Recent Developments, Challenges, and Environmental Benefits of Using Hermetia illucens for Bioenergy Production Within a Circular Economy Approach

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper presents valuable research on an innovative approach to bioenergy production, but there are several areas that could be improved:

1. Clarify Research Objectives: The introduction could more clearly articulate specific research questions and objectives. While general aims are mentioned, adding explicit research questions would strengthen the paper.
2. Experimental Design: The methodology section for the larvae rearing experiments could be more systematically presented. Consider using standardized formats for experimental designs, including clearer identification of independent and dependent variables.
3. Statistical Analysis: While statistical methods are mentioned, the paper would benefit from more detailed explanation of how statistical significance was determined, particularly for differences in larval weight and substrate consumption.
4. Process Design Integration: The connection between the experimental findings and the process design could be strengthened. It's not entirely clear how the specific results from the BSF experiments informed the parameters of the integrated process design.
5. Economic Feasibility: The paper acknowledges the need for future techno-economic analyses but could benefit from preliminary economic indicators or comparison with conventional bioenergy systems.
6. Regulatory Discussion: While regulatory constraints are mentioned, a more detailed discussion of specific regulatory pathways or potential policy recommendations would enhance the impact of the paper.
7. Tables and Figures: Some tables (particularly Table 1) are difficult to read due to formatting. Consider redesigning for better clarity, possibly splitting into multiple tables if needed.
8. Conclusions: The conclusions could more explicitly address the limitations of the current approach and provide more specific guidance for future research directions.

The paper presents innovative research with significant potential for advancing circular bioeconomy applications, but addressing these areas would strengthen its contribution to the field.

Author Response

COMMENT 1 [The introduction could more clearly articulate specific research questions and objectives. While general aims are mentioned, adding explicit research questions would strengthen the paper.]
RESPONSE 1 [Thank you for the suggestion, which helped us clarify the objectives by structuring them into three research questions outlined at the end of the introduction (Page 5, lines 182-208).]

COMMENT 2 [The methodology section for the larvae rearing experiments could be more systematically presented. Consider using standardized formats for experimental designs, including clearer identification of independent and dependent variables.]
RESPONSE 2 [Thank you for the suggestion. We have adjusted the section of experimental design (“2.2.1 Diets and experimental design”), splitting the description for each experiment and clearly declaring variables (Pages 7-8, lines 208-344).]

COMMENT 3 [While statistical methods are mentioned, the paper would benefit from more detailed explanation of how statistical significance was determined, particularly for differences in larval weight and substrate consumption.]
RESPONSE 3 [Thank you for the suggestion. We adjusted the statistical analysis section. As reported, a standard level of 0.05 was chosen to accept statistically significant differences among treatments for final weight of larvae and substrate (Page 8, lines 349-355).]

COMMENT 4 [The connection between the experimental findings and the process design could be strengthened. It's not entirely clear how the specific results from the BSF experiments informed the parameters of the integrated process design.]
RESPONSE 4 [We thank the reviewer for the useful remark. At the present stage of investigation, the design leverages on a larvae-based feedstock whose composition has been taken from the literature. Further experimental activity is required to (i) confirm the composition of the larvae-based feedstock fed with ham residues; (ii) assess the performance of the anaerobic digestion process with the new larvae-based feedstock. This activity is scheduled and will be published upon availability of data and consequent performance review. A dedicated discussion has been added at the end of Paragraph 3.3 and in the conclusion paragraph (Page 22, lines 708-718; Page 23, lines 756-759).]

COMMENT 5 [The paper acknowledges the need for future techno-economic analyses but could benefit from preliminary economic indicators or comparison with conventional bioenergy systems.]
RESPONSE 5 [We appreciate the suggestion, which allowed us to acknowledge in the conclusions of the current work that the economic dimension was not addressed and thus constitutes a limitation and a potential area for future research (Page 24, lines 777-780).]

COMMENT 6 [While regulatory constraints are mentioned, a more detailed discussion of specific regulatory pathways or potential policy recommendations would enhance the impact of the paper.]
RESPONSE 6 [Thank you for your comment. This is a critical aspect, and legal constraints are difficult to overcome. We have added a new paragraph and the end of the result section to discuss this topic (Pages 22-23, lines 720-729).]

COMMENT 7 [Some tables (particularly Table 1) are difficult to read due to formatting. Consider redesigning for better clarity, possibly splitting into multiple tables if needed.]
RESPONSE 7 [Thank you for the suggestion, but each entry in table 1 refers to a specific paper, and we prefer not to split information in different tables. We have reduced the height of the figures to avoid splitting of tables across different pages. We also split table 5 in two different tables to better organize the formatting of the manuscript (Figure 3 – Page 12, line 454; Figure 4 – Page 14, line 491; Figure 5 – Page 15, line 522; Table 5 – Page 20, line 667; Table 6 – Page 21, line 674).]

COMMENT 8 [The conclusions could more explicitly address the limitations of the current approach and provide more specific guidance for future research directions.
RESPONSE 8 [Thank you, we improved the conclusions in order to address the limitations explicitly (Pages 23-24, lines 764-776).]

Reviewer 2 Report

Comments and Suggestions for Authors

This study proposes a novel integrated biorefinery scheme for fat-rich food waste, combining black soldier fly (Hermetia illucens) larvae, anaerobic digestion (AD) and hydrothermal carbonization (HTC). The aim of this work is to enhance biogas yield and mitigate the inhibitory effect of lipids in AD systems. Bioconversion experiments using larvae resulted in efficient fat removal and enhanced potential for protein and biomass recovery. The integration of AD and HTC enables thermal self-sufficiency, increased energy efficiency and improved digestate dewatering. Recycling of HTC process water in the AD unit was evaluated considering its acidic nature and trading in biomethane production. A thermally integrated scheme is proposed that achieves efficient heat exchange and reduces energy costs. The system can produce various products including biogas, hydrochar and larval biomass, providing a sustainable pathway to achieve a circular economy. The conducted research provides opportunities for the creation of a synergistic process chain that maximizes energy recovery and resource efficiency from waste streams.

 

However, the work has the following comments:

1. The article lacks a detailed comparison of the proposed integrated approach (bioconversion with Hermetia illucens, anaerobic digestion, hydrothermal carbonization) with existing methods of processing fat-containing waste. It is necessary to add a table or section comparing the efficiency, economic and environmental indicators of different technologies.
2. Section "2.2 BSFL rearing experiments" does not contain enough information to conduct experiments. It is necessary to specify the breed, the conditions of their placement and maintenance, the characteristics of the equipment used (e.g. container sizes, ventilation type).
3. This article is devoted to the application of circular activity restriction in the field of waste processing, but the main focus of the economics is on food waste and bioenergy. As an example, we could consider mining waste (https://doi.org/10.1016/j.resourpol.2025.105513, https://doi.org/10.1016/j.rcradv.2024.200224). What they have in common is the desire for a closed production cycle and minimization of waste generation. The article proposes to use Hermetia illucens larvae for waste pre-treatment. The presented solution is aimed at optimizing the composition for further processing or disposal of waste, turning it into secondary resources for the production of bioenergy and other products, while minimizing the environmental impact. In some cases, activation by larvae can significantly improve the properties of the final product.
4. 4. Section 3.3 (provides estimated data on biomethane production (226 l/kg VS) and hydrochar (0.257 kg/kg waste). However, there is no information on the errors or variability of these parameters. It is necessary to clarify whether these values are average or obtained as a result of a single experiment.
5. Pre-treated meat waste and wheat were used in the experiments with Hermetia illucens larvae. However, the criteria for choosing these components and their proportions are not specified (e.g., why a ratio of 1:2 was chosen and not other options).
6. IBM SPSS Statistics was used for statistical analysis, but the version and parameters of the analysis are not specified (e.g., the significance level for the post-hoc test). It is also not described how the integrated process modeling was carried out (e.g., whether specialized programs such as Aspen Plus were used).
7. Section 2.2.2 mentions the use of Testo 174H temperature sensors, but there is no information on their accuracy and calibration. This is important for the assessment reliability of temperature and humidity data.
8. The experiments with larvae do not indicate whether control measurements of substrate pH and salinity were made after pre-treatment. These parameters are critical for interpreting the larval survival results.
9. Table 7 provides data on the energy consumption of the process, but does not provide a comparison with similar systems. For example, how much energy is spent on traditional methods of processing fat-containing waste?
10. Section 3.2 describes experiments with different proportions of substrates, but does not indicate how many replicates were performed for each condition. This is important for assessing the statistical significance of the results.
11. The article mentions legislative barriers to the use of larvae in waste processing, but does not analyze how these restrictions may affect the scaling of the technology.

Author Response

COMMENT 1 [The article lacks a detailed comparison of the proposed integrated approach (bioconversion with Hermetia illucens, anaerobic digestion, hydrothermal carbonization) with existing methods of processing fat-containing waste. It is necessary to add a table or section comparing the efficiency, economic and environmental indicators of different technologies.]
RESPONSE 1 [Thank you for the comment. We have clarified the objectives of the work to emphasise that the goal of this study is to assess whether larval digestion has the potential to be integrated into a biorefinery process, exploring pathways that are not yet present in the literature. A comparison of all possible valorisation routes for residual substrates with high lipid content is therefore outside the scope of the study, and this has been clarified in the text; however, we thank the reviewer for the recommendation, which will be investigated in future works already planned in the funding project (Page 5, lines 182-208).]

COMMENT 2 [Section "2.2 BSFL rearing experiments" does not contain enough information to conduct experiments. It is necessary to specify the breed, the conditions of their placement and maintenance, the characteristics of the equipment used (e.g. container sizes, ventilation type).]
RESPONSE 2 [Thank you for your suggestion. We adjusted the text of the section “2.2 BSFL rearing experiments" to better explain rearing condition of the Hermetia illucens strain used in the experiments (Pages 6-7, lines 256-275).]

COMMENT 3 [This article is devoted to the application of circular activity restriction in the field of waste processing, but the main focus of the economics is on food waste and bioenergy. As an example, we could consider mining waste (https://doi.org/10.1016/j.resourpol.2025.105513, https://doi.org/10.1016/j.rcradv.2024.200224). What they have in common is the desire for a closed production cycle and minimization of waste generation. The article proposes to use Hermetia illucens larvae for waste pre-treatment. The presented solution is aimed at optimizing the composition for further processing or disposal of waste, turning it into secondary resources for the production of bioenergy and other products, while minimizing the environmental impact. In some cases, activation by larvae can significantly improve the properties of the final product.]
RESPONSE 3 [Thank you. We have improved the discussion section by including one of the suggested references, which, similarly to our study, highlights the need to revise the legislative framework and the current governance model (Page 22, line 723).]

COMMENT 4 [Section 3.3 (provides estimated data on biomethane production (226 l/kg VS) and hydrochar (0.257 kg/kg waste). However, there is no information on the errors or variability of these parameters. It is necessary to clarify whether these values are average or obtained as a result of a single experiment.]
RESPONSE 4 [We thank the reviewer for the comment. The biomethane production reported is based on literature assumptions as experimental activity to process a larvae-based feedstock fed with ham residues is not available at the moment. The reported information is meant to represent a reasonable and preliminary design point. Further experimental activity is being designed to (i) confirm the composition of the larvae-based feedstock fed with ham residues; (ii) assess the performance of the anaerobic digestion process with the new larvae-based feedstock. This activity will be published upon the availability of data and the consequent performance review. A dedicated discussion has been added ad the end of Paragraph 3.3 and in the conclusion paragraph (Page 22, lines 708-718; Page 23, lines 756-759).]

COMMENT 5 [Pre-treated meat waste and wheat were used in the experiments with Hermetia illucens larvae. However, the criteria for choosing these components and their proportions are not specified (e.g., why a ratio of 1:2 was chosen and not other options).]
RESPONSE 5 [The choice of ham by-product was made because the meat treatment process produces waste equivalent to 25-55% of the initial animal's live weight, with high handling and disposal costs associated with potentially recoverable resources. Due to the economic importance of this sector for the region we are inserted, the objective is to recover material and energy, reducing the amount of waste and achieving cost containment in the production line. The experiments performed were a preliminary approach to understand the capacity of BSF larvae to transform ham by-product. As demonstrated by the results achieved, untreated ham is completely rejected by BSF. The idea initially was to feed larvae only with a ham diet. Because of that, we tested different pretreatment conditions of the ham. But even after pretreatment, the rearing conditions continue to be suboptimal. In view of this, we chose to mix the pretreated ham with wheat grains, due to its constant availability throughout the year. Also, wheat grains can be infected by phytopathogenic fungi, causing diseases like Fusarium Head Blight (FHB), being one more by-product to be discarded with recoverable potential. Therefore, we performed experiments with different ham/wheat ratios to verify the best conditions for the development of BSF larvae, since we did not have any preliminary data on the performance of larvae in this substrate mixture. We clarified in the section “2.2.1 Diets and experimental design” that these are preliminary experiments (Page 8, lines 331-332).]

COMMENT 6 [IBM SPSS Statistics was used for statistical analysis, but the version and parameters of the analysis are not specified (e.g., the significance level for the post-hoc test). It is also not described how the integrated process modeling was carried out (e.g., whether specialized programs such as Aspen Plus were used).]
RESPONSE 6 [Thank you for the suggestion. We adjusted the statistical analysis section. SPSS version is included (29.0.1.0 (171)) as well as significance levels adopted to evaluate the statistical significance of the results (Page 8, lines 349-355). With reference to the process design activity, an in-house black-box model was developed in Excel for process sizing, based on the assumptions tables already reported in the paper: the model solves mass and energy balance around three main process steps which are (i) bioconversion with H. illucens, (ii) anaerobic digestion (AD) and (iii) Hydrothermal carbonization (HTC). The model is capable to handle HTC process water stream recirculation, providing a preliminary estimation of relevant energy and performance indicators. This specification has been added in section “2.3 Integrated Refinery Process design” (Page 9, lines 364-371).]

COMMENT 7 [Section 2.2.2 mentions the use of Testo 174H temperature sensors, but there is no information on their accuracy and calibration. This is important for the assessment reliability of temperature and humidity data.]
RESPONSE 7 [Thank you for the suggestion. We added this information in the section “2.2.1 Diets and experimental design” (Page 7, lines 392-394).]

COMMENT 8 [The experiments with larvae do not indicate whether control measurements of substrate pH and salinity were made after pre-treatment. These parameters are critical for interpreting the larval survival results.]
RESPONSE 8 [In the preliminary phase, we did not perform specific substrate pH and salinity measurements after the pre-treatment. However, the initial substrate has a pH ranging from 5.40 to 6.30 and a salt content varying between 4% and 6%. These specifications are reported in the manuscript (Page 7, lines 284-286). We recognize the importance of monitoring these parameters for accurately interpreting larval survival results and plan to include such measurements in future experimental phases to ensure a more comprehensive assessment of environmental conditions.]

COMMENT 9 [Table 7 provides data on the energy consumption of the process, but does not provide a comparison with similar systems. For example, how much energy is spent on traditional methods of processing fat-containing waste?]
RESPONSE 9 [Thank you for the comment. As already answered before, we have clarified the objectives of the work to emphasise that the goal of this study is to assess whether larval digestion has the potential to be integrated into a biorefinery process, exploring pathways that are not yet present in the literature (Page 5, lines 182-208). A comparison of all possible valorisation routes for residual substrates with high lipid content is therefore outside the scope of the study, and this has been clarified in the text. Moreover, if the ham residue assessed in the present investigation is not used as in alternative processes, it may be downgraded to a waste stream which is generally co-incinerated. Direct comparison with mono-incineration in Waste-to-Energy is not directly representative of the directly fate of this stream.]

COMMENT 10 [Section 3.2 describes experiments with different proportions of substrates, but does not indicate how many replicates were performed for each condition. This is important for assessing the statistical significance of the results.]
RESPONSE 10 [The information about the number of replicates is reported in the Methodology section “2.2.1 Diets and experimental design”, that was adjusted to clarify all the experiments and conditions used (Page 7, lines 299-300).]

COMMENT 11 [The article mentions legislative barriers to the use of larvae in waste processing, but does not analyze how these restrictions may affect the scaling of the technology.]
 RESPONSE 11 [Thank you. This issue has been addressed in the conclusion section (Page 24, lines 783-787).]

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have worked to correct the comments. The article in its present form can be published in the journal.