Old Landfill Leachate and Municipal Wastewater Co-Treatment by Sequencing Batch Reactor Combined with Coagulation–Flocculation Using Novel Flocculant

Round 1

Reviewer 1 Report

In this study, Co-treatment of landfill leachate with a mixture of municipal wastewater was performed at 5%, 10%, 15% and 20% in SBR and effluent were treated by coagulation-flocculation. Some minor issues should be addressed:

(1) The authors should conform the references format in the text to meet the journal  standard;

(2)  Page 5, "As the landfill age increases, landfill enters anoxic/anaerobic phases where nitrogen cycle is inhibited mainly due to decreased rate of nitrification which results into ammonia accumulation",  Does the author have data to support this hypothesis that nitrogen cycle is inhibited mainly due to decreased rate of nitrification which results into ammonia accumulation?

(3)  Page 6, " The ammonia removal was mainly due to nitrification as evident from the concurrent increase in nitrate concentration", the authors are suggested to explain detaily the "nitrification". It's better to use some chemical equation or flow chart.

(4) Page 7, "Color removal by alum showed an insignificant difference(P>0.05) at 5% leachate while at 10%, 15% and 20% leachate significant difference (P<0.05) was observed", the reason for why significant difference is suggested to be discussed.

Overall, the paper is well organised and can be accepted after minor reversion.

Author Response

Authors' Response to Reviewer Comments:

 We thank the reviewers and the editorial team for taking efforts to critically evaluate the work, which has helped to improve the overall quality of the manuscript. The new additions and corrections in the manuscript are highlighted in blue font color. We have addressed all the comments and details of the same are mentioned below:

Reviewer 1:

1. The authors should conform the references format in the text to meet the journal  standard.

Authors Response:

References have been updated and formatted as per the requirements of the journal.

2. Page 5, "As the landfill age increases, landfill enters anoxic/anaerobic phases where nitrogen cycle is inhibited mainly due to decreased rate of nitrification which results into ammonia accumulation", Does the author have data to support this hypothesis that nitrogen cycle is inhibited mainly due to decreased rate of nitrification which results into ammonia accumulation?

Authors Response:

We thank the reviewer for bringing our attention to this point. We did not monitor the landfill for this study as it was not within the scope of the objectives of the study. However, we tested for ammonia and nitrate concentration in the leachate and the samples collected from SBR and coagulation-flocculation experiments. Further, for nitrification oxic conditions are needed whereas denitrification requires anoxic conditions. Another pathway for ammonia release from landfill is volatilisation at higher pH >8, however, it is least likely to occur in landfills. It has been reported that under methanogenic conditions (which is the case with old landfills) there are no known mechanisms for ammonia removal (please refer to Kjeldsen et al. 2002). We have added this reference to the statement to validate this discussion.

3. Page 6, " The ammonia removal was mainly due to nitrification as evident from the concurrent increase in nitrate concentration", the authors are suggested to explain detaily the "nitrification". It's better to use some chemical equation or flow chart.

Authors Response:

Ammonia removal pathways from wastewater are nitrification, anaerobic ammonia oxidation (Anammox), Single reactor High activity Ammonia Removal Over Nitrite (SHARON), ammonia stripping, ammonia precipitation etc. Among these, the reaction conditions in this study are conducive to nitrification.

As per the suggestion of the reviewer, we have shown the chemical equation of ammonia oxidation to nitrate.

4. Page 7, "Color removal by alum showed an insignificant difference(P>0.05) at 5% leachate while at 10%, 15% and 20% leachate significant difference (P<0.05) was observed", the reason for why significant difference is suggested to be discussed.

Authors Response:

We thank the reviewer for pointing this out. We have addressed this comment in the manuscript now.

Reviewer 2 Report

In this manuscript, authors stated an application of novel flocculant for landfill leachate and municipal wastewater co-treatment. There are issues need to be addressed before its acceptance.

1.     Authors please recheck your reference list, the references cited in context apparently not in the same order as in reference list, if they are same references.

2.    Authors have a lot of acronyms, not each one is denoted, MSW, BOD/COD, HRT, SRT, TSS, MLSS etc.

3.    Authors mentioned wastewater composition characterization in section 3.1, what & how was the test performed exactly? Please provide related analysis data.

4.    What are those refractory substances in the landfill leachate, and how do they inhibit the COD removal efficiency? Please clarify.

5.    Authors should elaborate on the analysis of COD, turbidity, TSS and color removal in detail. Detailed experiment procedures should be provided. What are the “alum to GGI-g-PAM” / “FeCl₃ to GGI-g-PAM ratio” ? ANOVA , F-test and post hoc test data could be provided as supporting information.

6.    Authors mentioned negative correlation between flocculant dosage and solid removal, from the plot, in both cases, lowest flocculant dosage output the highest removal at 20% leachate concentration. Why is it ? Will even lower flocculant dosage provide better performance ?  

Author Response

Authors' Response to Reviewer Comments:

 We thank the reviewers and the editorial team for taking efforts to critically evaluate the work, which has helped to improve the overall quality of the manuscript. The new additions and corrections in the manuscript are highlighted in blue font color. We have addressed all the comments and details of the same are mentioned below:

Reviewer 2:

1. Authors please recheck your reference list, the references cited in context apparently not in the same order as in the reference list, if they are same references.

Authors Response:

References have been updated and formatted as per the requirements of the journal.

2. Authors have a lot of acronyms, not each one is denoted, MSW, BOD/COD, HRT, SRT, TSS, MLSS etc.

Authors Response:

We thank the reviewer for pointing this out, we have addressed this in the manuscript now.

3. Authors mentioned wastewater composition characterization in section 3.1, what & how was the test performed exactly? Please provide related analysis data.

Authors Response:

We thank the reviewer for bringing up this point. We have addressed this in the manuscript now and added the analytical data. All the leachate and municipal wastewater analyses were performed per the standard protocols described in the reference APHA [29].

4. What are those refractory substances in the landfill leachate, and how do they inhibit the COD removal efficiency? Please clarify.

Authors Response:

The refractory compounds in the old landfill leachate will be fulvic-like and humic-like compounds which are difficult to be degraded by bacterial-based processes. We have addressed this comment in the manuscript with a reference.

5. Authors should elaborate on the analysis of COD, turbidity, TSS and color removal in detail. Detailed experiment procedures should be provided. What are the “alum to GGI-g-PAM” / “FeCl₃to GGI-g-PAM ratio”? ANOVA, F-test and post hoc test data could be provided as supporting information.

Authors Response:

We have provided the details of analytical procedures in the supplementary file. ANOVA, F-test and post hoc test data are also provided as a supplementary file.

6. Authors mentioned negative correlation between flocculant dosage and solid removal, from the plot, in both cases, lowest flocculant dosage output the highest removal at 20% leachate concentration. Why is it? Will even lower flocculant dosage provide better performance?

Authors Response:

We thank the reviewer for highlighting this point. In coagulation-flocculation the treatment efficiency gets lower as the coagulant/flocculant dosage increases. This is mainly due to the fact that charge reversal and destabilising of colloidal particles occur as the dosage increases. Lowering the dosage further also affects the coagulation-flocculation efficiency as the dosage will not be sufficient for charge neutralization, adsorption, inter-particle bridging and enmeshment of impurities in precipitate.