Microalgae-Enabled Wastewater Remediation and Nutrient Recovery through Membrane Photobioreactors: Recent Achievements and Future Perspective

Round 1

Reviewer 1 Report

This review provided a literature survey and discussion about wastewater remediation and nutrient recovery from microalgae using membrane photobioreactors and provides an outlook for future research. The authors offered detailed descriptions and overviews about the performance of microalgae-membrane photobioreactor, membrane fouling mechanisms, innovations in design and future perspectives. However, in the large amount of description content, there is a lack of author's own induction and summary, especially in the performance of MPBR. I suggest some modifications or addition of discussions for author to facilitate the understanding of readers as below:

1.     In Introduction, the authors briefly described the advantages of microalgal wastewater treatment and the benefits of membrane separation-based photobioreactors for wastewater treatment as well as nutrient recovery. However, the current content lacks relevant research progress as well as effectiveness of MPBR and failed to highlight the importance of this review.

2.     Page 3, Lines 103-104: the authors demonstrated that membrane separation is less energy intensive than traditional methods such as flocculation and sedimentation. What is the main source of energy consumption for traditional separation methods (flocculation and sedimentation) and why membrane separation consumes less energy than these methods? I suggest that you provide a comprehensive summary of the energy consumption of the different techniques in parallel so that the reader can better understand the superiority of membrane-based separation.

3.     The second part of this review is called “Wastewater as a Source of Nutrients for Microalgae”. I suggest that the author reduce the description of anaerobic digestion process in this section. Especially in Page 4, Lines 170-171: “Microalgal biomass can be attractively used as a feedstock for biogas production via anaerobic digestion”. These descriptions actually deviate from the topic of this section.

4.     In the second part, authors should state more details about the general design and operating parameters of microalgae-membrane photobioreactor.

5.     In Page 4, Line 181: “Through the interesting psycomediation approach……”. It seems a typo. What do you mean?

6.     Figure 1 summaries the merits and challenges of microalgae-enabled wastewater treatment. Since the topic of this review is “microalgae-enabled wastewater remediation and nutrient recovery through membrane photobioreactor”. It might be better to add some merits and challenges of membrane photobioreactor. Besides, the current Figure 1 still looks too simply and lacks key information, such as the carbon dioxide capture rate, or the specific high-value end products.

7.     In Page 6, Line 286: What dose “PMR” mean? Please check the text to make sure all abbreviations are marked with full names.

8.     The fourth part of the article focuses on the performances of micralgae-MPBR for wastewater treatment and nutrient recovery. However, the images in Figure 4 are still quoted from other studies. I hope the author can organize and summarize this content and draw your own schematic diagram (such as box diagram) based on the extensive description in this part.

9.     The layout in Figure 4 is messy and unattractive.

10.  As for the membrane fouling in microalgae-wastewater medium, the authors provided a detailed summary and description of the membrane mechanisms and fouling-contributing components in MPBR. I suggest that the authors add some cases about the flux decline or membrane resistance rise caused by membrane fouling during the operation of the MPBR.

Author Response

Reviewer #1

This review provided a literature survey and discussion about wastewater remediation and nutrient recovery from microalgae using membrane photobioreactors and provides an outlook for future research. The authors offered detailed descriptions and overviews about the performance of microalgae-membrane photobioreactor, membrane fouling mechanisms, innovations in design and future perspectives. However, in the large amount of description content, there is a lack of author's own induction and summary, especially in the performance of MPBR. I suggest some modifications or addition of discussions for author to facilitate the understanding of readers as below:

Thank you for the suggestions and comments. The changes made are marked in blue.

1. In Introduction, the authors briefly described the advantages of microalgal wastewater treatment and the benefits of membrane separation-based photobioreactors for wastewater treatment as well as nutrient recovery. However, the current content lacks relevant research progress as well as effectiveness of MPBR and failed to highlight the importance of this review.

The research progress of MPBR and its advantages are briefly described in page 1, line 103-110. The detailed discussion of the operation of MPBR is provided in Section 3. The importance of this review has been mentioned in page 1, line 122-128

2. Page 3, Lines 103-104: the authors demonstrated that membrane separation is less energy intensive than traditional methods such as flocculation and sedimentation. What is the main source of energy consumption for traditional separation methods (flocculation and sedimentation) and why membrane separation consumes less energy than these methods? I suggest that you provide a comprehensive summary of the energy consumption of the different techniques in parallel so that the reader can better understand the superiority of membrane-based separation.

Conventional technology such as sand filtration can be more expensive due to the use of chemicals for flocculation. Due to the difference of operating condition and types of wastewater, the direct comparison of energy usage among the wastewater treatment technology is difficult. The mentioned statement has been removed to avoid confusion.

3. The second part of this review is called “Wastewater as a Source of Nutrients for Microalgae”. I suggest that the author reduce the description of anaerobic digestion process in this section. Especially in Page 4, Lines 170-171: “Microalgal biomass can be attractively used as a feedstock for biogas production via anaerobic digestion”. These descriptions actually deviate from the topic of this section.

The description of anaerobic digestion process has been condensed. The process is briefly mentioned to highlight its limitation in nutrient recovery.

4. In the second part, authors should state more details about the general design and operating parameters of microalgae-membrane photobioreactor.

The general design of MPBR has been discussed in Section 3, Line 298-322. The important operating parameters have been discussed in Section 3, Line 342-358

5. In Page 4, Line 181: “Through the interesting psycomediation approach……”. It seems a typo. What do you mean?

The typo has been corrected: psycoremediation (which refers to use of algae to treat wastes or wastewaters)

6. Figure 1 summaries the merits and challenges of microalgae-enabled wastewater treatment. Since the topic of this review is “microalgae-enabled wastewater remediation and nutrient recovery through membrane photobioreactor”. It might be better to add some merits and challenges of membrane photobioreactor. Besides, the current Figure 1 still looks too simply and lacks key information, such as the carbon dioxide capture rate, or the specific high-value end products.

Figure 1 has been revised with the addition of quantitative information and merits & challenges of MPBR.

7. In Page 6, Line 286: What dose “PMR” mean? Please check the text to make sure all abbreviations are marked with full names.

The misspelled abbreviated term has been removed. A list of abbreviations has been added in the revised manuscript.

8. The fourth part of the article focuses on the performances of micralgae-MPBR for wastewater treatment and nutrient recovery. However, the images in Figure 4 are still quoted from other studies. I hope the author can organize and summarize this content and draw your own schematic diagram (such as box diagram) based on the extensive description in this part.

An illustration summarizing the contents of this review is added as Figure 5 as suggested.

9. The layout in Figure 4 is messy and unattractive.

The illustrations have been reorganized.

9. As for the membrane fouling in microalgae-wastewater medium, the authors provided a detailed summary and description of the membrane mechanisms and fouling-contributing components in MPBR. I suggest that the authors add some cases about the flux decline or membrane resistance rise caused by membrane fouling during the operation of the MPBR.

Two case studies were discussed, i.e. the effect of N ratio and the effect of HRT. Kindly refer to Section 5.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper is interesting, well written and organized. The following minor faults should be fixed:

Line 163 – “…, the process does not allow…”.

Line 176 – “The cultivation using…”.

Line 181 – The word “psycomediation” should be explained with a sentence.

Line 221 – “The typical industrial wastewater” should be referred to a specific industrial sector (food industry? beverages?) as some sectors may have much stronger wastewaters (e.g. tannery, slaughterhouse, feedstock, etc.).

Line 234 – Please specify “charge”.

Line 234-236 – The sentence is trivial, please check.

Line 265 – Please explain “…circulates wastewater in low energy, …”.

Line 286 – What is PMR?

Line 337 – “An MPBR that operates…”.

Lines 371-373 – Please report removal percentages.

Lines 388-389 - Please report removal percentages.

Lines 411—412 – Is there a reason for italics?

Line 429 – Please replace “was operated at” with “displayed”.

Line 556 – “Using synthetic wastewater rich in nitrogen…”.

Line 560 – “… and had poorer…”.

Line 564 – Please replace “imparted” with “caused”.

Line 582 – “… system was designed to enable the detection…”.

Line 584 – Please explain “reciprocal”.

Line 601 – E. Coli is not a bio-foulant. It is rather an indicator of fecal contamination widely used for the assessment of the hygienic safety of municipal wastewater effluents. Please change the sentence, as here the reference to E. Coli appears out of context.

Line 639 – “… reactions taking…”

Line 645 – “… chambers…”.

Line 700 – “… which are mainly…”

Line 704 – “… consortium composed of…”

Line 721 – “… layered channels in the fowling layer of the….”.

Line 790 – “… Chlorella vulgaris, many other important…”.

Line 804 – It is not clear if you refer to influent phenol concentration, please clarify.

Line 871 – The effect of co-cultivation on fouling is controversial, as mentioned at lines 722-724. Therefore here the word “worsens” should be changed, and the concept clarified.

Line 893 – “… in line…” remove capital I

Line 900 – Sentence “… for combines interesting…” should be improved.

Line 902-903 – Design of membranes is not in the scope of this paper, please correct.

Line 909 – “certain” is a bit too strong.

The “Concluding Remarks” should include a synthesis of the main aspects as a list of bullet points.

Author Response

Reviewer #2

The paper is interesting, well written and organized. The following minor faults should be fixed:

Thank you for the comments and suggestion. The changes are marked in green

Line 163 – “…, the process does not allow…”.

Phrase has been revised

Line 176 – “The cultivation using…”.

Phrase has been revised

Line 181 – The word “psycomediation” should be explained with a sentence.

The typo has been corrected: phycoremediation (which refers to use of algae to treat wastes or wastewaters)

Line 221 – “The typical industrial wastewater” should be referred to a specific industrial sector (food industry? beverages?) as some sectors may have much stronger wastewaters (e.g. tannery, slaughterhouse, feedstock, etc.).

The phrase has been revised: The typical sewage and agricultural wastewater contains

Line 234 – Please specify “charge”.

The phrase has been revised: The pH of the suspension and the surface charge of biomass

Line 234-236 – The sentence is trivial, please check.

The phrase has been revised: Therefore, maintaining low pH would be beneficial…

Line 265 – Please explain “…circulates wastewater in low energy, …”.

The phrase has been revised: treats wastewater

Line 286 – What is PMR?

The misspelled abbreviated term has been removed.

Line 337 – “An MPBR that operates…”.

The phrase has been revised

Lines 371-373 – Please report removal percentages.

The values are removal rate, therefore cannot be expressed in %

Lines 388-389 - Please report removal percentages.

The values are removal rate, therefore cannot be expressed in %

Lines 411—412 – Is there a reason for italics?

It is a formatting error and it has been corrected

Line 429 – Please replace “was operated at” with “displayed”.

The phrase has been revised: the OMPBR displayed higher operating

Line 556 – “Using synthetic wastewater rich in nitrogen…”.

The phrase has been revised.

Line 560 – “… and had poorer…”.

The phrase has been revised.

Line 564 – Please replace “imparted” with “caused”.

The word has been replaced accordingly

Line 582 – “… system was designed to enable the detection…”.

The phrase has been revised.

Line 584 – Please explain “reciprocal”.

It refers to the reversal of membrane condition through mechanical cleaning. To avoid confusion, the sentence has been simplified: The mechanical cleaning reduced the total hydraulic resistance

Line 601 – E. Coli is not a bio-foulant. It is rather an indicator of fecal contamination widely used for the assessment of the hygienic safety of municipal wastewater effluents. Please change the sentence, as here the reference to E. Coli appears out of context.

E-coli and S. aureus have been commonly used as biofoulant models in membrane biofouling studies. To avoid confusion, the sentence has been revised: the commonly found microorganisms in typical municipal wastewater.

Line 639 – “… reactions taking…”

The phrase has been revised.

Line 700 – “… which are mainly…”

The phrase has been revised.

Line 704 – “… consortium composed of…”

The phrase has been revised.

Line 721 – “… layered channels in the fowling layer of the….”.

The phrase has been revised.

Line 790 – “… Chlorella vulgaris, many other important…”.

The phrase has been revised.

Line 804 – It is not clear if you refer to influent phenol concentration, please clarify.

Yes, it refers to influent. The sentence has been revised: For influents containing high level

Line 871 – The effect of co-cultivation on fouling is controversial, as mentioned at lines 722-724. Therefore here the word “worsens” should be changed, and the concept clarified.

The statements have been clarified as: The implementation of co-cultivation of microalgae and bacteria is a double-edge sword. It may worsen the fouling of membrane when excessive EPS and soluble microbial products are produced as a result of environmental stress and competition between microalgae and bacteria. Therefore, the growth and ratio of microalgae and bacteria in the consortium should be carefully controlled.

Line 893 – “… in line…” remove capital I

Correction has been made

Line 900 – Sentence “… for combines interesting…” should be improved.

The sentence has been revised: This review discusses the recent development of microalgae-MPBR to simultaneously….

Line 902-903 – Design of membranes is not in the scope of this paper, please correct.

The development of nanocomposite membrane such as Ag/GO PVDF membrane can be known as membrane design.

Line 909 – “certain” is a bit too strong.

It has been revised as ‘expected’

The “Concluding Remarks” should include a synthesis of the main aspects as a list of bullet points.

      The authors decided to keep the concluding remark in paragraph form.

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript detailly reviewed wastewater treatment and a potential nutrient recovery via micro-algal system with membrane. When wastewater is treated, algae products can also be received, or used for anaerobic fermentation to produce biogas, so as to realize pollution control and resource recycling. However, it is difficult to separate microalgae from water because of their small size and light weight, so membrane separation is necessary to solve the problem. In addition to nitrogen and phosphorus, wastewater also contains organic carbon, which is bound to breed a large number of nitrifying bacteria. Nitrifying and denitrifying bacteria use the oxygen released by algae photosynthesis to convert some of the nitrogen into nitrogen. As such, only 30% of the ammonium could be captured by microalgae. If the authors want to emphasize nutrient recycling, more relevant content needs to be added, rather than simply emphasizing nitrogen and phosphorus uptake or adsorption by algae.

It is suggested to list all the abbreviations.

1 Introduction

Here, the author needs to explain how to do nutrient recovery.

Line 103, how do you determine that membrane separation technology is more energy efficient than flocculation precipitation?

Lines 108-117, it is difficult to detect nutrient recycling.

Lines 132-134, it needs to rewrite this sentence.

2. Wastewater as a Source of Nutrients for Microalgae

Lines 134-136, suggest to be deleted.

Lines 149-151, wastewater is difficult to use in agriculture.

Lines 163, does not?

Line 221-222, concentrations might be low for industrial wastewater.

Line 225-231, the authors need to further clarify these points.

4. Performances of Micralgae-MPBR for Wastewater Treatment and Nutrient Recovery

The author is advised to simplify this part of the content.

5. Membrane Fouling in Microalgae-Wastewater Medium

It is suggested that the authors summarize membrane cleaning methods.

6. Innovations in Membranes and Reactor Design for MPBR

Online backwashing, intermittent backwashing, and regular chemical cleaning, in-situ or off-line chemical cleaning are normal.

What are about chemical cleaning methods? Will chemical methods used to wash membrane be toxic to microalgae?

7. Microalgal-Bacterial Co-Cultivation in MPBR for Wastewater Treatment

This part is suggested to be deleted. Bacterial growth is inevitable in the treatment of wastewater. The author can merge the relevant content into other sections and simplify it.

 The authors were advised to simplify the content and remove some references

Author Response

Reviewer #3

This manuscript detailly reviewed wastewater treatment and a potential nutrient recovery via micro-algal system with membrane. When wastewater is treated, algae products can also be received, or used for anaerobic fermentation to produce biogas, so as to realize pollution control and resource recycling. However, it is difficult to separate microalgae from water because of their small size and light weight, so membrane separation is necessary to solve the problem. In addition to nitrogen and phosphorus, wastewater also contains organic carbon, which is bound to breed a large number of nitrifying bacteria. Nitrifying and denitrifying bacteria use the oxygen released by algae photosynthesis to convert some of the nitrogen into nitrogen. As such, only 30% of the ammonium could be captured by microalgae. If the authors want to emphasize nutrient recycling, more relevant content needs to be added, rather than simply emphasizing nitrogen and phosphorus uptake or adsorption by algae.

Thank you for the suggestions and comments. The changes made are marked in purple

It is suggested to list all the abbreviations.

List of abbreviation has been added

1 Introduction

Here, the author needs to explain how to do nutrient recovery.

The mechanisms of nutrient recovery by microalgal cells are briefly mentioned in the introduction with a reference cited for further reading. A more detailed discussion has been provided in Section 2.

Line 103, how do you determine that membrane separation technology is more energy efficient than flocculation precipitation?

Due to the difference of operating condition and types of wastewater, the direct comparison of energy usage among the wastewater treatment technology is difficult. The mentioned statement has been removed to avoid confusion.

Lines 108-117, it is difficult to detect nutrient recycling.

The ambiguous statement has been revised as: The concept of integrating membrane technology and microalgae for wastewater remediation has been established

Lines 132-134, it needs to rewrite this sentence.

The sentence has been revised as: Agricultural activities have considerably contributed to the surging amount of nitrogen and phosphorus in the receiving water bodies. Due to the extensive use of fertilizer in agricultural lands, majority of the reactive nitrogen compounds present in the fertilizers are also introduced lost to the water bodies

2. Wastewater as a Source of Nutrients for Microalgae

Lines 134-136, suggest to be deleted.

The sentence has been deleted

Lines 149-151, wastewater is difficult to use in agriculture.

Nutrient such as nitrogen-containing compounds recovered from wastewater can be used directly as fertilizer, not the wastewater

Lines 163, does not?

Yes, correction has been made

Line 221-222, concentrations might be low for industrial wastewater.

The sources have been clarified with references: Typical sewage and agricultural wastewater contains 15–90 mg/L of nitrogen and 4–20 mg/L of phosphorus…

Line 225-231, the authors need to further clarify these points.

The statements have been rechecked by referring to the references cited.

4. Performances of Micralgae-MPBR for Wastewater Treatment and Nutrient Recovery

The author is advised to simplify this part of the content.

Some sentences have been omitted/condensed.

5. Membrane Fouling in Microalgae-Wastewater Medium

It is suggested that the authors summarize membrane cleaning methods.

Membrane cleaning has been described.

….the fouling caused by cake layer build up is normally reversible and can be removed by physical cleaning such as online or intermittent backflushing. In addition to physical cleaning, mechanical and chemical cleaning methods have also been adapted to restore membrane performance [116]. Chemical cleaning which involves the soaking of membrane in a cleaning chemical has been commonly applied to address irreversible fouling. Regardless of the approaches used, the efficiency of membrane cleaning depends on multiple factors such as fouling nature, extent of fouling and cleaning frequency.

6. Innovations in Membranes and Reactor Design for MPBR

What are about chemical cleaning methods? Will chemical methods used to wash membrane be toxic to microalgae?

Based on our literature search, chemical cleaning of microalgae-fouled membrane has not been reported.

7. Microalgal-Bacterial Co-Cultivation in MPBR for Wastewater Treatment

This part is suggested to be deleted. Bacterial growth is inevitable in the treatment of wastewater. The author can merge the relevant content into other sections and simplify it. The authors were advised to simplify the content and remove some references

Section 7 has been removed. The discussion related to microalgal–bacterial consortium has been included in Section 5.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have basically addressed the reviewers' concerns.

Reviewer 3 Report

It can be accepted.