Microwave Treatment of Three Different Types of Sewage Sludge Based on Their Solar Drying Exposure Time: Effect on Microorganisms, Water Content and Agronomic Aspects

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for submitting the manuscript. Please address the following points for quality enhancement. 

1. Update the references. References from 2012 till 2018 should be revised as new research should reflect in your literature. 

2. Mass transfer analysis for different microwave conditions should be elaborated. 

3. Elaborate in methodology section the reason for selecting microwave conditions. A sludge is of complex nature. 

4. Explain the role of pH effecting the MW process as sludge A and C are quite different in nature. 

5. Microwave is fairly good technology, however, how authors can justify its suitability for large scale operations as it is energy intensive process. 

Author Response

Please see the attachment. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript titled: “Microwave treatment of three different types of sewage sludge based on their solar drying exposure time: effect on microorganisms, water content and agronomic aspects” by González-López et al examines the effects of microwave (MW) irradiation on sewage sludge, with particular attention to fecal pollutants, agronomic nutrient concentrations, and microbiological composition. The authors provide a thorough and organized analysis, describing the outcomes of many experimental setups and including pertinent references for background information. Important insights are gained from the examination of E. Coli levels, total coliforms, and the effectiveness of MW treatment for fecal pollutants. The results are presented in an understandable and structured manner. A comprehensive grasp of the technology's consequences is further enhanced by the investigation of MW effects on the carbon and nitrogen content of sludge and the comparative analysis of agronomic nutrients. The manuscript makes an excellent attempt to explore possible ramifications for agricultural usage and wastewater treatment.

Although the overall review is positive, several suggestions are included to improve the effect of the work. These include refining statistical analyses, providing more context, and expanding on specific findings. All things considered, the study makes a substantial addition to the area and, with minor adjustments, might greatly expand our knowledge of MW treatment in sewage sludge management.

Here are my constructive comments/suggestions:

Introduction

Please add some up-to-date references for sludge dehydration or sludge dewatering between lines 61 – 65. This is an important key for the manuscript. Please refer to the following suggestions:

·        https://doi.org/10.1021/acs.iecr.3c01553

·        https://doi.org/10.1007/s13399-022-03480-3

·        https://doi.org/10.1016/j.watres.2020.115912

Section 3.1.1

·        Explanation of the possible causes of E. Coli development in sludge A following a 15-day dehydration period would be beneficial. A brief discussion on how this proliferation affects future treatment procedures or agricultural usage would be appreciated.

Section 3.1.2

·        Consider highlighting the importance of these results for sludge management and provide further context on the observed variances in total coliform eradication after 15 days of drying.

Section 3.6

·        The insightful explanation relates to the temperature at which organic stuff begins to decompose. It would be helpful to elaborate on how this threshold corresponds with the MW treatment settings used in the investigation though. This would offer a more thorough comprehension of the experimental design and its consequences.

·        A short comparison or discussion of any findings that differ between the present study and the cited work, taking into account the reference to Mawioo et al. (2017), might improve the interpretation. For instance, are there any divergent results vs any published work in the literature? This would make it easier for readers to understand the subtleties and originality of the current research concerning earlier works.

Section 3.7

·        The practical consequences of the uniformity in agronomic nutrient concentrations before and after treatment should be further discussed by the authors. Consider the possible advantages or disadvantages of keeping certain nutrient levels for use in agriculture or the environment.

·        If there are any standards for agronomic nutrient levels in sludge intended for agricultural use, mentioning and discussing these guidelines would give readers important background information.

Conclusion

·        Any futuristic research attempts in this field, i.e., what is next? Consider adding 2 lines addressing your ongoing or future work.  

Technical Comment

 

·        Please adjust (*) to (×), also replace (,) with (.) in the scientific numbers for the whole manuscript. Example: the number in line 30: 46,27 * 10⁵ should appear as 46.27 × 10⁵ and so on. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This research investigates the application of microwave (MW) irradiation for sewage sludge treatment at the laboratory scale. The primary objective is to explore the impact of MW irradiation on microorganisms, water content, organic matter, and agronomic nutrients present in diverse types of sewage sludge obtained from a full-scale wastewater treatment plant (WWTP). The study addresses a significant and interesting topic for researchers in the fields of environmental science and wastewater treatment. However, certain improvements are necessary before publication. The manuscript would benefit from a major revision based on the following comments:

 

Main Comments:

Methodology

Chapter 2.1:

• To enhance comprehension of the experiment's course and results, as well as facilitate future utilization by other researchers, it is advisable to provide information about the municipal wastewater treatment plant generating the sludge. Include details such as location, sources of inflow (urban, industrial, etc.), treatment technology, pollutant load, disposal of treated wastewater, and handling of sludge.

Chapter 2.3:

• The manuscript lacks information about the manufacturer and country of origin of the ETHOS X MW device.

Chapter 2.3:

• The authors specify the application of four MW radiation powers: 555 Watts/g, 955 Watts/g, 1355 Watts/g, and 1500 Watts/g; and four different temperatures: 55, 75, 95, and 105 ºC. Given that MW radiation induces temperature increase and, if the container is closed, pressure as well, it seems that the specified temperatures were also achieved using MW radiation. This raises ambiguity; clarification is needed. For each variant, both the applied MW radiation power and the resulting temperature should be provided. Additionally, consider including a schematic representation of the experiments for better interpretation by other scientists.

Simultaneously, the authors state that microbiological results are presented for two variants: 75 °C - 1355W and 95 °C - 1355W. This is unclear. Results for other variants should either be included in the article's supplement or, if not,  these information should be specified in  the abstract to avoid misleading readers.

Chapter 2.4:

• Specify the applied physicochemical analysis methods.

Chapter 2.7:

• Provide information about the manufacturer and country of origin for the ACCULAR ATILON device. Were the studies conducted in replicates? If so, disclose the variability of the obtained results, e.g., by adding standard deviation values (Table 3; Fig. 3-4, 6-7, and in the text).

Additional Methodological Comments:

• In the supplement, the labeling of samples is inconsistent with the manuscript. This needs correction.

Results and Discussion:

• The manuscript lacks references to results obtained through alternative methods of sewage sludge management, such as methane fermentation, lime application for hygienization, drying, and incineration. It is crucial to address these methods.

Additional Comments:

• Format citations according to the journal's guidelines.
• Use a period as the decimal separator throughout the manuscript, including tables and figures; currently, a comma is used as the decimal separator.
• The text requires editorial corrections, including font size adjustments (e.g., line 238: "log.."), removal of extra spaces (e.g., line 235), and the addition of superscript indices (e.g., lines 310-312 for "-1").

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Please follow the Guide for Authors.

In my opinion, it is advisable to improve language.

15,18 – I do not recommend using abbreviations in Abstract

23 – “105 ℃” – no space.

105 “So, the objective of this work is” – I suggest using past tenses throughout the work.

129 "The ETHOS X MW (Figure 1) is one of the new MWs available on the market" - in my opinion, this is not a good thing to write.

137: „MW (MW) device” – 2 x MW?

149 "ºC" – please remove the underline

165 "E. Coli” – Apply italic text throughout.

198 "3.2 * 104 ± 4 *" Replace stars with dots throughout.

339 – improve quality Figure 7.

339 -I do not recommend finishing the Figure chapter.

Comments on the Quality of English Language

I suggest to improve language. 

Author Response

Please see the attachment. 

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

Reading the text allows the reader to formulate the following comments and objections:

 

1.     The text presents the results of several different experiments with many variables in a very chaotic way, which the authors did not fully control.

2.     The methodology of the conducted research is described very poorly and illegibly and does not make it possible for another team to reproduce the course of the research.

3.     It is not known why the authors performed some tests: e.g. in L146-148, it is written that "Four specific powers were used for this treatment: 555 Watts/g, 955 Watts/g, 1355 146 Watts/g, and 1500 Watts/g, with a constant frequency of 2.45 GHz for varying durations 147 ranging from 1 to 6 minutes" but nowhere are the results of the influence of different temperatures or processing times presented.

4.     Although the authors included a short list of symbols and abbreviations (L36-44), they left many unexplained or undeveloped abbreviations and symbols in the text (e.g. Eh, TS, ulog, n, or UFC, CFU - the latter are used interchangeably in the text).

5.     Description "MW device" from L129-135 does not contain any functional features but looks like an advertising brochure – just like Figure 1, which adds no value to the text.

6.     Lack of measurement methodology and equipment used in testing physicochemical properties from Chapter 2.4 (L158-163)

7.     The paragraph from chapter 2.7 L189-193 is a substantive error because, contrary to what the authors wrote, the difference in the mass of the sample before and after processing does not determine "mass of the actual sludge sample without water or moisture" but precisely the moisture content in the sample.

8.     In L208, the value of the determination was confused with the uncertainty of its determination.

9.     In Table 1, under "Standard deviation", the designations "pH" and "Cond" were omitted, while these values ​​are presented in Table 3.

10.  The inaccuracy values ​​of the UFC and Ulog size determinations from Table 2 are greater than those ​​of the determinations themselves and by at least one order! So, how useful could be measured values whose inaccuracy is ten times greater than their values!

11.  What do chapters 3.1.2, 3.1.3 and 3.2 add substantively, apart from repeating the values ​​from tables 2 and 3?

12.  How were graphs made from Figure 5a-5d? No measurement points are marked on them, only a dashed line (!) Does this line combine measurement data or approximate it?

13.  Charts from Figure 6a and 6b are methodologically incorrect because the description of the OX axis suggests something completely different than what is on the chart, no values ​​are presented on this axis, and the legend used only obscures the content of these charts.

14.  Instead of a conclusion, one (!) summary paragraph was presented. There is no analysis of the influence of solar and microwave treatment parameters (as the title suggests), synthetic conclusions, a description of the significance of the presented research, and the application of the obtained results.

 

To sum up:

The text is chaotic, contains rudimentary methodology, almost no description of the research equipment used, and the results are burdened with measurement uncertainties one order of magnitude higher than their values. The results are poorly described, misrepresented and misinterpreted. There is no conclusion but only a short summary.

Such a text is not suitable for publication.

 

Comments on the Quality of English Language

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Author Response

Please see the attachment. 

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

After reviewing the changes made by the authors, I have decided that the article can be accepted for publication.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have partly made changes. I request that they re-check and correct the publications. 

Examples of errors:

- unnecessary spaces in the Abbreviations section

- 14 notations need to be changed 

- 162 unit notations "Watts/g,"

- 164 , 167 "ºC" - why is the character underlined "-"?

- 181 "105°C" - once there is a space, other times there is no space. 

- 269 different table format e.g. Table S2 and Table S3.

I suggest you check and improve the paper in the English language context. 

Comments on the Quality of English Language

I suggest to improve language.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

The text has been significantly improved, and only two points (1 and 3) currently require significant clarification/improvement:

 

1.     Figure (previously 5, currently S4):
According to the authors, was the water content 100% at time t=0? (sic!)

2.     Moreover (the same Figure):

There are still only dashed lines without measurement points in the drawing - the points must be marked.

3.     In Table (previously 2, now S2), the authors have swapped the measurement values ​​with their inaccuracy values (standard deviations?)  -  the latter were larger than the former - currently, after the swapping, there are still cases when the values ​​​​are greater than their inaccuracy values. A standard deviation greater than the mean raises eyebrows for a group of positive values. Especially since there are only 12 measurement values, maybe it is worth supplementary materials to provide a set of these data.

4.     Why did all Figures and Tables receive the S prefix? (e.g. Figure 1S, Table 1S). Such prefixes suggest that they are supplementary material.

 

Comments on the Quality of English Language

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Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 3

Reviewer 4 Report

Comments and Suggestions for Authors

The authors only followed some of my comments. I suggest making further changes before the paper will be published.

Comments on the Quality of English Language

I recommend checking the English language again.

Reviewer 5 Report

Comments and Suggestions for Authors

Now that the suggested corrections and improvements have been made, the text is in a publishable form and content.
I accept the text in its present form.

Comments on the Quality of English Language

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