Utilization of Sewage Sludge in the Sustainable Manufacturing of Ceramic Bricks

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article is interesting, although it presents very little new scientific information, as this type of raw material is often used in the production of ceramic products.

I have a few main remarks that, in my opinion, should be addressed before the article can be considered for publication.

 

1. Such types of waste often contain significant amounts of heavy metals. However, the authors did not provide any data on the heavy metal content in the Sewage Sludge, despite discussing this issue in the literature section. In my opinion, it would also be important to conduct leaching tests of heavy metals from the ceramic products to ensure that the final construction product does not pose a risk to human health or the environment.
2. It is not necessary to provide well-known standard formulas, such as those for density, water absorption, etc. A reference to the relevant standard is sufficient.
3. It is very important to assess the plasticity of the clay and how it changes when Sewage Sludge is introduced into the clay mases (Atterberg limits).
4. The labeling of the forming mixtures is incorrect. It would be clearer to label them as 100:0, 95:5, etc. Mixture 90.00 (How to understand)?
5. The confidence intervals for the mechanical properties are very large, especially for clay. This is surprising, as well-formed and properly fired clay products usually exhibit relatively stable mechanical and physical properties.
6. Could the authors clarify how many specimens were tested per mixing mass? This information is important to assess the reliability of the presented results.
7. Please ask the authors to explain why the porosity indicator significantly decreases when 10% Sewage Sludge is introduced into the forming mass?

8.At the end of the introduction section, the authors should explain the scientific problem of using SS (sewage sludge) and clarify how this research differs from other studies. There must be an element of novelty.

9. The authors should significantly expand the discussion section. Currently, there is a lack of in-depth analysis explaining why the investigated properties change, as well as what structural (macro- and micro-level) transformations may have influenced the observed results. It is also essential to compare the obtained data with findings from other researchers, highlighting key similarities and differences. Without such a comparative approach and deeper interpretation, the results appear to be presented in a declarative manner, lacking sufficient scientific reasoning. To meet the quality standards of a peer-reviewed scientific journal, I strongly recommend substantially broadening the discussion section.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript tried to clarify the potential of sewage sludge as an alternative raw material in the production of clay-based ceramic bricks. The optimized addition of 10wt% sewage sludge provided the best value, maintaining a good level of compressive strength, water adsorption and density. However, the manuscript significantly lacks novelty and scientific insights. I don’t think it can be accepted by Sustainability. The issues need to be concerned are below.
1.    In this paper, there is no data about the frost resistance of ceramic bricks with varying sludge content.

2． Although the authors concluded in the Introduction section that “the heavy metals contained in the sewage sludge are not leached from the product”, it should be better to carry out the tests about the leaching of heavy metals, especially the harmful metals to the environment, such as Cr, Hg, et al.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

• lines 195-198: superfluous information;
• table 2: various errors including the incorrect formula for titanium oxide and sodium oxide repeated twice with different quantities;
• lines 269-271: same data repeated several times as in line 262;
• line 277: incorrect reference figure number;
• line 288: the description of the images in the caption appears to be reversed;
• in the laboratory results, was a release test carried out to verify the leachability of heavy metals present in the sludge?

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors All the concerns raised by the reviewers have been adequately addressed by the authors, and the manuscript is now suitable for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

Based on the revisions made, in my opinion, the article could be published.