Promotion of Microalgal Growth, CO₂ Fixation, and Pollutant Removal in Piggery Effluent by a Column Photobioreactor with Funnel-Shaped Spoilers

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Reviewer comments

 

Manuscript: Promotion of microalgal growth, CO2 fixation and pollutant removal in piggery effluent by column photobioreactor with funnel-shaped spoilers

 

ID: sustainability-3521217

 

Date: 2025-03-02

 

General comments

 

This manuscript presents a numerical and experimental analysis of a modification of a bubble column photobioreactor. The authors investigate how a funnel-shaped deflector improves the “quality” of the flow within the reactor. Then, confirm that it is translated to reality by leading biological experiments. Despite numerous shortcomings leading this reviewer to believe that this may (at least in part) be an MSc student work, the manuscript shows merit and deserves publication after extensive correction. The major concerns regard the description of the method used. In addition, while the English language is okay, it has to be improved. The same can be said for the layout. All in all, this reviewer advises on the status of minor revisions.

 

Major concerns

 

1. Please add line numbering!

 

2. In the introduction, microalgae are described as a manner to “sequestrate” CO2. This is wrong. They can be used to cycle CO2. Furthermore, the examples cited by the authors are related to cycling. Please revise the wording accordingly. Or provide a convincing rebuttal.

 

3. Section 2.2 is outrageously shallow. The reader is left with so many questions. The quality of the work and the results are dramatically hindered by this. Please describe the modeling and numerical methods better. Was it Euler-Euler? Euler-Lagrange? VoF? What is the coalescence and break-up model? Which species of microalgae is mimicked? Why choose 0.1 s for microalgae tracking (which coincides with PQ pool characteristic time)?

 

4. Equation 3 seems wrong. 1 – h0/h1 would be better suited?

 

5. Section 2.4 and 2.5. Please revise the English language. Indeed, these methods are known to the reviewer. Still, a regular reader might experience trouble understanding them.

 

6. Please provide the rationale for the use of 35% sulfuric acid. Indeed, for this type of test, the fluid should have the same viscosity as the medium (water). It may not be the case.

 

7. Section 2.5. Please indicate if and how biomass was washed prior to quantification and analysis.

 

8. Section 3.1. It is impossible to conclude cell growth at this point. Either present the relevant results or move this conclusion forward.

 

9. The claim on gas transfer efficiency is to be substantiated as no mention of it is made in the model description.

 

10. Please report turbulence intensity and not TKE.

 

11. Light and zones are mentioned without being introduced. Please correct.

 

12. KlA value is reported in 1/h. This is an outrageous mistake. The correct unit is m3/s. Please correct.

 

Minor concerns

 

Wording: Pool => Pond. Plate => Flat panel.

 

What/Who is JPP?

 

Figure 2 caption is misplaced.

 

“Flow rate” is often used instead of “velocity”. Please correct (text, Fig. 3B, Fig. 4….)

 

Figure 3 caption is misplaced.

 

Typos

If only lines were numbered.

There are many. Yet, I cannot report them…

Comments on the Quality of English Language

If only lines were numbered.

There are many. Yet, I cannot report them…

Author Response

请参阅附件。

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, the author has designed a column photobioreactor with funnel-shaped spoilers for microalgae culture. The design is interesting, but there are still some problems in the paper that need to be revised.

 

1 Numerous studies have focused on flow field optimization in vertical column reactors. However, the introduction of this paper examines only one optimized reactor configuration, which reportedly achieves a 200% increase in biomass yield - a performance level significantly higher than the optimization results presented in this study. This observation indicates that the introduction section fails to adequately summarize prior research achievements while insufficiently clarifying the innovative aspects and outstanding advantages of the current design.

2. The paper exhibits formatting inconsistencies. For example:

The text in the first paragraph of Section 2.4 unnecessarily uses italics.

The captions for Figures 2 and 3 should be placed immediately after the figures.

The font size of Figure 4’s caption is noticeably too small.

Figure 5 suffers from excessively low resolution.

3. Figure 2c appears overly cluttered. It is recommended to split this figure into multiple separate figures for clarity.
4. Error bars should be displayed in Figure 3 to demonstrate that differences in average bubble diameter, flow velocity, and other factors are attributable to the flow disturbance plate settings, rather than measurement errors.

5 3.4 “As illustrated in Fig. 6, two reactors were placed in an artificial climate chamber maintained at a temperature of 25°C, with a light intensity of 2,000 lux under continuous lighting. A total of 9 liters of culture effluent, with a concentration of 40% (v/v) and fenu-greek algae in the logarithmic growth phase, were added to the reactors. Additionally, CO2, with a volume fraction of 10%, was introduced into the reactor at a gas flow rate of 0.02 vvm.” This part should be in the method, not discussed here.

6. In Figure 7a, absorbance typically exhibits a linear relationship with microalgal biomass, and absorbance generally reflects biomass dry weight. Why do the trends of these two parameters differ significantly in this figure? Additionally, based on the growth curve, the microalgae have not reached the stationary phase, indicating an incomplete cultivation cycle.
7. Section 3.4 includes measurements of chlorophyll a, but lacks relevant discussion. For example:

Why was chlorophyll b not measured?

What is the purpose of quantifying chlorophyll a in this context?

What conclusions can be drawn from these results?

8. The differences in COD and TP removal efficiencies between the two systems are relatively minor. Error bars should be added to enhance the credibility of the data.
9. In Figure 6, why does the algal culture appear brown during the early cultivation phase but turn green in later stages?
10. Both the formatting and language of the paper require further revision.

Comments on the Quality of English Language

language of the paper require further revision.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript explores a column photobioreactor with funnel-shaped spoilers to improve microalgal growth and pollutant removal efficiency, which is of certain significance. However, before publication, there are still some issues that need to be addressed. The main problem is that many parts of the analysis are not deep enough, and in several sections, the discussions do not incorporate the results of the CFD simulations. I hope the following suggestions can be helpful to you:

Introduction

1. The first paragraph is too broad and needs to be more focused on the specific problem you aim to address.
2. You need to highlight the limitations of photobioreactors in other studies and the advantages of the one in this research.
3. The introduction to the CFD method is insufficient.

Materials and methods

1. The sample size of only 5 bubbles seems too small to accurately calculate the average velocity and diameter.
2. Additional details on the light source and cultivation duration are needed.

Result and discussion

1. Many sections lack analysis of the physical mechanisms. For instance, section 3.1 should explain how velocity direction changes and turbulent kinetic energy affect microalgae mixing and CO₂ transfer, rather than just presenting the data.
2. DPM simulation data is needed to support the conclusion on increased microalgae residence time in the light zone.
3. The decrease in bubble size should be analyzed with the CFD simulation data, not just speculated.
4. An analysis of how the decrease in bubble diameter leads to an increase in gas retention rate is needed.
5. Section 3.3 should cite the flow field simulation results to support that improved mixing time and mass transfer are due to “increased horizontal flow velocity” and “turbulent kinetic energy.”
6. What other factors, aside from microalgal growth, contribute to the improved pollutant removal with the addition of spoilers? Why is the TP removal rate most improved?

Conclusion

1. The conclusion overlaps heavily with the abstract. It should be more detailed, offering deeper insights into the findings and suggesting future research or improvements.

Comments on the Quality of English Language

The English could be improved.

Author Response

请参阅附件

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

Dear authors, the topic you raise in your work is extremely relevant. Biotechnology in the field of wastewater disposal from various enterprises, including livestock farms, requires development. However, the results you have presented are at a poor level. You will need to thoroughly review your work methods and results. Provide additional materials to reflect all the missing points in the work. I wish you a good luck and exciting experiments in future!

Here are my comments, questions and remarks:

1. Line 105: What experiment? In numerical calculation? Please add the supplementary material with all parameters of your calculation mode. The mesh structure, calculation boundary conditions and etc.
2. Line 112: It could be not fine mesh with such a low number of elements. Or it is the number of nodes? Please add to supplementary materials the mesh characteristics and the image of the mesh structure
3. Line 115: What criteria is taken for mesh quality evaluation? What mesh metrics? Shape checking? Skewness?
4. Line 133: This particle is analog to the Desmodesmus cells? You know that Desmodesmus makes a colony connecting into groups from 2 to 8 cells, turning into a kind of cage. Sizes would be really differed from 5 µm. Please add in supplementary materials an image of the cells from microscope and write the scale of the image.
5. Line 141: Show please a scheme of the capture system. How is camera installed and what areas are shoot? Add in supplementary materials (SM) a example of a photos from these shoots.
6. Line 150: Please add in SM the picture from ImageJ.
7. Line 186: You need to write the weights of all components that you used for preparing this medium. Of course it is a standard one, but we need to be sure that you took the right weights.
8. Line 189: What is a spectrum of the light? 2000 Lux is a very low intensity. You have to write in SI units (W/m² or µmol photons m^-2 s^-1). If it is warm white light with T=2600-3000K it would be about only 30 µmol photons m^-2 s^-1. You should also add the name of the lamp and its parameters.
9. Line 193: Are you sure about logarithmic phase? How was pre-cultivation done? In what conditions and in what vessels?
10. Line 199: Commonly the 750 nm are used to determine OD. Measurement on 620 nm doesn't gives you reliable results. You should measure at 680 nm and subtract the measurements at 750 nm.
11. Lines 210-214: How had you measured dissolved CO2 in a liquid phase?? You didn't write about dissolved CO2 sensor.
12. Figure 2 (a,b): As you wrote before on the bottom of the column the sprayer disk with a numerous holes is installed. Why the starting flow is modeled as inlet with just a one hole in the center?
13. Line 304: How did you measure bubble diameters so accurately? Provide statistics on the measurements and estimate the error. You can add it to SM.
14. Figure 3: On what point (what area) the measurement were done?
15. Line 336: Air of gas mixture bubbles?
16. Figure 7: Provide statistics please! How much experiments with PBR you have made? One cultivation case? If so it is not reliable results. Just because of low light a small difference in average light intensity on the PBR could make such a difference in growth rates and OD. You need to make at least 3 biological repeats. And try it on higher light intensities 100-400 µmol photons m^-2 s^-1. Why OD starts with -0.6? Because of better results viewing? Divide the graph on to figures.  

17. References: Why you don't consider this work that was made by your colleges? Long-zao Luo, Yu Shao, Shuang Luo, Fan-jian Zeng & Guang-ming Tian (2019) Nutrient removal from piggery wastewater by Desmodesmus CHX1 and its cultivation conditions optimization, Environmental Technology, 40:21, 2739-2746, DOI:10.1080/09593330.2018.1449903
18. References: There are several references without DOI. Please add the DOIs.

Author Response

请参阅附件。

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,
Thank you for taking the time to address all my comments. I do not have any major point anymore.
Have a great day,

Author Response

Thank you for your valuable feedback! 

Reviewer 2 Report

Comments and Suggestions for Authors

no more comments

Author Response

Thank you for your valuable feedback! 

Reviewer 4 Report

Comments and Suggestions for Authors

Dear authors, 

Thank you for the additions and clarifications to the work. But there are questions that are not answered well. Maybe I was not clear enough.

1. You had to show a central section of the mesh structure, where the mesh of a baffeles and the regions around them could be seen. Fig1. from SM represents only the surface mesh as I can conclude. 
2. You didn't answered on a main question: How much experiments with PBR you have made? One cultivation case? If so it is not reliable results. You have added error bars, what they represent? Technical repeats or biological?
3.  How can you measure the diameter of a bubble to within ten thousandths without error? "As the relative diameter increases, the bubble diameter at position Y1 rises from 0.9353 mm to 1.0121 mm."  
4. Measuring light characteristics and maintaining uniform illumination is also questioned by me. You do not describe how and by what the intensity of illumination inside the light chamber was measured to confirm the uniformity of illumination. Light is the most important growth factor for photosynthetic microorganisms, the influence of which can override all tricks with gas flows and mixing. At the same time, you have only one lighting option and at extremely low intensities. 

I advise you to make at least two more experiments (runs) under the same conditions for gas-air supply and flow rates, but at higher light intensities and confirm the positive effect of the presence of baffles inside the cylindrical reactors.

Wish you a good work!

Author Response

Please see attachment.

Author Response File: Author Response.pdf