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Cost Benefit Analysis of Applying Digestate to Agricultural Land in Wales Presented by: Ann Ballinger, Senior Consultant, Eunomia 19 th June 2014 Future of Food Waste Marketing
Cost Benefit Analysis of Applying
Digestate to Agricultural Land in
Wales
Presented by:
Ann Ballinger, Senior Consultant, Eunomia
19th June 2014
Future of Food Waste Marketing
Overview
– Project summary
– Options for digestate treatment
– Cost benefit analysis
– Results
– Concluding remarks
Anaerobic Digestion
ENERGY DIGESTATE
The project
– Started in May 2013, completed March
2014
– Funded by WRAP Cymru
– Project team
– Eunomia led the project and developed the
cost benefit model
– Technical support from Aqua Enviro
–Option development
–Data on processes obtained from industry
– Peer review by Cranfield University
– Considered EIGHT options for digestate
Option descriptions
Baselin
e
Whole digestate directly applied to agricultural land
Issues with using whole digestate
Issues with using whole digestate
– Potentially up to three tonnes digestate
produced per tonne of food waste treated
– Limited time period each year when
spreading the digestate to land is
possible
– = the potential for a
SUBSTANTIAL storage
problem
– Transport costs may be significant
Option descriptions
DIGESTATE
DEWATERING
END FATE:
LIQUID
FRACTIONTREATMENT
OF LIQUID
FRACTION
END FATE:
FIBRE
FRACTION
other
options
Option descriptions
Centrifugation
Discharge
to
watercourse
Nutrient
recovery
Fibre to
land
OPTION
1
Nutrient
recoveryBiological
oxidation
Recovered
phosphorus
to land
Option descriptions
Centrifugation
Discharge
to
sewer
Nutrient
recovery
Fibre to
land
OPTION
2
Nutrient
recoveryBiological
oxidation
Recovered
phosphorus
to land
Option descriptions
Centrifugation
Discharge
to
sewerNutrient
recovery
Fibre to
land
OPTION
3
Recovered
phosphorus
to land
Option descriptions
Centrifugation
Discharge
to
sewerBiological
oxidation
Fibre to
land
OPTION
4
Recovered
phosphorus
to land
Option descriptions
Centrifugation
Discharge
to
watercourseBiological
oxidation
Fibre to
land
OPTION
5
Recovered
phosphorus
to land
Option descriptions
Centrifugation
Discharge
to
sewer
Fibre to
land
OPTION
6
Recovered
phosphorus
to land
Option descriptions
Centrifugation
Liquor to
land
Fibre to
land
OPTION
7
Dewatering the digestate
– Aim is to separate the fibre from the
liquor
– A range of processes
– Study focused on CENTRIFUGATION
–High speed rotation of the whole digestate
– Other options (e.g. belt press; bucher press;
evaporation) not guaranteed to meet PAS110
and/or not yet commercially viable
– 90% of the digestate’s nitrogen content
remains in the liquor
Liquor treatment processes
– Nutrient recovery
– Soluble forms of ammonia and phosphorus
partially precipitated from the liquor by adding
magnesium chloride and sodium hydroxide
– Produces struvite – an easily transported form
of phosphorus (pellet form)
– Biological oxidation
– Conversion of ammonium-nitrogen into nitrate
– Nitrate can then be removed from liquor
– Some of the treated liquor can be returned to
feedstock as dilution water
Disposal of the liquor
– Two options considered
– Discharge to watercourse
–Plant operator must apply for consent to
discharge to the watercourse; charges are
applicable
– Discharge to sewer
–Plant operator must apply for a trade effluent
discharge consent; charges are applicable
–Charges are calculated using the Mogden formula
and are dependent on cost of treating the effluent
– In practice, some treatment of the liquor
is likely to be necessary prior to its
disposal
Options for fibre
– Study only considered direct application
of fibre to land
– Currently insufficient data in the UK context
to consider the impact of composting the
fibre
– Other fibre treatment processes (e.g. energy
recovery) not yet commercially viable
Cost Benefit Analysis (CBA)
– Calculation of financial costs
– Focus on comparative costs of the different
options for managing digestate
– Environmental analysis
– Uses life cycle assessment to develop
inventory of pollution
–Greenhouse gases and air pollutants (e.g. NOx)
– Impacts monetised using standard data from
DECC and HM Treasury
– Methodology recognised by UK Government
CBA – system boundaries
Digestion
process
Production
of
digestate
Biogas
utilisation
Digestate
treatment
Product
transport
Water
treatment
Applying
products
to soil
Results – dilution to 20% dry solids
Comparative costs per tonne of feedstock; digestate transport 80 km (round trip)
Financial costs
– Transport costs are relatively significant
– But at dilution to 20% and where digestate is
transported 80 km (round trip), the costs of other the
digestate treatment processes outweigh transport
costs
– General trend towards higher financial costs
with additional processing steps
– However the cost of discharge to sewer is
relatively high
– Use of biochemical oxidation offsets the otherwise
larger costs associated with discharging liquor to
sewer, through avoided Mogden charges
Results – dilution to 10% dry solids
Comparative costs per tonne of feedstock; digestate transport 80 km round trip
Saving of
around £12 per
tonne feedstock
on baseline
Results – round trip transport 280 km
Comparative costs per tonne of feedstock
Saving of
around £10 per
tonne feedstock
on baseline
Conclusions
– Best environmental outcome achieved through
applying whole digestate to land for all options
- but financial costs dominate the CBA results
– Applying whole digestate to land is the best
performing option overall where feedstock
diluted to 20%
– Biological oxidation with liquor discharged to
watercourse is the best performing option
– Where feedstock is diluted to 10%
– Where whole digestate needs to be
transported long distances to market
Full report available at:
http://www.wrapcymru.org.uk/sites/files/wrap/Assess
ing%20the%20Costs%20and%20Benefits%20for%2
0Production%20and%20Beneficial%20Application%
20of%20Anaerobic%20Digestate%20to%20Agricult
ural%20Land%20in%20Wales%202014.pdf
