ROC]], [[FIT]] and [[SEG]] subsidy schemes, which demonstrates sustainability of the [[biogas]] produced based upon:
The process produces a [[Biogas]] which is around 60% methane, 40% carbon dioxide and traces of other contaminant gas (depending on the feedstock being digested). [[Biogas]] can be combusted to provide heat, electricity, or both. Alternatively the [[Biogas]] can be 'upgraded' to pure methane, often called biomethane, by removing other gases. This pure stream of biomethane can then be injected it into the mains gas grid or used as a road fuel. One cubic metre of [[biogas]] at 60% methane content converts to 6.7 kWh energy<ref>http://www.biogas-info.co.uk/about/biogas/</ref>.
* meeting greenhouse gas criteria and is made from solid biomass (i.e. Farm AD based on a crop such as maize) that also meets the land criteria
* is made from feedstock which is waste (i.e. Food AD) or
* consists of both of the above (i.e. Farm AD that is multi-streamed).  
The banding levels shown for the various subsidies relate to different subsidy rates in each band and are referred to in this page and the associated subsidy pages and links.


===Digestate===
===The Chemical Process of AD===
[[Digestate]] is a nutrient-rich substance produced by AD that can be used as a fertiliser. It consists of left over indigestible material and dead micro-organisms - the volume of digestate will be around 90-95% of what was fed into the digester. Digestate is not compost, although it has some similar characteristics. In order for the material to no longer be considered as waste and be used as a fertiliser, the digestate must meet the standards set out in the Quality protocol and [[PAS110]]<ref>[http://www.wrap.org.uk/content/bsi-pas-110-specification-digestate [[WRAP]] Website With Link to Download]</ref> ([[SEPA]] position statement in Scotland<ref>http://www.biofertiliser.org.uk/pdf/SEPA-Position-Statement.pdf</ref>)
The process of AD occurs in several stages and requires a community of micro-organisms:
# Hydrolysis - large, complex polymers like carbohydrates, cellulose, proteins and fats are broken down by hydrolytic enzymes into simple sugars, amino acids and fatty acids.
# Acidogenesis - simple monomers are broken down into volatile fatty acids
# Acetogenesis - the products of acidogenesis are broken down into acetic acid, releasing hydrogen and carbon dioxide.
# Methanogenesis - bacteria called methane formers produce methane either by cleaving two acetic acid molecules to form carbon dioxide and methane, or by reduction of carbon hydroxide with hydrogen <ref>http://www.biogas-info.co.uk/about/ad/</ref>


==Process Options==
===AD Technology Options===
The process options are varied to manage the AD stages into the following classifications:
The process options are varied to manage the AD stages into the following classifications:
* Mesophilic or Thermophilic - operating at 25 to 45<sup>o</sup>C and 50 to 60<sup>o</sup>C respectively, the majority are Thermophilic in the UK
* Mesophilic or Thermophilic - operating at 25 to 45<sup>o</sup>C and 50 to 60<sup>o</sup>C respectively, the majority are Thermophilic in the UK
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