Development of a Continuous Hydrothermal Treatment Process for Efficient Dewatering of Industrial Wastewater Sludge

Round 1

Reviewer 1 Report

The most prominent and attracting work of this work is the pilot scale experiment of hydro-thermo conversion of sludge and the discussion based on it. It provide more message about the case of bio-sludge and fiber residue from paper mill. However, I still think the method lacks innovation and the work cover only the experiment rang. So , I strongly suggested that authors can give more quantitive assessment about the process, such as tech-economical data comparing with bath operation and conventional processes.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript presents  the report from experimental studies performed in microclave, autoclave and then in pilot scale. The main aim of this research was to improve the dewaterability of the bio-sludge from paper industry. In my opinion it is a very interesting and valuable research with high application potential. The experimental part is supported by the standard and instrumental methods. The supporting material is quite important. My comments are as follows:

1. The highlight and graphical abstract are not provided, but they are not required by journal.

2. The abstract presents all discussed issues and is supported by some results, but is a bit too long too and could be more detailed.

3. The introduction part is concise and supported by 34 up-to-date references.

4. The material and methods, as well as the experimental part, are well presented.

5. The results section is well presented. The discussion is very interesting based on the results performed by different instrumental techniques, but is rarely supported by literature positions. I think this part should be improved. My detailed comments:

Please explain why you suggest that pH is related with better dewatered cakes?

I suggest to take into consideration the polarity index  (O + N)/C presented by Li et al. (https://doi.org/ 10.1016/j.biortech.2018.04.066.), Rutherford et al. (https://doi.org/10.1021/ES00026A014.) and Sliz et al.(

 https://doi.org/10.1016/j.wasman.2022.07.029 ) which correlates hydrophobicity of hydrothermally carbonized cakes with dewaterability.

6. The conclusion are informative but should give more detailed information about the study.

 In concluding, I recommend to publish it in after minor revision.

Author Response

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Reviewer 3 Report

This article exposes the scaling-up of a continuous HTC plant for treating paper sludge, showing the methodologies and the characterization of the products. The work is very interesting and well-presented. I suggest its acceptance after a minor revision. The following considerations could help in improving the quality of the work:

 

Introduction

 

·         Line 63. Please correct the number of autogenous pressure corresponding to the temperature you are referring to ( 1-5 MPa).

·         Lines 63-64. You should expand this sentence because the “shift at milder conditions” depends on the sector you are working with.

·         Lines 65-66. It would be interesting to specify the temperature at which TORWASH operates.

·         Lines 69-98. I suggest shortening this general section of HTC since it exposes very well-known concepts from the literature. You could focus on the most relevant part of your work.

 

Material and methods

 

·         Line 144. It would be interesting to add the exact operating conditions you are referring to.

·         Lines 150-154. I suggest expanding the explanation of the CO2 countercurrent flow since it is interesting.

·         You could add the details about the heating and cooling system you are adopting in the pilot-scale plant.

 

Results and discussion

 

·         Section 3.1. You could add a sentence that justifies the very high content of metals in the slurry.

·         Figure 3. I should format the figure in a nicer way (for example, remove the external border).

·         Table 7. I suggest properly formatting the table according to the journal style.

·         To add value to the work, I suggest adding a section with some general considerations on the final usage of the cake and the filtrate. Also, how can you deal with a so high quantity of metals?

 

·         As a general suggestion, have you thought of increasing the exiting temperature to further improve the dewaterability of the slurry?    

Author Response

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Author Response File: Author Response.pdf