Reverse Osmosis Modeling Study of Lead and Arsenic Removal from Drinking Water in Tarija and La Paz, Bolivia

Round 1

Reviewer 1 Report

This is a very interesting study, due tonArsenic rejection by reverse osmosis is not a clear aspect even for the membranes manufacturers which do not introduced this concept in the specification sheets. Due to this I agree with the importance of this manuscript.

Regarding the possibilities to improve it, I would like to comment the following if it could help to the authors:

1. To highlight in the abstract the goal of the study, its value and the benefits of the results obtained.

2. To introduce more explications for each graphic presented in the results section.

3. To maintain a similar form of the presentation of the graphics shown in the results section. It looks as different forma to show them, maybe should be better to have a standard base of the graphics when it is possible, to look more similar configuration.

4. To show a section of discussions of the results with the references shown.

5. To introduce more reference related with arsenic rejection in reverse osmosis membranes.

Thanks so much and good job. I like this very much.

 

Author Response

Reviewer 1

MPDI

Procesess

Of my highest consideration,

Thank you for the review and the time spent, indicating that the comments and observations made have served to substantially improve the manuscript. All observations and comments have been considered in the manuscript.

To facilitate review, the answers are detailed one by one. The manuscript is sent with track changes to facilitate the review process.

Observation 1: To highlight in the abstract the goal of the study, its value and the benefits of the results obtained.

Response O1: Thank you very much for your important suggestion. The goals and importance of the study have been highlighted in the abstract, please check the lines 20-30

Observation 2: To introduce more explications for each graphic presented in the results section

Response O2: Thank you very much for your observation. Major comments have been made to the results graphs, please check the lines 351-353, 365-369, 384-388, in the lines 452-471 a greater explanation of the results is made.

• Figure 4: The figure shows the behavior of the permeate depending on the temperature variation, the parameters have allowed to obtain an adjustment factor applied to the experimental results, lines 351-353.
• Figure 5 and 6: Centrifugal pumps with capacities up to 30 bar were used. For the variation of the pressures, valves were installed at the inlet and outlet of the membrane and a frequency regulator. It is observed (figures 5 and 6)that with the increase in transmembrane pressure, the Jv increases linearly for the three concentrations. It is also observed that at higher concentration the Jv decreases, line 365-369
• Figure 7 and 8: The results in the rejection are expected in an RO process, however, the research shows that there is no physical compaction of the membrane at low concentrations but at high concentrations it is the CP phenomenon that governs the process. This is due to the increase in solute concentrations. Lines 384-388

Observation 3:  To maintain a similar form of the presentation of the graphics shown in the results section. It looks as different forma to show them, maybe should be better to have a standard base of the graphics when it is possible, to look more similar configuration.

Response O3: It has been standardized the presentation of graphs please check figures 7 and 8, lines 379-383 and figures 9 and 10, 395-397

Observation 4: To show a section of discussions of the results with the references shown.

Response O4: Section 4.5 of results discussion was added, please check lines 472-489

Observation 5: To introduce more reference related with arsenic rejection in reverse osmosis membranes Response O5: More references were added regarding the rejection of As, please check the lines, 128-135 and 677-682 in references

Kind regards,

Esteban Manuel Villena Martínez

Universidad Católica Boliviana - Universitat Politécnica de Valencia

Author Response File: Author Response.docx

Reviewer 2 Report

Uniform the font especially the color and underlined parts.

Comments for author File: Comments.pdf

Author Response

Reviewer 2

MPDI

Procesess

 

 

Of my highest consideration,

 

Thank you for the review and the time spent, indicating that the comments and observations made have served to substantially improve the manuscript. All observations and comments have been considered in the manuscript.

 

To facilitate review, the answers are detailed one by one. The manuscript is sent with track changes to facilitate the review process.

 

Observation 1: Uniform the font especially the color and underlined parts.

Response O1: Thank you very much for your observation, the texts have been standardized, please check references 21, 24, 28, 34, 37 and 38

Kind regards,

Esteban Manuel Villena Martínez

Universidad Católica Boliviana - Universitat Politécnica de Valencia

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript by Villena-Martínez et al. presents an effort in applying a mathematical model–considering the concentration-polarization phenomenon–that best describes lead and arsenic removal in drinking water during the reverse-osmosis filtration process. The major finding was that the inclusion of an Arrhenius temperature adjustment factor allows relatively accurate prediction of process performance.

 

1. What is novel about this study? The authors did not develop a new mathematical model. Data and information presented in this study are readily available in the literature.

 

2. How long were the membranes compacted for before each experiment to ensure stable performance? At what pressure? This information seems to be missing from the manuscript.

 

3. How many replicates were performed for each experiment? Please include error bars.

 

4. Why was the pH of the solution not maintained at a fixed value during the experiment?

 

5. Please double check units in graphs. Volumetric flux is typically presented in L/m².h. Permeability is typically presented in L/ m².h.bar.

 

6. Please double check values in Table 4 for medium concentration.

 

7. Please be consistent in presenting graphs. Make sure that they all look the same in terms of layout, format, legend, font, font size, and etc.

 

8. Figures 17 and 18, please label the different concentrations (i.e., low, medium, high) for the benefit of your readers.

Author Response

Reviewer 3

MPDI

Procesess

Of my highest consideration,

 

Thank you for the review and the time spent, indicating that the comments and observations made have served to substantially improve the manuscript. All observations and comments have been considered in the manuscript.

 

To facilitate review, the answers are detailed one by one. The manuscript is sent with track changes to facilitate the review process.

 

Observation 1: What is novel about this study? The authors did not develop a new mathematical model. Data and information presented in this study are readily available in the literature.

Response O1: Thank you very much for your comment. The research was carried out to solve serious problems of water contamination by heavy metals that are toxic to health in two basins in Bolivia. RO was studied in the removal of Pb and As. The main contribution of this research is the development and construction of a mathematical model based on the Spiegler-Kedem polarization concentration model using different concentrations of Pb and As. The model allows to design high conversion facilities (>80%) and optimize the process from the point of view of energy efficiency in future works. The model has been developed including an Arrhenius temperature adjustment factor that allows accurate prediction of process performance. The experimentation was carried out in two RO pilot plants using polyamide membranes. Among the benefits of the study, it allows guaranteeing the rejection of metals greater than 99%, even at low pressures. The novelty and the objectives are explained and developed in the manuscript, please check the lines 19-30, 136-148, 149-153, 509-510 and 523-525

Observation 2: How long were the membranes compacted for before each experiment to ensure stable performance? At what pressure? This information seems to be missing from the manuscript.

 

Response 2: Thank you very much for your observation. Before each test, the membrane was compacted at maximum pressure (10 bar) with the corresponding solution for 1 hour, without observing significant changes in the flow, since the behavior of the membrane was very stable in all the tests from the first 10 minutes, please check lines 332-335. In section 3.3, lines 301-307 and 328-331 the duration times of the experiments are detailed. The pressures are detailed in the lines 355-356 and 423-424

 

Observation 3: How many replicates were performed for each experiment? Please include error bars.

Response O3: Using a membrane module with an active surface of 2.5 m2, the flux density during the test stabilized in the first 10 minutes and remained practically stable during the 30 minutes of the test. Therefore, it has not been necessary to carry out repetition tests, unlike what happens with much smaller membrane modules (150 cm2).

Observation 4: Why was the pH of the solution not maintained at a fixed value during the experiment?

Response O4: The variation of the pH was due to the variation of the concentrations of the metal salts and NaCl. This explanation was included in the manuscript, please check lines 468-471

Observation 5: Please double check units in graphs. Volumetric flux is typically presented in L/m².h. Permeability is typically presented in L/ m².h.bar.

Response O5: Units have been changed.

Observation 6: Please double check values in Table 4 for medium concentration.

Response O6: The data has been verified

Observation 7: Please be consistent in presenting graphs. Make sure that they all look the same in terms of layout, format, legend, font, font size, and etc.

Response O7: The formats of the figures have been unified

Observation 8: Figures 17 and 18, please label the different concentrations (i.e., low, medium, high) for the benefit of your readers.

Response O8: Thank you for your observation, the graphs show the maximum errors obtained in the validation that correspond to high concentrations. For a better interpretation, the detail in the graphics has been improved. Please check de figures 17 and 18. in lines 455-457 this is explained.

 

Kind regards,

Esteban Manuel Villena Martínez

Universidad Católica Boliviana - Universitat Politécnica de Valencia

Author Response File: Author Response.docx

Reviewer 4 Report

This study represents an investigation of RO membrane technology to separate heavy metals (Lead and Arsenic) from the simulated drinking water in the two pilot plants. Overall, the treatment can achieve a good rejection. Different parameters such as the input concentration and the temperature were varied in the experimental work. The model considering the CP phenomenon matched the experimental data, which further proved that the CP phenomenon could decrease the membrane performance in these pilot plants.

        The authors did a lot of work; however, the current presenting quality is not very good, I would highly recommend the authors rewrite the manuscript. 1) Multiple typos and grammar mistakes exist throughout this manuscript; 2) The writing needs to be improved, the ideas jump so quickly to another; 3) there is not enough discussion in the results and discussion part. Please see the details.

Section 1.

The introduction part is not very clear. The several beginning paragraphs are ok. The author stated the toxicity of heavy metals, then followed by the treatment method-RO, the challenge in RO—concentration polarization (CP), and then the importance of mathematical modeling to describe the process to better predict the process. However, when it comes to the literature review and the motivation part, the authors did not clearly present the relationship between the previous work and the intent of the current study. For example, in the paragraph of Line 99-102, the author stated that “a mathematical model” was applied to do xxx. When I read this, I would like to ask two questions: 1. What type of mathematical model? 2. How did this previous work inspire your current work? No comments and links were given by the authors, and I did feel confused. The same problem is with the literature review paragraph of Line 107-119. And then the authors jump directly to their own work from Line 120.  Again, what is the relationship?

Line 47. No period at the end of the sentence. Same in Line 430.

Line 83. “for the optimization o the final design of the system, …” should be “of”

Line 111. Wrong grammar. “Abejón, et al. [40] through their research study, pointed out the economic and energy viability of arsenic removal by RO. ”

Line 133. “…..of the membrane. Being an important contribution to …” should be “….. , being an…” (still not good, maybe you should separate this sentence).

 

Section 2.

Line 163: “(Figure 2) schematizes the phenomenon of CP showing the increased concentration in the feed boundary layer due to the retention of solute.” Please remove the bracket. The same problem with Line 301, Line 318, Line 335, Line 355, Line 374, Line 378……… (so many, please check)

Line 205. Should provide how you calculated this mass transfer coefficient k based on Re and Sc number.

Line 212. Eqn 8. Did you assume cf’=cf?

Line 233. Eqn 12. You should state here what is “i”, because this is the first time this nomenclature appears.

Section 3.3

Table 2. Please state how you chose low, medium, and high concentrations of NaCl. Why are they different? The author said something in Line 289 to 290, but it is not clear enough. The grammar of this sentence is also not right.

Section 4

Line 325 to Line 330. There is no discussion for Figures 5 and 6.

Line 351 to 353. “……the research shows that there is no physical compaction of the membrane at low concentrations but at high concentrations, it is the CP phenomenon that governs the process. “There is no discussion on your results. How did you come up with this conclusion that there is no membrane compaction and why does CP govern this process? You should talk a little bit more.

Line 359. “These results agree with the work of [51], who ..” The author should say what is the work, or the researcher of the work, rather than the citation number.

Figure 11, 12, 13, 14, 15, 16. The figures were not presented well. What operating conditions caused the three steps in the figure? There is no description either in the figure or in the text.

Figures 17, 18. The author stated in Line 416 that the three steps are the flux behavior of three concentrations. You should find a way to put this information into the figure for easier understanding for the readers.

Section 4.5.

Line 432-433. How the model “made it possible…”? There is always no link between the experimental/modeling results between the authors’ conclusions, which does need to be improved.

Section 5.

The conclusion part should be a concise and clear summary of the entire article that helps the readers go over the important ideas quickly. However, the current conclusions are more like the results and discussion part. The authors did not discuss adequately in the discussion part.

Also, the conclusion of the author is not clear. For example, Line 462-464, I guess what the authors want to say is that--at a lower concentration, the mass transfers more easily than at a higher concentration because of the lower CP phenomenon. Actually, CP is also one type of “mass transfer”, you cannot say at lower concentration it is “mass transfer” and at higher concentration, it is not.  This is very confusing.

 

Author Response

Reviewer 4

MPDI

Procesess

Of my highest consideration,

Thank you for the review and the time spent, indicating that the comments and observations made have served to substantially improve the manuscript. All observations and comments have been considered in the manuscript.

To facilitate review, the answers are detailed one by one. The manuscript is sent with track changes to facilitate the review process.

Comment 1: Extensive editing of English language and style required

Response: Thanks for the suggestion and observation. The style and the English edition have been checked by a native English-speaking colleague. Please check the line 16-30, 35-54, 62-100, 118-124, 136-148, 155-162, 165-166, 181-182, 191-195, 498-501  and 503-529

Section 1

Observation 1: The introduction part is not very clear. The several beginning paragraphs are ok. The author stated the toxicity of heavy metals, then followed by the treatment method-RO, the challenge in RO—concentration polarization (CP), and then the importance of mathematical modeling to describe the process to better predict the process. However, when it comes to the literature review and the motivation part, the authors did not clearly present the relationship between the previous work and the intent of the current study. For example, in the paragraph of Line 99-102, the author stated that “a mathematical model” was applied to do xxx. When I read this, I would like to ask two questions: 1. What type of mathematical model? 2. How did this previous work inspire your current work? No comments and links were given by the authors, and I did feel confused. The same problem is with the literature review paragraph of Line 107-119. And then the authors jump directly to their own work from Line 120.  Again, what is the relationship?

Response O1: Thank you for your comments. The writing has been improved in all the indicated topics. Please check the lines 98-108, 113-117. The wording of line 125-135 has been improved.

Lines 128-138 incorporate more references on the rejection of arsenic by RO, whose investigations have been important for the development of the project.

 

Observation 2: Line 47. No period at the end of the sentence. Same in Line 430.

Response 2: Thanks for the observation, it has been corrected, please check the lines 47 and 469

Observation 3: Line 83. “for the optimization o the final design of the system, …” should be “of”

Response O3: It has been corrected, Check the line 82-83

Observation 4: Line 111. Wrong grammar. “Abejón, et al. [40] through their research study, pointed out the economic and energy viability of arsenic removal by RO. ”

Response O4: The wording has been improved, check the lines 118-120

Observation 5: Line 133. “…..of the membrane. Being an important contribution to …” should be “….. , being an…” (still not good, maybe you should separate this sentence).

Response O5: Thanks to your comments, the wording has been improved, check the lines 149-153

Section 2

Observation 6: Line 163: “(Figure 2) schematizes the phenomenon of CP showing the increased concentration in the feed boundary layer due to the retention of solute.” Please remove the bracket. The same problem with Line 301, Line 318, Line 335, Line 355, Line 374, Line 378……… (so many, please check)

Response O6: Brackets have been removed, please check the lines 182, 285, 321, 345, 356, 370, 390, 405, 412, 420-422

Observation 7: Line 205. Should provide how you calculated this mass transfer coefficient k based on Re and Sc number.

Response O7: Thanks for you observation. In this work, the mass transfer coefficient was not calculated by means of a function of Re and Sc, but rather as an adjustment parameter of the model as indicated in section 4.4 model validation. The reason is that a parametric study was not carried out with the rate of entry to the membrane module to have an adjusted equation of the mass transfer coefficient. The range of speeds used in the pilot plant was very narrow according to the specifications of the membrane manufacturer. Please check lines 222-226 this explanation has been included

 

Observation 8: Line 212. Eqn 8. Did you assume cf’=cf?

Response O8: These concentrations are not the same. Thanks for the observation, since it is an error in the writing of equations 8 and 9. The equations have been corrected, please check equations 8 and 9

 

Observation 9: Line 233. Eqn 12. You should state here what is “i”, because this is the first time this nomenclature appears.

Response O9: Thanks for the comment, the subscript “i” refers to the specific solute (NaCl, As, Pb), also equation 12 is corrected, please check the line 249-251

Section 3.3.

Observation 10: Table 2. Please state how you chose low, medium, and high concentrations of NaCl. Why are they different? The author said something in Line 289 to 290, but it is not clear enough. The grammar of this sentence is also not right.

Response O10: The grammatical wording has been improved giving a better explanation to the concentration ranges: The selection of concentrations is based on building a model that allows the design of installations with high conversion (>80%). With this it will be possible to optimize the process from the point of view of energy efficiency in future works. check lines 301, 307, 308-314 and 325-327

Section 4

Observation 11: Line 325 to Line 330. There is no discussion for Figures 5 and 6.

Response O11: Thanks to your comments, the discussion of the results shown in the figures was placed please check lines 365-369

Observation 12: Line 351 to 353. “……the research shows that there is no physical compaction of the membrane at low concentrations but at high concentrations, it is the CP phenomenon that governs the process. “There is no discussion on your results. How did you come up with this conclusion that there is no membrane compaction and why does CP govern this process? You should talk a little bit more.

Response O12: Thanks, the wording on the results of the graphs has been improved, check the lines 384-388

Observation 13: Line 359. “These results agree with the work of [51], who ..” The author should say what is the work, or the researcher of the work, rather than the citation number.

Response O13: Section 4.5 of discussion of results has been included in the manuscript with a better reference to the bibliography, please check lines 472-489

Observation 14: Figure 11, 12, 13, 14, 15, 16. The figures were not presented well. What operating conditions caused the three steps in the figure? There is no description either in the figure or in the text.

Response O14: Thanks to your comments, the explanation of the operating conditions of each step was added and the explanation in each figure was improved. Please check de lines 423-425 and the lines 452-457, please check de figures 11, 12, 13, 14, 15 and 16

Observation 15: Figures 17, 18. The author stated in Line 416 that the three steps are the flux behavior of three concentrations. You should find a way to put this information into the figure for easier understanding for the readers.

Response O15: The wording has been improved with respect to the indicated figures. The legends and references in figures 17 and 18 have also been improved. Please check the lines 455-457 and the figures 16 and 18

Section 4.5

Observation 16: Line 432-433. How the model “made it possible…”? There is always no link between the experimental/modeling results between the authors’ conclusions, which does need to be improved.

Response O16: The wording has been improved considering your important observation, check lines 491-493

Section 5

Observation 17: The conclusion part should be a concise and clear summary of the entire article that helps the readers go over the important ideas quickly. However, the current conclusions are more like the results and discussion part. The authors did not discuss adequately in the discussion part.

Response O17: In the section 4.5, 472-489 the discussion of results was improved

Observation 18: Also, the conclusion of the author is not clear. For example, Line 462-464, I guess what the authors want to say is that--at a lower concentration, the mass transfers more easily than at a higher concentration because of the lower CP phenomenon. Actually, CP is also one type of “mass transfer”, you cannot say at lower concentration it is “mass transfer” and at higher concentration, it is not.  This is very confusing.

Response O18: Thanks for the comments, the conclusions have been improved, making them clearer and more specific, please check lines 503-529

Kind regards,

Esteban Manuel Villena Martínez

Universidad Católica Boliviana - Universitat Politécnica de Valencia

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

I am satisfied with the authors' responses.

Author Response

Reviewer 3

MPDI

Procesess

Of my highest consideration,

Thank you very much for the time spent, your comments and suggestions have been important to improve the manuscript.

Kind regards,

 

Esteban Manuel Villena Martínez

Universidad Católica Boliviana - Universitat Politécnica de Valencia

 

Author Response File: Author Response.docx

Reviewer 4 Report

All the questions were addressed, except for a minor issue.

Line 265-266. The answer to observation 5 is not good. There is no need to separate the sentence into a new paragraph. Combine it back to Line 265. And just change the first word “Research” to “This”, or “The current research”. Otherwise, it is a weird sentence.

Author Response

Reviewer 4

MPDI

Procesess

Of my highest consideration,

Thank you for the review and the time spent, indicating that the observation has allowed us to improve the manuscript. The answer is detailed below. The manuscript is sent with changes control to facilitate the review process.

 

Observation 1: Line 265-266. The answer to observation 5 is not good. There is no need to separate the sentence into a new paragraph. Combine it back to Line 265. And just change the first word “Research” to “This”, or “The current research”. Otherwise, it is a weird sentence.

 Response O1: Thank you for your comments. The wording has been corrected, please check the lines 149-152

The results show that the operation of the RO at low pressures is effective in the rejection of Pb and As in drinking water, allowing energy savings without affecting the physical behavior of the membrane. The current research is an important contribution to guarantee safe water to the population in a sustainable way.

Kind regards,

 

Esteban Manuel Villena Martínez

Universidad Católica Boliviana - Universitat Politécnica de Valencia

 

Author Response File: Author Response.docx