Underestimation of Dry Matter of Anaerobic Media with High Bicarbonate Concentration

No.

Reviewer’s comments

Authors’ responses

Revised text

1

The manuscript describe an interesting investigation of the significant of thermal decomposition of bicarbonate in OM and DM measurements of anaerobic digestors.

the authors presents an important feature to be taken into account when performing these measurements, and present their hypothesis and finding in a clear and concise way.

I recommend to accept the manuscript for publication with the following minor corrections:

The authors thank the reviewers for these valuable comments and suggestions. The concerns raised by the reviewer were addressed in this letter and the corresponding revision was made. We hope that the revised paper, taking into account the kind suggestions of reviewers, could be considered by Applied Sciences.

Not applied.

2

line 116 -  “one with pHs 2.53 and 9.23.” should be corrected to  “with pHs of…”.

Corrected.

Lines 132-134: as Derikx et al. [14] did, only two conditions of pH and VFA form were tested, with pHs 2.53 and 9.23.

3

Line 126 -  the authors claim that at pH above 7 no significant loss of volatile acid should occur. This may be true ( as pH7 is 2.5 units about the pKa and the VA) but as the authors only tested this claim at pH9.23 – ergo, the claim should be stated as an assumption.

The sentence has been rephrased in a way to give the possible explanation instead of a statement.

Lines 141-142: It could be explained by the fact that volatile fatty acids, in their ionised form, i.e. for solutions with pH values above about 7, do not significantly interfere with the DM de-termination method.

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Line 135 – change rate to ratio.

Corrected.

Line 153: […] at a ratio of about 36 %

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Line 167 – change than to to

Corrected.

Line 185: […]are identical to the […]

6

Lines 233,259,275 - Caption of figures, remove the “:” character after the dot.

Corrected.

Line 252: Figure 1. Schematic DM and OM balances considering the bicarbonate decomposition.

Line 278: Figure 2. a). mass evolution of […]

Line 294: Figure 3. Temperature (blue line) and mass loss […]

7

Line 252 – temperature of 50 degrees was tested for 40 minutes, please revied the conclusions from 1 hour to 40 minutes.

Corrected.

Line 270: Figure 2.b. shows that at 50 °C in 40 min the mass loss is statically negligible, but it is rapid at 70 and 105 °C.

8

The authors are encouraged to improve figures 2 and 3 graphical appearance.

The graphical quality of figures 2 and 3 should have been good. Along this resubmission we upload the original PNG files in case that the exportation of pdf degrades the picture quality.

9

Figure 3, Y axis caption – please correct to “mass loss”.

Corrected.

Axis caption is now corrected in Figure 3.

10

Line 289 – please rephrase or remove the “.,” .

Corrected.

These tests show that it makes sense to correct the experimentally obtained DM values obtained with the standard method by considering the bicarbonate content, since the other ions do not interfere with the measurement.

No.

Reviewer’s comments

Authors’ responses

Revised text

1

This MS discusses the experimental estimation of dry matter of anaerobic biological process media and the effect of high Bicarbonate Concentration on the estimation.

The introduction and discussion sections needs to be bit more improved to indicate the research importance, the impact of findings, bit more discussion comparing with reported literature.

Introduction: Please discuss the where anaerobic sludge is practically applied, where the dry matter analysis is need etc. suing examples of previous research publications. Following is a potential list of publications. Suggest to explain bit of where the anaerobic sludge can be find (anaerobic wastewater treatment, anaerobic food waste digestion etc.). Then why the dry matter measurement is important with examples of anaerobic biological process.  

• Effect of organic loading rate on VFA production, organic matter removal and microbial activity of a two-stage thermophilic anaerobic membrane bioreactor.

 https://doi.org/10.1016/j.biortech.2010.12.081

• Insights into the ambient temperature startup of an anaerobic ceramic membrane bioreactor with thermally hydrolyzed inoculum for domestic wastewater treatment.

https://doi.org/10.1016/j.cscee.2021.100122

• Application of neural network prediction model to full-scale anaerobic sludge digestion.

https://doi.org/10.1002/jctb.2569

• Effect of pH on the behaviour of volatile compounds in organic manures during dry matter determination

https://doi.org/10.1016/0960-8524(94)90171-6

• Evaluation of different types of anaerobic seed sludge for the high rate anaerobic digestion of pig slurry in UASB reactors

https://doi.org/10.1016/j.biortech.2017.04.014

• Comparison of approaches for organic matter determination in relation to expression of bio-methane potentials

https://doi.org/10.1016/j.biombioe.2017.03.005

• Net and Gross Nitrogen Turnover in Soil Amended with Acidified and Differently Dried Solids from Biogas Digestate

https://doi.org/10.2136/sssaj2016.03.0059

We enhanced the introduction section by adding more references (Reference [1,3,4,6-13]), either recommended by the review or found by ourselves, so that the research importance and the impact of the findings are strengthened.

Lines 30-51: Anaerobic digestion (AD) is one promising biogas-producing process that performs wastewater or solid waste treatment as well. The characterization of the feedstock and the digestate in anaerobic digesters is the key to a successful realization of AD. In the an-aerobic digestion world, the dry matter (DM) and organic matter (OM) contents of an-aerobic samples are the major data for the design, the handling, the maintenance of the process and of biogas plants [1]. Many national and international standards or technical guidelines emphasize the importance of DM and OM measurement for the characterization of substrates, like French norm NF EN 15934 [2], German standard VDI 4630 [3] and ISO 20675:2018 [4]. The determination of DM and OM in biowaste like sewage sludge, wastewater, animal slurries and digestate in wastewater treatment plants and biogas plants is usually achieved according to standard protocols [2,5]. Dry matter content (also put as Total solids) is therefore determined by drying the samples at 105 °C until constant weight, usually expressed in the percentage of fresh weight. Organic matter content (also put as Volatile solids) is usually determined by burning dried samples in an oven at 550 °C until constant weight, expressed in the percentage of dry matter. Both DM and OM contents are used to realize mass balance check in anaerobic digesters [6]. In addition, the biochemical methane potential (BMP) of one specific substrate is always reported and expressed in the volume of methane produced by one gram, kilogram or tonne of DM or OM [7–11]. They are among the first and the most important parameters to be analyzed in biogas reactors at industrial scale [12,13]. Hence, an inaccurate measure of DM or OM could inevitably lead to wrongful interpretation of the experimental results and therefore, a potential failure of anaerobic digestion.

2

Figures:

Figure 3 has a typo (Y axis “Mass moss”), please revise it.

Corrected.

The axis caption is now corrected in Figure 3.

3

 Results and discussion

Please discuss while comparing with previous studies. 

The related sections have been revised.

Lines 141-144: It could be explained by the fact that volatile fatty acids, in their ionised form, i.e. for solutions with pH values above about 7, do not significantly interfere with the DM de-termination method. This confirms the previous findings of Derikx et al. (1994) about the loss of VFA during drying at various initial pH levels.

Lines 301-313: The decomposition of bicarbonate is a uniquely thermo-dependent process that manifests itself measurably in dry mass measurements as early as 70 °C, let alone 105 °C. The resulting loss of mass during measurement is of the order of 35%, thus leading to an underestimation of the DM and an indirect overestimation of the OM measurement in methanogenic reactors. For an anaerobic sample highly loaded with bicarbonate content, the drying at 105 °C applying standard protocol could give rise to a significantly wrongful results of DM and OM contents. This constitutes a major factor that should be considered when one wants to realize mass balance in anaerobic reactors or to report the specific methane potential of the sample concerned. It is also confirmed that no mass loss due to bicarbonate decomposition takes place at thermophilic conditions of anaerobic digestion.

These tests show that it makes sense to correct the experimentally obtained DM values obtained with the standard method by considering the bicarbonate content, since the other ions do not interfere with the measurement.

4

Figure 3 information, please add another analysis (temperature Vs mass loss graph). 

The related sections have been modified.

Lines 287-290: The apparatus was programmed to stabilise for one hour at 50 °C, rise to 55 °C in 5 minutes, stabilise again for one hour at 55 °C and so on in successive steps of 5 °C up to 105 °C, finally maintained for 2 hours (Figure 3).

Lines 282-284 were moved to Lines 299-301 below the Figure 3.

The figure shows that the mass does not vary up to a temperature of 70-75 °C and that the kinetics are relatively slow; the loss of mass is 32 % at 105 °C after 14 hours of this experiment.