Online headspace chromatographic method for measuring VFA in biogas reactorsKanokwan Boe, Damien John Batstone, Irini Angelidaki
Environment & Resources
Technical University of Denmark
Lyngby, Denmark
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
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 indicators
Accumulation of CO, a precursor of acetate formate CO concentration Accumulation of hydrogen, a key intermediate H2 concentration Degradation efficiency COD, BOD or VS reduction Accumulation of different VFA
(usually determined by gas chromatography) Individual VFA Total concentration of VFA
(usually determined by titration) Total VFA Detects changes in buffer capacity Alkalinity Decrease in pH due to accumulation of VFA pH Concentrations of CH4 and CO2 Biogas composition Specific gas production Biogas production Principle Indicator
Alternative indicators Methane production rate during degradation of different substrates can estimate the activity of the corresponding bacterial group Methanogenic activity Bacterial activity NADH activity is correlated with bacterial activity NADH F-420 is a characteristic co-enzyme found in methanogens and can quantify active methanogens F-420 Concentration of ATP indicates the general microbiological activity ATP Activity of specific enzymes can indicate substrate turn in the bacterial ecosystem Enzyme activity Special lipids can be used to identify different bacterial groups and their numbers Bacterial membrane lipids Biochemically based indicators Specific probes against 16S rRNA for specific groups or individual bacteria Genetic probing Molecular methods Numbers of different groups of bacteria Bacteria counting Bacterial numbers Principle Indicator
Background
Co-digestion plants
Need of on-line monitoring system
VFA as process indicator
Digested manure => problem with filtration
Anaerobic digestion for treatment of organic waste and biogas production is very well-known and has applications worldwide.
In Denmark there are 20 centralized biogas plants treating manure and organic waste from industries, producing biogas for electricity and district heating.
Denmark : 20 centralized biogas plants treating manure and organic waste from industries, producing biogas for electricity and district heating.
And more than 50 farm biogas plants
Biogas from farm biogas plant is used mainly for producing electricity and heat for farm facilities. Extra heat and electricy can also be sold to the district network.
The technologies of anaerobic digestion for treatment of organic waste and biogas production are well-defined but in real operation the processes are still un-optimized.
Well-defined technologies BUT un-optimized real operation.
One of limitations for advanced optimization and control is a lack of rapid monitoring system.
As we know that process intermediates such as volatile fatty acids are the excellent indicators of process instability. So in this study we tried to develop the online monitoring system for measuring VFA in the biogas reactor
Volatile fatty acids (VFA) are well-known as the excellent process indicators.
Online monitoring system for measuring VFA in the biogas reactor.
Due to the characteristic of manure-based wastewater which normally contain high solids content which are mainly straws.
Also when co-digested with the in...
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
The method is based on gas phase extraction, using principle of headspace chromatographic method.
To determine VFA concentration in liquid phase by measuring the concentration in the gas phase.
VFA in liquid sample are extracted into the gas phase under specific equilibrium condition and measured by GC
The method can handle sample with high solids such as digested manure or other kind of slurry waste without sample pretreatment or filtration needed.
Can assess individual volatile fatty acids in the sample
Gas-liquid extraction factor
No effect :
Gas/Liquid volume ratio
VFA concentration Strong effect :
pH
Temperature
Salt addition
In the method development, the batch experiment has been set up to find the equilibrium condition that have high extraction of VFA
The factors that have strong effect on the VFA extractions are pH, Temperature and addition of salt
And the volume ratio of gas/liquid in the system and the VFA concentration don’t have effect on the partitioning of VFA between two phase
KH = CL,i/Pi
Ideal gas law;
Pi = RTnG/VG = RTCG
The Henry’s coefficient will be
KH = CL,i/Pi
= CL,i/(RTCG,i)
= (CL,i/CG,i)*(1/RT)
= (ML,i/MG,i)*(VG/VCL)*(1/RT)
Concept
Equilibrium cell GC pH T Acid Salt Sample H
System setup
System setup
Gas chromatogram of digested manure
From liquid sample From gas sample Ac Ac Pr Pr But Iso-val But Iso-but Iso-but Iso-val Val Val
Test on standard VFA solution
Environment & Resources
Technical University of Denmark
20 mL sample + 1 mL 34% H3PO4, Equilibrium temp. 75 oC, Total Equilibrium time 6 min.
It takes about 5 min. to increase temperature from about 40 oC to 75 oC so the actual equilibrium time is only 1 min., final pH is about 2-2.5
Preliminary test online application
Acetic acid Propionic acid Iso-butyric acid Peak area from on-line measurement
VFA conc. in liquid phase from off-line measurement
11May
15.30 clean up TC&pH probe, open reactor
17:00 Start sampling every 2 hr
19:00 wash sampling tube&filter
19:30 add pulse sodium valerate 20 g in manure 90 mL
22:00 have condensate in the TC
12May
11:00 changed bulbs setting
12:00 - 20:00 V_GC leak
21:00 condensate in tubing
13May
10:00 clean sampling tube
14:00 clean sampling tube
19:00 clean up TC&pH probe, change sampling tube
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
In conclusion, a new method for online-measurement of VFA in biogas reactor has been developed based on gas-phase extraction.
this method can measure the individual VFA concentrations in the reactors which will provide more information of the process dynamics
The method is very suitable for samples with high solid contentations or dirty samples. Filters have problems of fouling or clogging like manure or slurry waste, without the need of sample pre-treatment or filtration.
The system is clean, it only needs a proper washing of equilibrium cell and flushing of the gas line. The gas chromatograph is always clean and need much less maintenance
And the future work will be minimization the effect of sample matrix and validation on different waste compositions.
System simplification. Make it more compact and more robust.
And real application in biogas plant
Acknowledgement
Technicians : Uwe Albert Walter
Majbrit Staun Jensen
Students : Elena Fernández Ordáz
Mickael Serres
In conclusion, a new method for online-measurement of VFA in biogas reactor has been developed based on gas-phase extraction.
this method can measure the individual VFA concentrations in the reactors which will provide more information of the process dynamic
The method is very suitable for the sample with high solid content or dirty sample all fine filter can have problem of fouling or clogging like manure or slurry waste, without the need of sample pre-treatment or filtration.
The system is clean, it only needs a proper washing of equilibrium cell and flushing of the gas line. The gas chromatograph is always clean and need much less maintenance
And the future work will be minimization the effect of sample matrix and validation on different waste compositions.
System simplification. Make it more compact and more robust.
And real application in biogas plant
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