Physical assessment of the environmental impacts of centralised anaerobic digestion SUMMARY

112 Pages·2005·4.56 MB·English

Physical assessment of the environmental impacts of centralised anaerobic
digestion

Trevor Cumby1, Daniel Sandars1, Elia Nigro1, Robert Sneath1, Graham Johnson2
and Chris Palmer2
1Silsoe Research Institute 2Holsworthy Biogas PLC

November 2005

SUMMARY

Introduction and approaches
Stored slurries on UK farms emit substantial amounts of methane. Previous MAFF-funded research
(e.g. CC 0222), has shown that farm-scale anaerobic digestion (AD) can reduce these emissions by 50
to 75% and generate useful amounts of heat and electrical energy whilst assisting with safe recycling
of wastes. However, despite these benefits, AD is not widely used in UK agriculture. Capital costs
and substantial management requirements are obvious dis-incentives to its adoption, although both can
be reduced substantially per unit volume of slurry treated by using much larger, centralised AD (CAD)
plants. For instance, co-processing with other wastes can generate revenues from gate fees.

CAD has already been adopted in other parts of Europe (e.g. Denmark and Germany), but so far, only
one CAD Plant has been built and commissioned in the UK, near Holsworthy in Devon. The aim of
this research was therefore to complete a Life Cycle Assessment (LCA) of this Plant to enable
comparison with other manure management strategies, and to help in assessing the possible impacts of
other CAD plants that may be proposed in the future. Accordingly, an emissions and biogas
monitoring system was designed, installed and commissioned in close collaboration with Holsworthy
Biogas PLC (the original owners of the plant) and the principal design and construction contractors,
Farmatic Biotech Energy UK Ltd. Other means of data acquisition were also arranged, including
several visits to farms enrolled in the CAD scheme. All monitoring was completed between January
2003 and April 2005.

CAD Plant design and operation
When monitored, the Plant treated approximately 277 m3 /day of input materials, comprising: 57%
farm slurry, 19% blood, 11% food waste, 8% chicken manure and 5 % other non-farm wastes. On
average, these inputs produced 10,085 m3/day of biogas when monitored between March 2004 and
April 2005, which generated approximately 1.32 MW of electricity, corresponding to 3.1 kWh/m3 of
biogas. Plant operation consumed 9.7% of the gross electricity produced by the Plant and the extra
road transport required, (mainly slurry and digestate) used the equivalent of 5.1% to 8.6%.

Handling of input materials and digestate in the Reception Hall (RH), was the main source of fugitive
emissions from the Plant. Although this included a ventilation system that exhausted 1.2 air changes
per hour (chg/hr), through a wet scrubber and biofilter (WSB), the total flow increased to 2.4 chg/hr
during working hours, as the RH doors opened for vehicles. Thus, although the WSB reduced the
concentrations of carbon dioxide (CO ), ammonia (NH ) and methane (CH ) by 84%, 87% and 44%
2 3 4
respectively, only half of the air from the RH passed through the WSB, leading to corresponding
annual emissions from the RH and WSB of 1091 t, 0.36 t and 21 t, respectively. The open doors
emitted 70% of the CO , 95% of the NH and 36% of the CH . Options for reducing the RH and WSB
2 3 4
emissions were reviewed.

Sample tracer gas (SF ) studies confirmed that biogas was not leaking from the digesters. The average
6
concentrations of CH , CO , and NH in the biogas were 51%, 28% and 0.05% (v/v), with most of the
4 2 3
remainder being water vapour. Thus, the ratio of CH to CO was 64% to 36%, as expected.
4 2
Modelling of air quality impacts from the Plant indicated minimal levels of ground level
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contamination from carbon monoxide, oxides of nitrogen, hydrogen sulphide, mercaptans and odours.
Nearby sites of wildlife and notable species were predicted to lie beyond the deposition envelopes.

Effects on farms enrolled with the CAD Plant
Enrolment with the Plant improved most on-farm slurry-handling facilities thus avoiding the need for
“emergency spreading” to prevent overflow. Most farmers benefited from access to new on-farm
digestate stores. Where these served outlying areas, farmers could use biologically sourced plant
nutrients (i.e. digestate) for the first time, and so use less inorganic fertiliser. These stores allowed
more use of energy-efficient umbilical handling systems (20.3 MJ/m3) replacing vacuum tankers (69.4
MJ/m3).

The mean total solids (TS) and total phosphate (P O ) concentrations in the digestate were between the
2 5
typical mean values for pig and cattle slurries. The total nitrogen (TN) concentration of digestate was
approximately 64% higher than cattle slurry, 33% higher than pig slurry and 43% lower than poultry
(layer) slurry, on a fresh weight basis. Hence, by using digestate, a sample set of three farms gained
the potential to reduce annual fertiliser purchases to around 12 kg [P O ]/ha and 167 kg [K O]/ha, for
2 5 2
intensive grassland production.

Many of the enrolled farms adopted new approaches to achieve light applications of digestate. Some
of these used band-spreaders, which reduced “scorch” (i.e. short-term destruction of chlorophyll
following surface application of digestate). Fresh digestate had an average pH of 8.2, which was
higher than the original farm slurries, (typically pH 7.5), and tended to increase NH emissions.
3
However, low-emission storage or spreading systems were not a priority issue amongst farmers,
except (rarely) where odours had caused complaints.

Implications for the wider community: Results of the Life Cycle Assessment
Overall, the CAD Plant was found to reduce greenhouse gas emissions by 12,100 tonnes CO eqv (100
2
year global warming potential, GWP) and if the enrolled farmers use a very high proportion of the
nitrogen in the digestate to replace inorganic fertiliser, nitrate eutrophication of water may be reduced
by 48 tonnes NO eqv. However, the Plant increases environmental acidification by emitting 310
3
tonnes SO eqv,. mostly as increased NH losses during the storage and use of digestate. These
2 3
emissions also increase nutrification by 59 tonnes PO eqv. Thus, for every 1000 tonnes of each total
4
Western European burden, the CAD Plant reduces GWP by 2.5 kg and eutrophication by 2.2 kg, but
increases acidification by 11.4 kg and nutrification of terrestrial habitats by 4.7 kg. If the elimination
of each burden carries the same priority, then the increases in acidification and nutrification
substantially outweigh the GWP and eutrophication benefits. Building the proposed 400kW district
heating system would improve the GWP benefit by 602 t CO eqv (i.e. 5%) and reduce acidification
2
by 3%.

Annually, the Plant’s atmospheric emissions plus the associated on-farm operations save 670 t CO ,
2
410 t CH and 2.8 t N O and 28t NO , but emit an extra 175t of NH . Nationally, these represent a
4 2 3 3
saving of 0.0001% CO , 0.02% CH and 0.002% N O, but an increase of 0.06% NH . In other words,
2 4 2 3
50 such CAD plants would be needed in the UK to reduce the national CH inventory by 1%.
4

Enrolled farmers appear to supply more P O than they receive in digestate. Specifically, when
2 5
recorded, that in the digestate accounted for approximately 65% of the P O in the slurry supplied to
2 5
the Plant. The difference could be due to progressive accumulations of solid matter in the digesters or
elsewhere.

The LCA included some important parameters that had to be assumed rather than measured. The
sensitivity of the LCA to these parameters was tested as follows.
• The Plant might produce between 43% and 66% of the maximum theoretical CH yield, thus
4
affecting subsequent methane losses from stored digestate. Acidification and eutrophication
were largely unaffected within this range, but higher yields increased the GWP benefit by 7%.
• The increased pH due to digestion was estimated to increase NH volatilisation during digestate
3
storage, by a factor between 1 and 4, compared with pig slurry. This leads to maximum
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increases in acidification and nitrification of 76% and 72% respectively. Nevertheless, this loss
of NH improved the reduction in eutrophication by 25-fold. GWP was largely unaffected.
3
• The enrolled farmers indicated that they used less inorganic nitrogen fertiliser, although the
amounts replaced with digestate were variable. If only 25% of the crop-available nitrogen in the
post-application digestate were utilised (as typical of many slurries), then the CAD Plant would
increase eutrophication by 388 t NO eqv. However, increasing this to 75% nitrogen utilisation
3
would reduce eutrophication by 48 t NO eqv. The “neutral point”, with no net change in
3
eutrophication, was 69.5%. GWP and acidification are largely unaffected by this factor.

Some environmental improvements could be achieved by operational changes, as follows.
• Increased abatement of NH emissions from digestate stores (e.g. by fitting covers); 95%
3
abatement would achieve net reductions in acidification and nitrification of 105 t SO eqv and
2
17 t PO eqv, respectively. However, eutrophication would increase to 193 t NO eqv due to
4 3
extra nitrate leaching.
• Increased abatement of NH emissions from land spreading of digestate (e.g. by deep injection);
3
85% abatement would yield net reductions in acidification and nitrification of 370 t SO eqv and
2
86 t PO eqv, respectively, but eutrophication would increase to 407 t NO eqv.
4 3
• Combined improvements: The GWP benefits of the CAD Plant can be achieved without any
acidification, eutrophication and nutrification disbenefits. Firstly, enrolled farmers need to use
at least 85% of the theoretically available nitrogen in the digestate to replace inorganic fertiliser.
Secondly, land spreading techniques need to abate 70% of the ammonia emissions compared
with low trajectory splash plate applicators. Thirdly, ammonia emissions from digestate stores
must be 50% of those from uncovered tanks. There is a trade-off between land spreading and
storage abatement. With 60% storage abatement, the minimum spreading abatement falls to
65%. Conversely, 35% storage abatement would require 80% land spreading abatement.

The establishment of further CAD systems in the UK might be guided by the following issues.
• Changes in the transport distances for slurry and digestate would have relatively small impacts,
e.g. tripling the transport distances at the Holsworthy CAD Plant would reduce the net GWP
benefit by 6%, increase net acidification and nutrification by 3% and 2% respectively, and leave
eutrophication largely unaffected. Therefore, there is some flexibility when considering new
CAD Plant locations, although the costs of road transport and other effects on local
infrastructure would impose greater constraints than the isolated implications of environmental
emissions.
• Use of digestate in arable systems; grassland systems and spring sown crops, such as maize,
provide good opportunities to utilise the highly available nitrogen in the digestate. However,
most arable crops are winter-sown so arable use of digestate brings disbenefits. For example,
4.8% of the Holsworthy digestate goes to arable land, if this increased to 80%, the
eutrophication benefit of 47 t would become a 240t additional NO eqv burden. Concurrently,
3
the other environmental burdens would deteriorate by 4 to 5%.
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CONTENTS

1 INTRODUCTION..........................................................................................................8
1.1 Background to Centralised Anaerobic Digestion (CAD)...............................................8
1.2 Purpose of this research..................................................................................................8
1.2.1 The CAD opportunity.....................................................................................................8
1.2.2 The application Life Cycle Assessment techniques.......................................................8
1.3 Strategic context of this research....................................................................................9
2 SCIENTIFIC OBJECTIVES..........................................................................................9
2.1 Objective 01...................................................................................................................9
2.2 Objective 02...................................................................................................................9
2.3 Objective 03...................................................................................................................9
2.4 Objective 04.................................................................................................................10
2.5 Objective 05.................................................................................................................10
2.6 Objective 06.................................................................................................................10
3 ACHIEVEMENT OF OBJECTIVES...........................................................................10
3.1 Objective 01.................................................................................................................10
3.1.1 Working plan................................................................................................................10
3.1.2 Life Cycle Assessment.................................................................................................10
3.2 Objective 02.................................................................................................................10
3.2.1 Installation....................................................................................................................11
3.2.2 Commissioning.............................................................................................................11
3.3 Objective 03.................................................................................................................11
3.3.1 Plant monitoring during start-up..................................................................................11
3.3.2 Plant monitoring during normal operation...................................................................11
3.4 Objective 04.................................................................................................................11
3.4.1 Mass balance analysis..................................................................................................12
3.4.2 Environmental benefits and disbenefits of CAD..........................................................12
3.5 Objective 05.................................................................................................................12
3.6 Objective 06.................................................................................................................12
4 APPARATUS AND METHODS.................................................................................12
4.1 CAD Plant concept, design and operation....................................................................12
4.1.1 Holsworthy CAD Plant location...................................................................................12
4.1.2 The Holsworthy CAD Plant concept............................................................................13
4.1.3 Overview of plant design............................................................................................13
4.1.4 Plant operation..............................................................................................................15
4.1.4.1 Delivery of input materials.....................................................................................15
4.1.4.2 Pre-processing and digestion..................................................................................15
4.1.4.3 Use of biogas and digestate.....................................................................................17
4.1.5 Overview of the envisaged economic performance of the plant................................17
4.2 Emissions and dispersion modelling............................................................................18
4.2.1 Aims of emissions and dispersion modelling..............................................................18
4.2.2 Likely key sources of gas and odour emissions from the Plant, and preventative
measures.......................................................................................................................18
4.2.3 Strength of emission sources........................................................................................19
4.2.4 Design and operation of the wet scrubber and biofilter................................................20
4.2.4.1 Wet Scrubbing Unit................................................................................................20
4.2.4.2 Biofilter...................................................................................................................20
4.2.4.3 Operation................................................................................................................21
4.2.5 Dispersion modelling...................................................................................................21
4.2.5.1 Locations studied....................................................................................................21
4.2.5.2 Meteorological Data................................................................................................22
4.2.5.3 Methods..................................................................................................................23
4.3 Process monitoring and sampling strategies for biogas and emissions........................24
4.3.1 CAD plant control data.................................................................................................24
4.3.2 Biogas and emissions sampling and analysis...............................................................25
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4.3.3 Principles and calibration of the gas analysis system...................................................28
4.3.3.1 Principles................................................................................................................28
4.3.3.2 Zero Point Calibration.............................................................................................28
4.3.3.3 Cross Interference Calibration................................................................................28
4.3.3.4 Span Calibration......................................................................................................29
4.4 Fugitive biogas emissions............................................................................................29
4.4.1 Specific biogas emission sources and methods of measurement.................................29
4.4.2 Biogas leakage: theoretical aspects..............................................................................29
4.4.3 Emissions from the Reception Hall..............................................................................30
4.5 Gathering data from farms involved with the CAD Plant............................................30
4.5.1 Supply of slurry and utilisation of digestate.................................................................30
4.5.2 Properties of digestate..................................................................................................31
4.6 Life Cycle Assessment production and analysis..........................................................31
4.6.1 Goal and scope.............................................................................................................31
4.6.2 Limiting assumptions...................................................................................................32
5 RESULTS AND DISCUSSION: SYSTEM MONITORING......................................33
5.1 CAD Plant performance...............................................................................................33
5.1.1 Mass balance................................................................................................................33
5.1.1.1 Flows within the Plant............................................................................................33
5.1.1.2 Month-by-month exchange of materials with local farms......................................33
5.1.1.3 Quantities of other input materials..........................................................................34
5.1.1.4 Biogas production...................................................................................................35
5.1.2 Energy balance.............................................................................................................36
5.1.2.1 Gross energy production.........................................................................................36
5.1.2.2 Net energy production.............................................................................................36
5.1.3 Sources of input materials............................................................................................37
5.1.4 Transport of materials..................................................................................................39
5.1.4.1 Overview of transport operations............................................................................39
5.1.4.2 Analysis of transport operations: proportion of return loads from farm sites.........40
5.1.4.3 Estimation of total transport distance *total load...................................................42
5.1.4.4 Analysis of operational radius of transport operations...........................................44
5.1.4.5 Fuel consumed for transport operations..................................................................49
5.1.5 Reception hall emissions..............................................................................................51
5.1.5.1 Determination of ventilation flows by tracer gas studies........................................51
5.1.5.2 Concentrations and emissions of methane, ammonia and carbon dioxide from the
Reception Hall and Biofilter...................................................................................54
5.1.5.3 Options for reducing odours and emissions from the Reception Hall and Biofilter57
5.1.6 Digester emissions: Results of tracer gas study using sulphur hexafluoride..............58
5.1.7 Dispersion Modelling Results......................................................................................59
5.1.7.1 Overview of modelling and results.........................................................................59
5.1.7.2 Oxides of Nitrogen..................................................................................................59
5.1.7.3 Carbon Monoxide...................................................................................................62
5.1.7.4 Hydrogen Sulphide.................................................................................................64
5.1.7.5 Mercaptans..............................................................................................................66
5.1.7.6 Odours.....................................................................................................................68
5.1.8 Effects on farms involved with the CAD Plant............................................................70
5.1.8.1 Properties of digestate.............................................................................................70
5.1.8.2 Effects on farm infrastructure and operations: slurry collection............................72
5.1.8.3 Effects on farm infrastructure and operations: digestate delivery and utilisation..74
5.2 Implications for the wider community.........................................................................78
5.2.1 Protection of the environment: slurry management on farms......................................78
5.2.2 Protection of the environment: digestate management on farms.................................79
6 RESULTS AND DISCUSSION: LIFE CYCLE ASSESSMENT...............................81
6.1 Determination of the Life Cycle Inventory..................................................................81
6.1.1 Transport of dairy slurries to Holsworthy and transport of digestate back to farms....81
6.1.2 Emission avoided from land fill sites...........................................................................81
6.1.3 The CAD plant and electricity generation....................................................................82
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6.1.4 Oil consumption for the Combined Heat and Power engines......................................82
6.1.5 Effects of changing from on farm storage of slurries and solid manures to storage of
digestate........................................................................................................................83
6.1.6 Effects of changing from land application of slurries to application of digestate........84
6.1.6.1 The changes observed.............................................................................................84
6.1.6.2 Energy for mixing store contents............................................................................84
6.1.6.3 Energy for land application.....................................................................................84
6.1.6.4 Emissions from land application.............................................................................85
6.1.7 Digestate assimilation by crop, soil, and environment compared with untreated slurries
85
6.1.7.1 Previous research....................................................................................................85
6.1.7.2 Timing of applications............................................................................................85
6.1.7.3 Reductions in the use of mineral fertilisers.............................................................86
6.1.7.4 Life Cycle Inventory – impact assessment.............................................................86
6.1.7.5 Impact normalisation..............................................................................................87
6.2 Life Cycle Assessment results and discussion.............................................................87
6.2.1 Nutrient balances on farms...........................................................................................87
6.2.2 Environmental Life Cycle Assessment of the Holsworthy CAD system.....................88
6.3 Sensitivity Analysis......................................................................................................88
6.3.1 Overview......................................................................................................................88
6.3.2 Methane yield...............................................................................................................88
6.3.3 Effect of pH on the volatilisation of ammonia in digestate storage.............................89
6.3.4 Nitrogen fertiliser saved as a proportion of the theoretical maximum.........................90
6.4 Improvement Analysis.................................................................................................91
6.4.1 Overview......................................................................................................................91
6.4.2 Emission Contributions from various parts of the Holsworthy Biogas scheme...........91
6.4.3 Identification of possible improvements......................................................................92
6.4.3.1 Ammonia emissions................................................................................................92
6.4.3.2 Nitrogen fertiliser use.............................................................................................93
6.4.3.3 Methane emissions..................................................................................................93
6.4.3.4 Nitrous oxide emissions..........................................................................................93
6.4.4 Improved ammonia abatement measures in digestate stores........................................94
6.4.5 Improved ammonia abatement measures during land spreading of digestate.............94
6.4.6 Comparative assessment of storage and land spreading abatement measures to reduce
ammonia emissions......................................................................................................95
6.4.7 Comparative assessment of abatement of land-spreading ammonia emissions and
increased replacement of inorganic nitrogen fertiliser with digestate..........................97
6.4.8 Combined improvements.............................................................................................99
6.5 Strategic issues for possible future CAD Plants.........................................................101
6.5.1 Carbon dioxide emissions and transport distances.....................................................101
6.5.2 Comparison of digestate utilisation in grassland and arable systems.........................102
7 IMPLICATIONS AND CONCLUSIONS.................................................................103
7.1 Holsworthy CAD Plant design and operation............................................................103
7.1.1 Overall performance of the CAD Plant......................................................................103
7.1.2 Transport of input materials and digestate.................................................................103
7.1.3 Fugitive emissions: input materials............................................................................103
7.1.4 Fugitive emissions: digester operation.......................................................................104
7.1.5 Fugitive emissions: dispersion modelling..................................................................104
7.2 Effects on farms enrolled with the CAD Plant...........................................................104
7.2.1 Management of slurry and digestate on-farms...........................................................104
7.2.2 Digestate utilisation: nutrient supply..........................................................................104
7.2.3 Digestate utilisation: application methods..................................................................104
7.3 Implications for the wider community: Results of the Life Cycle Assessment.........105
7.3.1 Key characteristics of the Holsworthy CAD Plant.....................................................105
7.3.2 Sensitivity analysis.....................................................................................................105
7.3.3 Improvement analysis.................................................................................................105
7.3.4 Strategic issues for possible future CAD Plants.........................................................106
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8 FUTURE WORK AND ACTIONS ARISING..........................................................106
8.1 Digestate Utilisation...................................................................................................106
8.2 Other issues................................................................................................................106
9 ACKNOWLEDGEMENTS.......................................................................................107
10 REFERENCES...........................................................................................................107
11 GLOSSARY...............................................................................................................109
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1 INTRODUCTION
1.1 Background to Centralised Anaerobic Digestion (CAD)
Stored slurry makes a substantial contribution to anthropogenic methane emissions in the UK.
Anaerobic digestion (AD) can bring about significant reductions in these emissions, as shown by a
previous MAFF funded project (CC 0222, Fugitive Emissions of Methane From Anaerobic Digestion).
During the AD process, anaerobic micro-organisms break-down organic compounds (including many
odorous ones) in the waste to produce mainly carbon dioxide and methane. This mixture of gases,
usually known as “biogas”, can be burned as a fuel, in boilers or in engine-driven generators to
produce heat and/or electricity. The major plant nutrients, nitrogen, phosphorus and potassium remain
in the digested product, termed “digestate”, so this material can be used as a biological fertiliser.

Intensive monitoring of commercial, on-farm digesters, completed as part of CC 0222, revealed
reductions in net methane emissions of between 50 and 75%. However, despite this advantage and the
other benefits (e.g. generating useful amounts of energy, and providing a means of sustainable waste
management by improving the plant-availability of nitrogen in slurries), AD is not widely used in UK
agriculture. Capital costs and the costs of maintaining a high level of management are obvious dis-
incentives to adoption of the technology. Both of these charges can be reduced substantially per unit
volume of slurry treated by using larger, centralised AD (CAD) plants. This uses very large digesters
to co-digest wastes from several sources, often including livestock wastes. Co-processing with other
wastes can generate revenue from gate fees, although diverse sources of input materials mean that
particular attention must be given to preventing the transfer of pathogens in the digestate. Hence,
potentially, CAD can achieve all of the benefits of using smaller AD plants on individual farms, but
also offers economic and management advantages through economies of scale, and can answer a wide
range of waste management problems.
1.2 Purpose of this research
1.2.1 The CAD opportunity
CAD has already been adopted in other parts of Europe (e.g. Denmark and Germany), but so far, only
one CAD plant has been built and commissioned in the UK, and is located near Holsworthy in Devon.
This plant was designed to process cow, pig and poultry manure from participating farmers, plus other
biological residues from food processors and other sources. The plant has the capacity to generate up
to 1.7 MW of electricity, and the digestate is returned to local farms as a biological fertiliser. The
plant also has the potential to export surplus hot water produced from cooling to a district heating
scheme. The construction of the Holsworthy Biogas Plant provided an excellent opportunity to extend
and apply the monitoring techniques previously used in CC 0222 to provide the data necessary to
produce a comprehensive and impartial environmental Life Cycle Assessment (LCA) of the Plant.

The main aim of this research was therefore to produce a comprehensive and impartial LCA of a
commercial CAD process to enable comparison with other manure management strategies, and to help
in assessing the possible impacts of other CAD plants that may be proposed in the future. This
assessment was based on plant throughput, energy production, measurements of fugitive methane and
other emissions at the digestion site, and on data from the associated upstream and downstream
transport and storage facilities.
1.2.2 The application Life Cycle Assessment techniques
LCA methods systematically follow mass and energy balances from ‘cradle to grave’ to ensure that
improvements at one stage correspond to an overall improvement and do not simply move problems
up or down the chain (SETAC, 1993). They are typically conducted using principles and guidelines
laid out in ISO 14040-14043. These break the analysis into sections: goal definition, system boundary,
inventory, and impact assessment. LCA studies stress the importance of expressing impacts per unit
of function and considering the whole production chain. This was achieved here by calculating the
Life Cycle Inventory (LCI) of emissions and the environmental burdens due to these emissions.
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The required LCA approaches were previously developed through MAFF project WA 0629 (Life
Cycle Assessment of Livestock Waste Production, Storage, Treatment and Disposal Systems). The
present LCA is intended to allow comparisons with other manure management strategies, and to assist
in assessing the possible impacts of other CAD plants that may be proposed in the future. The LCA
completed in this project was based on key data concerning plant throughput, energy production,
fugitive emissions at the digestion site, and on data from the associated upstream and downstream
transport, storage and digestate utilisation stages.

The work was completed in close consultation with Holsworthy Biogas PLC, the original owners of
the plant, and the principal design and construction contractors, Farmatic Biotech Energy UK Ltd.
SRI would like to acknowledge the considerable help and assistance provided by the staff of both
organisations.
1.3 Strategic context of this research
Overall, this project helped to meet DEFRA's policy needs for sound scientific evidence to identify
and support the best available techniques to reduce agricultural atmospheric emissions in line with
international agreements. In particular, this is relevant to meeting future targets that may follow from
the success achieved in meeting the 1992 UN Conference on Environment and Development
(UNCED) Framework Convention on Climate Change (UNFCCC), as projected under the UK Climate
Change Programme. Specifically, the targets set by the Kyoto Protocol and by the EU Burden Sharing
Agreement require research to investigate the cost-effectiveness of potential control options. Such
information will be particularly relevant to meeting the requirements of the Agriculture Working
Group within the European Climate Change Programme. Detailed research in connection with CAD
processes is highly appropriate in connection with this, following recent studies suggesting that “high
tech” anaerobic digestion has the potential to be one of the more cost-effective options for abating
methane emissions (Jarvis et al, 2001). This project helped to resolve some of the uncertainties about
CAD identified in previous MAFF-funded studies (Tipping, 1996).

This study has provided verified quantitative data on the overall environmental impact of a
commercial CAD scheme, as exemplified by the Holsworthy Biogas Plant, which can be compared
and contrasted with previous theoretical and other assessments (Baldwin, 1993; Meeks and Bates,
1999; Scott, et al., 1996; Tipping, 1996). In so doing, it began to establish a means of “bench
marking” future CAD processes. Hence, in future, it will help to formulate future Defra policies to
reduce anthropogenic methane emissions from agriculture and will assist the CAD industry in
achieving better environmental targets.

2 SCIENTIFIC OBJECTIVES
2.1 Objective 01
01 To establish a detailed working plan in conjunction with the owners, constructors and operators of
the CAD plant to be monitored. This will also include formulation of an outline LCA and the
definition of the key process and environmental measurements needed.
2.2 Objective 02
02 To install and commission the monitoring equipment necessary for the purposes of the LCA
aspects of the project.
2.3 Objective 03
03 To undertake a period of at least 18 months of plant monitoring, comprising three campaigns, each
of at least six months duration. This will include sufficient time for any start-up difficulties to be
resolved, and will thus establish a clear picture of true plant performance. The monitoring will also
include emissions from the peripheral activities such as collection, transport, storage and land
spreading. These will be undertaken on a periodic basis according to process schedules.
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2.4 Objective 04
04 To assess the environmental benefits and disbenefits of CAD, in particular its value in reducing
atmospheric emissions, by completing a “mass balance“ analysis of the whole process and of certain
subsidiary parts.
2.5 Objective 05
05 To assess the impacts of various scenarios of CAD implementation for UK emissions based on a
comparative LCA of the process. The LCA will be used to examine the sensitivity of the
environmental impact of CAD systems to key operational factors such as types of feedstock and
haulage distance. Any areas where emission reductions can be made will be identified and discussed
with the CAD Company. Where possible, the impacts of any resulting changes will be assessed. The
general implications of such changes on the costs of CAD operations can also be assessed.
2.6 Objective 06
06 To report findings including recommendations about best practice for CAD operation to reduce
emissions.

3 ACHIEVEMENT OF OBJECTIVES
3.1 Objective 01
3.1.1 Working plan
A formal “Collaboration Agreement” was signed by both Silsoe Research Institute and Holsworthy
Biogas PLC, the original owners of the plant in August 2001 (see CSG 15 Appendix 2: Collaboration
Agreement). This was supplemented with further documentation detailing the specific site inputs (see
CSG 15 Appendix 3: Technical Details and Queries – Installation of monitoring equipment: for on–
site discussion: 3/7/01 Updated 4/10/01). Together, these documents defined the key process and
environmental measurements needed, and were completed before the relevant stages of plant
construction and commissioning so that the installation of operational plant and SRI’s monitoring
equipment could be coordinated.
3.1.2 Life Cycle Assessment
LCA studies stress the importance of expressing impacts per unit of function and considering the
whole production chain. Therefore, an approach was formulated to produce the required LCA by
systematically following mass and energy balances from ‘cradle to grave’ to ensure that any
improvements at one stage would produce an overall improvement and not simply move problems up
or down the chain (SETAC, 1993). The planned LCA methods followed the principles and guidelines
defined in ISO 14040 (1997), ISO 14041 (1998), ISO 14042 (2000) and 14043 (2000). Hence the
analysis comprised four key sections:
• goal definition,
• system boundary,
• inventory and
• impact assessment.

During the first phase of the project a set of causal loop diagrams were used to develop an
understanding of the ‘system’ and its key components and interactions. These were used to develop
an outline LCA model and to give added focus to the design of the site monitoring and farm survey
activities. As expected, the initial LCA modelling helped to refine the data-collection goals of later
site visits to the farms involved with the CAD Plant. However, due to constraints of Plant operation
and other logistical and practical factors, detailed in CSG 15 Appendix 3, it was not feasible to use the
LCA modelling to refine the monitoring activities or plant management. Further details of the LCA
approaches used are provided in Section 4.6.
3.2 Objective 02
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