Accurate Estimation of Bicarbonate and Acetic Acid Concentrations with Wider Ranges in Anaerobic Media Using Classical FOS/TAC Titration Method

No.

Reviewer’s comments

Authors’ responses

Revised text

1

Line 22: sodium acetate

Corrected.

Line 22: […] sodium acetate and sodium bicarbonate contents […]

2

Line 91: Flüchtige Organische Säuren

Corrected.

Line 95: […] the VFAs concentration is called FOS as “Flüchtige Organische Säuren” in German […]

3

Line 190: By which method were these concentrations verified?

We did not do specific verification of the concentrations in the prepared solutions.

However, when seeing Figures 2 and 3A, one may observe the linearity of the points at the same acetic (Figure 2) or bicarbonate (Figure 3A) concentration. This confirms that the solutions are prepared at the desired concentrations so that their reciprocal effect on the determination of either FOS or TAC is systematic during titration.

Not applied.

4

Line 195: How many times each titration was repeated? 

We applied just one titration for each condition since the results show good accordance with the simulation.

Not applied.

5

Line 197: not sure whether the required FOS and TAC concentrations were valid before or after dilution (5 ml + 15 ml)

We agree with the reviewer’s comment that the dilution may have an impact on the acid-base equilibrium (different dissociation degree). This has also been studied (results not shown).

That is why we followed the same dilution procedure as described in the guidance of HACH titrator (5 ml + 15 ml) so that all of the correction made in the present study is based on the same dilution level.

One should apply the same dilution level when using our model.

We add several notes in Section 3.3.

Line 397: The dilution level may change the acid-base equilibrium of acetate and bicarbonate. Therefore, it has to be noted that one should apply the same dilution protocol as we did (5 mL of sample + 15 mL of water) when using the new formulas.

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Line 354: Consequently, we will use the results .........  better to say the results will be used ...

Corrected.

Line 360: Consequently, the results of the simulation instead of the experiment will be used to calibrate to […]

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Line 370: correct s Figures 4A and 4B

Corrected.

Line 377: Figures 4A and 4B show the recalculated values […]

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Line 420: any range for FOS?

As indicated in Section 2.4 Line 263, TAC and FOS were initially found at 11,009 ± 674 mg CaCO₃·L^-1 and 2,452 ± 64 mg HAc·L^-1, respectively. After acidification and degasification, they were reduced to 2,802 ± 181 mg CaCO₃·L^-1 and 1,152 mg HAc·L^-1. We revised the sentence to make it clearer.

Line 433: After acidification and degasification, the empirical TAC and FOS of the digestate were reduced from 11,009 mg CaCO₃·L^-1 to 2,802 ± 181 mg CaCO₃·L^-1 in terms of TAC and from 2,452 mg HAc·L^-1 to 1,152 mg HAc·L^-1 in terms of FOS.

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Line 467: even the real anaerobic media was partially synthetic

We thank the reviewer for pointing out this. We revised the sentence to make it unambiguous.

Line 498: […] is experimentally confirmed both in synthetic water solution and in anaerobic media.

10

Table S2: the second TACnew should be FOSnew

Corrected.

Cf Table S2 in supplementary material

No.

Reviewer’s comments

Authors’ responses

Revised text

1

Manuscript Number: applsci-1478738

The manuscript “Accurate estimation of bicarbonate and acetic acid concentrations with wider ranges in anaerobic media using classical FOS/TAC titration method” performs an improved method for an easy and accurate determination of total alkalinity content and volatile fatty acids by titration.

The work is well structured and written and provides an improved method to determine TAC and VFA (FOS) in anaerobic digesters. The results are related to the objectives and the conclusions fit with the overall aim of the manuscript and the special issue.

Just some comments to the authors:

The authors thank the reviewer for these valuables comments with which the paper has undoubtedly been improved.

Not applied.

2

Page 2, line 77. This is not the best ratio, but a limit that should not be surpassed to assure process stability.

We agree with this point and accordingly revised the sentence.

Line 77: […],the ratio should be generally less than 0.4.

3

Page 4, line 187. The experiment was performed with acetic as the only volatile fatty acid. What can we expect for the model for the rest of VFA?

Many expectations could be made for the consideration of the rest of VFA:

1)     The original Scilab code is available in the Supplementary material. One can take into account the acid-base equilibrium of propionic, butyric and valeric acids during the simulation. The pKa of each species is available in the Table S1. One can vary the concentrations of these VFA to see their respective impact on the final results.

2)     In fact, the pKa values of propionic, butyric and valeric acids are so close (around 4.7-4.8) that the consideration of these species in the titration process has limited effect on the output results (i.e. A and B values). However, one should be fully conscious of the fact that there exist different calculation methods to report the total VFA content: VFA could be calculated at first in molar concentration and converted to equivalent acetic acid mass concentration, which is generally more accepted; however, the total VFA content could also be a simple sum of all mass concentrations of the concerned acids, which is also practiced in many articles.

This is also true for the original Nordmann's equations.

Taking an example, a total VFA molar concentration is 0.083 mol/L, composed of 2/3 as acetic acid (60 g/mol) and 1/3 as propionic acid (74 g/mol). It corresponds to 5,000 mg HAc_eq/L using the first method. However, by doing the simple sum, we achieve 0.083*2/3*60+0.083*1/3*74=5367 mg/L. A difference of 7% could be obtained.

We add some comments on this issue in the text.

Line 524: The experiments were performed only with acetate. The inclusion of other VFA (propionic, butyric and valeric acids) in the simulation is possible. The original Scilab code has been given in Supplementary materials. One can vary the concentrations of these VFA to see their respective impact on the final results.

The pKa values of propionic, butyric and valeric acids are so close (around 4.7-4.8, cf Table S1) that the consideration of these species in the titration process has limited effect on the output results (i.e. A and B values). However, one should be fully conscious of the fact that there exist different calculation methods to report the total VFA content: VFA could be calculated at first in molar concentration and converted to equivalent acetic acid mass concentration; the total VFA content could also be a simple sum of all mass concentrations of the concerned acids. The first method does not modify the FOS calculation, for the original Nordmann's method as well as the proposed model, while the second the effect of the second one could be significant.

4

Page 5, line 203 - Equation 3. Where does the number 50045 come from?

The equation 3 comes from the It is a constant calculated from molar mass of CaCO₃ (100.09 g/moL) x 1000 / 2.

“1000” converts milliliters into liters.

“2” converts bicarbonate content into equivalent calcium carbonate content as each molecule of CaCO₃ can take up 2 H⁺.

Equation 3 is used in the Hach titrator to calculate the TAC values

Not applied.

5

Page 9, Fig. 4. Could the authors provide more information about how the data contained in the figures ?

In Figure 4, the newly calculated bicarbonate or acetic acid contents were introduced against the real concentrations in the medium at different acetic acid or bicarbonate levels.

The modeled results given by the Equations 8 and 9 are so good that all of the points at the same bicarbonate or acetic acid level are mixed up, superimposed and thus not visible. We added one sentence commenting on this issue.

Line 421: In these figures, all of the points at the same bicarbonate or acetic acid level are mixed up, superimposed, and thus not visible.