Sustainable Treatment of Landfill Leachate Using Sugar Lime Sludge for Irrigation and Nitrogen Recovery

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Add a graphical Abstract

 

1. Abstract:

• Lack of clarity on experimental design (treatment conditions, parameters analyzed, statistical approach).
• No mention of pollutant removal efficiency (BOD, COD, ammonia, heavy metals).
• Mention statistical analysis used to validate findings.

 

2. Introduction

• Incomplete background on landfill leachate composition—what are the key pollutants (e.g., heavy metals, nutrients, persistent organic pollutants)?
• No discussion on alternative treatment methods—how does this compare to biological, chemical, and advanced oxidation methods?
• No reference to prior studies on sugar lime sludge—has this material been used in wastewater treatment before?
• Compare existing leachate treatment technologies to justify the use of sugar lime sludge.

 

3. Methodology

• No baseline characterization of raw leachate—before treatment, what are the BOD, COD, ammonia, heavy metals, and microbial loads? Clearly characterize the raw leachate composition before treatment.
• No control experiments—untreated leachate should be tested over the same period to evaluate natural changes. Include control experiments (e.g., natural leachate aging, alternative treatments).
• Weak statistical approach—there are no details on replicates, standard deviations, p-values, or confidence intervals. Explain statistical methods used for data validation.
• Lack of explanation on nitrogen recovery mechanism—how does sugar lime sludge facilitate nitrogen retention in sediment? Provide a mechanistic explanation of nitrogen removal and transformation.
• No detailed phytotoxicity test methodology—what plant species were tested? What specific parameters were evaluated (e.g., root elongation, biomass production)? Specify phytotoxicity testing conditions and parameters.

 

4. Results

• No statistical validation—are the reductions in contaminants statistically significant? Conduct statistical tests (e.g., ANOVA, t-tests) to validate microbial reduction and pollutant removal.
• Limited pollutant removal analysis—only electrical conductivity and microbial load are discussed, but key water quality parameters (BOD, COD, ammonia, nitrate, heavy metals) are missing. Include detailed removal efficiency data for BOD, COD, ammonia, nitrate, heavy metals.
• Inconsistent microbial reduction results—fecal streptococci reduction varies between 93.1% and 99.1%, but no explanation is provided. Explain why microbial reduction varies across conditions.
• Lack of clear figures for reaction kinetics—was the removal efficiency time-dependent?
• No explanation of variability in nitrogen retention—what factors influenced nitrogen recovery in sediment?

 

5. Discussion

• Overstates the irrigation safety of treated leachate—there is no discussion on long-term soil and plant impact. Provide long-term soil impact data—does treated leachate affect soil pH, nutrient availability, or microbial health?
• No assessment of secondary pollution risks—does sugar lime sludge introduce trace contaminants or alter soil chemistry? Discuss potential risks of residual contaminants.
• No comparison to existing treatment efficiency benchmarks—how does this method compare to standard leachate treatment processes? Compare treatment efficiency to conventional methods (e.g., constructed wetlands, reverse osmosis).

 

6. Conclusion

• The conclusion is too short. Adequately summarize the work with proper quantitative data.
• Fails to address major limitations—does not discuss scalability, cost-effectiveness, or regulatory compliance.
• No discussion on long-term application risks—how does repeated irrigation with treated leachate impact crops and soil over time?

Author Response

Abstract

Comment: Lack of clarity on experimental design (treatment conditions, parameters analyzed, statistical approach).
Response: We revised the abstract to include clear treatment conditions (25%, 37%, 50% sugar lime sludge, 24–36 h contact), parameters analyzed (physicochemical, microbiological, and phytotoxicity), and statistical approach (ANOVA using Jamovi software).

Comment: No mention of pollutant removal efficiency (BOD, COD, ammonia, heavy metals).
Response: The abstract now briefly includes key efficiency indicators. For instance, fecal streptococci were reduced by up to 99.13%, and organic matter in the sediment increased significantly, indicating depollution. BOD and COD values are detailed in the main text.

Comment: Mention statistical analysis used to validate findings.
Response: One-way ANOVA was applied to determine statistical significance, and this is now clearly mentioned in the abstract.

Introduction

Comment: Incomplete background on landfill leachate composition—what are the key pollutants?
Response: The introduction now includes an expanded section on key pollutants, including xenobiotic organic compounds, dissolved organic matter, heavy metals, and inorganic macro-compounds (lines 61–78).

Comment: No discussion on alternative treatment methods.
Response: A new section with Table 1 outlines standard treatment techniques, including physicochemical, biological, membrane, electrocoagulation, and AOPs, with references (lines 80–91).

Comment: No reference to prior studies on sugar lime sludge.
Response: We added references (20 and 21) to previous uses of sugar lime sludge in our laboratory, particularly in composting and waste valorization.

Comment: Compare existing leachate treatment technologies to justify sugar lime sludge.
Response: In the Discussion section (lines 444–452), we compare our method to conventional approaches such as reverse osmosis and constructed wetlands, highlighting the cost-efficiency and sustainability of sugar lime sludge.

Methodology

Comment: No baseline characterization of raw leachate.
Response: Baseline physicochemical and microbiological properties of the raw leachate are now provided in Table 3 (lines 376–408), including pH, BOD, COD, and heavy metals.

Comment: No control experiments.
Response: Control (untreated leachate) was used in all experiments. This is reflected in all figures and tables (e.g., Figures 4–6, Tables 4–6). We clarified this point in the revised text.

Comment: Weak statistical approach.
Response: Statistical validation is now clearly described (lines 163–167). ANOVA and Student’s t-tests were performed with p < 0.05 as the significance threshold. Results are presented with standard deviations.

Comment: Lack of explanation on nitrogen recovery mechanism.
Response: We included a detailed explanation (lines 277–301) on how calcium in sugar lime sludge promotes nitrogen aggregation and stabilization during composting, supporting nitrogen recovery.

Comment: No detailed phytotoxicity test methodology.
Response: Phytotoxicity test procedures are now fully described (lines 152–162), including plant species (Zea mays), parameters (germination rate, root elongation), and calculation of the Germination Index.

Results

Comment: No statistical validation.
Response: All measured parameters (pH, conductivity, microbial load, OM) were statistically validated. Results from ANOVA and t-tests are provided (e.g., Table 4), and differences are statistically significant (p < 0.001).

Comment: Limited pollutant removal analysis.
Response: The composition of raw leachate (BOD, COD, heavy metals) is provided in Table 3. While removal efficiencies are not all presented in percentage terms, their evolution is reflected in the results and discussion. Future work will quantify full removal efficiencies.

Comment: Inconsistent microbial reduction results.
Response: The variability (93.1–99.1%) is discussed (lines 251–261) as resulting from differences in treatment concentration, contact time, and leachate variability. This is now clarified.

Comment: No clear figures for reaction kinetics.
Response: While a kinetic model was not used, time-dependent effects of 24 h vs. 36 h treatment are reported and analyzed, particularly for conductivity and organic matter removal (Figures 2 and 3).

Comment: No explanation of variability in nitrogen retention.
Response: This variability is now explained in section 3.6, focusing on the role of calcium in the flocculation and stabilization of nitrogen compounds (lines 293–297).

Discussion

Comment: Overstates irrigation safety of treated leachate—no long-term soil impact.
Response: We acknowledge this in the revised conclusion. Long-term effects on soil and plants will be evaluated in the second phase of our study (lines 471–474).

Comment: No assessment of secondary pollution risks.
Response: This limitation is now discussed. The possibility of residual contaminants from sugar lime sludge and their effects on soil chemistry is noted (lines 467–470).

Comment: No comparison to existing treatment efficiency benchmarks.
Response: A comparative discussion was added (lines 444–452), highlighting advantages and disadvantages of our method compared to advanced systems such as reverse osmosis and constructed wetlands.

Conclusion

Comment: Conclusion is too short.
Response: The conclusion has been expanded to include quantitative findings (e.g., reduction of organic matter from 10.43 to 40.84 g/L, microbial reduction rates), reinforcing the study’s contributions (lines 459–477).

Comment: Fails to address major limitations.
Response: Scalability, cost-effectiveness, and regulatory implications are now addressed as part of the future research agenda.

Comment: No discussion on long-term application risks.
Response: We now acknowledge the importance of long-term soil monitoring and crop safety, which will be included in the continuation of this project.

We thank the reviewers again for their valuable comments, which helped us significantly improve the clarity and quality of our manuscript. We hope that the revised version meets your expectations and remains a strong candidate for publication in Nitrogen.

Sincerely,
Tilila Baganna (on behalf of all authors)
PhD Candidate, University Cadi Ayyad, Morocco
Email: t.baganna.ced@uca.ac.ma

 

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents an interesting approach of utilizing sugar lime sludge for treating landfill leachate, aiming for irrigation reuse and nitrogen recovery. This subject is highly relevant, especially for arid regions facing water scarcity. Overall, the work is promising, but several areas need improvement before publication can be considered. Below are detailed suggestions and observations.

1. In the Abstract section, clearly state the primary research objective or hypothesis at the beginning to help readers immediately understand the purpose of the study.

2. In the Abstract, provide concise quantitative values for key results, such as specific final pH and electrical conductivity measurements, to better illustrate the effectiveness of the treatment.

3. In the Abstract, explicitly state the novelty or unique contribution of this research in comparison to previous landfill leachate treatments, emphasizing why sugar lime sludge was selected.

4. The Introduction section should be expanded by including a deeper review of recent literature (within the last 5 years) related to sustainable or alternative landfill leachate treatment methods, clearly highlighting gaps this research addresses.

5. In the Introduction, explicitly justify the choice of sugar lime sludge by discussing its specific advantages and previous evidence of effectiveness in similar contexts.

6. In the Materials and Methods, provide complete details of experimental procedures, clearly stating total volumes used, mixing conditions, temperature, containers, and agitation speed for reproducibility.

7. Include full standard citations (ISO, NF) with publication years in the Materials and Methods section, following MDPI guidelines, to ensure methodological clarity and precision.

8. Clearly describe composting parameters such as temperature control, humidity, turning frequency, and container or pile type in the Materials and Methods section to facilitate reproducibility.

9. Specify detailed conditions for phytotoxicity testing in the Materials and Methods section, including seed type, replicates, dilution ratios, controls, and interpretation criteria for the Zucconi method.

10. Improve grammar, clarity, and technical language throughout the Materials and Methods, particularly in the descriptions of calcination procedures and electrical conductivity measurements (including equipment details).

11. Verify and standardize units consistently across the Results section; the electrical conductivity (EC) value reported as "300 mS/cm" appears unrealistically high and likely should be expressed in µS/cm or clarified.

12. Enhance the clarity and readability of all figures (Figures 1–9) in the Results section, ensuring clear labels, legends, and descriptions to allow independent interpretation by readers.

13. Provide a deeper explanation in the Discussion section of why sugar lime sludge effectively reduced electrical conductivity, stabilized pH, reduced microbial contamination, and decreased phytotoxicity, addressing underlying chemical or biological mechanisms.

14. Explicitly compare findings in the Discussion with recent studies (preferably from the last 5 years), clearly highlighting similarities, differences, and the unique advantages of the presented method.

15. Clearly outline potential limitations in the Discussion, including practical scalability, long-term effectiveness, sludge variability, and implications for real-world implementation.

16. Clarify in the Discussion which further treatments or complementary methods may be necessary to fully meet irrigation standards for sensitive agricultural applications.

17. Conduct thorough proofreading to address grammatical errors, unclear sentence structure, and language inconsistencies across the entire manuscript to improve readability and scientific rigor.

18. Ensure references throughout the manuscript strictly adhere to MDPI formatting requirements, including proper numerical citation in-text and accurate full references in the Reference section.

19. In the Conclusion, clearly provide practical recommendations for implementing the described leachate treatment method in real-world agricultural or environmental management contexts, including potential benefits and constraints.

20. Suggest specific future research directions in the Conclusion, such as optimization of treatment parameters, economic feasibility studies, or investigations into long-term environmental impacts to guide subsequent research effectively.

 

 

Comments on the Quality of English Language

Improve grammar, clarity, and technical language throughout the Materials and Methods, particularly in the descriptions of calcination procedures and electrical conductivity measurements (including equipment details).

Author Response

Abstract

1. Clearly state the primary research objective or hypothesis at the beginning.
Response: We revised the beginning of the Abstract to clearly state the main research objective: evaluating the use of sugar lime sludge to treat landfill leachate for safe irrigation reuse and nitrogen recovery.

2. Provide concise quantitative values for key results (pH, EC, etc.).
Response: We added specific pH values (8.69–8.91) and electrical conductivity reductions to clearly demonstrate treatment effectiveness.

3. Explicitly state the novelty or unique contribution of the research.
Response: The Abstract now emphasizes that sugar lime sludge, an agricultural by-product, is investigated for the first time in Morocco as a dual-purpose agent for depollution and nutrient recovery.

Introduction

4. Include a deeper review of recent literature (last 5 years).
Response: The Introduction has been expanded to include recent references (2023–2024) on alternative treatment methods, including AOPs, electrocoagulation, and bio-adsorption techniques (lines 80–91; References 26–43).

5. Explicitly justify the choice of sugar lime sludge.
Response: We now provide a more detailed rationale for using sugar lime sludge, citing its availability, high calcium content, alkalinity, and prior valorization in composting (lines 88–91; References 20–21).

Materials and Methods

6. Provide complete details of experimental procedures (volume, agitation, containers, etc.).
Response: Additional information has been added: 40 liters of leachate, mixing in plastic drums, agitation at 150 rpm, and ambient room temperature (~22–25°C) are now clearly stated.

7. Include full ISO/NF standard citations with years.
Response: All ISO and NF standards are now cited with publication years and formatted according to MDPI guidelines (e.g., ISO 4833:2013, NF ISO 9308-1:2014).

8. Describe composting parameters (temperature control, humidity, turning, container type).
Response: Composting conditions are now clearly stated: performed in plastic bins, turned every 5 days, moisture maintained at 50–60%, temperature monitored daily (lines 267–271).

9. Specify conditions for phytotoxicity testing (replicates, dilutions, controls).
Response: The Zucconi method is described in detail (lines 152–162): 3 replicates per treatment, dilution ratios (1/4, 1/5, 1/6, 1/10), control with distilled water, and germination index as the evaluation metric.

10. Improve technical language in calcination and EC measurements.
Response: We clarified the calcination description (600°C in muffle furnace, 4 h) and specified the equipment for EC and pH (Biobase 900 multiparameter meter) for technical accuracy.

Results

11. Standardize units—verify EC units (“300 mS/cm” too high).
Response: Thank you for this observation. However, the value of ~300 mS/cm reflects the actual measurement obtained from the raw leachate sample collected in 2024 from the Marrakech landfill site.  Therefore, we retained this value as measured and clarified its origin and context in the manuscript to avoid confusion. Units have been double-checked to ensure consistency throughout the figures and text.

12. Improve figures: labels, legends, clarity.
Response: All figures have been revised for clarity. Labels, axes, and legends were improved (e.g., EC in µS/cm), and figure captions were rewritten to provide standalone understanding.

Discussion

13. Provide deeper explanation of treatment mechanisms.
Response: We expanded the discussion to explain that pH stabilization is due to calcium carbonate buffering, conductivity reduction results from precipitation/adsorption of ions, microbial reduction is linked to lime alkalinity, and phytotoxicity mitigation is tied to organic contaminant removal (lines 427–443).

14. Compare findings with recent literature (last 5 years).
Response: We added a comparison with multiple recent studies (2021–2024), highlighting that our method is cost-effective, locally sourced, and efficient compared to membrane or chemical oxidation methods (lines 444–452).

15. Outline potential limitations (scalability, sludge variability).
Response: Limitations are now explicitly addressed in the Discussion and Conclusion: including sludge composition variability, scaling challenges, and possible need for post-treatment (lines 467–474).

16. Clarify further treatments needed to meet irrigation standards.
Response: We mention that while sugar lime sludge reduces toxicity, additional steps (e.g., filtration, dilution, polishing) may be necessary to comply with irrigation norms for sensitive crops.

Language and Formatting

17. Proofread for grammar, sentence structure, clarity.
Response: The entire manuscript was carefully proofread. Language has been improved for clarity, precision, and fluency, particularly in technical descriptions and transitions.

18. Ensure references follow MDPI formatting.
Response: All references have been reformatted to comply with MDPI standards, including correct numbering, journal names, volumes, and DOI links where applicable.

Conclusion

19. Add practical recommendations for real-world implementation.
Response: The conclusion now outlines practical implementation paths, including use in green space irrigation, compost enhancement, and resource recovery in low-resource settings.

20. Suggest specific future research directions.
Response: The Conclusion now proposes future work: optimizing sludge concentration/time, conducting field-scale trials, assessing economic feasibility, and studying long-term environmental impacts.

We again express our appreciation to the reviewer for the thorough and constructive evaluation. We believe these revisions have substantially strengthened the manuscript and improved its scientific quality and applicability.

Sincerely,
Tilila Baganna (on behalf of all authors)
PhD Candidate, University Cadi Ayyad, Morocco

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for addressing all the comments

Reviewer 2 Report

Comments and Suggestions for Authors I have carefully evaluated the updated version of the article titled "Sustainable Treatment of Landfill Leachate Using Sugar Lime Sludge for Irrigation and Nitrogen Recovery". The authors have addressed the previous comments in a comprehensive and satisfactory manner. The manuscript now demonstrates improved scientific clarity, structure, and relevance. In my opinion, the revised version meets the standards for publication. I recommend the manuscript for publication in its current form.