Comparative Life Cycle Assessment of Pyrolysis and Hydrothermal Carbonization for Sewage Sludge Treatment in Colombia

Response to Reviewer 1 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find below our detailed responses, with all corresponding revisions indicated in track changes in the re-submitted files. We appreciate the reviewer’s evaluation and the recognition of the robustness of the work; the observations provided guided focused improvements in the revised version.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Is the content succinctly described and contextualized with respect to previous and present theoretical background and empirical research (if applicable) on the topic?

Yes

Are the research design, questions, hypotheses and methods clearly stated?

Can be improved

Improved according to the reviewer’s suggestion.

Are the arguments and discussion of findings coherent, balanced and compelling?

Yes

For empirical research, are the results clearly presented?

Yes

Is the article adequately referenced?

Yes

Are the conclusions thoroughly supported by the results presented in the article or referenced in secondary literature?

Can be improved

Improved according to the reviewer’s suggestion.

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: “First, the Abstract could be slightly condensed. Although it effectively summarizes the methodology and results, it may benefit from a more concise presentation of key findings, particularly the quantitative outcomes for the best-performing technology and their policy relevance. A clearer statement on how the results support circular economy strategies in developing countries would also improve its communicative value.”

Response 1: Agree. Thank you for this suggestion. The final sentence of the Abstract was rewritten to provide a clearer and more direct statement on how the results inform circular economy strategies in developing countries, addressing the reviewer’s comment (line 10 to 28):

“The sustainable management of sewage sludge (SS) requires comparative evaluations that capture both environmental impacts and the trade-offs associated with emerging and established treatment routes. This study applies life cycle assessment (LCA) to compare hydrothermal carbonization (HTC), rotary kiln pyrolysis, and incineration using SS from the Salitre WWTP in Bogotá, Colombia, based on a life cycle inventory that integrates experimental characterization, Aspen Plus simulations, and Ecoinvent datasets modeled in EASETECH. 13 ILCD midpoint impact categories were assessed, and uncertainty was evaluated through global sensitivity analysis (GSA) and Monte Carlo simulations. The results show that the three technologies distribute their impacts differently across categories: HTC yields reductions in several categories due to carbon storage and fertilizer substitution but presents toxicity-related impacts linked to heavy metal transfer to soils; pyrolysis produces a pyrochar with metal retention and nutrient recovery potential that influences climate and resource related categories while remaining sensitive to sludge composition; and incineration influences climate categories without the potential toxic effects of using chars in soils, reduces sludge volume, and facilitates subsequent nutrient recovery processes from ash, with lower uncertainty due to its technological stability. These results support circular economy strategies in developing countries by clarifying the environmental conditions under which carbonized materials or ash-derived recovery pathways can be incorporated into sludge treatment systems.”

Comments 2: “In the Introduction, the contextual background is comprehensive and well referenced, but the discussion could be enriched by briefly addressing the technological and regulatory challenges that may affect the implementation of pyrolysis and HTC in Colombia. Mentioning existing national or Latin American waste management policies would strengthen the connection between the study and regional decision-making contexts.”

Response 2: Agree. Thank you for this valuable suggestion. To address the reviewer’s comment, a paragraph was added in the Introduction (line 52 to 55) to strengthen the discussion on the technological and regulatory context of sewage sludge management in Colombia. This addition provides a concise overview of the current national framework, which promotes agricultural use while allowing energy valorization, and highlights the technological and economic barriers to large-scale implementation of thermal treatments. The revised text is as follows:

“In Colombia, SS is primarily applied as a soil amendment; however, national regulations also enable the energy recovery from these residues [10]. Incineration has been explored at an industrial pilot scale [11], whereas other thermal technologies remain constrained by limited technological maturity and high capital investment requirements”.

Comments 3: “Regarding the Methodology, the definition of system boundaries, functional unit, and inventory assumptions is commendable. However, some modeling assumptions (especially the 100% substitution rates for nitrogen, phosphorus, and potassium fertilizers) may be overly optimistic. Even though the manuscript acknowledges this simplification, it would be helpful to include a short quantitative discussion of how the results might vary if lower substitution rates were applied (for instance, at 50% or 75%)”.

Response 3: Agree. Thank you for this constructive comment. In response to the reviewer’s observation (and also to those made by the Reviewer 3) regarding the optimistic assumption of 100% substitution rates for nitrogen, phosphorus, and potassium, the methodological section was substantially revised to incorporate bibliographically grounded substitution values and to explicitly reduce uncertainty associated with this parameter.

Accordingly, Section 2.4 (Application of chars to soil) was updated to specify nutrient substitution assumptions using experimental data and literature-derived availability values for N, P, and K. The revised paragraph now reads as follows (line 250 to 270):

“The substitution of mineral fertilizers was modeled following standard stoichiometric conventions widely used in agronomic and LCA studies. Phosphorus substitution was expressed as P₂O₅, where 1 kg of P in char acorresponds to 2.29 kg of P₂O₅ fertilizer. Potassium substitution was expressed as K₂O, where 1 kg of K in char replaces 1.2 kg of K₂O fertilizer. Nitrogen substitution was expressed on a mass basis, assuming that 1 kg of N in char is equivalent to 1 kg of N fertilizer. Based on experimental data [38], [57], this assessment applied a substitution rate of 0 % for nitrogen in pyrochar and 3.5 % for nitrogen in hydrochar. Substitution percentages for phosphorus were set at 3.5 % for pyrochar and 5.0 % for hydrochar [48], [58], [59], [60], [61], while potassium substitution was set at 10 % following reported K availability in sludge-derived chars[61]. To incorporate uncertainty in nutrient availability, all substitution parameters were modeled using a uniform distribution ranging from 0 % to 25 %, which reflects the broad variability reported in the literature for nutrient release and plant uptake from pyrochar and hydrochar. These distributions were integrated into the uncertainty analysis to examine their influence on the variability of impact category results. In addition, a sensitivity analysis of fertilizer substitution rates is presented in Section 3.2, where substitution levels of 0 %, 10 %, and 100 % are evaluated to assess how this parameter affects the magnitude and direction of environmental outcomes. 100 % is selected as an ideal maximal value that cannot be surpassed, and 10 % as a mean typical value. Detailed information on the LCI framework and modeling assumptions is provided in the Supplementary Material (Section S2).”

In addition to the methodological refinement, a new Section 3.2 (“Fertilizer substitution rate analysis”) was incorporated into the Results to provide a quantitative evaluation of how variations in substitution rates affect the direction and magnitude of environmental impacts across the 13 ILCD categories. This section directly addresses the reviewer’s request by demonstrating the sensitivity of results under substitution levels of 0%, 10%, and 100% (line 486 to 516).

These changes ensure that the modeling assumptions are now fully supported by experimental evidence and literature, reduce uncertainty in nutrient substitution, and provide a robust quantitative analysis consistent with the reviewer’s recommendation.

Comments 4: “The Results and Discussion section is rich in detail but somewhat lengthy. The presentation of all thirteen impact categories could be streamlined by summarizing similar trends and emphasizing the most policy-relevant ones, such as climate change, toxicity, and resource depletion. Repetitive explanations could be shortened, and extensive numerical details might be moved to the Supplementary Material to improve readability”.

Response 4: Agree. Thank you for this observation. To improve clarity and reduce redundancy, the section “3.1. LCA results: process contribution to the impact categories” was reorganized by consolidating impact categories with related environmental mechanisms. In particular, the human toxicity (cancer and non-cancer) and freshwater ecotoxicity categories were unified into a single subsection (Section 3.2.3, line 347), enabling a more integrated discussion of toxicity-related outcomes. Similarly, the terrestrial, freshwater, and marine eutrophication categories were merged into a single consolidated subsection (Section 3.2.8, line 431) to present eutrophication impacts more coherently and avoid repetitive explanations across categories.

This restructuring streamlines the presentation of the thirteen ILCD impact categories, reduces descriptive repetition, and enhances the readability of the Results and Discussion section while preserving the analytical depth and technical rigor expected in an LCA study.

Comments 5: “The discussion of uncertainty and sensitivity analysis is excellent from a methodological perspective, but its practical implications are not fully articulated. It would strengthen the paper to include a short paragraph explaining which parameters are most critical to monitor or control in future research or operational settings—for example, the zinc content of sludge, energy demand in thermal processes, and the fertilizer substitution efficiency.”

Response 5: Agree. Thank you for this valuable comment. In response to the reviewer’s suggestion, the Results and Discussion section was strengthened to clarify the practical implications of the sensitivity and uncertainty assessments. An explicit sensitivity analysis of fertilizer substitution rates was incorporated in Section 3.2, Fertilizer substitution rate analysis (line 480). Additionally, the description of the most influential parameters was expanded in Sections 3.3, Uncertainty propagation, and 3.4, Comparative assessment based on discernibility analysis, in order to explain more clearly how fertilizer substitution efficiency, sludge composition and thermal energy requirements shape the variability and robustness of the environmental outcomes.

To explicitly highlight the relevance of these findings for public policy and decision-making processes, the following paragraph was added at line 566:

“These findings indicate that environmental performance varies according to the type of impact considered. No scenario performs consistently better across all categories. The interpretation of these results therefore requires attention to the specific environmental mechanisms involved rather than a single aggregated judgment. For public policy and project development, the discernibility analysis provides information that can support decisions aimed at reducing environmental risks or addressing particular policy objectives, such as reducing greenhouse gas emissions, improving nutrient management or limiting toxicity related impacts, without implying that one technology should be uniformly preferred over the others.”

These revisions address the reviewer’s recommendation by making the methodological results more directly connected to their practical and policy-oriented implications.

Response to Reviewer 2 Comments

1. Summary

Thank you very much for reviewing this manuscript. We appreciate the reviewer’s positive assessment of the study and the recommendation for publication pending revisions. Below we provide detailed responses to each comment, and all corresponding modifications are clearly indicated in track changes in the re-submitted files.

2. Point-by-point response to Comments and Suggestions for Authors

Comments 1: “Overall, the manuscript is well-written and contains interesting research findings. I recommend publishing this work after minor revisions.

A period, not a comma, should be used to separate decimal places”

Response 1: Thank you for this observation. The manuscript has been updated accordingly, and the decimal comma was replaced with the decimal point throughout the entire document to ensure consistency with the reviewer’s recommendation.

Comments 2: “Values are presented with standard deviation (±)" - add number of repetitions?.”

Response 2: Agree. Thank you for pointing this out. We agree with this comment. Therefore, we have modified the footnotes of Tables 1, 2, and 3 to specify that all measurements were performed in triplicate.

“Values are presented with standard deviation (±) from triplicate samples”.

Comments 3: “In section 2.2.1 "laboratory and pilot scale equipment" please specify the exact sample mass, batch volume, and number of repetitions.”

Response 3: Agree. Thank you for this comment. To address the reviewer’s observation, the paragraph in Section 2.2 (Experimental treatments and characterization of sewage sludge and products), line 157 on page 5, was revised to include the sample mass, batch volume, and number of repetitions for each experiment. The updated text now provides explicit experimental details for both HTC and pyrolysis. The revised paragraph reads as follows (lines 160 to 164):

“HTC was carried out in a 250 ml batch reactor at 200 °C with a residence time of 1 h, using a load of 14 g of SS and deionized water to achieve 15,00 % total solids. Experiments were performed in triplicate. Pyrolysis was conducted in a rotary kiln reactor operating continuously at 450 °C with a residence time of 30 min, using a single experimental run with a feed rate of 1 kg/h of dried SS.”

Comments 4: “The manuscript uses two terms: "Incineration" and "Combustion." Can just one of them be used instead of both interchangeably?”.

Response 4: Agree. Thank you for this comment. In the manuscript, the terms “incineration” and “combustion” are not used interchangeably. Incineration refers to the complete thermal treatment technology applied to sewage sludge, including drying, combustion, and flue gas cleaning stages, as defined in Scenario S0. In contrast, combustion represents a specific process block within the incineration scenario, corresponding to the oxidation of organic matter. To avoid ambiguity, Figure 2 was revised to clarify this distinction and ensure consistent terminology throughout the manuscript.

Comments 5: “In the "human toxicity" section of the analysis, only heavy metals were included, and PAHs, PCBs, and PFAS seemed to be omitted, even though they were listed by acronym. These acronyms don't appear in the manuscript, only in the table. Could you explain this?”.

Response 5: Agree. Thank you for this observation. In the human toxicity analysis, only heavy metals were included, as they represent the main contributors to potential toxic effects from sewage sludge under thermochemical treatment conditions. Other contaminant groups such as PAHs, PCBs, and PFAS were not included in the study because no experimental data were available for these compounds, and the life cycle inventory (LCI) was developed exclusively from measured concentrations of heavy metals. Consequently, the references to PAHs, PCBs, and PFAS were removed from the results section, and this modification was made in Section 3.2.3 (Human toxicity and freshwater ecotoxicity impacts), line 327 on page 10. Additionally, a statement was added in the Conclusions section recommending that future studies expand the analysis of toxicity categories to include a broader range of contaminants, given their potential relevance—particularly for the HTC scenario, where higher impacts were observed.

“3.1.3. Human toxicity and freshwater ecotoxicity impacts

Human toxicity and freshwater ecotoxicity categories quantify the potential adverse effects of chemical emissions on human health and aquatic ecosystems, expressed as CTUh/FU for cancer and non-cancer impacts and CTUe/FU for ecotoxicity. These indicators reflect the influence of heavy metal emissions and their redistribution across air, water, and soil pathways.

For human toxicity, cancer effects (HT_c), all scenarios show positive values: S0 at 7.29×10⁻⁶ CTUh/FU, S1 at 2.07×10⁻⁵ CTUh/FU, and S2 at 3.38×10⁻⁵ CTUh/FU. In S0, contributions stem from combustion residues and gas-cleaning materials, while in S1 they arise mainly from sludge drying (1.56×10⁻⁵ CTUh/FU) and pyrolysis reactor operation (3.46×10⁻⁶ CTUh/FU). In S2, HT_c is influenced by sludge drying (5.83×10⁻⁶ CTUh/FU), HTC reactor emissions (2.45×10⁻⁶ CTUh/FU), and land application, where metal transfer to soils plays a relevant role. Small offsets from fertilizer substitution are observed in both S1 (– 8.91×10⁻⁷ CTUh/FU) and S2 (–1.63×10⁻⁶ CTUh/FU), but they do not change the overall positive sign.

In the human toxicity, non-cancer effects (HT_nc) category, S2 shows the highest contribution at 3.98×10⁻² CTUh/FU, clearly above S1 (1.38×10⁻⁴ CTUh/FU) and S0 (2.52×10⁻⁵ CTUh/FU). The elevated value in S2 is associated with metal transfer during hydrochar land application, while S1 reflects lower exposure potential due to metal retention in pyrochar. These differences highlight the importance of metal immobilization mechanisms, which warrant further experimental validation across a broader range of contaminants [67], [68] [69].

Regarding freshwater ecotoxicity (ET_fw) [70], all scenarios yield positive results: S0 at 1.21×10³ CTUe/FU, S1 at 4.84×10³ CTUe/FU, and S2 at 2.17×10⁴ CTUe/FU. In S2, ET_fw is largely driven by metal releases (notably zinc and copper) during land application, whereas in S0 contributions originate mainly from combustion processes. In S1, major contributors include sludge drying (3.80×10³ CTUe/FU) and pyrolysis reactor operation (8.32×10² CTUe/FU). Fertilizer substitution provides partial offsets in S1 (– 2.44×10² CTUe/FU) and S2 (–4.44×10² CTUe/FU), though the regional nature of the ET_fw characterization limits the extent to which these credits influence the total outcome.

Overall, the results show differing toxicity profiles across the scenarios. S1 presents comparatively lower values in HT_nc and avoids the elevated ET_fw observed in S2, reflecting the relative stability of metals in pyrochar. S2 is influenced by greater mobility of metals during land application, resulting in higher contributions in all toxicity-related categories. S0 shows lower toxicity magnitudes than S2 but remains consistently positive across categories, in line with the controlled emission mechanisms typical of incineration systems.”

Comments 6: “I would recommend slightly refining this sentence, meaning there's another type of pyrolysis that's been omitted here, namely "intermediate." You can refer to Energies 2024, 17, 5082, for example.”

Response 6: Agree. Thank you for this observation. The sentence was revised to include the missing “intermediate” pyrolysis type, as suggested. This modification was made in the Introduction (line 69). The updated text in the manuscript now reads:

“Depending on operating conditions such as temperature, heating rate, and residence time, pyrolysis can be classified as slow, intermediate, fast, or flash [19].”

Comments 8: “Please check the abbreviation HTc, which stands for Human Toxicity, Cancer Effects. Two designations are used: HTc and HT,c, with a comma.”

Response 8: Agree. Thank you for pointing this out. We carefully reviewed the entire manuscript and adjusted the symbols for HT_c and the other impact categories to maintain uniform notation throughout the text. Revisions were made in Figures, the list of abbreviations, and Section 2.6 (Impact assessment methodology) to ensure consistency across the manuscript.

Response to Reviewer 3 Comments

1. Summary

Thank you very much for reviewing this manuscript. We appreciate the reviewer’s careful assessment and the constructive feedback provided. The corresponding clarifications and revisions are addressed in the responses presented below.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Is the content succinctly described and contextualized with respect to previous and present theoretical background and empirical research (if applicable) on the topic?

Yes

Are the research design, questions, hypotheses and methods clearly stated?

Yes

Are the arguments and discussion of findings coherent, balanced and compelling?

Must be improved

Improved according to the reviewer’s suggestion.

For empirical research, are the results clearly presented?

Can be improved

Improved according to the reviewer’s suggestion.

Is the article adequately referenced?

Yes

Are the conclusions thoroughly supported by the results presented in the article or referenced in secondary literature?

Can be improved

Improved according to the reviewer’s suggestion.

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: “The assumption of a 100% substitution rate for Phosphorus (P) and Potassium (K) in both chars, and 100% for Nitrogen (N) in hydrochar but 0% in pyrochar, is a massive oversimplification and the primary driver of the results. The nutrient availability from chars, especially P, is highly dependent on its speciation and soil chemistry. It is not equivalent to highly soluble mineral fertilizers. Assuming 100% substitution is not scientifically defensible and likely leads to a significant overestimation of the environmental benefits for S1 and S2.”

Response 1: Agree. Thank you for this important observation. In the original version of the manuscript, high substitution rates were used to represent theoretical upper‐bound conditions, allowing the establishment of maximum reference values for the potential benefits of nutrient recovery. These values were not intended to reflect realistic agronomic performance but rather to contextualize the ideal limits of environmental outcomes for comparative purposes. Following the reviewer’s comments, this aspect was substantially revised.

First, the literature review on nutrient availability from sludge-derived chars was expanded, and the substitution assumptions were updated accordingly. Based on the evidence reported in recent agronomic and LCA studies, all substitution rate parameters (Subs_K, Subs_N_HTC, Subs_N_HR, Subs_P_HTC, Subs_P_HR) were assigned a uniform distribution between 0% and 25%, reflecting the wide variability documented for nutrient release and plant uptake from pyrochar and hydrochar. These revised ranges were incorporated into the uncertainty analysis to quantify their contribution to the variability of impact category results.

Second, the sensitivity analysis was redesigned to evaluate three representative substitution levels: 0% (no substitution), 10% (a feasible and realistic value supported by the literature), and 100% (the ideal upper limit). This framework enables the identification of minimum, feasible, and maximum environmental outcomes without implying that full substitution is achievable in practical soil applications.

Accordingly, Section 2.4 (Application of chars to soil) was rewritten to clearly define the updated substitution assumptions and the incorporation of uncertainty. The revised paragraph now reads (line 250 to 270):

“The substitution of mineral fertilizers was modeled following standard stoichiometric conventions widely used in agronomic and LCA studies. Phosphorus substitution was expressed as P₂O₅, where 1 kg of P in char acorresponds to 2.29 kg of P₂O₅ fertilizer. Potassium substitution was expressed as K₂O, where 1 kg of K in char replaces 1.2 kg of K₂O fertilizer. Nitrogen substitution was expressed on a mass basis, assuming that 1 kg of N in char is equivalent to 1 kg of N fertilizer. Based on experimental data [38], [57], this assessment applied a substitution rate of 0 % for nitrogen in pyrochar and 3.5 % for nitrogen in hydrochar. Substitution percentages for phosphorus were set at 3.5 % for pyrochar and 5.0 % for hydrochar [48], [58], [59], [60], [61], while potassium substitution was set at 10 % following reported K availability in sludge-derived chars[61]. To incorporate uncertainty in nutrient availability, all substitution parameters were modeled using a uniform distribution ranging from 0 % to 25 %, which reflects the broad variability reported in the literature for nutrient release and plant uptake from pyrochar and hydrochar. These distributions were integrated into the uncertainty analysis to examine their influence on the variability of impact category results. In addition, a sensitivity analysis of fertilizer substitution rates is presented in Section 3.2, where substitution levels of 0 %, 10 %, and 100 % are evaluated to assess how this parameter affects the magnitude and direction of environmental outcomes. 100 % is selected as an ideal maximal value that cannot be surpassed, and 10 % as a mean typical value. Detailed information on the LCI framework and modeling assumptions is provided in the Supplementary Material (Section S2).”

Finally, a dedicated sensitivity analysis of the substitution-rate parameter was included in Section 3.2 (line 489 to 520), Fertilizer substitution rate analysis, where substitution levels of 0%, 10%, and 100% were examined. This analysis highlights the decisive influence of nutrient substitution efficiency on the direction and magnitude of environmental outcomes, demonstrating its importance as a critical parameter in LCA studies of sludge-derived chars.

Comments 2: “The results show S2 (HTC) has dramatically higher human toxicity (non-cancer) and freshwater ecotoxicity impacts than S0 and S1, primarily due to heavy metal transfer to soil. The authors note this as a constraint but do not sufficiently grapple with its severity.”

Response 2: Agree. Thank you for this important comment. The discussion of toxicity-related categories was substantially revised to address the severity of the human toxicity (non-cancer) and freshwater ecotoxicity impacts observed for HTC (S2). The updated Section 3.2.3, Human toxicity and freshwater ecotoxicity impacts (line 353 to 389) now provides a more explicit and detailed interpretation of these results, based on the distribution of heavy metals across the different pathways considered in the LCA.

In the revised text, the magnitude of the HT_nc and ET_fw impacts in S2 is explained by the higher mobility of metals such as zinc and copper during hydrochar land application, which leads to significantly larger toxicity contributions than those observed for incineration (S0) and pyrolysis (S1). The section now clarifies that these outcomes constitute a key environmental constraint of HTC rather than a modeling artifact, and that they should be considered a critical factor in evaluating its applicability at scale. The discussion further contrasts the behavior of hydrochar and pyrochar, emphasizing that metal immobilization is more effective in S1, resulting in substantially lower toxicity potentials. By presenting these mechanisms clearly, the revised version directly addresses the reviewer’s concern and strengthens the interpretation of the comparative results. The new text reads as follows (line 353 to 388):

“3.1.3. Human toxicity and freshwater ecotoxicity impacts

Human toxicity and freshwater ecotoxicity categories quantify the potential adverse effects of chemical emissions on human health and aquatic ecosystems, expressed as CTUh/FU for cancer and non-cancer impacts and CTUe/FU for ecotoxicity. These indicators reflect the influence of heavy metal emissions and their redistribution across air, water, and soil pathways.

For human toxicity, cancer effects (HT_c), all scenarios show positive values: S0 at 7.29×10⁻⁶ CTUh/FU, S1 at 2.07×10⁻⁵ CTUh/FU, and S2 at 3.38×10⁻⁵ CTUh/FU. In S0, contributions stem from combustion residues and gas-cleaning materials, while in S1 they arise mainly from sludge drying (1.56×10⁻⁵ CTUh/FU) and pyrolysis reactor operation (3.46×10⁻⁶ CTUh/FU). In S2, HT_c is influenced by sludge drying (5.83×10⁻⁶ CTUh/FU), HTC reactor emissions (2.45×10⁻⁶ CTUh/FU), and land application, where metal transfer to soils plays a relevant role. Small offsets from fertilizer substitution are observed in both S1 (– 8.91×10⁻⁷ CTUh/FU) and S2 (–1.63×10⁻⁶ CTUh/FU), but they do not change the overall positive sign.

In the human toxicity, non-cancer effects (HT_nc) category, S2 shows the highest contribution at 3.98×10⁻² CTUh/FU, clearly above S1 (1.38×10⁻⁴ CTUh/FU) and S0 (2.52×10⁻⁵ CTUh/FU). The elevated value in S2 is associated with metal transfer during hydrochar land application, while S1 reflects lower exposure potential due to metal retention in pyrochar. These differences highlight the importance of metal immobilization mechanisms, which warrant further experimental validation across a broader range of contaminants [67], [68] [69].

Regarding freshwater ecotoxicity (ET_fw) [70], all scenarios yield positive results: S0 at 1.21×10³ CTUe/FU, S1 at 4.84×10³ CTUe/FU, and S2 at 2.17×10⁴ CTUe/FU. In S2, ET_fw is largely driven by metal releases (notably zinc and copper) during land application, whereas in S0 contributions originate mainly from combustion processes. In S1, major contributors include sludge drying (3.80×10³ CTUe/FU) and pyrolysis reactor operation (8.32×10² CTUe/FU). Fertilizer substitution provides partial offsets in S1 (– 2.44×10² CTUe/FU) and S2 (–4.44×10² CTUe/FU), though the regional nature of the ET_fw characterization limits the extent to which these credits influence the total outcome.

Overall, the results show differing toxicity profiles across the scenarios. S1 presents comparatively lower values in HT_nc and avoids the elevated ET_fw observed in S2, reflecting the relative stability of metals in pyrochar. S2 is influenced by greater mobility of metals during land application, resulting in higher contributions in all toxicity-related categories. S0 shows lower toxicity magnitudes than S2 but remains consistently positive across categories, in line with the controlled emission mechanisms typical of incineration systems.”

Comments 3: “The manuscript dismisses incineration as not leading in any category but fails to acknowledge its primary advantage that it is a proven, large-scale, and often economically viable technology for mass sludge reduction. In contrast, continuous, large-scale HTC for sewage sludge is noted as being in its infancy (only one commercial provider identified).”.

Response 3: Agree. Thank you for this valuable and constructive comment. We appreciate the reviewer’s observation and have strengthened the balance and clarity of the comparative assessment accordingly. The manuscript already acknowledges in the Introduction that incineration is a well-established and widely implemented treatment option for sewage sludge, noting its role in stabilization, volume reduction and the possibility of recovering phosphorus from ashes. This context positions incineration as the reference technology against which the emerging thermochemical alternatives are compared.

To address the reviewer’s suggestion more explicitly, the revised manuscript clarifies that the objective of the comparative analysis is to examine how pyrolysis (S1) and HTC (S2) redistribute environmental impacts when their products are valorized through land application, while incineration (S0) serves as the benchmark scenario. This framing ensures that the comparison presents differences across impact categories without implying technological superiority over incineration, whose operational maturity and stability are well recognized.

In the Results, Section 3.2.3 has been refined to highlight that incineration shows lower magnitudes in toxicity-related categories compared with the valorization routes. This addition provides a clearer interpretation of the mechanisms underlying the higher toxicity values observed for S1 and especially S2.

Finally, the Conclusions reinforce the role of incineration within the comparative framework by noting its capacity for substantial sludge volume reduction, its performance below 1 PE in most categories, and its suitability as a basis for subsequent nutrient recovery from ashes. These revisions ensure a balanced interpretation of the three thermochemical routes while acknowledging incineration as a proven and reliable option in sludge management.

Comments 4: “The methodology for calculating carbon storage in the soil from char application is not clearly described. The rates used for pyrochar and hydrochar should be explicitly stated and justified with references, as the stability of the carbon pools differs between the two materials.”.

Response 4: Agree. Thank you for this valuable comment. We agree on the importance of making the information regarding carbon storage in soil explicit in the manuscript. Accordingly, a clarifying statement was added in Section 2.4 (Application of chars to soil), in the paragraph at line 268 on page 8, specifying the storage rates and their experimental source. The added text reads as follows (line 271 to 274):

“The carbon storage in soil resulting from char application was also considered in the assessment. Fixed storage rates of 98.4 % for biochar and 88.5 % for hydrochar were applied, as reported in Table S5 of the Supplementary Material and based on experimental data from [61].”

Comments 5: “The results indicate a severe trade-off: HTC performs well on global metrics like climate change but very poorly on toxicity impacts. How should a decision-maker weigh a significant reduction in global CO2eq emissions against a substantial increase in the potential for local soil and water contamination? Does this trade-off challenge the conclusion that HTC has the "best overall environmental performance"?”

Response 5: Agree. Thank you for this thoughtful comment. The manuscript was revised to clearly reflect the severity of the trade-offs associated with HTC. Section 3.2.3 (Human toxicity and freshwater ecotoxicity impacts) now provides a detailed explanation of the substantially higher toxicity contributions observed for S2, which result from the greater mobility of metals such as zinc and copper during hydrochar land application. This section clarifies that these mechanisms lead to elevated values in both human toxicity (non-cancer) and freshwater ecotoxicity, in contrast with the lower magnitudes observed for S0 and the metal retention behavior of pyrochar in S1 (line 283 to 388).

To reinforce these findings, Section 3.3 (Uncertainty propagation) describes that S2 presents the largest uncertainty ranges precisely in the toxicity-related categories, indicating high sensitivity to parameters governing hydrochar behavior in soil (line 531 to 551). The text explains that the composition of the sewage sludge, particularly its zinc content, is a dominant contributor to this variability, underscoring the importance of metal mobility as a critical limitation for HTC under real operating conditions.

The Conclusions (line 607 to 621) were also strengthened to highlight this issue. The revised text states explicitly that HTC achieves reductions in several impact categories but is affected by the transfer of heavy metals into soil, which results in toxicity-related impacts. The Conclusions further indicate that no scenario performs best across all categories and that decision-making should balance HTC’s advantages in global metrics with the local risks associated with toxicity.

These revisions ensure that the manuscript presents a balanced interpretation of the environmental performance of HTC by acknowledging its benefits in climate-related categories while clearly articulating the magnitude and significance of its toxicity impacts, allowing decision-makers to weigh global benefits against localized environmental risks.