Comparative TEA–LCA of CHP, Biomethane, and Hybrid Biogas Utilization Pathways for Poultry Manure with Fruit and Vegetable Waste Co-Digestion Systems

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study presents a combined techno-economic analysis (TEA) and life-cycle assessment (LCA) of a hypothetical 50-tonnes-per-day farm-based anaerobic digestion (AD) plant in South Africa, co-digesting poultry manure (PM) and fruit/vegetable waste (FVW). It evaluates five substrate blends and three biogas utilization pathways (100% electricity via CHP, 50/50 CHP-biomethane, and 100% biomethane). The research aims to fill gaps in farm-scale, co-digestion TEA-LCA for the South African context, explicitly considering digestate nutrient credits and the trade-offs in a coal-dominated grid. Below are my comments:

1. Table 1 has unclear column headers (e.g., "CO₂ (Nm3/t) off-gas (kg/t) CO₂ (kg/t)") and seems to contain a formatting error, making the data difficult to interpret.
2. The description of Figure 3 is confusing: *"Left=Right within each scenario: 100% E, 50/50, 100% B"*. What does "Left=Right" mean? The figure caption should clearly state that each group of three stacked bars represents the three energy routes for a given blend.
3. The treatment of FVW feedstock with a negative cost (gate fee of -ZAR 500/t) is a significant driver of project economics. Is this fee guaranteed, market-standard, or a best-case scenario for a subsidy? Sensitivity analysis on this parameter is crucial.
4. For a wet, continuous stirred-tank reactor (CSTR) system, the Hydraulic Retention Time (HRT) determines the volume of the digester. A 50 t/d feed with 30-day HRT implies a digester volume of ~1,500 m³ (assuming ~1 kg/L density). This is an enormous, likely unrealistic volume for a "farm-scale" plant in the South African context. 
5. The digestate emission factors are stated as coming from literature but are not quantified. What were the assumed NH₃ volatilization and N₂O emission factors (e.g., % of N applied)? These are the most important parameters for the main LCA conclusion, but are hidden.
6. The decision to exclude CO₂ and sulfur revenue is defensible but should be framed as a conservative assumption that makes upgrading pathways look worse.
7. The paper identifies digestate as the dominant emission source but offers only generic mitigation advice. A scenario analysis within the LCA is needed: What if digestate storage is covered? What if it's injected? Quantifying the potential GWP/AP/EP reduction from these improvements would make the study vastly more impactful for designers.
8. The study assumes the displaced electricity is from the average South African coal-heavy grid. How sensitive are the GWP results—and thus the apparent climate benefit of the CHP pathway—to the future decarbonization of the South African grid (e.g., increased renewables)? Does this potential shift alter the long-term strategic recommendation between electricity and biomethane production?
9. Some sentences are long and convoluted (e.g., the last sentence of the Introduction on Page 3). The manuscript would benefit from careful proofreading.

Author Response

Comment 1: Table 1 has unclear column headers (e.g., "CO2 (Nm3/t) off-gas (kg/t) CO2 (kg/t)") and seems to contain a formatting error, making the data difficult to interpret. Response 1: We have now removed the reporting of CO2 in mass terms and maintained its reporting in volume units per ton of substrate which is consistent with all values reported in the Table. Comment 2: The description of Figure 3 is confusing: *"Left=Right within each scenario: 100% E, 50/50, 100% B"*. What does "Left=Right" mean? The figure caption should clearly state that each group of three stacked bars represents the three energy routes for a given blend. Response 2: Thank you for the suggestion to revise figure captions. The captions have now been expanded to be self-explanatory and to clearly label subfigures. Comment 3:The treatment of FVW feedstock with a negative cost (gate fee of -ZAR 500/t) is a significant driver of project economics. Is this fee guaranteed, market-standard, or a best-case scenario for a subsidy? Sensitivity analysis on this parameter is crucial. Response 3:Clarification of the feedstock cost assumptions has been added. It is agreed that gate fee assumptions can significantly affect project economics. A sensitivity analysis was conducted by varying the FVW tipping fee, and the impact on IRR was reported.These are shown in methodology (section 2.3.3) and the Results (section 3.1) sections. Comment 4: For a wet, continuous stirred-tank reactor (CSTR) system, the Hydraulic Retention Time (HRT) determines the volume of the digester. A 50 t/d feed with 30-day HRT implies a digester volume of ~1,500 m³ (assuming ~1 kg/L density). This is an enormous, likely unrealistic volume for a "farm-scale" plant in the South African context. Response 4: The resulting working volume (1,500 m³) is intentional and consistent with reported full-scale agricultural digesters. Maluta et al. (2023) explicitly analyse a stirred anaerobic digester with an industrial reference volume of 1,500 m³, which is described as representative of typical agricultural biogas plants. In addition, Ahlberg-Eliasson et al. (2021) report farm-scale manure digesters with active volumes of 1,000–1,300 m³ operating at HRTs of 22–30 days. These studies confirm that the assumed digester size and HRT are technically realistic and aligned with current practice. This has been clarified in the revised manuscript. Comment 5: The digestate emission factors are stated as coming from literature but are not quantified. What were the assumed NH₃ volatilization and N₂O emission factors (e.g., % of N applied)? These are the most important parameters for the main LCA conclusion, but are hidden. Response 5: In the revised manuscript, the emission factors used for digestate handling have been clearly quantified and reported based on IPCC Tier 1 guidance. These values are now explicitly stated in Section 2.4.4. In addition, to ensure full transparency and reproducibility, a step-by-step worked numerical example has been provided in Appendix D, demonstrating how NH₃ and N₂O emissions are calculated per functional unit and how they propagate into GWP, acidification, and eutrophication impacts. A cross-reference to this Appendix has also been added in the Results section (Section 3.2). Comment 6: The decision to exclude CO₂ and sulfur revenue is defensible but should be framed as a conservative assumption that makes upgrading pathways look worse. Response 6: We thank the Reviewer for this guidance. The exclusion of CO₂ and sulfur revenues is now explicitly described as a conservative modelling choice (Section 2.3.3). Comment 7: The paper identifies digestate as the dominant emission source but offers only generic mitigation advice. A scenario analysis within the LCA is needed: What if digestate storage is covered? What if it's injected? Quantifying the potential GWP/AP/EP reduction from these improvements would make the study vastly more impactful for designers. Response 7: The revised manuscript now includes an explicit digestate mitigation scenario analysis within the LCA. Baseline digestate emission factors (NH₃ and N₂O) are specified in Section 2.4.4, and a worked numerical calculation is provided in Appendix D. In addition, Table 6 (Appendix E) quantitatively compares covered storage, digestate acidification, and soil injection. The results are discussed in Section 3.2. It was further clarified that while improved digestate management substantially lowers absolute impacts, it does not change the relative ranking of the biogas utilization pathways considered. Comment 8: The study assumes the displaced electricity is from the average South African coal-heavy grid. How sensitive are the GWP results—and thus the apparent climate benefit of the CHP pathway—to the future decarbonization of the South African grid (e.g., increased renewables)? Does this potential shift alter the long-term strategic recommendation between electricity and biomethane production? Response 8: In the revised manuscript, the sensitivity of GWP results to future grid decarbonization has been explicitly addressed in the Section 3.2. It was also clarified that the large climate benefit of the CHP pathway in the base case is partly driven by the displacement of the current coal-dominated South African electricity mix. Comment 9: Some sentences are long and convoluted (e.g., the last sentence of the Introduction on Page 3). The manuscript would benefit from careful proofreading. Response 9: A thorough language edit has been performed throughout the manuscript. Long or convoluted sentences were shortened or split, inconsistent spacing and minor typographic errors were corrected, and abbreviations were defined at first use.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a comparative TEA and LCA of multiple biogas utilisation pathways (CHP, biomethane, hybrid) for a 50 t/day co-digestion system processing poultry manure and fruit/vegetable waste. Overall, the article is scientifically sound, methodologically thorough, and highly relevant for circular-economy and renewable-energy policy design in South Africa. The integration of TEA and LCA is particularly strong; the results are well supported by detailed tables, system diagrams, and methodological transparency throughout sections 2 and 3. The main findings, namely that CHP-only is the most financeable route, while digestate management dominates environmental impacts, are clearly articulated and novel in the regional context. The novelty of the paper lies in (i) modelling five substrate blends with explicit nutrient-crediting for digestate, (ii) evaluating three biogas-use pathways, (iii) quantifying bankability metrics (IRR, DSCR) that are rarely included in LCA-centered studies, and (iv) situating the findings within a coal-intensive national grid. As the first farm-scale, PM-FVW TEA-LCA integration for South Africa, the contribution is meaningful and worth publishing after major revisions.

1. While the background section thoroughly addresses anaerobic digestion, co-digestion synergies, and South African energy context, a major class of organic-waste valorisation studies is missing. Recent literature on agro-waste upcycling into high-value bioproducts, particularly cellulose fiber extraction from agricultural residues, should be incorporated. Such studies are important because they illustrate how organic wastes can simultaneously serve the energy sector and the materials sector, strengthening circular-economy logic beyond energy recovery alone. Example: ChemEngineering 8(6), 112. https://doi.org/10.3390/chemengineering8060112. This paper shows that agricultural residues contain valuable bioactive compounds (cellulose fibers) that can be extracted and used in textiles and other non-food applications, thus presenting alternative or complementary valorisation routes to anaerobic digestion. Including this work will reinforce the argument that waste streams, such as FVW or crop residues, should be evaluated not only for biogas potential but also within broader cascaded-use strategies. This improves the manuscript’s conceptual framing of circularity and highlights why feedstock prioritisation (e.g., high-FVW blends) has consequences beyond energy yields.
2. Your LCA results unambiguously show that digestate handling is the dominant hotspot across GWP, acidification, and eutrophication. However, the discussion would benefit from:A) a more detailed explanation of why PM-rich blends produce disproportionately higher digestate burdens (e.g., N-rich content, higher NH₃ volatilisation potential).B) Also, quantitative comparison of mitigation potential (covered storage, acidification, injection) using values from the literature (e.g., reductions up to 90 % as mentioned in Tan et al.). C) An explicit statement on whether improved digestate management would change the relative ranking of pathways (e.g., could biomethane become competitive if digestate emissions were minimised?). Given that digestate dominates environmental indicators, this section should be strengthened to reflect its practical and policy relevance.

3. The manuscript provides base-case results for IRR, DSCR, and NPV, but no systematic sensitivity analysis is presented for key parameters such as: Biomethane selling price, Electricity tariff volatility, Methane slip during upgrading, Digestate nutrient concentration assumptions, Discount rate. Because the conclusion that CHP-only is ‘most bankable’ relies heavily on present energy prices, the robustness of this conclusion cannot be assessed without sensitivities. Considering the high volatility in South Africa’s electricity prices and gas markets, a tornado diagram or multi-parameter sensitivity matrix should be added.
4. Tables and appendices (e.g., nutrient composition Table 5), list digestate nutrient concentrations, but the methodology must clarify whether credits were based on plant-available fractions or total nutrient content (important for N, given volatilisation), How spatial variability of digestate quality was considered, Whether environmental credits might diminish if digestate requires post-treatment (e.g., dewatering, composting). A brief justification for using direct substitution credits in the South African context is needed to strengthen methodological transparency.
5. Figure 1 clearly shows the three process chains, but the narrative in section 2.2 could be improved by adding explicit inclusions/exclusions (e.g., why CO₂ valorisation, sulfur recovery, and logistics beyond 20 km are excluded), clarifying whether parasitic electrical loads differ between CHP and upgrading cases (currently implied, not stated). Also, explaining whether thermal energy from CHP is considered fully utilised or partially wasted. This will make the boundary conditions and assumptions more reproducible for future TEA-LCA work.
6. The conclusions mention SDGs, but the discussion should further explore trade-offs between financial performance and environmental performance (e.g., CHP better financially but upgrading may reduce some local pollutants), whether high-FVW blends face logistical constraints, since FVW supply may be seasonal and geographically uneven. The implications of using FVW for biogas instead of potential material-valorisation pathways.
7. Minor revisions.a) Ensure consistent significant figures across tables (e.g., Table 1 and Table 4).b) Improve caption detail for Figures 2-7 (e.g., specify FU = 1 t substrate, state ‘per ton as-received’).c) Some abbreviations appear before being defined; unify them.d) Ensure spelling and formatting conform to journal standards (some spacing issues are apparent).

Author Response

Comment 1: While the background section thoroughly addresses anaerobic digestion, co-digestion synergies, and South African energy context, a major class of organic-waste valorisation studies is missing. Recent literature on agro-waste upcycling into high-value bioproducts, particularly cellulose fiber extraction from agricultural residues, should be incorporated. Such studies are important because they illustrate how organic wastes can simultaneously serve the energy sector and the materials sector, strengthening circular-economy logic beyond energy recovery alone. Example: ChemEngineering 8(6), 112. https://doi.org/10.3390/chemengineering8060112. This paper shows that agricultural residues contain valuable bioactive compounds (cellulose fibers) that can be extracted and used in textiles and other non-food applications, thus presenting alternative or complementary valorisation routes to anaerobic digestion. Including this work will reinforce the argument that waste streams, such as FVW or crop residues, should be evaluated not only for biogas potential but also within broader cascaded-use strategies. This improves the manuscript’s conceptual framing of circularity and highlights why feedstock prioritisation (e.g., high-FVW blends) has consequences beyond energy yields. Response 1: Thank you for this valuable suggestion. The Background section has been revised to include recent literature on agro-waste upcycling into higher-value cellulosic materials, highlighting that certain agricultural and food-related residues may be valorised through material-recovery pathways in addition to energy production. The added text explicitly recognises these cascaded-use strategies while clarifying their practical limitations for mixed, high-moisture food and vegetable waste streams, thereby justifying the continued focus on anaerobic digestion within the present study. This revision strengthens the circular-economy framing and contextualises feedstock prioritisation decisions without expanding the analytical scope. Comment 2:Your LCA results unambiguously show that digestate handling is the dominant hotspot across GWP, acidification, and eutrophication. However, the discussion would benefit from:A) a more detailed explanation of why PM-rich blends produce disproportionately higher digestate burdens (e.g., N-rich content, higher NH₃ volatilisation potential).B) Also, quantitative comparison of mitigation potential (covered storage, acidification, injection) using values from the literature (e.g., reductions up to 90 % as mentioned in Tan et al.). C) An explicit statement on whether improved digestate management would change the relative ranking of pathways (e.g., could biomethane become competitive if digestate emissions were minimised?). Given that digestate dominates environmental indicators, this section should be strengthened to reflect its practical and policy relevance. Response 2: The manuscript has been revised to explicitly report digestate emission factors (NH₃ = 25% of applied N; direct N₂O = 1% of applied N; indirect N₂O = 1% of volatilized NH₃–N) in Section 2.4.4. A worked numerical example is also included in Appendix D, which shows how these factors are applied and how mitigation scaling is calculated. A description of why poultry-manure-rich blends drive higher digestate impacts and to quantify mitigation effects has been provided in Section 3.2. The manuscript now also explicitly states that these mitigation options do not change the relative LCA ranking of CHP vs biomethane (they reduce absolute impacts across all routes equally). (See also Comment number 7 under Reviewer 1). Comment 3: The manuscript provides base-case results for IRR, DSCR, and NPV, but no systematic sensitivity analysis is presented for key parameters such as: Biomethane selling price, Electricity tariff volatility, Methane slip during upgrading, Digestate nutrient concentration assumptions, Discount rate. Because the conclusion that CHP-only is ‘most bankable’ relies heavily on present energy prices, the robustness of this conclusion cannot be assessed without sensitivities. Considering the high volatility in South Africa’s electricity prices and gas markets, a tornado diagram or multi-parameter sensitivity matrix should be added. Response 3: A systematic sensitivity analysis has been added to the methodology (Section 2.3.5) and results (Section 3.2). A one-at-a-time sensitivity framework was implemented for the key economic parameters (electricity tariff, biomethane selling price, and discount rate), and the results were visualised using pathway-specific tornado diagrams (Figure 5). The sensitivity analysis demonstrates that NPV is dominated by revenue-related parameters, with electricity tariffs driving CHP performance and biomethane prices governing upgrading-focused pathways. These results confirm that the conclusion regarding the relative bankability of the CHP pathway is conditional on prevailing energy prices, and that shifts in electricity or gas markets could materially alter pathway rankings. Parameters such as methane slip during upgrading and digestate nutrient composition were not included in the economic tornado analysis because they do not directly affect project cash flows; instead, their influence on system performance is captured through the LCA and digestate management scenario analysis (Sections 3.3 and Appendix E). This integrated approach ensures that both financial robustness and environmental trade-offs are transparently addressed. Comment 4: Tables and appendices (e.g., nutrient composition Table 5), list digestate nutrient concentrations, but the methodology must clarify whether credits were based on plant-available fractions or total nutrient content (important for N, given volatilisation), How spatial variability of digestate quality was considered, Whether environmental credits might diminish if digestate requires post-treatment (e.g., dewatering, composting). A brief justification for using direct substitution credits in the South African context is needed to strengthen methodological transparency. Response 4: An explicit justification in the Methodology section has been added to clarify that nutrient credits are calculated using plant-available fractions rather than total nutrient content, with nitrogen treated conservatively to account for volatilisation and handling losses, while phosphorus and potassium are assumed to be largely plant-available following land application. The revised manuscript also explains how spatial variability and uncertainty in digestate quality are represented through blend-specific nutrient concentrations and bounded availability ranges applied consistently across scenarios. Finally, the use of direct substitution credits in the South African context was justified by noting the common practice of applying digestate on nearby agricultural land to offset purchased mineral fertilisers, making avoided fertiliser production and application an appropriate and transparent representation of environmental benefit within an attributional LCA framework. Comment 5: Figure 1 clearly shows the three process chains, but the narrative in section 2.2 could be improved by adding explicit inclusions/exclusions (e.g., why CO₂ valorisation, sulfur recovery, and logistics beyond 20 km are excluded), clarifying whether parasitic electrical loads differ between CHP and upgrading cases (currently implied, not stated). Also, explaining whether thermal energy from CHP is considered fully utilised or partially wasted. This will make the boundary conditions and assumptions more reproducible for future TEA-LCA work. Response 5: Section 2.2 has been revised to explicitly clarify the system boundary inclusions and exclusions, as well as the rationale behind them. It has now been stated why CO₂ valorisation, sulfur recovery, and transport logistics beyond 20 km are excluded, noting that their omission represents a conservative assumption and improves comparability across pathways. It has also been explicitly clarified that parasitic electricity demands differ between CHP and upgrading configurations, with upgrading pathways including additional electricity consumption for gas separation and compression. Furthermore, it’s stated that thermal energy from CHP is only partially utilised for on-site digester heating, with surplus heat treated as unrecovered and not credited. Comment 6: The conclusions mention SDGs, but the discussion should further explore trade-offs between financial performance and environmental performance (e.g., CHP better financially but upgrading may reduce some local pollutants), whether high-FVW blends face logistical constraints, since FVW supply may be seasonal and geographically uneven. The implications of using FVW for biogas instead of potential material-valorisation pathways. Response 6: Thank you for this comment. Section 3.4 has been revised to explicitly discuss trade-offs between financial and environmental performance, noting that CHP configurations are financially favourable under current electricity prices, while upgrading pathways may offer environmental advantages in certain impact categories. It has also included discussion on logistical constraints associated with high food and vegetable waste blends, including seasonality and spatial variability, as well as the system-level implications of diverting FVW from alternative material-valorisation pathways. Comment 7: Minor revisions.a) Ensure consistent significant figures across tables (e.g., Table 1 and Table 4).b) Improve caption detail for Figures 2-7 (e.g., specify FU = 1 t substrate, state ‘per ton as-received’).c) Some abbreviations appear before being defined; unify them.d) Ensure spelling and formatting conform to journal standards (some spacing issues are apparent). Response 7 :The manuscript has been revised to ensure consistent use of significant figures across all tables, expanded figure captions to clearly state the functional unit (FU = 1 t of substrate, as received), consistent definition and use of abbreviations at first occurrence, and correction of spelling, spacing, and formatting issues to align with the journal’s style guidelines.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I thank the authors for their careful and comprehensive revisions. They have successfully addressed the majority of the issues raised, and the manuscript has been substantially strengthened. I have no further substantive criticisms and recommend it for publication.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have adequately addressed previous comments and the revised manuscript is improved. The additional context in the Introduction and the clarifications regarding digestate-related assumptions and system boundaries strengthen the work. The sensitivity analysis is a useful addition and supports the robustness of the techno-economic results.