Sustainable Solid Biofuel Production: Transforming Sewage Sludge and Pinus sp. Sawdust into Resources for the Circular Economy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The current manuscript provides a feasible way to covert the sewage sludge into resource. However, minor revision is needed before it can be accepted.

(1) Page 4, line 123, please provides the manufacturer, type of the hydraulic press, climate chamber

(2) Table 2, is the data refer to weight percent?

(3) Figure 1, there is a peak at 900 oC, please also explain the weight loss at the 900 oC

(4)Figure 4, the figure legend font is different with Figure 1,2,3, please also add error bar.

(5) Page 8, line 235, Tablea 3, typo.

 

Author Response

Reviewer A

All changes suggested by reviewer A are highlighted in red.

All changes in writing suggested by the reviewer were accepted.

• Page 4, line 123, please provides the manufacturer, type of the hydraulic press, climate chamber

The information has been added. The hydraulic press model is Marconi MA-098/CP0 and climate chamber is Marconi MA835/988ABCP

• Table 2, is the data refer to weight percent?

Yes, the table caption has been changed

“Table 2. Elemental composition of sewage sludge and Pinus sp sawdust in relation to dry mass”

(3) Figure 1, there is a peak at 900^oC, please also explain the weight loss at the 900^oC

(4)Figure 4, the figure legend font is different with Figure 1,2,3, please also add error bar.

(5) Page 8, line 235, Tablea 3, typo.

the error has been corrected (table 3)

Reviewer 2 Report

Comments and Suggestions for Authors

Comments for author File: Comments.docx

Comments on the Quality of English Language

Author Response

 

Reviewer B

All changes suggested by reviewer B are highlighted in green.

Major concerns

1. In the Experimental section, the authors used caliper to determine the dimensions of the briquettes. A more accurate method would have been to add each briquette to a graduated cylinder containing a know volume of water and then work out the dimensions of the briquettes by volume difference. This approach does not need the shape of the briquettes and is more accurate. The approach used by these authors assumes all briquettes are cylindrical in shape, but this is not necessarily true for all briquettes.

The use of the caliper was the best solution among the available options for this stage of the experiment. Diameter and height measurements were taken at various points to minimize possible errors.

 

2. The statistical analyses data presented in Tables 2 and 3 are ridiculous! For example, relative standard deviation values of about 50% (Table 2 H (%): 3.51 and standard deviation of 1.8) and another of over 1000% (Table 3 Ash (%): 0.531 and standard deviation of 5.6). How can these be? The authors need to seriously look at their analyses and fix these numbers: they do not make sense!

We apologize for the confusion. The work did not use standard deviation; instead, it utilized the coefficient of variation. This error has been corrected.

 

3. In section 3.3, the authors stated that the addition of sewage sludge (L) influenced the bulk density and energy density of the briquettes, but the contrary seems to be the case as shown in Figure 4. As L decreases (S increases) both bulk and energy densities decrease, which suggests that these parameters are strongly influenced by the %L. The authors need to clarify this point.

Although sewage sludge has a lower calorific value, its density is much higher, resulting in greater energy density. This information was highlighted to the manuscript.

 

4.Following from my point #4 above, the authors explained in the discussion section that high N content and ash content in the sewage sludge is a limiting factor in producing bioresource solid fuels. However, Figure 4 shows that lower sewage sludge content leads to lower energy density. How can you reconcile these two?

The choice of the best treatment in briquette production must consider a series of factors. High levels of ash, nitrogen, and sulfur make it impractical for large-scale use, despite its high energy density. Therefore, for environmental reasons, its proportion should be low in briquette production.

5.Also, the higher heating value obtained for pine sawdust compared to sewage sludge is inconsistent with the data presented in Figure 4, where decreasing the amount of sewage sludge causes a corresponding decrease in the energy density. The authors should explain/clarify this.

The energy density is a result of both the calorific value and the density of the material. Although the calorific value of wood is higher, its utilization significantly reduces the density of the briquette, resulting in lower energy density.

6. In the conclusion section, the authors recommend a 5% sewage sludge in briquettes. However, this recommendation will give fairly low energy density according to Figure 4. The authors need to justify this.

Although the addition of sawdust has reduced the energy density of the material, its values are acceptable for briquette trading. Furthermore, wood sawdust has low levels of N and S, which emit pollutants when burned. Therefore, for environmental reasons, the proportion of sawdust in briquette production should be high.

 

Minor comments

 

 

7. Line 2/61: Punctuation missing after the citation. Please correct this throughout the manuscript.

The suggestion was accepted, the changes are marked in green

8. Units of HCV in Table 3 should be MJ/Kg (j should be capitalized).

The suggestion was accepted, the changes are marked in green

9. Table 4: * and ** were not described. What do these refer to?

The suggestion was accepted, the '*' was removed because it was placed by mistake. The changes are marked in green

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

Your paper is overall of very high quality and provides significant insights into the characteristics of sewage sludge and Pinus sp. sawdust. However, I have noticed several areas where minor revisions are needed to improve the clarity and accuracy of your results and conclusions. Please pay attention to the following points:

Lines 70-74: Analyze how this work fits into the broader picture of the circular economy and sustainable development. Propose further research to explore how waste materials can be integrated into other industrial processes and how additional value can be created from waste.

Lines 75-77: Suggest further research to explore the potential application of the produced briquettes in various sectors, including industry, households, and agriculture. This could include analyzing energy efficiency, costs and benefits, and the environmental impact of using biofuels.

Lines 78-104: It seems that the details of the methodology are well-described. However, further consideration should be given to the impact of factors such as moisture, temperature, and processing time on the quality of the briquettes to ensure that experimental conditions are as standardized and reproducible as possible.

Lines 106-134: The description of briquette production is thoroughly presented. However, it is advisable to add more information about the raw material drying process before pressing and how this phase may affect the final quality of the briquettes. Additionally, it could be beneficial to provide a more detailed description of the measurement and analysis methods for determining the moisture content, density, and energy density of the briquettes.

Lines 209-212: Adding more detailed information about the mechanisms influencing energy generation related to carbon and hydrogen content and referencing relevant literature to strengthen the scientific foundation of the discussion.

Lines 215-218: Expanding the analysis of potential strategies for mitigating sulfur emissions and their impact on equipment corrosion and environmental pollution to deepen understanding in that area.

Lines 220-227: Providing additional insights into the mechanisms of nitrogen oxide formation and measures for its reduction to further enrich the analysis of nitrogen content implications for energy generation.

Lines 229-232: Offering a more detailed explanation of the significance of fixed carbon and volatile matter in relation to combustion behavior and energy yield.

Lines 233-240: Enriching the discussion on challenges related to ash disposal and methods for mitigating adverse effects on boiler operation and equipment longevity.

 

Lines 241-246: Including additional information on the variability of higher heating values within different biomass feedstocks and factors influencing these variations to deepen the scientific discussion .

Author Response

Reviewer C

All changes suggested by reviewer C are highlighted in gray.

Lines 70-74: Analyze how this work fits into the broader picture of the circular economy and sustainable development. Propose further research to explore how waste materials can be integrated into other industrial processes and how additional value can be created from waste.

This work fits into the context of the circular economy and sustainable development by proposing a model that utilizes recycled materials or industrial waste as raw materials. By doing so, it reduces the demand for virgin natural resources and minimizes the amount of waste sent to landfills, thus promoting environmental sustainability. Additionally, by integrating waste into industrial processes, the work demonstrates an approach that seeks to close the material lifecycle, maximizing its value over time and reducing the environmental impacts associated with the extraction and production of new materials.

Proposing further research to explore how waste materials can be integrated into other industrial processes and how additional value can be created from waste would be a natural extension of this work. Such research could include investigations into more efficient recycling techniques, the development of new materials from waste, optimization of energy recovery processes from waste, and studies of the economic feasibility of these approaches. Additionally, it would be important to consider the social and environmental aspects of the proposed solutions, ensuring that they not only generate economic value but also contribute to social inclusion and environmental preservation.

These informations were added in the manuscript

Lines 75-77: Suggest further research to explore the potential application of the produced briquettes in various sectors, including industry, households, and agriculture. This could include analyzing energy efficiency, costs and benefits, and the environmental impact of using biofuels.

These pieces of information are found in the paragraph between lines 74 and 82.

The paragraph has been rewritten, the changes are marked in gray.

“The utilization of sludge from Water Treatment Plants (WTP) for energy generation presents promising prospects; this idea fits into the context of the circular economy and sustainable development by proposing a model that utilizes recycled materials as raw materials. By doing so, it reduces the demand for virgin natural resources and minimizes the amount of waste sent to landfills, thus promoting environmental sustainability. nonetheless, its quality necessitates its incorporation into a blend with another raw mate-rial for the production of briquettes. Hence, the aim of this research endeavor was to con-tribute towards offering a sustainable and economically viable solution for the disposal of sewage sludge by crafting briquettes tailored for energy generation purposes. The over-arching expectation is that the outcomes derived from this study will bolster the feasibility of water treatment operations by harnessing the principal byproduct of the process and fostering an alternative avenue for the fabrication of environmentally-friendly solid biofuel.”

 

 

 

 

Lines 78-104: It seems that the details of the methodology are well-described. However, further consideration should be given to the impact of factors such as moisture, temperature, and processing time on the quality of the briquettes to ensure that experimental conditions are as standardized and reproducible as possible.

These parameters were standardized in the experiment to avoid interfering with the results, and this control is cited in the standards used.

 

Lines 106-134: The description of briquette production is thoroughly presented. However, it is advisable to add more information about the raw material drying process before pressing and how this phase may affect the final quality of the briquettes. Additionally, it could be beneficial to provide a more detailed description of the measurement and analysis methods for determining the moisture content, density, and energy density of the briquettes.

This information is described in the standards used. However, to facilitate understanding of the beds, we have also added some details to the text. The changes are marked in gray.

Lines 209-212: Adding more detailed information about the mechanisms influencing energy generation related to carbon and hydrogen content and referencing relevant literature to strengthen the scientific foundation of the discussion.

The paragraph has been rewritten

The carbon and hydrogen content is directly related to the calorific value of biomass [23,24] (table 2). Pine sawdust demonstrates superior quality in this regard, likely due to its favorable composition in these elements. While both carbon and hydrogen contribute positively to energy generation, hydrogen exhibits greater energy potential. Therefore, maintaining a lower C/H ratio is advantageous for maximizing the calorific value of bio-fuels [25]. Conversely, the higher oxygen content found in pine sawdust poses a drawback for energy generation purposes, as it diminishes the material's calorific value [26].

 

Lines 215-218: Expanding the analysis of potential strategies for mitigating sulfur emissions and their impact on equipment corrosion and environmental pollution to deepen understanding in that area.

The paragraph has been rewritten

Table 2 reveals that the sulfur content exhibited a higher concentration in sewage sludge compared to other materials. During combustion, sulfur undergoes a reaction with atmospheric oxygen, resulting in the production of sulfur oxides (SOx). These oxides pose significant environmental hazards due to their toxic nature. Additionally, they possess a relatively low condensation point, approximately 150°C, facilitating the formation of acidic compounds upon cooling. Consequently, these acids can instigate corrosion in equipment utilized for combustion processes. Recognizing the elevated sulfur levels in sewage sludge underscores the importance of implementing appropriate mitigation strategies to minimize the release of SOx into the environment and mitigate potential ad-verse effects on both human health and infrastructure integrity. Therefore, comprehensive measures, such as the production of briquettes by mixing sewage sludge with wood, a material with a low content of this element, should be adopted to reduce sulfur emissions, ensuring sustainable and environmentally responsible practices in energy generation processes

 

 

 

 

Lines 229-232: Offering a more detailed explanation of the significance of fixed carbon and volatile matter in relation to combustion behavior and energy yield.

The paragraph has been rewritten

Table 3 illustrates that the fixed carbon content in Pinus sp. wood is twofold higher than that found in sewage sludge. This metric signifies the proportion of material that combusts in a solid state, rendering it well-suited for energy production by facilitating sustained and consistent combustion [29]. Furthermore, Pinus sp. sawdust exhibits a su-perior volatile matter content compared to sewage sludge. Volatile matter combusts in a gaseous state, playing a crucial role in ignition during combustion processes. These find-ings underscore the advantageous properties of Pinus sp. wood, particularly in terms of its composition conducive to efficient energy generation. The elevated fixed carbon content ensures prolonged combustion, while the heightened volatile matter content enhances ig-nition efficiency, collectively contributing to the suitability of Pinus sp. wood as a viable feedstock for sustainable energy production endeavors [11].

The high ash content of sewage sludge compromises its use in briquette production (Table 3); this material is composed of minerals and does not combust, requiring removal after burning. This diminishes boiler operation time and causes equipment wear. There-fore, to make its use feasible, lower proportions of sewage sludge need to be used in bri-quettes [30]. Standards require that ash content in briquettes does not exceed 5%. Thus, the low ash content in pine sawdust allows briquettes to have approximately 8% sewage sludge in their composition. The high ash content was also reported for sewage sludge in China, between 33 and 62% [12].