Anaerobic Digestion in Orkney Pros and Cons - oref.co.uk€¦ · Reception and Treatment • Mixes...
Transcript of Anaerobic Digestion in Orkney Pros and Cons - oref.co.uk€¦ · Reception and Treatment • Mixes...
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Anaerobic Digestion in Orkney
Pros and Cons
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What is Anaerobic Digestion?
• Anaerobic digestion (AD) is a managed
biological process in which biodegradable
waste is broken down by naturally occurring
micro-organisms in the absence of oxygen to
produce a stabilised residue
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Process Description
• The anaerobic digestion process is broken
into various stages:
• Pre-treatment
• Digestion process
• Outputs and their treatment
• Biogas
• Digestate
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Generic Process Flow
Exhaust AirPurification
Reception and treatmentDigestion
Biogas treatmentand Use
Waste WaterPurification
Dewatering
Digestate
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ANAEROBIC DIGESTION PROCESSES AND
EMISSIONS
EMISSIONS:
Carbohydrate/
Proteins/Fats
Sugars/
Amino
Acids/
Fatty
acids
Carbon
based
acids &
Alcohols/
Ammonia/
CO2 and
H2
Acetic
acid
Carbon
dioxide
Hydrogen
BIOGAS =
Methane
CO2/H2S/
Condensate
HYDROLYSIS ACIDOGENESIS ACETOGENESIS METHANOGENESIS
Enzyme processing of
solids to soluble matter
Bacterial conversion to
VFAs etc
H2S Formed in this stage
Bacterial conversion to
acetic acidBacterial conversion of
acetic to methane + CO2
and
Bacterial conversion of
hydrogen and CO2 to
methane
Acidogenic bacteria multiply relatively quickly and
thrive in low pH environments.
Methanogens are slow to multiply and need stable
neutral pH and Temperature. Process interruptions
and changes are not well tolerated.
FEEDSTOCK
DIGESTER
SOLIDS &
LIQUID =
DIGESTATE
GAS
ENGINES/
WASTE
HEAT
BOILER/
FLARE
DEWATERED
DIGESTATE
PAS 110?
e.g Sewage
sludge/Distillery
waste/Food waste
Liquid waste
(High in
Ammonia/
BOD/COD)
Pre-Processing –
(size reduction/
feed mix prep/
pasteurisation*
*pasteurisation is to meet
ABP rules & can be
before or after
EM
ISS
ION
S:
E.g. ODOURCONTAMINATED
WASH WATERS/
DUST/
BIOAEROSOLS?
Buffer
Storage?
OUTPUT
OUTPUTDe-watering
Digestate
Storage
Biogas clean
up e.g De-
sulphurisation
by treating
with caustic?
EM
ISS
ION
S:
ODOUR from H2S or
SOx/NOx/CO
EM
ISS
ION
S:
ODOUR/
CONDENSATE
HIGH IN BOD/
COD
OUTPUT
EM
ISS
ION
S
Odour/dust
Pressure
relief
valvesEMISSIONS:
ODOUR &
BIOGAS
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80 m3 Farm Biogas Plant
Reception
Tank
Plant
Room
Anaerobic
Digester
Digestate
Storage
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Suitable Feedstocks
AD is suitable for treating many wastes
• Sewage sludge
• Organic farm wastes
• Municipal solid waste
• Green wastes & Food wastes
• Organic Industrial Wastes (inc liquids)
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Reception and Treatment
• Mixes different feedstock, adds water or
removes undesirable materials such as large
items and inert materials (e.g. plastics, glass)
• Allows for a better digestate quality and a
more efficient digestion process
• Pre-conditioning – pasteurisation, sterilisation,
filtration, grit removal etc
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Key AD Parameters
Feedstock Type & Quality
• Substrate Quality C:N ratio 30:1
• Particle size
• Moisture Content
Ensure suitable feed quality check systems
are in place and in use
Gas production can be affected by reactor
upsets – nitrification, digestate quality, solids
separation, clarification
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Digestion Process: Design
Options
• Temperature
• Mesophilic range 29 – 41oC, ideal ~35oC
• Thermophilic range 50 – 70oC, ideal 60-65oC
• Moisture
• Wet (5-15% Total Solids)
• Dry (25-35% Total Solids)
• Process
• One-stage
• Two-stage
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Key AD Parameters –
pH/Temperature
• pH (and alkalinity)
• pH control via feed addition rate and acid/base pH addition
Effective pH control must be in place – trends
• AD Reactor Temperature
• Mesophilic or Thermophilic
• Feeds require to be heated or cooled to give optimum reactor temperature control
• Too low or too high - Biogas production rate falls, microbiological activity suppressed/dies
• Thermophilic systems are less temperature stable
Effective temperature control must be in place
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Outputs of AD
• Biogas
• Generation of
renewable energy
• Vehicle fuel
• Gas to grid
• Powers the AD
process
• Nutrient rich
digestate
• Good fertiliser
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Biogas
• Biogas has a composition of 55-70% of methane, with the higher concentrations in the wet digestion systems
• Upgrading needed if it is to be used to create a source of income
• Hydrogen sulphide and water vapour need to be removed for boilers and combined heat and power units
• Removal of carbon dioxideis required if the gas is to be used as natural gas or vehicle fuel
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Digestate
• Digestate has the ability to improve the humus
balance of the soil
• Contains beneficial nutrients for plant growth
• But the properties of digestate are different to
compost
• In order to obtain a high quality soil
amendment, AD is often combined with
composting, adding value to the digestate
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PAS 110• Inputs
• Separately collected
biowastes inc ABP
• Sewage sludge not
allowed
• Process Controls
• Quality Management
System
• Pasteurization
• Outputs
• Quality requirements
• Information and record keeping
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Licensing Options
• PPC Permit
• Waste Management Licence
• Exemption
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Planning Stage Considerations
• Acceptability of the development as a land
use
• Location
• Design and layout including stack height
• Consentability
• General description of the process,
techniques and technology
• Impacts on sensitive receptors
• Heat and power plan
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Sources of Guidance
• National Operations Waste Unit intranet site
• Odour Guidance: www.sepa.org.uk/air/odour.aspx
• Useful Websites:
• www.biogas-info.co.uk: “the official information portal” on all things AD
• www.adbiogas.co.uk: the website of the Anaerobic Digestion and Biogas Association
• www.biogas.org.uk: trade
association for the biogas
industry
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Pros & Cons Summary
• Production of biogas
• Replaces fertiliser
• Nutrients bio-available
• Plenty potential
feedstock in Orkney
• Reduce waste disposal
costs for some
industries
• Sensitive process
• Odour control
• Corrosive environment
• Regulated process
• Cost to build