Performance of the Pressure Assisted Forward Osmosis-MSF Hybrid Desalination Plant

Round 1

Reviewer 1 Report

The short communication entitled "Performance of the Pressure Assisted Forward Osmosis-MSF Hybrid Desalination Plant" written by Daoud Khanafer et al. is interesting and propose the pressure assisted FO as a pre-treatment of MSF desalination plants to reduce the scaling tendency on heat exchanger of less soluble salts in water. However, this paper should be revised very carefully before being considered to be published in the journal water. I recommend the following comments to improve the document:

1. There is an extra blank line in the abstract. Please, check it.
2. Third line of the abstract, authors wrote FO without abbreviation and later in the same line abbreviation was done, please correct it.
3. Section introduction, fourth line. I think it should technology instead of technologies.
4. At the beginning of the section introduction, the authors only mentioned reverse osmosis in one sentence and then the remarked the advantages of thermal processes such as MSF and MED. The authors should remark that RO is the most extended technology in desalination mainly because Compared to RO, thermal desalination technologies require higher energy demands and maintenance costs which make them less attractive. Following paper should be included:
   1. Energy for desalination: A state-of-the-art review

They also should mention that the energy consumption of RO technology significantly increases if the feedwater has high salinity. The following papers analyzed the specific energy consumption considering different feedwater salinities:

1. Performance assessment of SWRO spiral-wound membrane modules with different feed spacer dimensions
2. Application of two-stage reverse osmosis system for desalination of high-salinity and high-temperature seawater with improved stability and performance

The authors also should mention that the main disadvantages of RO technology are fouling, boron rejection and scaling is also an issue in brackish water when high recovery rates are desirable. I think some papers should be included to support these statements. I suggest the following ones:

Fouling

1. Reverse osmosis desalination: A state-of-the-art review
2. Engineering antifouling reverse osmosis membranes: A review

Boron rejection

1. Comparison analysis of different technologies for the removal of boron from seawater: A review
2. Experimental investigation of forward osmosis process for boron removal from water
3. Different boron rejection behavior in two RO membranes installed in the same full-scale SWRO desalination plant

Scaling

1. Antiscalants in RO membrane scaling control
2. Antiscalant cost and maximum water recovery in reverse osmosis for different inorganic composition of groundwater

And then the authors can remark that thermal processes has the advantages the mention including deslinate high salinity feedwaters, etc..

5. Section introduction, page 1 the salts such as CaCO₃, etc are written in italics, please change it. Same when authors write ions like Ca²⁺, etc (Page 2 and 3). Check the entire document.
6. In most of the manuscript the symbol of grades (°) is written ion the middle instead of the top. Please revise the entire manuscript.
7. The citation format is not according with the style of the journal. Please change to numeric as it appears in the references.
8. Page 3, section 2.1. thin-film composite was already abbreviated please, use the abbreviation.
9. Any reason why the authors kept the page 4 almost blank?
10. Could the authors reduce the size of the equations. (too large).
11. Was the Table 1 taken from the manuscript published by Madsen, 2017 (in that case include the citation in the caption of the Table)? Did the authors any calculation of table 1? Equation 2 is not the same used by Madsen, mass transfer coefficient due to concentration polarization phenomena is missing.
12. Was the Figure 2 taken from other work? In that case include the citation in the caption please.
13. Page 7, why the authors wrote the units of permeation flux in capital letters? It should be Lmh.
14. Page 7. After Equation 3, square meter is written in italics, please, changed it.
15. If the authors used Scanning electron microscope (SEM) to check fouling and cleaning efficiency on membrane surface they had to include this in the material and methods section.
16. Could the authors remark what is the novelty of this work in comparison with the published by Madsen, 2017? As much I understand the authors here used two membrane that were already tested by Madsen et al. carrying out a similar experimental work.
17. Page 7, feed solution was already abbreviated as FS in page 2. Please check the entire document.
18. Page 14. ICP was already abbreviated, please just write ICP.
19. In Figure 10, Please write the 4 of SO4 as subscript.
20. In Figure 11 the authors showed the energy consumption. I would call specific energy consumption as it is divided by m³. Well, it is shown the trend with the pressure difference but, from figures 4 and 5 a reduction of around 50% of permeate flux can be observed. Basically, it means that in a few hours the SEC was doubled. I do not see the increase of SEC with the operating time. Beside this the authors should take into consideration that a cell membrane was tested, not a dull-scale membrane and the operating time is short. In real operation (years) the flux decline could be decrease more.
21. Why the authors used deionized water to clean the membrane? why didn't they use chemical cleaning products?
22. This is a general comment, considering PRO, FO or PAFO are emerging technologies and therefore the number of publications related with these technology is very high, the number of references in this paper are low. I recommend the authors to revise the following papers that I think it can be included by extending the introduction section:
    1. Current status and challenges of fabricating thin film composite forward osmosis membrane: A comprehensive roadmap
    2. Forward osmosis technology for water treatment: Recent advances and future perspectives
    3. Strategies in forward osmosis membrane substrate fabrication and modification: A review
    4. Forward osmosis as concentration process: Review of opportunities and challenges
    5. Seawater desalination: A review of forward osmosis technique, its challenges, and future prospects
    6. Forward osmosis membranes and processes: A comprehensive review of research trends and future outlook
    7. Forward osmosis: A critical review
    8. Research on forward osmosis membrane technology still needs improvement in water recovery and wastewater treatment
    9. Forward osmosis research trends in desalination and wastewater treatment: A review of research trends over the past decade
    10. The status of forward osmosis technology implementation
    11. Water desalination by forward osmosis: draw solutes and recovery methods–review
    12. Performance Comparison of Spiral-Wound and Plate-and-Frame Forward Osmosis Membrane Module
    13. Etc…

Author Response

Response to the reviewer 1 comments

Dear Reviewer,

Thank you for the comments. We are grateful that these valuable suggestions helped in improving the quality of the manuscript. We have addressed all the comments and response sheet is attached below.

We are looking forward to receive your positive reply.

Reviewer 1:

The short communication entitled "Performance of the Pressure Assisted Forward Osmosis-MSF Hybrid Desalination Plant" written by Daoud Khanafer et al. is interesting and propose the pressure assisted FO as a pre-treatment of MSF desalination plants to reduce the scaling tendency on heat exchanger of less soluble salts in water. However, this paper should be revised very carefully before being considered to be published in the journal water. I recommend the following comments to improve the document:

1. There is an extra blank line in the abstract. Please, check it.

 Answer: Thank you, the extra blank removed.

1. Third line of the abstract, authors wrote FO without abbreviation and later in the same line abbreviation was done, please correct it.

Answer: Thank you, corrected.

1. Section introduction, fourth line. I think it should technology instead of technologies.

Answer: Thank you, corrected.

1. At the beginning of the section introduction, the authors only mentioned reverse osmosis in one sentence and then the remarked the advantages of thermal processes such as MSF and MED. The authors should remark that RO is the most extended technology in desalination mainly because Compared to RO, thermal desalination technologies require higher energy demands and maintenance costs which make them less attractive. Following paper should be included:
1. Energy for desalination: A state-of-the-art review

Answer: Additional information is added in the manuscript that cover the reviewer feedback. Please note that it is well known that RO is the main desalination technology worldwide; however, our study is targeting the existing thermal desalination in the Gulf countries, not comparing the RO with thermal technologies. The aim is to find a cost efficient pretreatment step prior to thermal plants.

They also should mention that the energy consumption of RO technology significantly increases if the feedwater has high salinity. The following papers analyzed the specific energy consumption considering different feedwater salinities:

1. Performance assessment of SWRO spiral-wound membrane modules with different feed spacer dimensions
2. Application of two-stage reverse osmosis system for desalination of high-salinity and high-temperature seawater with improved stability and performance.

Answer: The energy of RO is around 2.5 kWh/m³ for 35 g/L seawater salinity; energy for pretreatment is not included in this power consumption (S. W. Srinivas Veerapaneni et al. Technical Report, Water Research Foundation, 20110. The focus in the paper is on the pretreatment step not the actual energy of the desalination process.

The authors also should mention that the main disadvantages of RO technology are fouling, boron rejection and scaling is also an issue in brackish water when high recovery rates are desirable. I think some papers should be included to support these statements. I suggest the following ones:

Fouling

1. Reverse osmosis desalination: A state-of-the-art review
2. Engineering antifouling reverse osmosis membranes: A review

Boron rejection

1. Comparison analysis of different technologies for the removal of boron from seawater: A review
2. Experimental investigation of forward osmosis process for boron removal from water
3. Different boron rejection behavior in two RO membranes installed in the same full-scale SWRO desalination plant

    Answer: Although boron is a serious problem in RO process, it is of less importance in thermal processes due to the purify product water. The manuscript objective is to provide FO pretreatment for seawater to MSF plant in order to reduce non-alkaline scaling. The RO technology in entirely different desalination method to the objective of the manuscript

Scaling

1. Antiscalants in RO membrane scaling control
2. Antiscalant cost and maximum water recovery in reverse osmosis for different inorganic composition of groundwater

Answer: thanks, please check our reply mentioned above about the RO membranes. The FO process is a low fouling technology and antiscalant is not an option since it uses osmotic pressure instead of hydraulic pressure in operation.  

And then the authors can remark that thermal processes has the advantages the mention including deslinate high salinity feedwaters, etc..

1. Section introduction, page 1 the salts such as CaCO₃, etc are written in italics, please change it. Same when authors write ions like Ca²⁺, etc (Page 2 and 3). Check the entire document.

            Answer: Thank you, the entire document checked for italics.

1. In most of the manuscript the symbol of grades (°) is written ion the middle instead of the top. Please revise the entire manuscript.

             Answer: Thank you, the entire manuscript revised and the symbol fixed.

1. The citation format is not according with the style of the journal. Please change to numeric as it appears in the references.

Answer: Thank you, the citation changed to MDPI style.

1. Page 3, section 2.1. thin-film composite was already abbreviated please, use the abbreviation.

Answer: Thank you, the TFC abbreviations checked and fixed.

1. Any reason why the authors kept the page 4 almost blank?

Answer: Thank you, the blank page removed.

1. Could the authors reduce the size of the equations. (too large).

Answer: Thank you, the size of the equations reduced.

1. Was the Table 1 taken from the manuscript published by Madsen, 2017 (in that case include the citation in the caption of the Table)? Did the authors any calculation of table 1? Equation 2 is not the same used by Madsen, mass transfer coefficient due to concentration polarization phenomena is missing.

            Answer: Thank you, all the values in table 1 were calculated by the authors. And regarding the mass transfer coefficient, it assumed that in FO, Expj_w/k=1 for pure water feed solution.

1. Was the Figure 2 taken from other work? In that case include the citation in the caption please.

            Answer: The measurements were conducted in the laboratory by the author.

1. Page 7, why the authors wrote the units of permeation flux in capital letters? It should be Lmh.

Answer: Thank you, the unit is written in capital letters in previous literature; however, it was changed to Lmh in the manuscript, as per the reviewer’s recommendation.

1. Page 7. After Equation 3, square meter is written in italics, please, changed it.

Answer: Thank you, the unit font changed.

1. If the authors used Scanning electron microscope (SEM) to check fouling and cleaning efficiency on membrane surface they had to include this in the material and methods section.

Answer: Thank you, SEM information included in the manuscript Section 2.

1. Could the authors remark what is the novelty of this work in comparison with the published by Madsen, 2017? As much I understand the authors here used two membrane that were already tested by Madsen et al. carrying out a similar experimental work.

Answer: Thank you, our study is not comparing RO and FO, the main objective is the pretreatment of seawater by the FO membrane for the removal of divalent ions that are responsible for scale formation in the MSF plant forward osmosis and pressure assisted forward osmosis at 4 bar.

1. Page 7, feed solution was already abbreviated as FS in page 2. Please check the entire document.

Answer: Thank you, feed solution checked in the manuscript.

1. Page 14. ICP was already abbreviated, please just write ICP.

Answer: Thank you, ICP used in the document.

1. In Figure 10, Please write the 4 of SO4 as subscript.

Answer: Thank you, SO₄ checked and changed.

1. In Figure 11 the authors showed the energy consumption. I would call specific energy consumption as it is divided by m³. Well, it is shown the trend with the pressure difference but, from figures 4 and 5 a reduction of around 50% of permeate flux can be observed. Basically, it means that in a few hours the SEC was doubled. I do not see the increase of SEC with the operating time. Beside this the authors should take into consideration that a cell membrane was tested, not a dull-scale membrane and the operating time is short. In real operation (years) the flux decline could be decrease more.

Answer: The specific power consumption was calculated for the permeate flow at end of the filtration process. As shown in equation 5:

          [5]

Q_p is the permeate flow at end of the test. Permeate flow in lab scale tests is low due to the limited membrane area and hence total permeate flow was used in equation 5 (ref 13 in the manuscript).

1. Why the authors used deionized water to clean the membrane? why didn't they use chemical cleaning products?

Answer: Thank you, the use of DI water in this paper was to evaluate fouling reversibility without the introduction of any chemical into the experiments. Taking into consideration that FO is a low fouling process.

1. This is a general comment, considering PRO, FO or PAFO are emerging technologies and therefore the number of publications related with these technology is very high, the number of references in this paper are low. I recommend the authors to revise the following papers that I think it can be included by extending the introduction section:
1. Current status and challenges of fabricating thin film composite forward osmosis membrane: A comprehensive roadmap
2. Forward osmosis technology for water treatment: Recent advances and future perspectives
3. Strategies in forward osmosis membrane substrate fabrication and modification: A review
4. Forward osmosis as concentration process: Review of opportunities and challenges
5. Seawater desalination: A review of forward osmosis technique, its challenges, and future prospects
6. Forward osmosis membranes and processes: A comprehensive review of research trends and future outlook
7. Forward osmosis: A critical review
8. Research on forward osmosis membrane technology still needs improvement in water recovery and wastewater treatment
9. Forward osmosis research trends in desalination and wastewater treatment: A review of research trends over the past decade
10. The status of forward osmosis technology implementation
11. Water desalination by forward osmosis: draw solutes and recovery methods–review
12. Performance Comparison of Spiral-Wound and Plate-and-Frame Forward Osmosis Membrane Module
13. Etc…

                   Answer: thank you. Relevant research papers are cited in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

1. The Introduction was too long, it should be shortened.
2. In Table 1, the unit of Zeta potential should be added.
3. In the equation (1), the Aw should be A.
4. In figure 2, the measurement of contact angle should be displayed in the figure, especially Figure 11. Otherwise, the scale should be added in the axis in the figure 4-7,9-11.
5. The Figures were not exquisite and they were needed to revise to deserve an International Journal.
6. The References were probably old, please cite some more references in recent years.

Author Response

Response to the reviewer 2 comments

Dear Reviewer,

Thank you for the comments. We are grateful that these valuable suggestions helped in improving the quality of the manuscript. We have addressed all the comments and response sheet is attached below.

We are looking forward to receive your positive reply.

Reviewer 2:

1. The Introduction was too long, it should be shortened.

Answer: Thank you, introduction shortened to the maximum; however, sentences regarding RO process were added as per the reviewer 1 recommendations.

1. In Table 1, the unit of Zeta potential should be added.

                Answer: Thank you, the unit of zeta potential added to the table.

1. In the equation (1), the Aw should be A.

Answer: Thank you, A replaced Aw.

1. In figure 2, the measurement of contact angle should be displayed in the figure, especially Figure 11. Otherwise, the scale should be added in the axis in the figure 4-7,9-11.

Answer: Thank you, Scale displayed in all figures.

1. The Figures were not exquisite and they were needed to revise to deserve an International Journal.

Answer: Thank you; all the figures were updated to suit the Journal.

1. The References were probably old, please cite some more references in recent years.

             Answer: Thank you, newly published references were added to the reference list.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The paper has been improved.

Page 3, second paragraph is in a different font than rest of the manuscript. Please, check the document.

Author Response

Dear Reviewer,

Thank you for the comments. We are grateful that these valuable suggestions helped in improving the quality of the manuscript. We have addressed all the comments, and the response sheet is attached below.

Reviewer 1:

1. Page 3, second paragraph is in a different font than rest of the manuscript. Please, check the document.

Answer: thank you, the font size has been corrected to size 12

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper had a significant improvement as the author revised according to the reviewer's comment. Thus, the paper was recommended for publication in its current version.

Author Response

Dear Reviewer,

Thank you very much for your help