Circular Perspective for Utilization of Industrial Wastewaters via Phytoremediation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents an evaluation of the treatability of wastewater from a composting plant through the phytoremediation technique using Miscanthus x giganteus, a plant that not only has phytoremediation capacity but also produces biomass usable for energy generation under a circular economy model. The approach is novel, the manuscript is well-structured and provides consistent results; however, it should improve the interpretation of results and address the study’s limitations. Additionally, the component of circularity, as well as the applications and recommendations derived from the findings, should be better developed. The following revisions are recommended:

• Include quantitative results in the abstract.
• In Table 1, add a caption with definitions of the abbreviations.
• Lines 158–163: Was the substrate’s water retention capacity determined? How was the dosed volume controlled in relation to field capacity to prevent saturation or leaching?
• Lines 168–169: What was the average temperature during the experiment?
• Line 190: Clarify the HCl (1:1 v/v) treatment procedure. Was it a digestion process? For how long and at what temperature?
• Section 2.8: Complement the information regarding quality control, limits of quantification, and recovery of reference materials used in the methods (this can be included as a supplementary table).
• Line 396: Check superscripts.
• Table 3: Specify the units for the data in each column (plant, substrate).
• Improve figure presentation: in Figures 1 and 2, increase text size — currently it is illegible. Improve the resolution of Figure 3 — the legend cannot be read. Check superscripts, subscripts, and units in Figure 4. Ensure consistency in unit notation throughout the manuscript. Improve the resolution of Figure 5. 
• Line 536: Check subscripts.
• Discussion: The study lacks soil quality indicators, microbial activity, metal bioavailability, ecotoxicity, and leaching assessments. The proposed model needs an environemental impact assessment and specific indicators to demonstrate the contribution to circular economy. These limitations should be explicitly discussed in the interpretation of results and as recommendations for future research, considering the scalability and real-scale applicability of the proposed approach. Create a section on limitations and recommendations analyzing the practical implications of the findings and the feasibility of implementing this circularity approach.
• Conclusions: Avoid repeating results and present a concise conclusion aligned with the research hypothesis.

Author Response

Manuscript: Circular perspective for utilization of industrial wastewaters via phytoremediation

Manuscript number: sustainability-3924270

Response to comments given by Reviewer #1

The Authors (A) are grateful to the Reviewer (R) for evaluating our manuscript and providing valuable comments and suggestions. Please find below the Authors’ replies to the comments. All the related changes and corrections in the text of the manuscript are highlighted in yellow.

R: The manuscript presents an evaluation of the treatability of wastewater from a composting plant through the phytoremediation technique using Miscanthus x giganteus, a plant that not only has phytoremediation capacity but also produces biomass usable for energy generation under a circular economy model. The approach is novel, the manuscript is well-structured and provides consistent results; however, it should improve the interpretation of results and address the study’s limitations. Additionally, the component of circularity, as well as the applications and recommendations derived from the findings, should be better developed.

A: Thank You for Your positive opinion on our work. During the revisions, we improved the interpretation of results, highlighted the limitations of the study, and improved the recommendations from the performed research.

R: (1) Include quantitative results in the abstract.

A: Thank You for this comment. The abstract has been revised and now includes quantitative results.

R: (2) In Table 1, add a caption with definitions of the abbreviations.

A: Definitions of abbreviations have been added to caption of Table 1.

R: (3) Lines 158–163: Was the substrate’s water retention capacity determined? How was the dosed volume controlled in relation to field capacity to prevent saturation or leaching?

A: Thank You for this remark. During the experiments, the water content was kept at the level of 60% of maximum water retention capacity. When necessary water losses were replenished by distilled water to a constant mass of the substrate.

R: (4) Lines 168–169: What was the average temperature during the experiment?

A: During the experiment, the average temperature inside the greenhouse was about 28 ± 5^oC.

R: (5) Line 190: Clarify the HCl (1:1 v/v) treatment procedure. Was it a digestion process? For how long and at what temperature?

A: The description of the digestion process in the methodology section was supplemented with detailed information, according to the Reviewer’s comment (Lines 206-213). 

R: (6) Section 2.8: Complement the information regarding quality control, limits of quantification, and recovery of reference materials used in the methods (this can be included as a supplementary table).

A: Element concentration in plant and soil materials were analyzed by ICP-OES methods, according to EN 13656:2020 and EN 16170:2016. Quality assurance protocols included analysis of certified reference materials and processing blanks in parallel with samples. Method validation showed recovery rates exceeding 95%, with method detection limits established at 0.1 μg·kg^-1 for all analyzed elements. The limit of quantification was three times higher than the limit of detection.

R: (7) Line 396: Check superscripts.

A: The superscripts were corrected, thank You.

R: (8) Table 3: Specify the units for the data in each column (plant, substrate).

A: The unit was specified and put in the Table caption.

R: (9) Improve figure presentation: in Figures 1 and 2, increase text size — currently it is illegible. Improve the resolution of Figure 3 — the legend cannot be read. Check superscripts, subscripts, and units in Figure 4. Ensure consistency in unit notation throughout the manuscript. Improve the resolution of Figure 5.

A: Thank You for the correction suggestions. The quality and readability of all figures have been improved, in addition we decided to join together previous Figs. 3 and 4. In addition, symbols instead of full names of parameters (especially for physiological parameters) were used in the figures, with explanations in the footer. We believe that now the symbols and units are uniform throughout the article.

R: (10) Line 536: Check subscripts.

A: Thank You, the subscript has been corrected.

R: (11) Discussion: The study lacks soil quality indicators, microbial activity, metal bioavailability, ecotoxicity, and leaching assessments. The proposed model needs an environmental impact assessment and specific indicators to demonstrate the contribution to circular economy. These limitations should be explicitly discussed in the interpretation of results and as recommendations for future research, considering the scalability and real-scale applicability of the proposed approach. Create a section on limitations and recommendations analyzing the practical implications of the findings and the feasibility of implementing this circularity approach.

A: Thank You very much for Your comments. We are aware of the limitations of this study, performed in model, greenhouse conditions. According to the Reviewer’s comment, we added a short paragraph (p. 3.9), discussing the limitations of the study, pointing out proposals on the future research directions.

R: (12) Conclusions: Avoid repeating results and present a concise conclusion aligned with the research hypothesis.

A: Thank You. We have revised the conclusion section and added a concise conclusion of the conducted research (final paragraph of the conclusion section).

 

The Authors are grateful for the comments and suggestions obtained from the Reviewer. We do hope that the responses as well as the corrections and improvements included in the revised version of the manuscript will be acceptable and satisfactory.

On behalf of the Authors,

Piotr Rybarczyk

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript titled ‘Circular perspective for utilization of industrial wastewaters via phytoremediation’ presents an interesting and valuable study exploring the use of Miscanthus x giganteus for phytoremediation and valorization of industrial wastewaters, while at the same time producing biomass with energy potential. The work is well-motivated, experimentally solid, and addresses issues highly relevant to circular economy and sustainable waste management. The title is clear and accurately reflects the scope of the study. Overall, the study makes a meaningful contribution and is worthy of publication after minor revisions.

1. The abstract is well-structured, but it could highlight more explicitly the novelty of testing real post-composting wastewaters combined with soil improver, compared to previous phytoremediation studies.
2. Consider quantifying the main results in the abstract (e.g., % increase in biomass yield and combustion heat).
3. The introduction is comprehensive, setting the environmental and circular economy context clearly. However, the literature review could be tightened. Currently, it lists many examples; instead, emphasize the knowledge gap this work addresses. To broaden the scope, I recommend check studies like Process Safety and Environmental Protection, 142, pp.317-324, that demonstating another biological approach for wastewater treatment, focused on microbial denitrification, and can provide an interesting contrast to your plant-based system. Positioning your work alongside such alternative biological strategies will highlight the originality of phytoremediation in a circular economy framework.
4. The methodology is well-documented and replicable. The inclusion of details such as soil improver characterization (Table A.2) and wastewater properties (Table A.1) is excellent. The description of pot experiments and irrigation scheme (Table 1) is very clear. One suggestion: the statistical analysis section could be streamlined. While all steps are described, some repetition of instruments and standards could be shortened.
5. The results are logically presented and supported by data. Biomass yield, tolerance index, and calorific value are convincing indicators of the benefits of wastewater application. Tables and figures are generally clear and informative. Table 3 and Table 4 are strong, but could benefit from highlighting statistically significant differences in bold for easier interpretation.
6. Figures are well-chosen, but the legends are sometimes too short, consider clarifying experimental conditions directly in the captions. Also, Figures resolution and readability (axis labels, font size) should be checked for the final version.
7. The conclusions are supported by results and summarize the main findings clearly. It would be beneficial to add one or two sentences on scalability and real-world implementation challenges (e.g., variability of wastewater composition, logistics of biomass use).

Author Response

Manuscript: Circular perspective for utilization of industrial wastewaters via phytoremediation

Manuscript number: sustainability-3924270

Response to comments given by Reviewer #2

The Authors (A) are grateful to the Reviewer (R) for evaluating our manuscript and providing valuable comments and suggestions. Please find below the Authors’ replies to the comments. All the related changes and corrections in the text of the manuscript are highlighted in yellow.

R: The manuscript titled ‘Circular perspective for utilization of industrial wastewaters via phytoremediation’ presents an interesting and valuable study exploring the use of Miscanthus x giganteus for phytoremediation and valorization of industrial wastewaters, while at the same time producing biomass with energy potential. The work is well-motivated, experimentally solid, and addresses issues highly relevant to circular economy and sustainable waste management. The title is clear and accurately reflects the scope of the study. Overall, the study makes a meaningful contribution and is worthy of publication after minor revisions.

A: Thank You for Your positive opinion on our manuscript. We believe that inclusion of the Reviewer’s suggestions in the revised version of our manuscript will further enhance it quality and interest to the readers.

R: (1) The abstract is well-structured, but it could highlight more explicitly the novelty of testing real post-composting wastewaters combined with soil improver, compared to previous phytoremediation studies.

A: Thank You for this suggestion, the abstract has been improved.

R: (2) Consider quantifying the main results in the abstract (e.g., % increase in biomass yield and combustion heat).

A: According to the Reviewer’s suggestion, the abstract section has been supplemented with quantitative data.

R: (3) The introduction is comprehensive, setting the environmental and circular economy context clearly. However, the literature review could be tightened. Currently, it lists many examples; instead, emphasize the knowledge gap this work addresses. To broaden the scope, I recommend check studies like Process Safety and Environmental Protection, 142, pp.317-324, that demonstating another biological approach for wastewater treatment, focused on microbial denitrification, and can provide an interesting contrast to your plant-based system. Positioning your work alongside such alternative biological strategies will highlight the originality of phytoremediation in a circular economy framework.

A: Thank You, the introduction section has been improved according to the comment, including the suggested citation regarding new solutions for the wastewater treatment.

R: (4) The methodology is well-documented and replicable. The inclusion of details such as soil improver characterization (Table A.2) and wastewater properties (Table A.1) is excellent. The description of pot experiments and irrigation scheme (Table 1) is very clear. One suggestion: the statistical analysis section could be streamlined. While all steps are described, some repetition of instruments and standards could be shortened.

A: Thank You for Your opinion and suggestions. We believe that the detailed description of the methods used in the study is crucial for possible repetition and development of new research in similar areas. In addition, we believe that the statistical analysis of the results is on acceptable level, but for increasing the clarity of data presentation, we have added information on the statistically significant results among all other presented.

R: (5) The results are logically presented and supported by data. Biomass yield, tolerance index, and calorific value are convincing indicators of the benefits of wastewater application. Tables and figures are generally clear and informative. Table 3 and Table 4 are strong, but could benefit from highlighting statistically significant differences in bold for easier interpretation.

A: Thank You for this suggestion. As suggested in the response to comment (4), we highlighted statistically significant differences by caption “S”.

R: (6) Figures are well-chosen, but the legends are sometimes too short, consider clarifying experimental conditions directly in the captions. Also, Figures resolution and readability (axis labels, font size) should be checked for the final version.

A: Thank You for this comment. The quality and readability of all the figures have been improved, including the use of unified symbols with clear explanations in the figure captions.

R: (7) The conclusions are supported by results and summarize the main findings clearly. It would be beneficial to add one or two sentences on scalability and real-world implementation challenges (e.g., variability of wastewater composition, logistics of biomass use).

A: Thank You for this comment. Our investigations show the potential for including the post-industrial / post-composting wastewaters for irrigation of landfill territories as well as plants, for beneficial recovery of nutrients and microelements by plants, and especially energy plants. The produced biomass can be regarded as energy and material source, in the framework of circular economy. The conclusion section has been improved and its clarity for the reader was also enhanced.

 

The Authors are grateful for the comments and suggestions obtained from the Reviewer. We do hope that the responses as well as the corrections and improvements included in the revised version of the manuscript will be acceptable and satisfactory.

On behalf of the Authors,

Piotr Rybarczyk

Reviewer 3 Report

Comments and Suggestions for Authors

Review Report:

The manuscript addresses the reuse of highly saline industrial wastewater and compost leachate for irrigating Miscanthus x giganteus, evaluating plant growth, physiological responses, and metal accumulation. The topic is timely and relevant for circular economy strategies and sustainable water management. The study is thorough and presents solid experimental work, but several points require clarification and deeper discussion to strengthen its scientific contribution and applicability.

1. The motivation and objectives are clearly stated, but the research gap could be better emphasized; the novelty compared to existing studies on Miscanthus irrigation with saline or nutrient-rich wastewater should be explicitly discussed.

2. The characteristics of the applied wastewater (extremely high EC, TOC, and metal content) warrant a more detailed risk assessment; the rationale behind the chosen dilution levels and irrigation doses should be explained more precisely.

3. The pot experiment provides valuable insight but has inherent limitations; the authors should discuss how field conditions (e.g., leaching, drainage, salt accumulation, evapotranspiration) might affect results and scalability.

4. The improvement in biomass yield and calorific value under higher wastewater doses is interesting, but the authors should disentangle the positive effects of nutrients from potential osmotic or toxic stress caused by salinity.

5. The role and composition of the soil improver (SK-9) are described, yet its contribution to nutrient and metal input should be quantitatively separated from that of the wastewater to clarify source effects.

6. The analysis of heavy metal uptake (BCF and U) is appropriate, though interpretation could go deeper; low BCF values do not necessarily imply low environmental risk if total biomass and metal removal are high.

7. The potential risks associated with the use of biomass containing metals for combustion or energy production are not sufficiently discussed; evaluation of ash composition and emission risks would be highly relevant.

8. The study should explicitly compare measured concentrations of metals and salinity in soil and biomass with legal or recommended limits for agricultural reuse and biomass valorization in the EU context.

9. The statistical analysis is acceptable but could be more transparent; the authors should provide F and p values, confidence intervals, and sample size information to ensure reproducibility.

10. The PCA and clustering analyses are well presented, but interpretation should connect more directly to physiological and biochemical responses, clarifying which variables drive the observed groupings.

11. The environmental and operational safety aspects of using such concentrated wastewater are underrepresented; guidelines for monitoring salinity, pH, and heavy metal accumulation during long-term application should be proposed.

12. The study would benefit from a section discussing practical implications, such as treatment pre-steps, irrigation management strategies, or potential coupling with other crops, to guide real-world implementation.

Author Response

Manuscript: Circular perspective for utilization of industrial wastewaters via phytoremediation

Manuscript number: sustainability-3924270

Response to comments given by Reviewer #3

The Authors (A) are grateful to the Reviewer (R) for evaluating our manuscript and providing valuable comments and suggestions. Please find below the Authors’ replies to the comments. All the related changes and corrections in the text of the manuscript are highlighted in yellow.

R: The manuscript addresses the reuse of highly saline industrial wastewater and compost leachate for irrigating Miscanthus x giganteus, evaluating plant growth, physiological responses, and metal accumulation. The topic is timely and relevant for circular economy strategies and sustainable water management. The study is thorough and presents solid experimental work, but several points require clarification and deeper discussion to strengthen its scientific contribution and applicability.

A: Thank You for Your positive opinion on our manuscript. We hope that the improvements included in the revised version of the manuscript will be satisfactory.

R: (1) The motivation and objectives are clearly stated, but the research gap could be better emphasized; the novelty compared to existing studies on Miscanthus irrigation with saline or nutrient-rich wastewater should be explicitly discussed.

A: The novelty of the research consists in the attempt of combining the nutrient recovery from post-industrial / post-composting wastewater streams for irrigation of giant miscanthus with its simultaneous valorization for energy recovery from biomass. The statement of novelty was included in the introduction section.

R: (2) The characteristics of the applied wastewater (extremely high EC, TOC, and metal content) warrant a more detailed risk assessment; the rationale behind the chosen dilution levels and irrigation doses should be explained more precisely.

A: Thank You for this comment. In the greenhouse experiments, the wastewater doses of 50, 100 and 200 mL (diluted with water to 500 mL) per week per pot were applied. The dose of the wastewater is related to nutritional requirements of miscanthus. This plant requires high doses of nutrients due to its high biomass production. In addition, the applied wastewater streams were diluted with distilled water in order to limit high salinity of the waste streams which can hinder the nutrient uptake from the solutions used for miscanthus irrigation.

R: (3) The pot experiment provides valuable insight but has inherent limitations; the authors should discuss how field conditions (e.g., leaching, drainage, salt accumulation, evapotranspiration) might affect results and scalability.

A: Thank You for this comment. In the reclamation of lands used for e.g. landfilling purposes, there is a possibility of the use of wastewaters to irrigate and fertilize the reclamation plants. However, a considerable limitation is posed by high substrate salinity, as well as the accumulation of toxic substances, which hinder the development and growth of the plants [53, 54; citations as they appear in the manuscript]. Therefore, it is recommended to perform research in open-field conditions of a real landfill site to test the efficiency of the proposed remediation ideas. A short section on limitations of the study was added in the revised manuscript.

R: (4) The improvement in biomass yield and calorific value under higher wastewater doses is interesting, but the authors should disentangle the positive effects of nutrients from potential osmotic or toxic stress caused by salinity.

A: Thank You for this comment. The increase in miscanthus biomass yield with increased doses of wastewater is likely related to the plant's high ability to overcome osmotic stress caused by high salinity. The effect was, however, positive within the studied range of concentrations, but became toxic at higher concentrations.

R: (5) The role and composition of the soil improver (SK-9) are described, yet its contribution to nutrient and metal input should be quantitatively separated from that of the wastewater to clarify source effects.

A: Thank You for this remark. It is true that the differentiation between the nutrient input either from the soil improver or the wastewaters would be valuable, however it is hard to assess based on the collected results. It is recommended for the future research to provide a nutrient balance and show the amounts of nutrients introduced with the soil improver and the wastewaters, and evaluate the nutrient uptake from these materials.

R: (6) The analysis of heavy metal uptake (BCF and U) is appropriate, though interpretation could go deeper; low BCF values do not necessarily imply low environmental risk if total biomass and metal removal are high.

A: Low BCF values can indicate a potential environmental risk due to high biomass yield and high metal uptake from the substrate. However, in our research the BCF coefficient was mainly used for emphasizing the waste management approach, showing that the conditions of object 1 (sand as a substrate), that is, without the soil improver, offered no availability of nutrients and toxic substances, and thus no increase of BCF was observed.

R: (7) The potential risks associated with the use of biomass containing metals for combustion or energy production are not sufficiently discussed; evaluation of ash composition and emission risks would be highly relevant.

A: The combustion of metal-rich biomass can be dangerous for both the environment and the construction materials of the stove. Combustion of such a biomass results in the formation of alkaline ash, leading to the stove corrosion. This information was added to the discussion in p. 3.4.

R: (8) The study should explicitly compare measured concentrations of metals and salinity in soil and biomass with legal or recommended limits for agricultural reuse and biomass valorization in the EU context.

A: Thank You for this remark. In our approach, the use of miscanthus for environmental reclamation and then for energy recovery is not a typical agricultural practice. This is why we decided not to evaluate the limits of salinity valid for greenhouse cultivation of plants on agricultural substrates.

R: (9) The statistical analysis is acceptable but could be more transparent; the authors should provide F and p values, confidence intervals, and sample size information to ensure reproducibility.

A: Thank You for this suggestion. We have included information on the statistically significant differences between the results obtained for different parameters and cultivation conditions. In our opinion the statistical analysis as it is presented in the manuscript is acceptable.

R: (10) The PCA and clustering analyses are well presented, but interpretation should connect more directly to physiological and biochemical responses, clarifying which variables drive the observed groupings.

A: According to the Reviewer’s comment, the discussion on the PCA analysis was improved.

R: (11) The environmental and operational safety aspects of using such concentrated wastewater are underrepresented; guidelines for monitoring salinity, pH, and heavy metal accumulation during long-term application should be proposed.

A: According to the existing regulations for landfill management, there is a need for continuous monitoring of physicochemical parameters of both the plant substrates and the wastewaters. When the allowable limits are exceeded, the wastewaters must be directed to a wastewater treatment plant in order to prevent secondary pollution of the environment [1; citation number as it appears in the manuscript].

R: (12) The study would benefit from a section discussing practical implications, such as treatment pre-steps, irrigation management strategies, or potential coupling with other crops, to guide real-world implementation.

A: Thank You for this suggestion. However, this study aimed to provide preliminary data that lay the groundwork for future research and application practices in plant irrigation and the recycling of wastewater streams at landfills or wastewater treatment facilities. The Authors are aware of the limitations of the study, and as indicated in the response to comment (3), an additional section on the limitations has been added.

 

The Authors are grateful for the comments and suggestions obtained from the Reviewer. We do hope that the responses as well as the corrections and improvements included in the revised version of the manuscript will be acceptable and satisfactory.

On behalf of the Authors,

Piotr Rybarczyk

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The new version satisfactorily incorporates the requested revisions.

Reviewer 3 Report

Comments and Suggestions for Authors

All my comments have been properly addressed. Accept in current form.