Determination of VFAs in sludge fermentation by gas chromatography
VFAs sample processing
The qualitative and quantitative analysis of various VFAs is performed by gas chromatography. Before the measurement, the sample was first filtered with qualitative filter paper at medium speed, and then filtered with 0.45μm filter membrane. The filtrate was collected in a 1.5mL brown vial for gas chromatography, and then 50-150μL of each was added to each vial 3% H3PO4 to ensure that the pH of each sample is less than 6.0 [119].
Instrument configuration and conditions of use
Gas chromatograph (FID hydrogen flame detector)
Column DB-WAX30m × 0.32mm × 0.25mm
Carrier gas nitrogen (flow rate is 50mL / min, no split setting)
The temperature of the injector and detector is 200 ℃ and 220 ℃.
The oven is programmed to run at 110 ° C for 2 minutes, and the temperature is increased to 180 ° C at a rate of 10 ° C / min.
Qualitative analysis
Using retention time qualitative and pre-analysis determination, the remaining sludge hydrolyzed and acidified products in the test are mainly six short-chain fatty acids. Figure 2.1 shows the corresponding peak time. The peak around 2min in the figure is a miscellaneous peak. In addition, the six peaks from left to right are acetic acid, propionic acid, isobutyric acid, n-butyric acid, isovaleric acid, n-valeric acid, and their corresponding retention times They are about 4.63min, 5.59min, 5.96min, 6.71min, 7.24min and 8.11min, respectively. This figure is the chromatogram of the standard sample. The peak size of each organic acid in the water sample is different, and the peak time is basically Within the retention time range above.
Quantitative analysis
The external standard method was used for quantitative analysis, and the concentration of various acids in the water sample was calculated according to the peak area [120]. First, each standard acid is formulated into a series of standard concentrations according to its concentration and content, and the standard curve of six acids corresponding to different concentrations and corresponding peak areas is measured from gas chromatography; then the water samples are corresponding to different retention The peak area of ​​time is substituted into the corresponding standard curve to obtain the concentration values ​​of various organic acids; finally, the various organic acids are added in units of COD to obtain the total VFAs. Formulas (1) and (2) are formulas for calculating the concentration of various acids. Table 1 shows the concentration and content of six standard acids and their boiling points under standard conditions. Table 2 shows the names and corresponding COD values ​​of the six short-chain fatty acids.
Table 1 Density, content and boiling point of various organic acids (under standard conditions)
Table2.1 Density, content, and boiling point of different VFAs (at standard state)
name
Drug grade
Density (g / mL)
content(%)
Boiling point (℃)
Acetic acid analytical purity 1.05 99.5 118.1 Propionic acid analytical purity 0.991 ～ 0.995 99.5 137 ～ 141 Isobutyric acid chemical purity 0.955 ～ 0.961 99.0 154.5 n-butyric acid chemical purity 0.946 ～ 0.950 98.5 161 ～ 165 Isovaleric acid chemical purity 0.936 ～ 0.942 100 173 ～ 176 n-valeric acid chemically pure 0.929 ～ 0.937 98.0 184 ～ 187Table 2 Characteristics of 6 organic acids produced by sludge fermentation
Table 2.2 Characteristics of 6 VAFs produced in excess activated sludge fermentation
Product number of carbon atoms Molecular weight (MW) (g / mol) COD (gCOD / mol) COD / MW (gCOD / mol) Acetic acid 2 60.05 64 1.07 Propionic acid 3 74.08 112 1.51 Isobutyric acid 4 88.11 160 1.82 N-butyric acid 4 88.11 160 1.82 Isovaleric acid 5 102.13 208 2.04 N-valeric acid 5 102.13 208 2.04Ci · Vi / Cs · Cs = Ai · As
(1) Because Vi = Vs = 1μL in the experiment, equation (2.1) can be written as: Ci / Cs = Ai · As
(2) In the formula: Ci—the concentration of organic acid in the sample, mg / L Vi—the sample injection volume, 1.0 μL is taken in the test
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