Cadmium and COD Removal from Municipal Wastewater Using Chlorella sp. Biomass in Microbial Fuel Cells

Round 1

Reviewer 1 Report

Agüero-Quiñones et al. have presented the research on Cd removal and Chemical Oxidation Demand reduction in the wastewater accompanied by simultaneous generation of electricity. It was demonstrated that biological activity of Chlorella sp. decreases the concertation of Cd and COD upon usage of microbial fuel cell in the first part of the manuscript. The second part of manuscript was devoted to generation of electricity and investigation of the carbon electrode used in the cell.

I cannot recommend the manuscript for publication in the present state. Moreover, I’d rather suggest to divide the manuscript in two separate works on Cd removal and COD reduction, and electricity generation with subsequent resubmission of the former work to «Sustainability».

MAJOR:

-          Introduction part does not provide sufficient information on the selection of Chlorella sp. as biological agent, i.e. no comparison to activity of other bacteria or plants. No sufficient information on generation of electricity with the bacteria or plants. Likewise, Cd is not the only dangerous heavy metal, why were not Pb or Hg selected?

-          COD includes wide range of organic compounds. Those compounds, in principle, can be different in sewage water from various locations. It’d be quite informative to know which compounds were in these water samples before and after treatment.

-          Regarding the water quality, would it be possible to use the water for, at least, technical purposes or further treatment is necessary?

-          As it was indicated in the conclusions, it would be more informative to talk about biochemical oxygen demand rather than COD, as the biological treatment was stated in the manuscript.

MINOR:

-          The manuscript definitely requires well written discussion section with the discussion of mechanism of Chlorella sp. activity and electricity generation. This moment, I can only find some comparisons to other research articles.

-          Why Pseudomonas putida and Klebsiella oxytoca were found on the electrodes? I did not understand from the manuscript if they were deliberately added or contained in the wastewater before

TYPOS and STYLE:

All text: watch the font and line space for the body of text

Page 1 line 35: the letter «E» is excessive

Page 3 Table 1: the acronyms are quite unexpected, e.g. COD already stands for Chemical Oxygen Demand

Page 4 line 124: «5.2 ± 1 cm in diameter. diameter, 0.7 ± and 0.1 cm thick.», the « diameter.» and «and» are excessive

Page 4 line 125: «cm2» should be « cm²»

Page 4 line 129: « end.» the full stop is excessive

Table 3 seems to useless as the cells contain same data

Table 5,6 and 7 can be combined in one or two tables, as the contain the repetitive cells

In table 6: what F.V. means?; the error is extremely high

Page 7 figure 5: please indicate number of the figure in the text, do not use words «The following figure shows». Figure 5 caption contains mistake, this figure shows the effect of microalgae

Figures 5 and 6 should be combined in the single one for better visibility

Table 10: please use the same acronyms throughout your manuscript

Page 20 lines 596-598: «cm-1» and «NO2» should be «cm^-1» and «NO₂»

Author Response

Dear colleague, I hope you are in good health. The authors appreciate the comments made to improve the manuscript. So we have also made the suggested changes and have made the responses to each of the comments made:

MAJOR:
- Introduction part does not provide sufficient information on the selection of Chlorella sp. as biological agent, i.e. no comparison to activity of other bacteria or plants. Not enough information on generation of electricity with the bacteria or plants. Likewise, Cd is not the only dangerous heavy metal, why weren't Pb or Hg selected?
Ans. Added justification for Chlorella sp. (lines 70-81), and only Cd was studied because the initial shift of the sample yielded a higher concentration of Cd.
- COD includes a wide range of organic compounds. Those compounds, in principle, can be different in sewage water from various locations. It'd be quite informative to know which compounds were in these water samples before and after treatment.
Ans. The initial values of the sample are in Table 1, and the monitor up to its final values are in Figure 8.

- Regarding the water quality, would it be possible to use the water for, at least, technical purposes or further treatment is necessary?
Ans. It would not be possible in its current state, since there is still residue of other metals or microorganisms, which has been studied in this report. But more work will be done to obtain this product.
- As it was indicated in the conclusions, it would be more informative to talk about biochemical oxygen demand rather than COD, as the biological treatment was stated in the manuscript.
Ans. Exactly for a future work, these missing factors will be taken into account, because we have had limitations in the use of this equipment for maintenance.
MINOR:
- The manuscript definitely requires well written discussion section with the discussion of mechanism of Chlorella sp. activity and electricity generation. This moment, I can only find some comparisons to other research articles.
Ans. A paragraph was placed in the introduction, exclusively for this (line 70-81)
- Why Pseudomonas putida and Klebsiella oxytoca were found on the electrodes? I did not understand from the manuscript if they were deliberately added or contained in the wastewater before
Ans. These bacteria are microbiota of the effluents, so they were isolated from the anode electrodes since they are bacteria that formed as biofilms.
 
TYPES and STYLE:
All text: watch the font and line space for the body of text
Page 1 line 35: the letter «E» is excessive
Ans. was fixed
Table 1: the acronyms are quite unexpected, e.g. COD already stands for Chemical Oxygen Demand
Ans. was fixed
Page 4 line 124: «5.2 ± 1 cm in diameter. diameter, 0.7 ± and 0.1 cm thick.», the « diameter.» and "and" are excessive
Ans. was fixed
Page 4 line 125: «cm2» should be «cm2»
Ans. was fixed

Page 4 line 129: «end.» the full stop is excessive
Ans. was fixed

Table 3 seems to be useless as the cells contain the same data
Ans. Was eliminated
Table 5,6 and 7 can be combined in one or two tables, as the contain the repetitive cells
Ans. The authors believe that the tables should not be combined in order to have a more complete understanding and avoid errors.

In table 6: what F.V. means?; the error is extremely high

Ans. F.V. means favorite variable, in the statistical program Infostat
Page 7 figure 5: please indicate number of the figure in the text, do not use words «The following figure shows». Figure 5 caption contains mistake, this figure shows the effect of microalgae
Ans. done
Figures 5 and 6 should be combined in the single one for better visibility.
Ans. The authors agree that the combination of these two figures can mislead readers.
Table 10: please use the same acronyms throughout your manuscript
Ans. Done
Page 20 lines 596-598: «cm-1» and «NO2» should be «cm-1» and «NO2»
Ans. Done

I hope you have a nice day.
kind regards

Author Response File: Author Response.docx

Reviewer 2 Report

The paper is good but it contains major mistakes

Comments for author File: Comments.pdf

The quality of English language is good

Author Response

Dear colleague, I hope you are in good health. The authors appreciate the comments made to improve the manuscript. So we have also made the suggested changes and have made the responses to each of the comments made:

The paper is good but there are mistakes such as
1-The name of microorganism must be Italic in all paper
Ans. donate
2-In introduction: remove letter E in line35 and remove word (environment) in line40.
Ans. donate
3-In materials and methods:
In line 96 write(s) capital letter in (µs/cm), change COD into COD in table 1
Ans. donate
In line 112: change the phrase (to divide it into 4 1000ml flasks)
Ans. OK.
In line 124: what did you mean diameter, 0.7 ± and 0.1 cm thick?
Ans. corrected
In line139: why did the sample divide into 2400, 3400 and 2400ml?
ans. it was an error because it was only 2400 ml, which was the volume of the container
In line 178: the temp is -22ᵒC or 22ᵒC.
ans. 22°C
In line 209: Did you mean the bacteria was approximately 6 to 8 hrs or what?
ans. it must have been 6 hours
The materials and methods do not contain reference, add references.
Ans. corrected
4-In results and discussion, in all results and discussion: write all references as number
In line 386: write the name of organism Scenedesmeus italic
Ans. Corrected.
In lines 426,427 and 456,457: rephrase the phrases.
Ans. Corrected, line 440-441 and 468-469.
In line 446 and 447 the phrase is not understood.
Ans, corrected, line 458-459
In line 461: the volume of work was greater than or what?
Ans. The volume was increased, due to the change of the cells.
In lines 514 and 515: remove the brackets
Ans. They are from the references.
In line520: why was the ratio of elimination of turbidity by Tee et al., is it better than your work?
Ans. It was removed to avoid confusion.
In Table 10 change COD into COD.
Ans. corrected
Inline 543: write the name of Pseudomonas putida and Klebsiella oxytoca into Italic.
Ans. corrected
In line 552: The figure is 17 or 14.
Ans. 14
Inline 598: The C-H bond belongs to what?
Ans. To the alkanes, whose compounds were found in the substrate.

I hope you have a nice day.
kind regards

Author Response File: Author Response.docx

Reviewer 3 Report

I have carefully read the review article entitled “ Cadmium and COD removal from municipal wastewater using Chlorella sp. Biomass in microbial fuel cells” and find the work relevant is fitting the scope of the journal. Went through the manuscript in detail. However, needs minor revisions to be considered as publishable in the journal.

In order to improve the quality of the manuscript, authors may take the following recommendations and comments into consideration:

1.     English needs to be polished and grammar improved considerably throughout the whole manuscript.

2.     Abstract should be more specific and mention what was the achievement.

4.     “Keywords” must be rewritten in good manner.

3.     Chlorella sp. should be italic

5.     Authors should establish a table for abbreviations at the end of the articles to avoid confusion of the readers.

Moderate editing of English language required

Author Response

Dear colleague, I hope you are in good health. The authors appreciate the comments made to improve the manuscript. So we have also made the suggested changes and have made the responses to each of the comments made:

In order to improve the quality of the manuscript, authors may take the following recommendations and comments into consideration:
1. English needs to be polished and grammar improved considerably throughout the whole manuscript.
Ans. corrected
2. Abstract should be more specific and mention what was the achievement.
Ans. corrected
4. “Keywords” must be rewritten in good manner.
  Ans. Some words were corrected
3. Chlorella sp. should be italic
Ans. corrected
5. Authors should establish a table for abbreviations at the end of the articles to avoid confusion of the readers.
Ans. It has been noted, but the number of tables in the manuscript can be confusing, but the abbreviations have been placed in the methodology part, as another reviewer recommended.

I hope you have a nice day.
kind regards

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors,

I read your responses to my questions, but I still cannot recommend this manuscript for publication in the present state. The reason is that you even didn't entirely addressed my minor comments. 

TYPOS:

keywords: should be "microalgae"

Fig. 18:  axis titles aren't in English

Author Response

Dear colleague, I hope you are in good health. Apologies for those two errors found, which have been corrected. In the response provided at the beginning of the review, the authors believe that we have modified it. If you have another suggestion, let us know so that the manuscript can be published.
have a nice day.
Best regards

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Dear Authors,

Thank you very much for your response. I wasn't able to find your modifications in the text as they were not highlighted. 

Author Response

Dear colleague, I hope you are in good health. The authors appreciate the comments made to improve the manuscript. So we have also made the suggested changes and have made the responses to each of the comments made:

MAJOR:
- Introduction part does not provide sufficient information on the selection of Chlorella sp. as biological agent, i.e. no comparison to activity of other bacteria or plants. Not enough information on generation of electricity with the bacteria or plants. Likewise, Cd is not the only dangerous heavy metal, why weren't Pb or Hg selected?
Ans. Added justification for Chlorella sp. (lines 68-79), and only Cd was studied because the initial shift of the sample yielded a higher concentration of Cd.
- COD includes a wide range of organic compounds. Those compounds, in principle, can be different in sewage water from various locations. It'd be quite informative to know which compounds were in these water samples before and after treatment.
Ans. The initial values of the sample are in Table 1, and the monitor up to its final values are in Figure 8.

- Regarding the water quality, would it be possible to use the water for, at least, technical purposes or further treatment is necessary?
Ans. It would not be possible in its current state, since there is still residue of other metals or microorganisms, which has been studied in this report. But more work will be done to obtain this product.
- As it was indicated in the conclusions, it would be more informative to talk about biochemical oxygen demand rather than COD, as the biological treatment was stated in the manuscript.
Ans. Exactly for a future work, these missing factors will be taken into account, because we have had limitations in the use of this equipment for maintenance.
MINOR:
- The manuscript definitely requires well written discussion section with the discussion of mechanism of Chlorella sp. activity and electricity generation. This moment, I can only find some comparisons to other research articles.
Ans. A paragraph was placed in the introduction, exclusively for this (line 68-79)
- Why Pseudomonas putida and Klebsiella oxytoca were found on the electrodes? I did not understand from the manuscript if they were deliberately added or contained in the wastewater before
Ans. These bacteria are microbiota of the effluents, so they were isolated from the anode electrodes since they are bacteria that formed as biofilms.
 
TYPES and STYLE:
All text: watch the font and line space for the body of text
Page 1 line 35: the letter «E» is excessive
Ans. was fixed (line 48)
Table 1: the acronyms are quite unexpected, e.g. COD already stands for Chemical Oxygen Demand
Ans. was fixed (line 118)
Page 4 line 124: «5.2 ± 1 cm in diameter. diameter, 0.7 ± and 0.1 cm thick.», the « diameter.» and "and" are excessive
Ans. was fixed (line 135)
Page 4 line 125: «cm2» should be «cm2»
Ans. was fixed (line 138)

Page 4 line 129: «end.» the full stop is excessive
Ans. was fixed

Table 3 seems to be useless as the cells contain the same data
Ans. Was eliminated
Table 5,6 and 7 can be combined in one or two tables, as the contain the repetitive cells
Ans.now they are tables 4, 5 and 6,  The authors believe that the tables should not be combined in order to have a more complete understanding and avoid errors.

In table 6: what F.V. means?; the error is extremely high

Ans. F.V. means favorite variable, in the statistical program Infostat
Page 7 figure 5: please indicate number of the figure in the text, do not use words «The following figure shows». Figure 5 caption contains mistake, this figure shows the effect of microalgae
Ans. done (linea 243)
Figures 5 and 6 should be combined in the single one for better visibility.
Ans. The authors agree that the combination of these two figures can mislead readers.
Table 10: please use the same acronyms throughout your manuscript
Ans. Done
Page 20 lines 596-598: «cm-1» and «NO2» should be «cm-1» and «NO2»
Ans. Done (line 599-601)

best regards

Author Response File: Author Response.docx