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[[Category:Technologies & Solutions]]
[[Category:Technologies & Solutions]]
'''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__
<br clear='left'/>
__TOC__


==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 (electric and heat), or refined and used as a 'biomethane', and [[Digestate]] is produced which can be used as a 'biofertiliser'.  
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>
[[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>
<br clear='left'/>
<br clear='left'/>


===Feedstock===
===Biogas===
Anaerobic Digestion uses biodegradable waste materials from a range of different sources:  
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>.
* [[Food Waste]]  
 
* [[Agricultural Waste]] such as manure and slurry
===Digestate===
* [[Sewage Sludge]]
[[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>)
* Crops (including maize, grass silage, energy beet and whole crop cereals).
 
===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.
 
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) :
 
{|class="wikitable"
![[BEIS]]!![[NNFCC]]!![[ADBA]]!!Feedstock
|-
|rowspan=5| Anaerobic Digestion||rowspan=3|Farm AD||rowspan=3|Agriculture||[[Agricultural Waste]] (incl. manures and slurries)
|-
|Crops (incl. silage and whole crops)
|-
|[[Food Waste]] ([[MSW]] and [[Commercial Waste]] sources)
|-
|rowspan=2|Food AD||rowspace=1|MSW/Commercial Waste||[[Food Waste]] ([[MSW]] and [[Commercial Waste]] sources)
|-
|Industrial||[[Food Waste]] (at the origin of the waste, incl. [[Industrial Waste]])
|-
|Sewage Sludge Digestion||Sewage Sludge AD||Sewage Sludge|| [[Sewage Sludge]] (at water treatment site)
|}
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 (broadly by [[ADBA]] on their on-line map<ref name="Map">[http://adbioresources.org/map ADBA AD Map]</ref>, but primarily by [[Ofgem]] via the different subsidy scheme requirements, but also [[BEIS]] via the Digest of UK Energy Statistics<ref name = "DUKES">[https://www.gov.uk/government/statistics/digest-of-uk-energy-statistics-dukes-2019 National Statistics: Digest of UK Energy Statistics (DUKES) 2019]</ref> and the [[DOT]] via the reporting on Road Transport Fuels<ref>[https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/790538/rtfo-annual-report-2017-2018-web.pdf RTFO Annual Report 2017-18]</ref>):


Where the process utilises [[Animal By-product Material]], it must comply with the [[Animal By-Products (Enforcement) (England) Regulations 2011]] requirements.
{|class="wikitable"
!Grouping!!Output!!Application!!Subsidy!!Band||rowspan=14|&nbsp; !!Sustainability
|-
|rowspan=8|[[CHP]]||rowspan=4|Electricity||rowspan=2|National Grid||[[ROC]] (now [[CFD]])||>5MW||rowspan=3| =>25MW
|-
|rowspan=3|[[FIT]] (now [[SEG]])||500kW-5MW
|-
|rowspan=2|'Local Wire'||250kW-500kW
|-
|<=250kW ||rowspan=7| <25MW >=1MW
|-
|rowspan=4|Heat||rowspan=2|Building(s)||rowspan=8|[[RHI]]||=>600kWth
|-
|<600kWth
|-
|rowspan=2|Heat Network||>=200kWth
|-
|<200kWth
|-
|rowspan=5| ||rowspan=5|Biomethane||rowspan=4|Gas Grid|| >80,000MWh
|-
|<=80,000MWh
|-
|> 40,000MWh || rowspan=3 |<1MW
|-
|<=40,000MWh
|-
|Transport||[[RTFO]]||-
|}


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:
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:
* 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
* 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  
* 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).  
* 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.


===Feedstock Sectors===
The Statutory Guidance for AD facilities including the use of the resultant biogas can be found here<ref>[https://www.gov.uk/government/publications/sr2021-no-6-anaerobic-digestion-facility-including-use-of-the-resultant-biogas-installations?utm_medium=email&utm_campaign=govuk-notifications-topic&utm_source=9ca610e6-b14a-4823-bb2b-77175d62080c&utm_content=daily SR2021 No.6: anaerobic digestion facility, including use of the resultant biogas - installations]</ref>.
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


===The Chemical Process of AD===
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>


'''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.
===AD Technology Options===
 
===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>)
 
==Process 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
Line 56: Line 94:


==Growth in Anaerobic Digestion==
==Growth in Anaerobic Digestion==
Whilst the introduction of the [[Renewables Obligation]] in 2002 started to stimulate growth in larger AD projects (including 'Food AD' and 'Sewage Sludge AD') it was only really with the introduction of The [[FIT]] scheme in 2010 that the AD market really started to grow, stimulating smaller projects and especially 'Farm AD' based on crops, [[Agricultural Waste]] and in some cases [[Food Waste]]. This was also enabled by investment tax benefits for investing in these projects ([[EIS]] and [[SEIS]] especially, which reduced in impact with changes to the schemes in 2015/16 leading to less investment stimulus.
Whilst the introduction of the [[Renewables Obligation]] in 2002 started to stimulate growth in larger AD projects (including 'Food AD' and 'Sewage Sludge AD') it was only really with the introduction of The [[FIT]] scheme in 2010 that the AD market really started to grow, stimulating smaller projects and especially 'Farm AD' based on crops, [[Agricultural Waste]] and in some cases [[Food Waste]]. This was also enabled by investment tax benefits for investing in these projects ([[EIS]] and [[SEIS]] especially, which reduced in impact with changes to the schemes in 2015/16 leading to less investment stimulus).
With greater focus on the collection of separate [[Food Waste]] from 2011 onwards this also stimulated the market at the time by making materials available with gate fees payable. The roll-out of [[Food Waste]] collection, however, slowed and the stimulation provided by the [[FIT]] scheme was reduced in 2017, leading to projects relying on the creation of biomethane, which in turn relied on the [[RHI]] (for injection to the gas grid) and [[RTFO]] (for use in vehicles).
This has led to a slower growth in the market, and depending on the view taken on load factors in the market (i.e. the amount of power/gas production vs the plated capacity) there is presently an excess of AD capacity in relation to the feedstocks available to the plants, especially for separately collected [[Food Waste]].


An analysis of the data provided by National Statistics<ref>https://www.gov.uk/government/statistics/monthly-small-scale-renewable-deployment</ref> for the monthly [[FIT]] commissioned installations over the period of the schemes existence shows an increase from 1 plant prior to 2010 to 427 plants by March 2019, generating 294.9MW of power.
With greater focus on the collection of separate [[Food Waste]] from 2011 onwards this also stimulated the market at the time by making materials available with gate fees payable. The roll-out of [[Food Waste]] collection, however, slowed and the stimulation provided by the [[FIT]] scheme was reduced in 2017, leading to projects relying on the creation of biomethane, which in turn relied on the [[RHI]] (for injection to the gas grid) and [[RTFO]] (for use in vehicles) which has generally led to a slowing in the growth of the market.
 
Depending on the view taken on load factors in operational plants (i.e. the amount of power/gas production vs the plated capacity) there is presently an excess of AD capacity in relation to the feedstocks available to the plants, especially for separately collected [[Food Waste]].
 
An analysis of the data provided by Office for National Statistics (ONS)<ref>[https://www.gov.uk/government/statistics/monthly-small-scale-renewable-deployment National Statistics: Monthly Small Scale Renewables]</ref> for the monthly [[FIT]] commissioned installations over the period of the schemes existence shows an increase from 1 plant prior to 2010 to 427 plants by March 2019, generating 294.9MW of power.
{| class="wikitable"
{| class="wikitable"
|-
|-
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| Total || 427||100%||294.9||100%
| Total || 427||100%||294.9||100%
|}
|}
The table shows that by 2019, whilst the size of plants were relatively evenly distributed amongst the 3 bands of measurement/scales of plant, the larger scale projects had a disproportionally large installed capacity.   
The table shows that by 2019, whilst the size of plants were relatively evenly distributed amongst the 3 bands of measurement/scales of plant, the larger scale projects had a dis-proportionally large installed capacity.   


==Number of UK Sites==
==Number of UK Sites==
At December 2018 National Statistics reported the following key statistics for the number of AD sites generating electricity (extract from table 6F)<ref>[https://www.gov.uk/government/statistics/renewable-sources-of-energy-chapter-6-digest-of-united-kingdom-energy-statistics-dukes:DUKES 2019 Chapter 6:Renewable Sources of Energy]</ref>.
At December 2018 the ONS reported the following key statistics for the number of AD sites generating electricity (extract from table 6F)<ref name = "DUKES"/>:
{| class="wikitable"
{| class="wikitable"
|-
|-
Line 92: Line 132:
|Total||419<sup>1</sup>||393||812
|Total||419<sup>1</sup>||393||812
|}
|}
# noted as 427 by the [[Ofgem]] reported figures above (these figures appear to be reporting December 2016 figures despite saying December 2018 in the table)
# noted as 427 by the [[Ofgem]] reported figures above (these ONS figures appear to be reporting December 2016 figures for FIT projects, despite stating December 2018 in the table)
 
[[ADBA]] maintain the national database of AD plants in the UK as an online tool<ref name="Map"/>. The online tool captures those plants which are based on four different feedstock configurations and three different power output configurations. A summary of the total plants listed is set out below, with an extract in the map to the right of MSW/Commercial [[Food Waste]] AD facilities generating electricity:
 
[[File:AD Map Biogas-info.png|800px|thumb|right|AD Map from Biogas-info.co.uk ]]


The table below is reproduced from table 4.3 in the Renewable Obligation Annual Report 2017-18 by [[Ofgem]] and shows the consignments reported by AD against the sustainability criteria required by the [[Biomass Sustainability]] reporting, although those with <1MW are not formally linked to a [[ROC]] issue<ref>[https://www.ofgem.gov.uk/system/files/docs/2019/03/ro_annual_report_2017-18_final.pdf Renewable Obligation Annual Report 2017-18]</ref> :
{| class="wikitable"
{| class="wikitable"
|-
|-
!  !! !! AD Stations <1MW !! AD Stations>1MW !! AD Stations >25MW  
!  !!  Electricity !! Heat !! Biomethane !! Column Total !! Matching Total
|-
| Agricultural || 314||30  || 67 || 411 || 374
|-
| MSW/Commercial (Food Waste) || 95 ||8 ||27 || 130 || 113
|-
| Industrial || 40 ||6 ||11 ||57 ||48
|-
| Sewage Sludge || 163 ||0 ||9 ||172 ||163
|-
| Row Total || 612 ||44 ||114 ||  ||
|-
| Matching Total|| 503 || 42 ||102 || ||
|}
[[ABDA]] most recently suggested<ref>Article in January 2021 for [[MRW]] magazine referring to data</ref> that in 2021 there were 686 [[AD]] plants in operation in the UK and a further 269 in development.
 
==Feedstock==
The table below is reproduced from table 4.3 in the Renewable Obligation Annual Report 2017-18 by [[Ofgem]] and shows the consignments (i.e. the feedstock flows reported to AD plants, not the number of AD plants) reported as AD against the sustainability criteria required by the [[Biomass Sustainability]] reporting, although those with <1MW are not formally linked to a [[ROC]] issue<ref>[https://www.ofgem.gov.uk/system/files/docs/2019/03/ro_annual_report_2017-18_final.pdf Renewable Obligation Annual Report 2017-18]</ref> :
{| class="wikitable"
|-
!  !! !! AD Stations <1MW !! AD Stations ≥1MW !! AD Stations >25MW  
|-
|-
| Meets the Land Criteria || Yes || 111 || 134|| 0
| Meets the Land Criteria || Yes || 111 || 134|| 0
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|Total ||1133 ||332||789||12
|Total ||1133 ||332||789||12
|}
|}
There are currently 100 AD stations with a Greenhouse Gas Emission  installed caapcity less than 1MW
There are currently 100 AD stations with a Greenhouse Gas Emission  installed capacity less than 1MW
The difference betwen the National Statistics data and the Ofgem data above, i.e. 221 sites, can be inferred to be those AD plants that are producing biomethane rather than electricity (assuming their datasets align).  
The difference between the ONS data and the [[Ofgem]] data above, i.e. 213 sites (adjusted from the 221 sites with the time error data highlighted above), can be inferred to be those AD plants that are producing biomethane rather than electricity (assuming their other datasets align).  
 
{| class="wikitable"
|-
! colspan="2"| AD Feedstock (by Volume of Gas Burnt) in AD <1MW but >50kW
|-
| colspan="2"|{{Graph:MaterialPie|Width=200|Height=200|size=200|offset=175|val1=1|val2=1|val3=20|val4=73|val5=2|val6=3}}
|-
! colspan="2"|Data Table % Split
|-
| Abbatoir Waste (AW)
| style="text-align:right;"| 1
|-
| Food Waste (FW)
| style="text-align:right;"| 1
|-
| Manures and Slurries (MS)
| style="text-align:right;" | 20
|-
| Silage (Si)
| style="text-align:right;" | 73
|-
| Whole Crops (WC)
| style="text-align:right;" | 2
|-
| Other (Ot)
| style="text-align:right;" | 3
|}
 
 


The same [[Ofgem]] data shows that the smaller projects are primarily fed by silage, manures and slurries (i.e. Farm based AD).


==References==
==References==
<references />
<references />
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