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Anaerobic Digestion:Anaerobic Digestion:turning organic waste into a resourceturning organic waste into a resource
Charles BanksCharles Banks
ENERGY from WASTE
Sherkin Island Marine Station25th Annual Environmental Conference
Thursday 7th and Friday 8th May 2009
GasometerDigester
Heat exchanger
Gas mixer
Spark or compression
ignition engine
ELECTRICITY
Generator
Biogas to: engine, heat exchanger and mixer
Biogas
SLUDGE IN
Heat exchanger
HEATFUEL FIBRE
Separator
LIQUOR
Upgrading
What is anaerobic digestion?
BIODEGRADABLE ORGANIC MATERIAL(CARBOHYDRATES, FATS, PROTEINS)
HYDROLYSIS SIMPLE SOLUBLE ORGANICS
ACID FERMENTATION PROPIONIC ACIDBUTYRIC ACIDLONG CHAIN VFA
ACETIC ACID
ACETOGENESIS H2 + CO2ACETIC ACID
ACETOCLASTIC METHANOGENIC
BACTERIA
HYDROGEN-USING METHANOGENIC
BACTERIA
CH4 + CO2
METHANOGENESIS
Some anaerobic digesters
Are all anaerobic digesters the same?
Wet
Thermophilic
One stage Multi-stage
Dry
Mesophilic
Wet digester Dry digester
mesophilic
thermophilic
Instant recognition!Instant recognition!
Dry digester
• Mixed household waste after pre-processing –(BMW)
• Source-segregated organic fraction of household waste
• Kitchen waste
• Garden waste
• Market and supermarket wastes
• Waste from certain urban-based industries eg. food processors and packagers
Wastes that can be digested
Dry anaerobic digestion for residual waste
Residual waste pre-treatment
Digester
The digester must be part of an integrated package
Digester residue
treatment
Residual waste treatmentResidual waste treatment
Recovery or disposal
BMW
DRANCODRANCO
Dranco flow diagram - mixed waste
Kompogas
liquor recirculation
Plug flow digesterSolid/liquid separationwalking floor
Kompogas mass balance for 1000 kg BMWBMW
Magnetic separator
Shredder
Thermal treatment
400kg
Drying
ScreenBiogas
60-80 kg
Digester
compost
Compaction
Disposal 300kg
H2O100-130kg
Metals10 kg
CO2
80-100 kg
40 -80mm>80mm
Wet anaerobic digestion for residual waste
Compost
Electricity
DIGESTER
Biogas storage
Gas Engine CHP
Centrifuge
Flare
Pelletisationplant
RDF
Press
Magnetic separator
Hammer mill
Drum screen
Premix. densiometric
separator
Air classifier
Aerobic maturation
CITECCITEC
Wet single phase complete mix digester
Tipping pit
Mass balance for 1000 kg of waste
Based on a 230,000 tonne/year plant at Vargon, NL
Household waste (MSW)
Refuse derived fuel 420 kg
‘Biodegradable waste’fraction (BMW) 400 kg
Paper / plastic150 kg
Metals30kg
Washing Process water
Sand39kg
BMW261 kg
Coarse inerts 100kg
Digestion
Digestate100 kg
Process water161 kg
Biogas (per T of BMW)142 m3
Biogas (per T of MSW)37m3
Summary• These are all Mechanical Biological Treatment (MBT)
processes in which the biological part is anaerobic
• Designed to operate with residual or only partially kerbside segregated waste
• Process is operated at a high organic loading rate
• In the UK the ‘compost’ from this sort of plant can not be applied to agricultural land for growing food crops
• It can be used for land reclamation, landfill cover, as a means of stabilising waste destined for landfill, or for further energy recovery through incineration
BMW digestate characteristics
400440850260Zn (mg kg-1 TS)200170270160Pb (mg kg-1 TS)5060120100Ni (mg kg-1 TS)1.0--0.18Hg (mg kg-1 TS)200150290110Cu (mg kg-1 TS)10064140260Cr (mg kg-1 TS)1.51.42.51.5Cd (mg kg-1 TS)
3.44.74.3TP (g kg-1 TS)3.9182.2TK (g kg-1 TS)164914TN (g kg-1 TS)
PAS110 proposed
limit
Digestate fibre
Digestate liquorBMW
Pre-treatment and post treatment are minimised
Green wasteMarket wasteKitchen waste
Food processing waste
Digester
Final product
Source segregated waste treatmentSource segregated waste treatment
Benefits of Source Segregation• Removes wet and putrescible waste from the
rest of the dry waste
• Reduces vermin and smell problems
• Recycling of dry wastes easier
• Biofertiliser is free of contaminants and can be applied to agricultural land
• Anticipates the EU Biowaste Directive
• Keeps the process simple
South Shropshire foodwastedigestion demonstration plant
Concept based on collection of domestic catering waste
• Separated in the household in separate lined bin
• Uncooked and cooked food• Vegetables and meat• Bones• NO garden waste• Moisture content > 75%• 200 kg per household-year
BiogenBiogen--GreenfinchGreenfinch
Feed preparation
Macerating pump
Mesophilic digester
Gas Collector
Digestate storage
digestate recycle
Food waste addition
PasteurisationFeed buffer tank
Fibre product
Energy products
CHP
Separator
Liquor product
DOMESTIC CATERING WASTE (23%TS)
230 kg TS, 770 kg H2O
DIGESTER
75% destruction of organic material
BIOFERTILISER (7.8% TS)
60 kg TS, 770 kg H2O
830 kg Total
BIOGAS
60 kg CH4, 110 kg CO2
170 kg Total (140m3
biogas)
CHPUNIT
ELECTRICITY
275 kWhr Gross
250 kWhr Net
HEAT
440 kWhr Gross
220 kWhr Net
Mass balance for 1000 kg of food wasteMass balance for 1000 kg of food waste
Food waste digestate characteristics
BDL: below the detection limit. Cd-1mg kg-1 TS, Hg-0.01mg kg-1 TS, Pb-10mg kg-1 TS
40013013016016050Zn (mg kg-1 TS)200BDLBDLBDLBDLBDLPb (mg kg-1 TS)501010404015Ni (mg kg-1 TS)1.0----BDLHg (mg kg-1 TS)200202040407Cu (mg kg-1 TS)1001010505030Cr (mg kg-1 TS)1.5BDLBDLBDLBDLBDLCd (mg kg-1 TS)
111214TP (g kg-1 TS)18475.4TK (g kg-1 TS)5511534TN (g kg-1 TS)
PAS110 upper limit
Digestate fibre
Digestate liquor
Food waste
Kompogas – mixed food & garden waste
Dry single-phase plug flow digestion
Bekon
Source segregated (biowaste) and residual waste digesters
WetContinuous
Batch
Mixed
Unmixed
Mesophilic
Mesophilic
Psychrophilic
LandfillFarm 'bags'
Storage tanks and lagoons
Single phase
Plug flow Mesophilic
CITEC, KrugerFarmatic, Greenfinch, Others
OPL
Dry
Mesophilic
Thermophilic
Mesophilic
ThermophilicSingle phase
Continuous
Batch
Plug flow
Mixed
Mesophilic
ThermophilicPercolating
Dranco, Linde BRVKompogas
Valorga
None
Valorga
None
Biocel, BekonFarm leach bedLandfill bioreactor
Two phase Continuous
Inter-phase solids separation
No inter-phase solids separation
Wet Mesophilic
Thermophilic
Mesophilic
Thermophilic
BTA
Linde KCA
None
None
Dry Two phase
Batch and Continuous
No inter-phase solids separation
Inter-phase solids separation
Mesophilic
Thermophilic
Mesophilic
Thermophilic
None
BiopercolatePortagesteBiothane - AN
None
ROMopur?
One and two phase digesters
Wet and dry digesters
Thermophilic and mesophilic digesters
Summary of digester types and their performance
25140 (pilot)
Wet complete mix mesophilic
4.0Biogen -Greenfinch
23150 (average)
Dry horizontal plug flow thermophilic
10.8Kompogas
25100-200Dry vertical plug flow thermophilic
11.4Dranco
20100-150Wet complete mix mesophilic
5.0Citec
Retention Retention time time (Days)(Days)
Biogas Biogas mm33/Tonne /Tonne waste (wet)waste (wet)
Process configuration Process configuration and operating and operating temperaturetemperature
Process Process Load (Kg Load (Kg TS/mTS/m33/ day/ day
Process Process technologytechnologysupplier or supplier or operatoroperator
Important factors in decision making
• Choice of feedstock = quality of product
• Energy balance
• Fertiliser value and markets for digestate
• Odour generation and gas cleaning
• Downstream processing, storage, and maturation of digestate
• Compliance with ABPR
• Proximity principal
Optimisation through co-digestion
50 tonnes /day
ODM 8%
Biogas = 1250 m3
= 750m3 CH4
750 kWh potential energy
250 kWh electricity
375 kWh surplus heat
50 kWh parasitic heat
Volume 1500 m3
30 day HRT
Load 2.7 kg ODM m-3 d-
1
Volumetric gas production
= 0.83 m3 m-3
biogas potential 25 m3 / tonne
50 tonnes /day
ODM 8%
Biogas = 2850 m3
= 1710 m3 CH4
1710 kWh potential energy
570 kWh electricity
850 kWh surplus heat
50 kWh parasitic heat
Volume 1500 m3
25 day HRT
Load 4.5 kg ODM m-3 d-
1
food waste
10 tonnes / day
ODM 28%
Volumetric gas production = 1.9 m3 m-3
biogas potential 25 m3 / tonnebiogas potential
160 m3 / tonne
Single feedstock and co-digestion digesters
Summary points for different approaches
Rural development
Improved nutrient balance
Improved material recovery
Maximum volumetric gas productivity
Maximum bioresource use
Maximum landfill diversion
Product quality
Co-digestionSource segregation
MBT
