Current Insights into Growing Microalgae for Municipal Wastewater Treatment and Biomass Generation
Round 1
Reviewer 1 Report
Dear Authors,
I admire your efforts in exploring the possibilities for growing microalgae utilizing wastewater treatment plant effluents, harvesting it, and utilizing it for commercial purposes. Unquestionably, this study addresses a crucial part of sustainable solutions and a circular economy strategy. After carefully reading your proposal, I think some significant changes are required to improve the content's cohesion and clarity.
In introduction:
1. Adding a sentence or two about the advantages of using microalgae over other methods could enrich the entry.
2. It may also be useful to include an explanation of the role of microalgae in wastewater treatment.
3. When referring to technical terms such as the Redfield ratio and N:P ratio, a brief explanation of what these terms are would be appropriate.
4. Figure 1 is not necessary. You may omit. In fact, the y-axis does not have any explanation.
5. 2.1. Municipal Wastewater Treatment Station: Here, you can change the title to “Municipal Wastewater Treatment Plants”
6. It would be appropriate for this paper to present the municipal wastewater treatment stages/types and the quality of the treated water obtained from these stages in Table 1 by relating the microalgae cultivation studies in the literature to this water quality.
7. . It is recommended that information on the factors affecting the cost of these treatment options be presented and discussed in relation to the table. In addition, the advantages, and challenges of transitioning to a circular economy can also be addressed in this section.
2.2. Microalgae :
8. The data presented in Table 2 should be discussed more effectively in the text. This is particularly important because the types of microalgae that can be cultivated and their contents determine the options for their utilization. For example, microalgae with high oil content can be used in specific applications such as biofuel production. In this context, discussing studies on microalgae contents and utilization possibilities by linking them in the text can make the article a more helpful resource for readers.
3. Factors Affecting the Microalgae Growth
9. In this review article, I suggest that you present the factors affecting microalgae growth in a table. Within the table, each factor can include relevant literature findings and references, thus providing a more comprehensive and organized summary of the topic. You can briefly discuss the important information in the table.
10. It is useful to make a clear distinction in the use of the terms "aeration" and "mixing". You could more clearly emphasize the effects and importance of gas transfer and mixing in the context of photobioreactor and HRAP.
11. To enhance the cohesiveness of your study's presentation, it is advisable to reorganize your section headings in a manner that incorporates comments on municipal wastewater and microalgae. The structure that has been suggested is as follows:
2. Municipal Wastewater Treatment and Microalgae
2.1. Municipal Wastewater Characterization
2.2. Microalgae and Their Growth Factors
2.3. Municipal Wastewater Treatment Plant Effluents and Microalgae Growth
This style provides readers with a coherent comprehension of the characterization of municipal wastewater, followed by pertinent details regarding microalgae and their interaction with wastewater effluents. This arrangement improves the overall coherence and cohesion of your paper.
12. 4.1. Non-potable Water Reuse subheading can be changed as “Reuse of Treated Municipal Wastewater.”
13. In the '”4.2. Recovery of Microalgae Biomass” section, eliminate duplicated information. Refine the structural organization for a coherent presentation of information. Ensure proper placement and referencing of 'Table 4.' Consider adding an introduction and a conclusion that emphasize the significance of recovering microalgae biomass.
14. For “4.3. Potential Applications of Algal Biomass”
The warning about elevated heavy metal concentration is critical. It would help to clarify this issue. Discussing pre-treatment techniques or quality control steps to ensure safe use or referencing studies on the topic may provide an improved understanding.
15. You could clarify the use of microalgae for Municipal Wastewater (MWW) treatment by categorizing challenges (e.g., environmental, technological, economic), emphasizing the challenges of transitioning from pilot studies to industrial applications, describing the pros and cons of microbial interactions and pretreatment, and comparing CO2 sequestration costs to conventional methods.
Comments for author File: 
Comments.pdf
There is a need to check the English for fluency and the coherence of the paper.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 2 Report
The manuscript review the benefits offered, as well as the challenges faced in using microalgae to treat municipal wastewater and produce algal biomass. The manuscript is written in a correct and understandable form and is well structured.
Certainly, the subject matter is interesting and current, however there are some aspects of the manuscript that need to be improved.
In section 2.1 "Municipal Wastewater Treatment Station": the authors should justify why they only talk about the activated sludge system, when there are several wastewater systems.
Since it is a review manuscript, the authors should inclue in the table footnote and figure footnote the source of the information in Table 1, Table 2 and Figure 1. Likewise, in Figure 1 they should define OD in the figure caption.
The authors refer to both microalgae and cyanobacteria as microalgae, which can lead to some inaccuracies. For example, cyanobacterias have the ability to fix nitrogen, and it is not mentioned when the use of nitrogen by microalgae is discussed. The authors should explain why nitrogen content diminished in cyanobacteria-treated efluents (Table 3).
In Table 3 (p. 12), the information about Haematococcus pluvialis is missing.
It is necessary to deepen more about the content of heavy metals which could be accumulated by algal biomass and limit its subsecuent use.
I think the manuscript is generally well writen.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Manuscript ID: Resources-2607040
Title: Current insights into growing microalgae for municipal wastewater treatment and biomass generation
This work was mainly aimed at reviewing the use of municipal wastewater for microalgae cultivation. The manuscript deals about an interesting topic; however, some aspects should be improved before publishing it. I include some comments below.
- Some information about the main conclusions obtained in the work should be included in the abstract, instead of only describing the importance of this issue.
- Lines 68-73: please clarify when you are talking about natural water mass or wastewater, this paragraph is confusing.
- Lines 82: be careful about your statements, conventional treatments are not expensive, for example, activated sludge is well known as an affordable a affordable system. In addition, you claim that “biological processes have stood out among these technologies because of their low cost and high efficiency”, so the entire paragraph is disconcerting.
- Line 88: certainly, microalgae are able to remove nutrients and other compounds from wastewater; however, in my opinion, it should be remarked that heavy metals are in any case degraded, but absorbed.
- “Coliforms” is not written in italics, since it is not a genus of bacteria; please correct this in the entire manuscript.
- Lines 92-93: comment the possible mechanisms of inactivating pathogens.
- Lines 95-97: this is not clear.
- Lines 107-112: I am afraid that I cannot see the relation of El Wali et al.’s work with microalgae in wastewater treatment, the importance of phosphorous recycling is not clear about microalgae performance in WWTPs. Clarify this paragraph or remove this reference.
- Line 112-114: this sentence is out of context in this paragraph.
- Lines 116-117: there is no information about pretreatment or primary treatment processes, you only talk about secondary treatment “abruptly”.
- Table 1: where are the references of these data?
- Line 124: what about Europe? It is very important to include information about European regulation.
- Line 131: you claim that “WWTP hardly include tertiary treatment”, this is not true, at least in Europe, please revise this statement.
- Table 2: where are the references of these data?
- Line 115: algae are primary producers; they are not at the top of the food chain.
- Line 161: please clarify this sentence.
- Line 166-170: what about heavy metals adsorbed in the biomass? For example, Hg.
- Line 198: if you are talking about municipal wastewater is very unusual to have a pH of 9.
- Figure 1: where are the reference of these data? what is control?
- Line 205: I don’t think that phosphorus is limiting in wastewater.
- Line 218: NTK acronym is incorrect; I supposed that you mean TNK (total nitrogen Kjeldahl).
- Line 222: if you are talking about municipal wastewater is impossible to have a pH of 11.
- Line 236: how are pathogens inhibited?
- Lines 249-250: please clarify/complete.
- Lines 289-291: this is confusing, since in lines 198-201 you indicated that ammonia is consumed as nutrient by microalgae, so is it toxic or not? In addition, a reference is missing in this paragraph.
- Line 272: what about the anaerobic heterotrophic microorganisms?
- Lines 383-391: what has happened with the format of this text?
- Table 3: some text is in bold without any reason, pH 7.8 (instead of 7,8), Scenedesmus sp. and…? (this is incomplete), what do you mean with “mixture = 0.2-0.3 m.s-1”? Correct these mistakes.
- Line 416: justify the importance of solar radiation.
- Lines 423-426: please clarify.
- Table 4: where are the references of these data?
- Line 456: please pay attention when you talk about coagulation/flocculation and sedimentation, these processes are not the same! Sedimentation procedure does not entails toxicity; the toxicity can be originated by the reactive employed as coagulant/floculant. Be accurate when you talk about these processes.
- Line 465: I suppose that Tanfloc SG is a commercial reactive, anyway, give more information about it. There is no information about aluminium sulphate performance, so the comparison between both coagulants/floculants is not clear.
- Lines 520-525: a reference is needed in this paragraph.
- Revise the grammar of the entire manuscript, specifically: lines 171-172, line 189, lines 196-198, lines 211-213, line 248, line 399, line 402 and lines 510-512.
I recommend a revision of the grammar of the entire manuscript.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 4 Report
This review paper delves deep into the intricacies of using municipal wastewater for microalgae cultivation and emphasizing the hurdles in biomass harvesting. Results showed that effluent microalgal consortiums present a promising strategy due to their adaptability and superior purification capabilities. The broader application of this technology still confronts various challenges. There is a growing emphasis on integrating advanced biotechnological tools, exploring alternative harvesting methodologies, and developing scalable systems that can address global wastewater treatment needs. Although some interesting results were found, this manuscript cannot be considered published at the present form. Specific major and minor comments are shown as following for further improving the manuscript.
- Both of microalgae and algae were used throughout the manuscript, please make them consistent in the file.
- Related references should be added for all species of microalgae in Table 2.
- Related references should be added to all the published figures including figures 1, 2 and 3.
- Unreadable information in figure 3 due to small fonts and low figure quality, please further improve it.
- What are the removal mechanisms of dominant pollutants in municipal wastewater using microalgae-based technologies? More related information should be added.
- Reference in the introduction should be updated using the following relate papers: Water Research 244 (2023) 120461; Water Research 122 (2017) 355-362; Water Research 126 (2017) 189-196; Chemical Engineering Journal 454 (2023) 140336; ACS Sustainable Chemistry & Engineering 2019, 7, 2127-2133; Chemosphere 218 (2019) 147-156; Algal Research 30 (2018) 94-100; Algal Research 34 (2018) 250-255.
- How about the potential of the practical application of the findings to prove municipal wastewater treatment efficiency and resources utilization potential using microalgae-based technologies? More related discussion should be added.
Comments for author File: Comments.pdf
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I have reviewed the revised version of your manuscript, "Current Insights into Growing Microalgae for Municipal Wastewater Treatment and Biomass Production". I value your efforts to resolve the comments and concerns raised during the review process. Your alterations and enhancements have significantly enhanced the manuscript.
Now, I am in favor of publishing the manuscript in its current form.
Regards.
Reviewer 2 Report
The manuscript improved substancially, I congratulated you for that.
Reviewer 3 Report
The manuscript has been notably improved, I only want to make a minor comment, please writte "total coliforms" instead of "Coliforms" in line 107.
Reviewer 4 Report
As the authors further improved the manuscript based on all the reviewers comments and suggestions, this paper could be consider published in this journal at the present version.
