Thermochemical Characteristics of Anaerobic Dairy Digestate and Its Pyrolysis Conversion for Producing Porous Carbon Materials

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The document is a well-written manuscript regarding a pyrolysis study of digestate, providing insight into this valorization option.

The document should take better advantage of the work performed by comparing the present results with other char-type materials derived from lignocellulosic biomass and reported by other authors.

Please highlight the urgent need to find a solution for processing a large amount of digestates throughout the year.

The following are minor corrections to the document.

1. Please add to the introduction section a brief description of the amount of digestate produced annually by a biogas plant (use suitable references to support the statement)
2. L176-180: This information is better placed in Table 1 since it is part of digestate characterization.
3. If authors performed several heating rates, please apply non-isothermal kinetic analysis to provide data to the literature regarding activation energy.
4. Table 2 should follow the journal's standard. Please add as a footnote the meaning of the "h" call, which is missing.
5. Figures 3, 4, and 5 should be reduced in size and presented in a single figure using a), b), and c) calls, so it would be easy for the reader to have a general portrait while reading the document.
6. From the methodological description, it appears that the authors performed elemental analysis on the different char samples from digestates obtained at various temperatures. If so, please add this information to a table, rather than reporting only for a single sample (L272-274).
7.  Compared the results obtained with those reported by different authors when performing pyrolysis of lignocellulosic biomass so the reader can have a clear picture of char material derived from digestate and the differences expected with the one from lignocellulosic biomass.
8. Please add a deeper analysis to the results and discussion section regarding the possible applications of the char material obtained, given its limitation of having a high ash content. Please indicate what authors consider to be the most suitable applications of this material, given the characteristics evaluated.

Author Response

Q1. Please add to the introduction section a brief description of the amount of digestate produced annually by a biogas plant (use suitable references to support the statement)

Reply: We thank the reviewer for this excellent suggestion.  The brief information about the amount of digestate produced annually by a biogas plant has been added to the first paragraph of the Introduction.

“…………. It was reported that an estimated 10,000 tonnes of solid digestate will be annually generated from a biogas plant with an installed capacity of 500 kW [3]. ……. ”

 

Q2. L176-180: This information is better placed in Table 1 since it is part of digestate characterization.

Reply: As suggested by the reviewer, the data on the elemental compositions by the EDS analysis (L.176-180) has been incorporated into Table 1.

 

Q3. If authors performed several heating rates, please apply non-isothermal kinetic analysis to provide data to the literature regarding activation energy.

Reply: In this work, we performed the TGA tests under four heating rates (i.e., 5, 10, 15, and 20°C/min, which aimed at evaluating the pyrolysis conditions based on the thermal decomposition variations.  Regarding the non-isothermal kinetic analysis for determining activation energy, it can be obtained from the model-free isoconveersional approaches like Flynn-Wall-Ozawa method, but we did not perform these calculations due to out of the study scope.

 

Q4. Table 2 should follow the journal's standard. Please add as a footnote the meaning of the "h" call, which is missing.

Reply: As suggested by the reviewer, Table 2 has been rearranged to follow the journal's standard and also make it readable.

 

Q5. Figures 3, 4, and 5 should be reduced in size and presented in a single figure using a), b), and c) calls, so it would be easy for the reader to have a general portrait while reading the document.

Reply: As suggested by the reviewer, Figures 3, 4, and 5 have been combined into a single figure (Figure 3) using (a), (b), and (c).   

 

Q6. From the methodological description, it appears that the authors performed elemental analysis on the different char samples from digestates obtained at various temperatures. If so, please add this information to a table, rather than reporting only for a single sample (L272-274).

Reply: A new table (Table 3) has been added to summarize the elemental compositions (by EDS analysis) of resulting biochars obtained at various temperatures.

 

Q7. Compared the results obtained with those reported by different authors when performing pyrolysis of lignocellulosic biomass so the reader can have a clear picture of char material derived from digestate and the differences expected with the one from lignocellulosic biomass.

Reply: We thank the reviewer for this excellent suggestion.  The discussions on the pore properties of digestate-based biochars in this work and other results reported by different authors when performing pyrolysis of dairy- digestate have been added to the Sec. 3.2.

“……  On the other hand, the carbon contents of resulting digestate-based biochars were significantly lower than those (typically ranging from about 50 wt% to nearly 90 wt%) produced from other biomass-based biochars [37], reflecting that the anaerobic digestate contained high contents of inorganic elements.”

 

Q8. Please add a deeper analysis to the results and discussion section regarding the possible applications of the char material obtained, given its limitation of having a high ash content. Please indicate what authors consider to be the most suitable applications of this material, given the characteristics evaluated.

Reply: We thank the reviewer for this excellent suggestion.  The discussions on the possible applications of resulting digestate-based biochars with high ash content have been added to the Sec. 3.3.

“…….. Accordingly, the surface of digestate-derived biochar was inferred to carry a net negative charge at pH values above this point, thereby enhancing its adsorption capacity for cationic species (e.g., heavy metal ions, methylene blue dye) through electrostatic attraction [39, 40]. In contrast, the surface modifications of resulting digestate-based biochar material for enhancing the affinity of anionic (e.g., acid red 18 dye) or acidic species (e.g., CO₂) may be achieved by doping nitrogen-containing compounds (e.g., aniline) as the nitrogen atoms will provide positive charges or binding sites [41, 42]. When reusing the digestate-based biochar as an adsorbent, it may be deashed to enhance its adsorption capacity.”

Author Response File: Author Response.doc

Reviewer 2 Report

Comments and Suggestions for Authors

In this work, a slurry digestate was employed as a novel precursor to produce porous biochar materials through pyrolysis at high temperatures. I would consider this paper to be accepted after the authors have addressed the following points to improve the manuscript.

1.What are the possible effects of various dairy feedstock or digestion conditions to the carbon content and structure of the digestate?

2. Can the carbon material produce from this study be further tailored for specific applications? Please give more details and include it in the introduction or discussion section.

3. How does the produced carbon from dairy digestate set apart from other biomass sources? Describe its unique characteristics from other biomass sources.

4. Does the pyrolysis process employed in this study more beneficial in terms of lower environmental impact and production cost as compared to conventional disposal methods?

5. Considering the zeta potential measurements, what alternative strategies can be employed to modify the surface charge of the carbon material to enhance its affinity for anionic species? please add discussion if possible.

Author Response

Q1. What are the possible effects of various dairy feedstock or digestion conditions to the carbon content and structure of the digestate?

Reply: We thank the reviewer for this excellent suggestion.  The description about the possible effects of various dairy feedstock or digestion conditions to the carbon content and structure of the digestate has been added to the second paragraph of the Sec. 3.1.

“….. Table 1 also listed the preliminary values of elemental compositions for the dried digestate, revealing substantial amounts of carbon (40.93 wt%) and oxygen (38.85 wt%), along with notable concentrations of inorganic elements like calcium (8.31 wt%) and silicon (2.58 wt%). It should be noted that the diversified values and high ash content in Table 1 were related to the dairy feedstocks and digestion conditions. For example, the carbon content of anaerobic digestate will be reduced when extending the AD duration, leading to more carbon fractions being converting into methane. Thus, these results ..……..”

 

Q2. Can the carbon material produce from this study be further tailored for specific applications? Please give more details and include it in the introduction or discussion section.

Reply: As suggested by the reviewer, the discussion on the potential applications of resulting digestate-based biochars with high ash content has been added to the Sec. 3.3.

“…….. Accordingly, the surface of digestate-derived biochar was inferred to carry a net negative charge at pH values above this point, thereby enhancing its adsorption capacity for cationic species (e.g., heavy metal ions, methylene blue dye) through electrostatic attraction [39, 40]. In contrast, the surface modifications of resulting digestate-based biochar material for enhancing the affinity of anionic (e.g., acid red 18 dye) or acidic species (e.g., CO₂) may be achieved by doping nitrogen-containing compounds (e.g., aniline) as the nitrogen atoms will provide positive charges or binding sites [41, 42]. When reusing the digestate-based biochar as an adsorbent, it may be deashed to enhance its adsorption capacity.”

 

Q3. How does the produced carbon from dairy digestate set apart from other biomass sources? Describe its unique characteristics from other biomass sources.

Reply: We thank the reviewer for this excellent suggestion.  The discussion on the difference between the carbon contents of digestate-based biochars and other biomass-based biochars has been added to the second paragraph of the Sec. 3.3.

“……  On the other hand, the carbon contents of resulting digestate-based biochars were significantly lower than those (typically ranging from about 50 wt% to nearly 90 wt%) produced from other biomass-based biochars [37], reflecting that the anaerobic digestate contained high contents of inorganic elements.”

 

Q4. Does the pyrolysis process employed in this study more beneficial in terms of lower environmental impact and production cost as compared to conventional disposal methods?

Reply: We thank the reviewer for this excellent suggestion.  Regarding the beneficial features of pyrolysis process employed in this study as compared to conventional disposal methods, they have been described in the first paragraph of the Introduction.

“…………Among the available valorization strategies, pyrolysis, one of thermochemical processes, offers significant benefits over conventional disposal methods (e.g., landfilling, incineration) by converting AD-derived digestate into valuable products, thus reducing environmental impacts GHG emissions and water contamination. Therefore, the production of porous biochar materials through thermochemical processes has been identified as a promising option. ……”

 

Q5. Considering the zeta potential measurements, what alternative strategies can be employed to modify the surface charge of the carbon material to enhance its affinity for anionic species? please add discussion if possible.

Reply: As suggested by the reviewer, the discussion on the surface modifications of resulting biochar material for enhancing the affinity of anionic species has been added to the last paragraph of the Sec. 3.3.

“………………… In contrast, the surface modifications of resulting digestate-based biochar material for enhancing the affinity of anionic or acidic species may be achieved by doping nitrogen-containing compounds (e.g., aniline) as the nitrogen atoms will provide positive charges or binding sites [41, 42]. …..”

Author Response File: Author Response.doc

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have thoroughly addressed all the questions and made thoughtful revisions to the manuscript, resulting in a significantly improved version. I believe it is now suitable for publication.

Author Response

Comment 1: The authors have thoroughly addressed all the questions and made thoughtful revisions to the manuscript, resulting in a significantly improved version. I believe it is now suitable for publication.

 

Response 1: Thanks for the reviewer's positive comments.