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Online headspace chromatographic method
for measuring VFA in biogas reactors
Kanokwan Boe, Damien John Batstone, Irini Angelidaki
Environment & Resources
Technical University of Denmark
Lyngby, Denmark
Traditional monitoring
Gas
Optimum
Brave
Careful
Time
Suppliers
Pre-storage
Reactor
After-storage
Operatør
Optimal monitoring
Gas
optimum
Time
Suppliers
Pre-storage
After-storage
Reactor
Datareg.
Model
PLC
Operator
Good indicator
• To indicate the actual status of the
process
• To give an early warning
• To give a secure warning
• To always predict imbalance
Types of disturbances
• Hydralic overload
• Organic overload
• Toxic compounds
• Temperature changes
• pH change
• New substrates
Traditional indicators
Indicator
Principle
Biogas production
Specific gas production
Biogas composition
Concentrations of CH4 and CO2
pH
Decrease in pH due to accumulation of VFA
Alkalinity
Detects changes in buffer capacity
Total VFA
Total concentration of VFA
(usually determined by titration)
Individual VFA
Accumulation of different VFA
(usually determined by gas chromatography)
COD,
BOD
reduction
or
VS
Degradation efficiency
H2 concentration
Accumulation of hydrogen, a key intermediate
CO concentration
Accumulation of CO, a precursor of acetate formate
Alternative indicators
Indicator
Principle
Bacterial numbers
Bacteria counting
Numbers of different groups of bacteria
Molecular methods
Genetic probing
Specific probes against 16S rRNA for specific groups or individual bacteria
Biochemically based indicators
Bacterial membrane lipids
Special lipids can be used to identify different bacterial groups and their numbers
Enzyme activity
Activity of specific enzymes can indicate substrate turn in the bacterial ecosystem
ATP
Concentration of ATP indicates the general microbiological activity
F-420
F-420 is a characteristic co-enzyme found in methanogens and can quantify active
methanogens
NADH
NADH activity is correlated with bacterial activity
Bacterial activity
Methanogenic activity
Methane production rate during degradation of different substrates can estimate the
activity of the corresponding bacterial group
Background
• Co-digestion plants
• Need of on-line monitoring system
• VFA as process indicator
• Digested manure => problem with filtration
Concept
• Gas phase extraction
• Liquid sample => measure in gas phase
• High solid sample / slurry waste
• No pretreatment or filtration needed
• Individual VFA in liquid phase
Gas-liquid extraction factor
• Strong effect :
– pH
– Temperature
– Salt addition
• No effect :
– Gas/Liquid volume ratio
– VFA concentration
Concept
Sample
GC
Acid
Salt
H
pH
T
Equilibrium cell
System setup
System setup
Gas chromatogram of digested manure
Iso-but
Pr
Ac
But
Iso-val
Val
But
Iso-val
Iso-but
Pr
Ac
From liquid sample
From gas sample
Val
Test on standard VFA solution
Peak area
250000
Acetic
200000
Propionic
150000
Iso-butyric
Butyric
100000
Iso-valeric
50000
Valeric
0
0
10
20
30
VFA concentration (mM)
40
120000
Acetic
Peak area
100000
Propionic
80000
Iso-butyric
60000
Butyric
40000
Iso-valeric
20000
Valeric
0
0
1
2
3
4
VFA concentration (mM)
5
Environment & Resources
Technical University of Denmark
Preliminary test online application
25000
85
20000
80
15000
75
10000
70
5000
65
0
07May
08May
09May
10May
11May
12May
34
33
20000
32
15000
31
28
07May
6.1
5.6
5.1
07May
08May
09May
10May
11May
12May
13May
Peak area
14000
12000
10000
8000
6000
4000
2000
0
0
08May
09May
10May
11May
12May
13May
Peak area from on-line
measurement
Iso-butyric acid
6.6
5000
29
13May
7.1
10000
30
VFA conc. in liquid phase
from off-line measurement
Peak area
90
Peak area
30000
Propionic acid (mM)
Propionic acid
95
Iso-butyric acid (mM)
Acetic acid (mM)
Acetic acid
Conclusion
• New online-VFA by gas extraction from liquid sample
• pH < 2, Temperature > 65oC
• Linear calibration curve in application range
• Measure individual VFA
• No pretreatment or filtration needed
Acknowledgement
• Technicians :
Uwe Albert Walter
Majbrit Staun Jensen
• Students :
Elena Fernández Ordáz
Mickael Serres