Review Reports - The Assessment of Sewage Sludge Utilization in Closed-Loop Economy from an Environmental Perspective

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Reviews for a study focusing on "The Assessment of Sewage Sludge Utilization in Closed-Loop Economy from an Environmental Perspective". The paper is generally interesting and its is eloquent, technically sound, relevant, and coherent. However, these are areas of improvement before the paper could be published:

-Sources of individual chemicals in the sludge.

-Sources of heavy metals in the sludge need to be emphasized.

-Difference between centralised and decentralised systems and their metals concentration.

-Heavy metals are interesting, however, is it possible for authors to consider emerging contaminants? This will make the study more interesting.

-The abstract is very weak and require some reinforcement and results to distill a clear summary of the contents therein.

-What are the basis of confining the study to only heavy metals in the sludge. There are undesired materials and why are we not considering those as well?

-Adding an off-grid system will strengthen the study since it will also look in the feed streams and catchment activities.

-Quality assurance and creditability of the results needs to be emphasized. Add error bars and SDV.

-Merge results and discussions. Conclusion need to be refined in light of the ultimate objectives of the study.

-Add recommendations and future perspectives.

-This is a very interesting study but a lot need to be done to refine the content and crystallize its objectives.

Comments on the Quality of English Language

Language is good

Author Response

-Sources of individual chemicals in the sludge.

-Sources of heavy metals in the sludge need to be emphasized.

Thank you for your attention. In the introduction, information on sources of chemicals in sewage sludge has been added and a Table 2 showing sources of heavy metals in sludge has been introduced.

-Difference between centralised and decentralised systems and their metals concentration.

Thank you for your attention. In centralised systems, the concentration of metals in the collected wastewater can vary depending on the sources and mixing of industrial, commercial and residential wastewater. Conversely, in some cases in non-centralised systems, especially for less advanced or smaller treatment plants, the efficiency of heavy metal removal may be lower compared to centralised systems. Our study was based only on sludge taken from centralised systems, as this is a system operating on a larger scale. We did not compare the quality of the sludge taken from the different systems, however, this is an indication for us for further research in future work.

-Heavy metals are interesting, however, is it possible for authors to consider emerging contaminants? This will make the study more interesting.

Thank you for your attention. Many countries and regions have strict regulations regarding the amount and types of heavy metals that may be present in sludge that can be used agriculturally. Monitoring the heavy metal content is essential to meet the legal requirements for wastewater treatment. The aim of our study was to show that a high metal content does not always exclude the possibility of using metals due to their combination in stable forms without a tendency to migrate in the soil. However, we are planning future research into the content of other contaminants in sludge.

-The abstract is very weak and require some reinforcement and results to distill a clear summary of the contents therein.

Thank you for your attention. The abstract has been corrected.

-What are the basis of confining the study to only heavy metals in the sludge. There are undesired materials and why are we not considering those as well?

As above. Many countries and regions have strict regulations regarding the amount and types of heavy metals that may be present in sludge that can be used agriculturally. Monitoring the heavy metal content is essential to meet the legal requirements for wastewater treatment. The aim of our study was to show that a high metal content does not always exclude the possibility of using metals due to their combination in stable forms without a tendency to migrate in the soil. However, we are planning future research into the content of other contaminants in sludge.

-Quality assurance and creditability of the results needs to be emphasized. Add error bars and SDV.

Each graph has error bars and SDV.

-Merge results and discussions. Conclusion need to be refined in light of the ultimate objectives of the study.

Thank you for your attention. has been corrected.

-Add recommendations and future perspectives.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper was written well with only a few minor editorial errors which are listed below. Overall, the paper is sound and I recommend publishing with minor changes and additions.

The paper could be improved by providing some commentary on how the differences in soil properties (permeability and depth to groundwater) for the application sites may also influence risk with regards to water contamination from leaching metals.

Some discussion regarding how the different wastewater treatment technologies may have influenced the metal content of the sludge would be good.

The authors should provide a comparison of metal content in the sludge they observed at the 4 treatment plants to what has been previously reported in other studies.

The editorial errors/comments include:

Use a capital P for “Pollution value” in the heading for Table 5 to be consistent with other tables.

Use a capital R for “Risk Value” in the heading for Table 6 to be consistent with other tables.

In Figure 4 use a capital L for “Limit of fulfillment” and a capital M for “Moderate pollution or high” for consistency

Remove the outside border in Figure 5 for consistency with other figures

In Figures 7 and 8 capitalize “Limit” in “Limit of fulfillment”

Author Response

Thank you for your attention. Information on differences in soil properties that may affect the risk of water contamination has been added in the introduction.

Some discussion regarding how the different wastewater treatment technologies may have influenced the metal content of the sludge would be good.

Thank you for your attention. A new paragraph 2.2 on the impact of different wastewater treatment technologies on the metal content of sludge has been introduced.

The authors should provide a comparison of metal content in the sludge they observed at the 4 treatment plants to what has been previously reported in other studies.

Thank you for your attention. Other authors who have performed speciation analysis of metal forms in sludge have obtained similar results, however, never the same, sludge is a heterogeneous material and will never obtain identical results. Our study focused on risk analysis through the use of risk indicators.

Use a capital P for “Pollution value” in the heading for Table 5 to be consistent with other tables.