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Oaktree Environmental Limited Unit 5, Oasis Park, 19 Road One, Winsford, Cheshire CW7 3RY
5.1 MODEL DESCRIPTION ....................................................................................................................................... 12 5.2 MODEL INPUTS.................................................................................................................................................. 12 5.3 MODEL VERIFICATION ...................................................................................................................................... 19 5.4 MODEL SCENARIOS ........................................................................................................................................... 19 5.5 MODEL UNCERTAINTY ..................................................................................................................................... 20
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Glossary AD Anaerobic Digestion ADMS Atmospheric Dispersion Modelling System AMS American Meteorological Society BPIP Building Input Profile Program CH4 Methane CO2 Carbon Dioxide DEFRA Department for Environment, Food and Rural Affairs EA Environment Agency ELV Emission Limit Value EPA Environmental Protection Agency EPR Environmental Permitting (England and Wales) Regulations 2010 LA Local Authority NGR National Grid Reference OE Oaktree Environmental Ltd. OS Ordnance Survey OU Odour Unit UK United Kingdom US United States
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1 Non Technical Summary
Detailed dispersion modelling has been undertaken to identify potential impacts as a
result of odour emissions from a proposed Anaerobic Digestion plant to be located off
Goostrey Lane, Twemlow, Holmes Chapel. Resulting odour concentrations have been
predicted to be significantly below the worst case assessment criteria at all identified
sensitive receptor locations. Furthermore, no exceedences of the worst case assessment
criteria for odour have been predicted at any location surrounding the proposed plant.
Therefore, negligible odour impacts have been predicted as a result of plant operation.
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2 Introduction 2.1 Scope of Work
2.1.1 An odour modelling assessment has been undertaken in support of a planning
application being submitted for the proposed operation of an Anaerobic Digestion
(AD) plant to be located off Goostrey Lane, Twemlow, Holmes Chapel. The
assessment has been undertaken to predict the potential odour impacts at surrounding
human receptor locations as a result of the proposed operations. Detailed dispersion
modelling has been undertaken to predict likely resulting ground level odour
concentrations surrounding the proposed plant, which have been compared with the
relevant assessment criteria for odour. This has enabled a judgement to be made over
whether the plant is likely to have potential to cause statutory nuisance in terms of
odour.
2.2 Site Location
2.2.1 The site is located at approximate National Grid Reference (NGR) 378067, 368995.
Reference should be made to Appendix 1 Figure 1 for a map illustrating the site
location and surrounding area.
2.3 Proposed Activities
2.3.1 CRES Biogas Ltd are applying for planning permission to develop a Community
Renewable Energy Scheme. This will include development of an AD Plant.
2.3.2 AD is a biological process, which breaks down organic matter within biodegradable
wastes in the absence of oxygen, through the actions of a variety of micro-organisms.
The result of these processes is the production of biogas, which consists predominantly
of methane (CH4) and carbon dioxide (CO2) and a useable digestate product which has
environmental benefits when used in place of fertilisers. It is proposed to utilise the
biogas to power internal combustion engines for the production of electricity and heat.
The electricity produced will be exported to the National Grid. Once operational, the
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applicant will investigate possible uses for heat produced from the process, if a
suitable use can be found.
2.3.3 The feedstocks to be used will include agricultural wastes including dairy slurry and
poultry litter, maize, waste grass and food wastes. It is currently proposed to develop
the plant in two phases. During the initial phase, the wastes used will include dairy
slurry, maize, grass and poultry litter. During the second phase, the plant will also
utilise food wastes. It is currently anticipated that once fully developed, the plant will
accept the following quantities of waste:
• 15,000 tonnes cattle slurry;
• 4,000 tonnes poultry manure;
• 1,800 tonnes maize;
• 2,000 tonnes grass; and,
• 24,000 tonnes food waste.
2.3.4 Assessment of potential odour impacts within this report has been undertaken on the
basis of the fully developed plant, in order to ensure a worst case scenario.
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3 Odour Legislation and Relevant Guidance
3.1 Consulted Documents
3.1.1 The following documentation has been consulted for the purpose of this current
assessment:
• Additional Guidance for H4 Odour Management: How to Comply with your
O1 Residential receptor on Goostrey Lane 378138 368869
O2 Residential receptor on Goostrey Lane 378138 368841
O3 Residential receptor on Goostrey Lane 378145 368802
O4 Residential receptor on Goostrey Lane 378120 368792
O5 Residential receptor on Goostrey Lane 378104 368806
O6 Residential receptor on Goostrey Lane 378132 368756
O7 Residential receptor on Goostrey Lane 378139 368781
O8 Residential receptor on Twemlow Lane 378095 368757
O9 Residential receptor on Twemlow Lane 378076 368716
O10 Residential receptor on Twemlow Lane 378058 368726
O11 Residential receptor on Twemlow Lane 378045 368738
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Odour
Receptor
Identifier
Odour Sensitive Receptor Description
National Grid Reference (m)
X Y
O12 Residential receptor on Twemlow Lane 378028 368750
O13 Residential receptor on Twemlow Lane 378013 368761
O14 Residential receptor on Twemlow Lane 377990 368769
O15 Residential receptor on Twemlow Lane 377970 368780
O16 Residential receptor on Twemlow Lane 377954 368798
O17 Blue Slate Farm 377771 368833
O18 Residential receptor on Station Road 378285 369614
O19 New Farm 378896 369248
O20 Blackden Manor Farm 378882 369188
O21 Blackyard Farm 378836 368944
O22 Thornfields Cottage 378275 368746
O23 Hiverley Cottage 378416 368739
O24 Hiverley (Residential receptor) 378460 368756
O25 Hermitage Turkey Farm 376773 369074
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5 Modelling Methodology 5.1 Model Description
5.1.1 The potential air quality impact that may arise through emissions of odour during
operation of the AD plant has been quantified using AERMOD, which is a steady
state, next generation, dispersion model. AERMOD was developed jointly by the
American Meteorological Society (AMS) and the United States (US) Environmental
Protection Agency (EPA) Regulatory Model Improvement Committee. The AERMOD
model is a development from the ISC(Industrial Source Complex) 3 dispersion model
and incorporates improved dispersion algorithms and pre-processors to integrate the
impact of meteorology and topography within the modelling output, and is approved
for use in the UK by the EA. The version of AERMOD that has been used for this
current assessment is Lakes Environmental ISC-AERMOD View Version 6.7.1. The
model has been run using the most recent version of the AERMOD executable file,
11103.
5.2 Model Inputs
5.2.1 Emission Sources
5.2.1.1 Reference should be made to Appendix 1 Figure 2 for a graphical representation of the
proposed site layout. The AD process itself is sealed and therefore there is limited
potential for odour to escape from the process. The main operations which have the
potential to create odour include the delivery and handling of wastes prior to entry to
the AD process and the storage and handling of digestate and biogas evolved from the
AD process.
5.2.1.2 There are no emission limit values for odour and since the site is not yet operational, it
is not possible to monitor site odour emissions. In the absence of such information,
estimations of odour emissions from the various identified sources have to be made.
However, it should be noted that the estimations made are based upon a combination
of odour monitoring data reported elsewhere at similar plants and in the case of the
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biofilter, design residual odour concentrations provided by the technology provider for
the site.
5.2.1.3 Plant efficiency and odour control are directly interrelated parameters. Therefore,
since it is in operator interest to maximise efficiency of the plant, this in turn means
that odour generation is minimised. The principal odour abatement measures which
will be used at the plant are as follows:
• The use of a two stage digestion process with a lengthy 114.3 day retention time
ensures minimal residual gas content of digestate, whilst also providing good
opportunity for the complete stabilisation of the digestate to ensure that sulphur
containing compounds are broken down to minimise potential for generation of
odourous compounds such as hydrogen sulphide;
• Gas flow to engines will be carefully controlled to maximise combustion
efficiency and ensure biogas is not directly released to atmosphere;
• Use of the latest, most efficient technology for methane extraction to ensure
highest possible capture of methane;
• Food and poultry waste will be delivered to and stored within an enclosed
reception building, operated under negative pressure, with odour controlled using
a biofiltration system;
• Cattle slurry will be stored within an enclosed tank, only vented during slurry
delivery;
• Storage of dewatered digestate will be within an enclosed building;
• Storage of liquid digestate will be within an enclosed storage tank;
• Covering of inactive silage clamp surfaces with suitable sheeting; and,
• An Odour Management Plan (OMP) will be implemented on site, as part of the
conditions of the permit to operate.
5.2.1.4 The waste reception building will be operated under negative pressure and fitted with a
biofiltration system which will control odour before release to atmosphere. A residual
odour concentration of 2000OU.m-3 has been used to derive the odour emission rate
from the biofilter stack, which is the maximum odour concentration which the
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technology provider for the site has indicated can be achieved from the biofilter1. This
is considered to provide a worst case assessment for odour emissions likely to arise
from the waste reception building. It is currently anticipated that there will be up to 3
air changes per hour within the waste reception building, which will result in up to
35,712m3 of air being exhausted from the biofilter stack each hour, which equates to
an exit flow rate of 9.92m3.s-1. Based upon the biofilter odour emission concentration
of 2000OU.m3, an odour emission rate of 19,831.7OU.s-1 has been calculated for the
biofilter stack. It is currently anticipated that the stack internal diameter will be 0.65m,
whilst an efflux temperature of 298K has been assumed.
5.2.1.5 The cattle slurry tank will be enclosed. However, there is potential for odour emission
during the unloading of slurry into the tank within vented displaced air. Based upon
values reported elsewhere2, an emission factor of 200OU.m-3 is considered to provide
a worst case assessment. It is currently expected that slurry will be delivered to site
within 12 tonne capacity tankers, up to 4 times each day. Each delivery is expected to
take approximately 10 minutes. Based upon this data, the peak odour emission rate
during slurry delivery has been calculated to be 4OU.s-1. It has been assumed that
odours will be continuously released at this concentration in order to ensure a
conservative, worst case assessment.
5.2.1.6 It is currently anticipated that the dewatering and storage of solid digestate will be
undertaken within an enclosed building. Liquid digestate will be stored within an
enclosed tank. As such, it is not considered that odour emissions from these
operations will be significant. However, it is not expected that the dewatered solid
substrate or liquid digestate will have significant potential for odour generation, once it
has undergone two stages of digestion.
5.2.1.7 The AD process involves breaking down of odour-making substances and organisms
within the digestate, whilst the majority of the methane within the waste ends up
within the biogas which is captured and burnt within the engines to produce electricity.
It has been reported by measurements elsewhere that the end substrate (digestate) from 1 Email communication from David Castle, ATS, dated 8th August 2011. 2 Odour and Bioaerosol Risk Assessment, Proposed Anaerobic Digestion, Whitwell, Derbyshire, Alkane Energy
PLC, 2010.
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AD is not significantly odourous3. The retention time of waste within the digestion
process is expected to be 114.3 days across a two stage digestion process which is
considered significantly longer duration than is required to sufficiently minimise odour
potential of the digestate. On this issue, reference should be made to the previous
Department for Environment, Food and Rural Affairs (DEFRA) Code of Good
Agricultural Practice for the Protection of Air4, which indicates that odours from
digestate can be reduced by up to 80%, after a 12-20 day retention time. It is currently
anticipated that the retention time at the proposed plant will be over 5 times longer
than this in duration. The proposed two stage, 114.3 day digestion process will ensure
a high level of methane removal whilst providing good opportunity for complete
break-down of odour-making substances within the digestate, such as sulphur
containing compounds. As such, potential odour emissions from digestate storage,
handling and transfer off-site are considered to be negligible and have not been
considered further in this assessment.
5.2.1.8 Maize and grass may be stored within a silage clamp near to the waste reception
building. If silage is stored on site, the top surface and inactive face of the clamp
would be covered at all times. There would be potential for odour from the open face
of the silage clamp and during transfer of material into and out of the clamp. However,
the magnitude of such odour emissions would be limited since the area of silage which
would be disturbed each day would be relatively small. Furthermore, maize and grass
silage would be described to have an ‘agricultural’ smell, and would be no more
significant than on commercial farms which use silage as animal forage. As such, the
potential for odour impact from the silage, should this be stored on site, would be
considered to be negligible. Therefore, odour emissions from silage have not been
considered further in this assessment.
5.2.1.9 The biogas used to power the Combined Heat and Power (CHP) units will be stored
within a regulated vessel. Flow of the biogas to the CHP units will be carefully
3 Air Quality Assessment of Proposed Agricultural Anaerobic Digester at Denham Estate, Near Bury St Edmunds,
Suffolk, ADAS, 2009. 4 Code of Good Agricultural Practice for the Protection of Air, Ministry of Agriculture, Fisheries and Food, Welsh
Office Agriculture Department, 1998.
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regulated to prevent gas being vented to atmosphere. Therefore, no odour emissions
are likely to arise from the biogas used to power the CHP units.
5.2.1.10 Table 2 and Table 3 contain the emission parameters used as model inputs for each
emission source modelled. Reference should be made to the site layout drawing in
Appendix 1 for an illustration of the location of emission points modelled.
Table 2 Emission Source Parameters for Biofilter Stack Process Parameter Value
Stack internal diameter (m) 0.65
Stack height (m) 12.2
Expected stack volumetric flowrate (m3.s-1) 9.92
Expected stack efflux velocity (m.s-1) 29.9
Expected stack efflux temperature (K) 298 Odour concentration in biofilter exhaust gas (OU.m-3) 2,000
Odour emission rate from biofilter Stack (OU.s-1) 19,831.7
Table 3 Emission Source Parameters for Cattle Slurry Tank During Slurry Delivery Process Parameter Value