Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Journal:Sustainability (ISSN 2071-1050)

Manuscript ID: sustainability-3944207
Title : " Manure Application and Energy Potential in Southeastern

China"
Author(s): Yufeng Wang, Hua Deng and Chih-Chun Kung

 

General remark:

 

I consider that paper “Manure Application and Energy Potential in Southeastern China” addresses a current global problem —manure management and climate change mitigation— with direct environmental, energy, and social impact. It tries to be useful to climate change and emission offset. However, it does not present a solid experimental design and statistical validation, more over, conversion errors are detected, inconsistencies between GWh ↔ kWh ↔ US$, and miss link between costs, revenues, and manure mass, so economic data are not traceable and between others afirmations like “mesophilic system brings more agronomic benefits” lacks measurements of NPK, pH, C/N, or germination bioassays.

Therefore, it is necessary to make major corrections before it could be accepted.

 

 

Abstract:

Comment #1. On line 23, authors say that “0.98 million metric tons of emission offset).” However, it is not clear whether it is CO₂, CH₄, or CO₂ equivalent,  nor does it contextualize the total climate impact (net reduction of carbon footprint or contribution to carbon neutrality).

Introduction

Comment #2. On line 111  to 113. Authors say “investigating the carbon offset potential from various manure utilization possibilities is crucial for China's energy sustainability, social development, and environmental management”. In other words, it explains why it is worth studying the topic, not what is expected to be demonstrated with the study. Please include your work hypothesis.

Comment #3. On line 135 authors say “in which biogas contains various gases such as methane (CH4), hydrogen (H), carbon dioxide (CO2), and nitrous oxide (NOx), and digestate is known as the biogas residual”. First you should say H₂ not H, but also on biogas from anaerobic digestion rarely contains nitrous oxide (N₂O) or NOₓ in meaningful amounts. Please you should include a GC analysis of the gas composition of the biogas used in this study.

Comment #4. Authors make a great effort to include a full 2×4 factorial design implemented to evaluate the effect of digestion mode (mesophilic vs. thermophilic) and manure type (swine, bovine, poultry, ovine) on process performance and agronomic co-benefits. And for process robustness, a 2×3 sub-DOE on organic loading rate (OLR: 1.0, 1.5, 2.0 kgVS·m⁻³·d⁻¹) crossed with mode (meso/thermo) was included using a fractional factorial (Resolution IV) when equipment availability was limited. However, it is necessary to go deeper! Please include a factorial experimental design to validate the differences between mesophilic and thermophilic modes and types of manure.  Please a detailed paragraphs on the analytical methodology of biogas (GC-TCD/FID and quality control) which will be useful. And finally, please  it is necessary to include how the climate impact will be worked  in terms of carbon footprint (CO₂) and contribution to carbon neutrality.

Comment #5. On table 3 authors give information about 12 regions. It is necessary to national averages, which prevents verifying the total of 515 GWh cited in the abstract; please it is not clear if data is corrected for the 5% CH₄ loss; please include deviations or errors  or uncertainties from modeling or replication.

Economic analysis under techno-economic analysis

Comment #6. Please you should use money dollars values because it is almost impossible to evaluate if your data is consistent (on line 359). for example, electricity cost (Table 5) Costs range from 16–37 ¢ kWh⁻¹ (1 ¢=0.01 US$), equivalent to 160–370 US$ MWh⁻¹, which far exceeds the actual generation cost (≈ 70 US$ MWh⁻¹). This suggests a scaling error or that co-benefits were not discounted. (But of course value money of your study probably is not the same today). Then it is necessary to include the year of your data.

Comment #7.  To convert miles to kilometres, multiply by 1.60934. While the authors employ a conversion factor of “100 ×” to account for this difference, its usage is neither documented nor supported by dimensional analysis.

Comment #8.  From equation 2, the factor '0.4714' is based on a model using miles, but your calculations use kilometres, which suggests there may be a mix-up between imperial and metric units.  Then authors should verify the dimensional consistency of units, clearly state whether distances were expressed in kilometers or miles, and recalculate Table 4

Comment #9. On Tables 3, please give the total on each column and make it clear if your data in annual. Of course your total must be 515 GWh (on line 22). On table 6,  when checking to back-calculate the total manure mass using the revenues from Table 6 and the energy price, values are up to 3 times higher than those reported in Table 2. Please check your calculations or show the equations used to make that calculation.

Comment #10. In section 3.2 Economic Analysis (TEA), the author combines costs and revenues without distinguishing CAPEX from OPEX, presenting data per ton of manure or per kWh but omitting a time frame, and appears to calculate profit outside standard economic methods.  Please check your results on table 10

Comment #11.  In the section The Greenhouse Gas Offset subsection, authors lack quantitative rigor and methodological transparency. Authors argue with strong claims about the thermophilic system’s contribution to carbon neutrality without presenting underlying data, emission factors, or baseline scenarios. In the abstract authors offset of 0.98 Mt CO₂. However, this important data is unsupported by a transparent life-cycle calculation (e.g., IPCC 2019, GHG Protocol). Please in this section should include detailed emission inventories (CH₄, CO₂, N₂O), the reference system, and uncertainty analysis, without these data, the conclusions appear overstated and scientifically unsubstantiated.

Comment #12.  On abstract authors say “the mesophilic mode can yield greater agronomic benefits with digestate application.” On line 358, the cost per kWh from hog manure ranges from 16.78 ₵ to 18.89 ₵ under mesophilic mode and 17.27 ₵ to 18.93 ₵ under thermophilic Data is overlapping! On line 364, “a higher per kWh power cost would occur if the mesophilic mode were used.”  From lines 495 to 497 “The profit from the mesophilic mode is between $13.2 and $23.75, while that of the thermophilic system ranges from $10.93-$36.22. The thermophilic process seems more profitable because it yields higher power, but the mesophilic system generally brings more agronomic benefits.” However,  authors do not give support any agronomic physicochemical variables: available N, P, K;  assimilable nutrients for crops (g·kg⁻¹)or % dry matter C/N ratio;  stability of organic material; maturity dimensionless Electrical conductivity (EC) Salt level; crop tolerance (dS·m⁻¹);pH Soil condition when applying digestate —Humic substances / TOC; or  structural improvement of soil and water retention %Germination index / phytotoxicity. Authors must give a full comparative analysis of mesophilic vs. thermophilic digestates and perform an ANOVA statistical test.

Author Response

Thank you very much for your comments. We have uploaded a PDF response to address your concerns. Thank you very much.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Review Report

1. Summary of the Manuscript and Key Contributions

The manuscript investigates the potential of livestock manure as a renewable energy source in Southeastern China, focusing on provinces such as Anhui, Fujian, Guangdong, and Zhejiang. Using Techno-Economic Analysis (TEA) and Lifecycle Assessment (LCA), the study evaluates the feasibility, profitability, and environmental impacts of anaerobic digestion under mesophilic and thermophilic processes.

Key contributions include, A regional comparison of energy generation potential from different manure types (hog, cattle, poultry, sheep). Also, an integrated assessment combining energy output, economic costs, and greenhouse gas offsets, and identification of the economic role of digestate as biofertilizer in promoting sustainable agriculture. Policy implications for renewable energy development and carbon reduction strategies in China. Overall, the paper offers a comprehensive quantitative assessment linking energy policy, environmental management, and agricultural sustainability.

2. Evaluation of Methodology, Analyses, and Conclusions

• Methodology:

The methodological framework is clearly structured and rigorous. The combination of TEA and LCA is well justified, and the authors appropriately define system boundaries (Figure 1) and calculation equations for biogas production and economic evaluation. However, some assumptions could be better justified with references to regional datasets or sensitivity analyses. The use of constant values for costs and emission prices may not capture market variability; a scenario analysis would strengthen the robustness of the findings. It would be beneficial to include a validation or comparison with existing pilot projects or case studies in China or similar regions.

• Analyses:

The analytical results are detailed and logically presented. Tables 3–10 provide extensive quantitative data on power potential, transportation cost, and profit margins. However, the text sometimes repeats numerical details from the tables without sufficient interpretation. Figures 1 and 2 are informative, but Figure 2 could be improved—its subpanels (2A1–2B4) are not well described in the caption, and the color scale or legend is missing, making it hard to interpret the regional energy potential visually. Consider including a summary or synthesis figure that integrates energy, economic, and environmental outcomes for clearer policy insights.

• Conclusions:

The conclusions are consistent with the analysis and highlight important insights, thermophilic digestion offers higher energy yields but with higher cost and emissions. Mesophilic systems are more stable and feasible for smaller operators. Biofertilizer application adds economic and environmental value. However, the discussion could benefit from a clearer distinction between scientific implications (for energy research) and policy implications (for regional planning). Additionally, uncertainty factors, especially those tied to climate variability and local soil conditions, should be more explicitly addressed.

3. Constructive Feedback and Questions for the Authors

• Improve Figure 2 by clarifying what each subpanel (A1–B4) represents, adding units, and providing a color legend. Readers currently struggle to interpret regional patterns.
• Consider merging some of the numerous tables (Tables 5–10) or summarizing them graphically to enhance readability.
• The captions for figures should be more descriptive, explaining data sources and key observations.
• Provide more detail on data sources for manure distribution (Table 2). Were the values derived from official statistics, surveys, or estimates?
• Include a sensitivity analysis for key economic parameters (e.g., electricity price, emission value, transport cost).
• Clarify whether GHG offsets include methane leakage adjustments and how uncertainties were propagated.
• Compare the estimated biogas yields and profits with other studies or national benchmarks to contextualize findings.
• Discuss possible technological improvements (e.g., co-digestion, waste heat recovery) that could alter cost-effectiveness.
• Expand on the policy recommendations—especially how local governments could facilitate market access for small producers.
• Edit for grammar and stylistic conciseness—some sections are lengthy and repetitive.
• Update references where possible; some citations refer to older emission data.
• Include uncertainty ranges or confidence intervals in key quantitative results.

4. Overall Assessment

The manuscript is well structured and methodologically sound, addressing an important sustainability issue with practical policy relevance. It contributes valuable quantitative insights into the economic and environmental feasibility of manure-to-energy systems in China. With improved figure presentation, clearer explanation of assumptions, and expanded contextual discussion, the paper will be significantly strengthened and better aligned with the standards of Sustainability.

Recommendation: Minor to moderate revision.

Author Response

Thank you very much for your comments. We have uploaded a PDF response to address your concerns. Thank you very much.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

General remark:

 

I consider that authors of the  paper “Manure Application and Energy Potential in Southeastern China” make a few  corrections. But it is necessary to make minor correction before it could be accepted.

 

On line 279: authors insist to “The coefficient 100 is the conversion factor between miles and kilometer”. This is a mistake.

 

Comment #1. Although the authors have done a significant amount of work to improve the document, some important details are missing, such as introducing an equation that allows establishing a relationship between Biogas (m³) × kWh/m³ × (1 GWh / 10⁶ kWh) × (US$/kWh) = Income (US$). This would allow for an evaluation of the results proposed in Tables 3 and 

Author Response

Thank you very much for your constructive comments. We have modified the Equation (2) that convertis miles to kilometers and added an Equation (5) to address how income is calculated from methane production and utilization. The original Euqation (5) is renumbered to Equation (6).

Please see attached responses to verify the expression.

Thank you very much.

Author Response File: Author Response.pdf