Pretreatment in Vortex Layer Apparatus Boosts Dark Fermentative Hydrogen Production from Cheese Whey
, Polina D. Biryuchkova 4,5, Elena A. Zhuravleva 4, Anastasia V. Vishnyakova 4 and Yuriy V. Litti 4,*Round 1
Reviewer 1 Report
Mikheeva et al. have written an interesting paper focused on pre-treatment in vortex layer apparatus to enhance biohydrogen production from dark fermentation of cheese whey, obtaining interesting results in terms of process efficiency improvement.
In my humble opinion the topic studied, the set of methods used and the results are clear and well written. However, I report some of my minor considerations by scrolling the text. Thanks for your attention and if you want to consider them.
INTRODUCTION
105) What does “(reference)” mean? Are missing the referenced paper number?
1st) When at the beginning you talk about all the pre-treatments that have been studied and can be carried out, I would also add a few lines (or a papers for references, e.g. LCA) of considerations on the environmental impact and global sustainability that these pre-treatments, that require input and/or energy, have on the scale-up process. Substantially, I mean: "Is the game worth the candle?" If you agree with this environmental perspective, clearly.
2nd) Also, since you often talk about anaerobic digestion, I would clarify the relationship and the differences between anaerobic digestion and dark fermentation.
3rd) Since you talk about dark fermentation for biohydrogen production, you can consider, if you clearly agree with the topics, also mentioning these papers:
Florio, C., Pirozzi, D., Ausiello, A., Micoli, L., Pasquale, V., Toscano, G., ... & Dumontet, S. (2017). Effect of inoculum/substrate ratio on dark fermentation for biohydrogen production from organic fraction of municipal solid waste. Chemical Engineering Transactions, 57, 175-180. (that also deepens the effect of Inoculum/Substrate Ratio to increase the biohydrogen production).
Ausiello, A., Micoli, L., Turco, M., Toscano, G., Florio, C., & Pirozzi, D. (2017). Biohydrogen production by dark fermentation of Arundo donax using a new methodology for selection of H2-producing bacteria. International Journal of Hydrogen Energy, 42(52), 30599-30612. (that also deepens the pre-treatment on the inoculum to increase the biohydrogen production to the detriment of methane production).
MATERIALS AND METHODS
123) Why did you prefer a thermophilic bacterium and in particular this species (see also comment on 137)? I would specify this in the text.
133-134) Have you calculated the contribution of the inoculum to the total production? Are the volumes you wrote in the results already net of the inoculum contribution?
133) Why did you use a mineral solution? To balance the C/N or provide other nutrients not obtainable from the feedstock? I would specify this in the text.
135) Why did you choose this pH value? In the literature it is sometimes reported that a more acidic pH can help the biohydrogen production. E.g.: “This can be achieved simply controlling the pH to a value promoting BioH2 production (Khanal et al., 2004). H2-producing microorganism’s activity found its optimum at pH from 4.5 to 6 (Fan et al., 2004; Zhu et al., 2006), while the optimum pH for methanogenic microorganisms is between 6.0 and 7.5 (Chandrasekhar et al., 2015). Although it seems to me that I understand that the pH is 4.7-6.1 inside the dark fermentation reactor, right?
137) Why without agitation? In addition, as you know the thermophilic process requires more energy input than the mesophilic one, why did you choose to work in thermophilic conditions? I would also specify this in the text. Again, is a wet process in a batch (not mixed) mode, right? I would also specify this in the text.
RESULTS & DISCUSSION
180) It is probably a typo or formatting error, but I would insert the space between the number and the °C symbol, e.g. 30 °C. If you accept, I would check all the text.
192) Please, add the numeric reference of “Leano and Babel”.
301) Please, add the numeric reference of “Lee et al”.
CONCLUSIONS
"This study showed the significant potential of using VLA in the enhancement 361 of dark fermentative H2 production and sustainable biorefineries" is certainly true on the basis of your results, but I would add a small consideration as a future development to also evaluate the environmental sustainability of the overall process with the addition of this pre-treatment; if you agree with this environmental interpretation, clearly.
Author Response
Mikheeva et al. have written an interesting paper focused on pre-treatment in vortex layer apparatus to enhance biohydrogen production from dark fermentation of cheese whey, obtaining interesting results in terms of process efficiency improvement.
In my humble opinion the topic studied, the set of methods used and the results are clear and well written. However, I report some of my minor considerations by scrolling the text. Thanks for your attention and if you want to consider them.
The authors express their gratitude to the reviewer for his positive response and appreciation of our work. His comments were very helpful in improving the manuscript.
INTRODUCTION
105) What does “(reference)” mean? Are missing the referenced paper number?
Yes, that's the missing reference, thanks. It has been added.
1st) When at the beginning you talk about all the pre-treatments that have been studied and can be carried out, I would also add a few lines (or a papers for references, e.g. LCA) of considerations on the environmental impact and global sustainability that these pre-treatments, that require input and/or energy, have on the scale-up process. Substantially, I mean: "Is the game worth the candle?" If you agree with this environmental perspective, clearly.
We thank the reviewer for this valuable comment. The need for a life cycle assessment of pretreatment methods was highlighted in the Introduction and Conclusion sections.
2nd) Also, since you often talk about anaerobic digestion, I would clarify the relationship and the differences between anaerobic digestion and dark fermentation.
Now, dark fermentation technology, its relationship and difference from AD are discussed better in the Introduction section
3rd) Since you talk about dark fermentation for biohydrogen production, you can consider, if you clearly agree with the topics, also mentioning these papers:
Florio, C., Pirozzi, D., Ausiello, A., Micoli, L., Pasquale, V., Toscano, G., ... & Dumontet, S. (2017). Effect of inoculum/substrate ratio on dark fermentation for biohydrogen production from organic fraction of municipal solid waste. Chemical Engineering Transactions, 57, 175-180. (that also deepens the effect of Inoculum/Substrate Ratio to increase the biohydrogen production).
Ausiello, A., Micoli, L., Turco, M., Toscano, G., Florio, C., & Pirozzi, D. (2017). Biohydrogen production by dark fermentation of Arundo donax using a new methodology for selection of H2-producing bacteria. International Journal of Hydrogen Energy, 42(52), 30599-30612. (that also deepens the pre-treatment on the inoculum to increase the biohydrogen production to the detriment of methane production).
These references are now included in the Introduction section
MATERIALS AND METHODS
123) Why did you prefer a thermophilic bacterium and in particular this species (see also comment on 137)? I would specify this in the text.
As a rule, the rate of biochemical reactions is higher in the thermophilic mode. Secondly, this bacterium was found to be very tolerant of low pH values, for example, it dominated the microbial community (and was isolated from this microbial community) in the thermophilic DF reactor, which produced 2 mol H2/mol hexose at pH 4. At this low value pH hydrogen-producing bacteria usually yield less hydrogen as they change their metabolism to the solventogenesis pathway. Thus, since we did not control the pH in the batches (which will inevitably lead to acidification of the medium), this bacterium was preferred.
This explanation was added in Section 2.1.
133-134) Have you calculated the contribution of the inoculum to the total production? Are the volumes you wrote in the results already net of the inoculum contribution?
Yes, the endogenous H2 production was extracted from experimental H2 production.
133) Why did you use a mineral solution? To balance the C/N or provide other nutrients not obtainable from the feedstock? I would specify this in the text.
It was used to prevent potential nutritional deficiency.
The explanation was added in Section 2.3.
135) Why did you choose this pH value? In the literature it is sometimes reported that a more acidic pH can help the biohydrogen production. E.g.: “This can be achieved simply controlling the pH to a value promoting BioH2 production (Khanal et al., 2004). H2-producing microorganism’s activity found its optimum at pH from 4.5 to 6 (Fan et al., 2004; Zhu et al., 2006), while the optimum pH for methanogenic microorganisms is between 6.0 and 7.5 (Chandrasekhar et al., 2015). Although it seems to me that I understand that the pH is 4.7-6.1 inside the dark fermentation reactor, right?
Yes, the reviewer is right. pH is usually set lower, primarily to suppress H2 scavengers. However, the optimal pH values for H2 producers are close to neutral values, which is also confirmed by reduced lag-phase.
Some information and reference were added in Section 2.3.
137) Why without agitation? In addition, as you know the thermophilic process requires more energy input than the mesophilic one, why did you choose to work in thermophilic conditions? I would also specify this in the text. Again, is a wet process in a batch (not mixed) mode, right? I would also specify this in the text.
Thermophilic mode was chosen primarily to meet requirements for the growth of the thermophilic bacterium T. thermosacharoliticum SP-H2. Some explanation on this issue is given in Section 2.1. Lack of agitation is also specified in Section 2.3.
RESULTS & DISCUSSION
180) It is probably a typo or formatting error, but I would insert the space between the number and the °C symbol, e.g. 30 °C. If you accept, I would check all the text.
The text on this issue has been checked.
192) Please, add the numeric reference of “Leano and Babel”.
Added.
301) Please, add the numeric reference of “Lee et al”.
Added.
CONCLUSIONS
"This study showed the significant potential of using VLA in the enhancement 361 of dark fermentative H2 production and sustainable biorefineries" is certainly true on the basis of your results, but I would add a small consideration as a future development to also evaluate the environmental sustainability of the overall process with the addition of this pre-treatment; if you agree with this environmental interpretation, clearly.
The section "Conclusion" is supplemented by future research at the suggestion of the reviewer.
Reviewer 2 Report
Manuscript ID: fermentation-2049928
Fermentation
Pretreatment in Vortex Layer Apparatus Boosts Dark Fermentative Hydrogen Production from Cheese Whey
The manuscript is of general interest. The following comments should help further improve the quality of the work:
1-Authors should read the journal`s guides for authors regarding the number of keywords allowed; usually, 6 is allowed while there are 7 keywords listed in the present work.
2-Please avoid the usage of first-person pronouns (e.g., we and our).
3-Reference lumping can be observed on some occasions. Please cite references where they exactly belong; this will prevent reference lumping.
4-The novelty of the present work against the existing literature in this domain should be more effectively highlighted.
5-All Equations should be properly referred to (mentioned) in the text and before they appear.
6-Please reduce the significant figures of the reported data to three. Here`s an example of significant figures (sig figs):
- 10082 (5 sig figs)
- 70,000 (1 sig fig)
- 0.0025 (2 sig figs)
- 0.000309 (3 sig figs)
- 50010.000 (8 sig figs)
- 0.0040030 (5 sig figs)
7-When a term is abbreviated, it should not appear in full on subsequent occasions nor be redefined again, and only the abbreviated form should be used. This has not been observed in the manuscript. See “cheese whey (CW)” for instance. Please scan through the manuscript and make sure this has been observed.
8-Please add an uncertainty analysis.
9-Please make sure all the units will be presented in compliance with the SI System. For instance, please use "mL" instead of "ml", etc.
10-Using too short paragraphs should be avoided.
11-It is not advisable to have headings after headings with no explanation.
12-The quality of Figure 1 is low; it is blurred in its present form. Please improve it.
13-The quality of Figure 2 is low; it is blurred in its present form. Please improve it.
14-The quality of Figure 3 is low; it is blurred in its present form. Please improve it.
15-In Figure 4, “2” in “H2” should be subscripted. Please scan the whole manuscript for such errors and get them fixed. The standard presentation of the unit “Day” is “d”. Moreover, "Day" is not a label; it should be Time (d).
16-In the labels located on the axes of the figures: please place the units in parentheses.
17-The Discussion section should be improved by more effectively comparing the results obtained with those reported in the literature and by presenting more critical discussions.
18-Please add an uncertainty analysis.
19-Application of sustainability assessment tools such as life cycle assessment and exergy analysis, as elaborated in a recent review “The role of sustainability assessment tools in realizing bioenergy and bioproduct systems” is expected to lead to interesting outcomes considering the sustainability aspects of the findings obtained. Authors can consider including the mentioned work to highlight the importance of such assessments and to direct future studies.
20-The limitations of the study should also be explained.
21-The practical implication of the present study should be included and discussed as well.
22-Please change "5. Conclusions" to "5. Conclusions and prospects". Accordingly, please elaborate on the future research needs in this domain.
Author Response
The manuscript is of general interest. The following comments should help further improve the quality of the work:
The authors express their gratitude to the reviewer for his positive response and appreciation of our work. The comments helped a lot to improve the quality of the work.
1-Authors should read the journal`s guides for authors regarding the number of keywords allowed; usually, 6 is allowed while there are 7 keywords listed in the present work.
The number of keywords has been reduced.
2-Please avoid the usage of first-person pronouns (e.g., we and our).
First-person pronouns were removed
3-Reference lumping can be observed on some occasions. Please cite references where they exactly belong; this will prevent reference lumping.
Checked
4-The novelty of the present work against the existing literature in this domain should be more effectively highlighted.
Comparison with recent research in this area has been substantially improved in Section 3.6 and Table 4.
5-All Equations should be properly referred to (mentioned) in the text and before they appear.
Some of the equations have been removed, while the rest were referred to in the text and before they appeared.
6-Please reduce the significant figures of the reported data to three. Here`s an example of significant figures (sig figs):
- 10082 (5 sig figs)
- 70,000 (1 sig fig)
- 0.0025 (2 sig figs)
- 0.000309 (3 sig figs)
- 50010.000 (8 sig figs)
- 0.0040030 (5 sig figs)
Done
7-When a term is abbreviated, it should not appear in full on subsequent occasions nor be redefined again, and only the abbreviated form should be used. This has not been observed in the manuscript. See “cheese whey (CW)” for instance. Please scan through the manuscript and make sure this has been observed.
Checked
8 (and 18)-Please add an uncertainty analysis.
Thank you for the comment. Indeed, uncertainty analysis is a well-established probabilistic approach to determine the output uncertainty that relies on the assessment of the statistical characteristics of the final distribution of output. We will keep in mind to carry out this analysis in future work, and it was mentioned in sections "3.6" and "4. Conclusions and prospects».
9-Please make sure all the units will be presented in compliance with the SI System. For instance, please use "mL" instead of "ml", etc.
Done
10-Using too short paragraphs should be avoided.
Short paragraphs have been removed.
11-It is not advisable to have headings after headings with no explanation.
This comment has been addressed as much as possible. Only the heading “3. Results and Discussion” was left.
12-The quality of Figure 1 is low; it is blurred in its present form. Please improve it.
The quality of Figure 1 has been improved
13-The quality of Figure 2 is low; it is blurred in its present form. Please improve it.
The quality of Figure 2 has been improved
14-The quality of Figure 3 is low; it is blurred in its present form. Please improve it.
The quality of Figure 3 has been improved
15-In Figure 4, “2” in “H2” should be subscripted. Please scan the whole manuscript for such errors and get them fixed. The standard presentation of the unit “Day” is “d”. Moreover, "Day" is not a label; it should be Time (d).
Checked and improved as per reviewer suggestion
16-In the labels located on the axes of the figures: please place the units in parentheses.
Done
17-The Discussion section should be improved by more effectively comparing the results obtained with those reported in the literature and by presenting more critical discussions.
Comparison with recent research in this area has been substantially improved in Section 3.6 and Table 4.
19-Application of sustainability assessment tools such as life cycle assessment and exergy analysis, as elaborated in a recent review “The role of sustainability assessment tools in realizing bioenergy and bioproduct systems” is expected to lead to interesting outcomes considering the sustainability aspects of the findings obtained. Authors can consider including the mentioned work to highlight the importance of such assessments and to direct future studies.
Thanks for the comment. We stressed the need for such a study in future work. The mentioned article has been added to Reference list
20-The limitations of the study should also be explained.
Limitations of the study was discussed in Section 3.6
21-The practical implication of the present study should be included and discussed as well.
Some discussion on practical implementation was added to Section 3.6
22-Please change "5. Conclusions" to "5. Conclusions and prospects". Accordingly, please elaborate on the future research needs in this domain.
Done
