From Waste to Energy – The Bio-Chemical Process
Biogas Compact Workshop
Project Planning
for
Biodigesters in Developing
and
Industrialized Countries
Postgraduate
Programme
Renewable
Energy (PPRE)
26 – 28 April, 2011
University of Oldenburg, Germany
LeAF
Henri Spanjers
Lettinga Associates Foundation
Content
• History
• Definitions
• Biochemical processes
–
–
–
–
Hydrolysis
Acidogenesis
Acetogenesis
Methanogenesis
• Reactors
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Natural Anaerobic Environments
20% O2
20% O2
AIR
Interface
8 ppm O2
8 ppm O2
Water
0 ppm O2
+ organic matter
- organic matter
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Carbon cycle
(CH2O)n
Photosynthesis:
Algae
Green -plants
Cyanobacteria
Organic compounds
Methane-oxidizing
bacteria
Respiration:
plants, animals,
microorganisms
Aerobic
(CO2)
CH4
Anaerobic
Methanogenic
bacteria
sedimentation
(Methyl
compounds)
Anaerobic respiration
Fermentation
Phototrophic
bacteria
Organic compounds
(CH2O)n
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Some history
First to report about natural methane production
Hey, there is a flammable
gas coming out of rotting
marshes and swamps!!!
ALESSANDRO VOLTA
Italy
1770
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Figure 1 Natural gas and the chemist. John Dalton (1766–1844) collecting marsh gas by poking a
stick into pond sediments (Picture by Ford Madox Brown). Marsh gas (methane, CH4, the
simplest alkane) is the main component of natural gas. As shown by the work of Zengler et al.
new aspects of this economically important bacterial process are still being revealed.
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Fermentation
Fermentation: process of deriving energy from the oxidation
of organic compounds, such as carbohydrates, and using
an endogenous electron acceptor, usually an organic
compound
German chemist and zymologist, Eduard Buchner, winner of
the 1907 Nobel Prize in chemistry, determined that
fermentation is actually caused by a yeast secretion that
he termed zymase.
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Fermentation Products
• Microbial cells or biomass (single cell protein, bakers
yeast, lactobacillus, E. coli, etc.)
• Microbial enzymes: catalase, amylase, protease,
pectinase, glucose isomerase, cellulase, hemicellulase,
lipase, lactase, streptokinase, etc.
• Microbial metabolites:
– Primary metabolites – ethanol, citric acid, glutamic acid, lysine,
vitamins, polysaccharides etc.
– Secondary metabolites: all antibiotic fermentation
• Recombinant products: insulin, HBV, interferon, GCSF,
streptokinase
• Biotransformations: phenyl acetyl carbinol, steroid
biotransformation, etc.
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Fermentation Products (cont’d)
• Alcoholic fermentation: sugars (glucose, fructose,
sucrose) are converted into Ethanol and carbon dioxide
• Dark fermentation is the fermentative conversion of
organic substrate to H2
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Anaerobic biodegradation
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Anaerobic biodegradation
• Anaerobic biodegradation or anaerobic digestion: very
complex biological and biochemical process performed by
various different species of bacteria working together
• End products of anaerobic digestion are “biogas” and
more bacteria that grow out of the consumed organic
matter
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living organisms
eukaryotes
prokaryotes
archaebacteria
Methanogenic
bacteria
extreme halophiles
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Archaebacteria were the
first living organisms on
earth. Before there was
oxygen in the atmosphere!
eubacteria
thermoacidophiles
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Comparison Aerobic - Anaerobic
Characteristic
Aerobic
Anaerobic
Reaction
C6H12O6 + 6O2 → 6CO2 + 6H2O
C6H12O6 → 3CO2 + 3CH4
Energy release
∆G°’ = -2840 kJ/mol glucose
∆G°’ = -393 kJ/mol
Carbon balance
50%
50%
60%
40%
Energy balance
Biomass
production
→
→
→
→
CO2
biomassa
biomassa
heat production
Fast growth of biomass,
Resulting in a sewage sludge
problem
glucose
95% → CH4 + CO2 (= biogas)
5% → biomassa
90% retained in CH4
5% → biomassa
5% → heat production
Slow growth of biomass
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Anaerobic digestion
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Scheme anaerobic biodegradation
Polymers
(proteins, polysaccharides, lipids)
h
Monomers
(sugars, amino acids, peptides)
Methanogenic
Consortium
1
Propionate
butyrate
2
2
1
H2 + CO2
1
acetate
3
CH4 + CO2
4
4
3 Homoacetogenic bacteria
h Hydrolytic enzymes
1 Fermentative bacteria
4 Methanogens
2 Syntrophic acetogenic bacteria
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From www.uasb.org
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ADM1 Model Structure Overview
Complex particulate waste and Inactive
biomass
Inert particulate
Carbohydr.
Proteins
Lipids
Inert soluble
Sugars
Amino acids
LCFA
2
1
3
Propionate
HVa, HBu
5
4
Acetate
H
2
Death
6
7
CH
4
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Anaerobic Conversion of Organic Matter
Organic Polymers
proteins carbohydrates lipids
Hydrolysis
Hydrolytic enzymes
Mono- and oligomers
amino acids, sugars, fatty acids
Acidogenesis
Fermentative bacteria
Volatile Fatty Acids
Lactate
Ethanol
H2 / CO2
Homoacetogenic bacteria
Acetogenesis
Syntrophic acetogenic bacteria
Acetate
Methanogenesis
Methanogens
CH4 / CO2
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Hydrolysis
Organic Polymers
proteins carbohydrates lipids
Mono- and oligomers
amino acids, sugars, fatty acids
Volatile Fatty Acids
Lactate
Ethanol
H2 / CO2
Homoacetogenic bacteria
Acetate
CH4 / CO2
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Hydrolysis: Characteristics
• Polymeric compounds monomer or dimeric
components
• By extra-cellular enzymes
• Slow process (rate limiting): dS/dt = -Kh•S
• Retention time and particle size rate determining
• Optimum pH = 6
• Cellulose/hemicellulose degradation depends on lignin
fraction
• Hydrolysis of fats hardly proceeds <15-20°C (rate limit ing)
• (Product) inhibition by: LCFA. NH3, amino acids, H2?
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Hydrolysis: Enzymes
• Hydrolysis of suspended solids is surface-related process
• The more specific surface, the faster the process
• Individual enzymes work at constant rate (constant T, pH)
• Number of enzymes determines the total rate
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Hydrolysis: Surface related
Hydrolysis as a surface-related process
More enzymes
“attack” the
substrate
Rate
increases
Particle
breakdown
or “lysis”
From: Wendy Sanders
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Hydrolytic enzymes
Long Chain Fatty Acids (LCFA)
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Hydrolysis: Carbohydrates
• Cellulose is hydrolysed by cellulase (mixture of exoglucanases, endo-glucanases and cellobiases)
• The hydrolysis of starch is performed by a mixture of
amylases that is able to hydrolyse the α-1,4 bonds and α1,6 bonds of the amylose and amylopectin.
From Dr. Wendy Sanders
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Hydrolysis: Proteins
• Proteinases (Peptidases + Proteases)
• Protein polypeptides peptides amino acids
From Dr. Wendy Sanders
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Hydrolysis: Lipids
most lipids in waste(water) are present as triacylglycerides
From Dr. Wendy Sanders
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Bio-degradation of cellulitic matter versus lignin content
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Digestible part of wood
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Acidogenesis
Organic Polymers
proteins carbohydrates lipids
Mono- and oligomers
amino acids, sugars, fatty acids
Volatile Fatty Acids
Lactate
Ethanol
Acetate
H2 / CO2
CH4 / CO2
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Acidogenesis: Sugars
•
Release of protons (H+) and reaction products (proton acceptors)
•
H2 formation (catalyzed by the enzyme hydrogenase)
•
Performed by a very large group of bacteria (about 1% of all bacteria
facultative fermenters)
C12H22O11 + 9 H2O → 4 CH3COO- + 4 HCO3- + 8 H+ + 8 H2
C12H22O11 + 5 H2O → 2 CH3CH2CH2COO- + 4 HCO3- + 6 H+ + 4 H2
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Acidogenesis: Sugars
End products depend on circumstances, e.g.:
• Glucose fermentation in a two-step system
– more reduced products like ethanol, lactate, propionate, butyrate,
CO2 and H2
• Glucose fermentation in a one-step system
– acetate, H2 and CO2
• Production of acids proceeds up to pH = 4
(product inhibition)
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Acidogenesis of sugars: most rapid step!
Kinetic Properties Acidifiers / Methanogens
Process
Rx
Y
gCOD/gVSS/d g VSS/g COD
Ks
µ-max
Td
mg COD/l
day-1
days
Acidogenesis
13
0.15
200
2.0
0.35
Methanogenesis
3
0.03
30
0.12
5.8
Overall
2
0.03 –0.18
-
0.12
5.8
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Substrate availability and bacterial growth
Monod’s equation:
µ = µmax ⋅
S
Ks + S
µ
µmax
At S = Ks → µ = ½ µmax
Ks
dX
= µ⋅X
dt
dS
dX
= −Y ⋅
dt
dt
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Where: dS/dt = substrate utilisation rate
Y = yield coefficient
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Acidogenesis: Acidification
Methane
Capacity
Exceeded
Poor
Buffering
Capacity
Methanogenic Toxicity
Increasing
VFA
increases
pH
decreases
Unionized VFA
increasing
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Acidogenesis: Proteins
• Organically bound N (amino acids) is released as NH4+
(Stickland reaction: oxidation-reduction)
Alanine: CH3CHNH2COO- + 3 H2O → CH3COO- + HCO3- + NH4+ + 2 H2
Glycine: 2 CH2NH2COO- + 2 H2 → 2 CH3COO- + 2 NH3
alanine + glycine + 3 H2O → 3 acetate + 2 NH3 + NH4+ + HCO3(2 NH3 + 2 H2O + 2 CO2 → 2 NH4+ + 2 HCO3-)
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Stickland reaction
Alanine
ATP
Glycine
4 e(ATP)
acetate, CO2, NH4
2 acetate, 2 NH4
Oxidative branch
Reductive branch
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Acidogenesis: Long Chain Fatty Acids
•
Anaerobic degradation of LCFA proceeds via β-oxidation
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COO-
•
Palmitic acid:
CH3-(CH2)14-COO- + 14 H2O → 8 CH3COO- + 7 H+ + 14 H2
•
With uneven numbers: acetate + propionate are formed:
CH3-(CH2)14-CH2COO- + 14 H2O → 7 CH3COO- + CH3CH2COO- 7 H+
+ 14 H2
•
Unsaturated LCFA are firstly hydrogenated before degradation
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Acetogenesis
Organic Polymers
proteins carbohydrates lipids
Mono- and oligomers
amino acids, sugars, fatty acids
Volatile Fatty Acids
Lactate
Ethanol
H2 / CO2
Acetate
Homoacetogenic bacteria
CH4 / CO2
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Acetogenesis (Acetate formation)
•
Conversion of fermentation products into acetic acid, CO2, and H2
•
Mainly from propionic acid, butyric acid and ethanol
propionate- + 3H2O → acetate- + HCO3- + H+ + 3H2
∆ G0’ = + 76.1 kJ/mole
butyrate- + 2H2O → 2 acetate- + H+ + 2H2
∆ G0’ = + 48.1 kJ/mole
→ acetate- + H+ + 2H2
∆ G0’ = + 9.6 kJ/mole
_______________________________________________________________________________________________
ethanol + 2H2O
4 H2 + CO2
→
CH4 + 2H2O
∆ G0’ = -138.9 kJ/mole
Need for syntrophic associations !!!
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Impact of pH2 on thermodynamics
∆G ' = ∆G '0 + RT ln
∆G’ (kJ/mole)
-100
[C ]c ⋅ [ D]d
[ A]a ⋅ [ B ]b
propionate- + 3H2O → acetate- + HCO3- + H+ + 3H2
-50
Reaction
possible
butyrate- + 2H2O → 2 acetate- + H+ + 2H2
Methanogenic niche
0
Reaction
impossible
4 H2 + CO2 → CH4 + 2H2O
50
2
4
6
8
pH2=-log (H2)
High H2
pressure
Low H2
pressure
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Methanogenesis
Organic Polymers
proteins carbohydrates lipids
Mono- and oligomers
amino acids, sugars, fatty acids
Volatile Fatty Acids
Lactate
Ethanol
Acetate
H2 / CO2
CH4 / CO2
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Methanogenesis
• Aceticlastic methanogenesis (70%):
CH3COOH → CH4 + CO2
• Hydrogenotrophic methanogenesis (30%):
CO2 + H2 → CH4 + CO2
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Methanogenesis: Substrates
∆G0 (kJ/mole CH4)
4H2 + CO2
=> CH4 + 2H2O
-130.4
4HCOOH
=> CH4 + 3CO2 + 2H2O
-119.5
4CO + 2H2O
=> CH4 + 3CO2
-185.5
4CH3OH
=> 3CH4 + CO2 + 2H2O
-103.0
CH3OH + H2
=> CH4 + H2O
-112.5
4CH3NH3 + 2 H2O
=> 3CH4 + CO2 +
+
4NH4
- 74.0
+
2(CH3) 2NH2 + 2H2O => 9CH4 + 3CO2 + 4NH4 - 74.0
CH3COOH
=> CH4 + CO2
- 32.5
Most important substrates: hydrogen and acetate
Furthermore: formate, carbon monoxyde, methanol and methylamines
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Maximum Production of Biogas
1,40
production of biogas
production of methane
1,20
[Nm³/kg]
1,00
0,80
0,60
0,86
0,40
0,20
0,50
0,40
0,00
Lipids
Carbohydrates
Proteins
(ATV-DVWK M 363)
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Animal manure
Biogas Practice Area
Inorganic
Organic
Phosphorous
Nitrogenous
Proteines
Inositiol
phosphate,
phospholipids,
nucleic acids,
ATP
Carbonaceous
Sulphurous
Carbohydrates
Lipids
Peptides
Sugars
Glycerol
Fibers
Amino acids
Alcohols
Fatty acids
Compounds
of Cu, P, K,
Zn, Mn, Co,
Ca, Fe, H, O
Sulphites
Volatile acids
Inorganic
phosphates
Biochemical
processes
andFoundation
biogas
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Associates
Cellulose
N, NH4
H2 O
CH4
CO2
lignin
Hs S
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Take-home message
• Anaerobic microbial conversion differs from aerobic
• Anaerobic digestion is a complex process
• Ultimate COD removal via production of CH4
• Anaerobic bacteria have a narrow substrate spectrum:
complex consortia are needed for complete COD removal
• Environmental factors affect the process
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Reactors
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Different Technologies of Process Management
Water content in the wet
dry
reactor
(TS <= 10%)
(TS = 30-35%)
Process
management
Continuous Stirred
Plug-Flow
Temperature
Mesophilic
(35-37ºC)
Thermophilic (55-60ºC)
Steps of process
Single-stage
Multi-Stage
Operational mode
Semi-continuous
Discontinuous (Batch)
Form of reactor
vertical (conventional, oval, etc.)
horizontal
Mixing process
Agitation
Circulation
Percolation
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Fixed Dome Domestic Digester
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Digester: Schematic
Digester with rubber membrane cover
all digesters
> 50 % of
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Biogas plant in the UK
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Development of “high-rate” anaerobic treatment systems
Completely mixed
Physical retention
Immobilised
biomass
Enhanced
contact
(Bio)gas
influent
effluent
Relative
capacity: 1
Relative
capacity: 5
Relative
capacity: 25
Relative
capacity: 75
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UASB and EGSB
Auto immobilization / granulation
EGSB
UASB
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Reactor Technologies for Liquids
gas
effluent
gas
effluent
gas
effluent
second stage
first stage
influent
influent
influent
UASB-Reactor
IC-Reactor
gas
Biobed-Reactor
gas
gas
effluent
Anaerobic Contact Reactor
influent
Fixed bed Reactor
recirculation
influent
influent
effluent
(loop)
recirculation
effluent
(loop)
recirculation
sludge bed
Fluidized bed Reactor
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UASB Reactor: Sewage
Bucaramanga, Colombia, 12000 m3/d
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UASB reactors: Sewage
Mirzapur, India, 14 m3/d plant
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UASB: Sewage
Accra, Ghana
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Anaerobic Industrial Wastewater Treatment
Anaerobic UASB-Reactor CSM
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IC-Reactor: Distillery Hanover
(Kraul & Wilkening u. Stelling)
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