Online headspace chromatographic method for measuring VFA in biogas reactors Kanokwan Boe, Damien...

Environment & Resources

Technical University of Denmark

Online headspace chromatographic method for measuring VFA in biogas reactors

Kanokwan Boe, Damien John Batstone, Irini Angelidaki



Lyngby, Denmark



Suppliers

Pre-storage After-storageReactor

Operatør

Time

Gas

Careful

BraveOptimum

Traditional monitoring



SuppliersPre-storage After-storageReactor

Operator

Time

Gas

Datareg.Model

PLC

optimum

Optimal monitoring



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 indicatorsIndicator 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 or VS reduction

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



• Strong effect : – pH

– Temperature

– Salt addition

Gas-liquid extraction factor

• No effect :– Gas/Liquid volume ratio

– VFA concentration



Concept

Equilibrium cell

GC

pH T

Acid

Salt

Sample

H



System setup



Gas chromatogram of digested manure

From liquid sample From gas sample

Ac

Ac

Pr

Pr

But Iso-val

But

Iso-but

Iso-butIso-val

Val

Val



Test on standard VFA solution

0

50000

100000

150000

200000

250000

0 10 20 30 40VFA concentration (mM)

Pea

k ar

ea

Acetic

Propionic

Iso-butyric

Butyric

Iso-valeric

Valeric

0

20000

40000

60000

80000

100000

120000

0 1 2 3 4 5VFA concentration (mM)

Peak a

rea

Acetic

Propionic

Iso-butyric

Butyric

Iso-valeric

Valeric

Environment & ResourcesTechnical University of Denmark



Preliminary test online application

65

70

75

80

85

90

95

07-May

08-May

09-May

10-May

11-May

12-May

13-May

Ace

tic a

cid

(mM

)

0

5000

10000

15000

20000

25000

30000

Pea

k ar

ea

28

29

30

31

32

33

34

07-May

08-May

09-May

10-May

11-May

12-May

13-May

Pro

pio

nic

ac

id (

mM

)

0

5000

10000

15000

20000

Pe

ak

are

a

5.1

5.6

6.1

6.6

7.1

07-May

08-May

09-May

10-May

11-May

12-May

13-May

Iso

-bu

tyri

c a

cid

(m

M)

02000400060008000100001200014000

Pe

ak

are

a

Acetic acid Propionic acid

Iso-butyric acid Peak area from on-line measurement

VFA conc. in liquid phase from off-line measurement



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 WalterMajbrit Staun Jensen

• Students : Elena Fernández OrdázMickael Serres

Online headspace chromatographic method for measuring VFA in biogas reactors Kanokwan Boe, Damien...

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