Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is well structured, with a clear aim and methodology.
However, some clarifications are needed.

In the Introduction (lines 81–102), different data on CO₂ emissions from university waste are analysed. However, there is a lack of commentary, as well as missing information on the reference year, the size of the universities, and other contextual details. I propose developing a table to present more detailed and comparable data.

In the Materials and Methods section, there is a detailed description of the case study (including the object of analysis and data collection methods) as well as stoichiometric calculations, which appear realistic and are based on different scenario modelling.

Could you clearly specify the functional unit and the system boundaries? This is recommended for LCA and carbon footprint (CF) calculations and should be defined at the beginning of the methodology section.

I also recommend adding a dedicated Discussion section. Currently, the manuscript includes only a short conclusion and limitations section, but lacks a discussion in relation to existing literature, methodological choices (e.g. how CO₂e is calculated), and experiences from other universities. There is also a lack of further recommendations for future research and policy implications.

Author Response

Comment 1 — Introduction: lack of commentary and missing context (reference year, university size, and comparable details); suggestion to develop a table.

Response: We agree that cross-study comparison requires explicit contextual information. To address this, we (i) strengthened the narrative in the Introduction to clarify why published HEI carbon footprint / GHG inventory results are often not directly comparable (differences in reference year, institutional size/activity indicators, scope/boundary definitions, and whether waste is disaggregated), and (ii) added a dedicated summary table to present the requested contextual details in a compact, comparable format.

Revision made: We inserted Table 1 in the Introduction, summarizing selected HEI studies with explicit reporting of reference year(s), campus size/population indicators, waste-related scope (or whether waste is disaggregated), the key reported metric, and boundary notes. We also revised the surrounding paragraphs to avoid redundancy and to interpret the heterogeneity highlighted by the table, concluding with a clear statement that sanitary waste is rarely treated as a distinct stream and that SW-specific emission factors remain scarce—thereby motivating the contribution of this study.

 

Comment 2 — Materials and Methods: detailed and realistic case study description and stoichiometric calculations; request to clearly specify functional unit and system boundaries (recommended for CF/LCA).

Response: Thank you for highlighting the need to state these elements explicitly. We added a clear “System boundary” description and defined the functional unit at the beginning of the methodology to ensure transparency and reproducibility, while also clarifying that this work is not a full LCA.

Revision made: At the start of Materials and Methods, we added a dedicated subsection titled “System boundary” that defines the study boundary as scenario-based and process-focused (end-of-life management of sanitary waste). We explicitly state the functional unit (FU) as 1 tonne of sanitary waste (1 t SW), used to express emission factors and to scale annual scenario totals using the annual reference mass (ARM). We also clarified the reporting convention consistent with this revision (“total climate indicator” defined as CO₂ (factor 1 within the climate indicator) + CH₄-derived CO₂e (GWP100)), and we explicitly note that the study should not be interpreted as a life cycle assessment (LCA) because upstream production burdens and operational logistics/infrastructure processes are excluded. To make the boundary fully transparent, we added a short included/excluded process summary table (see next item).

 

Comment 3 — Recommendation to add a dedicated Discussion section (currently short conclusion/limitations; discussion needed versus literature, methodological choices, and experiences from other universities).

Response: We agree and have added a dedicated Discussion section to interpret the scenario results and to situate the findings relative to published HEI carbon footprint / inventory literature.

Revision made: We created a standalone “Discussion” section that:

(i) interprets why the degradation pathway controls the climate signal (with CH₄ dominating the climate indicator under anaerobic conditions),

(ii) contrasts our scenario-based results with published HEI inventories and reporting practices (including boundary/scope heterogeneity and normalization choices, supported by Table 1), and

(iii) explains the methodological implication of reporting a total climate indicator rather than a single “Total CO₂e,” improving interpretability and consistency in the context of sanitary waste as a distinct stream.

 

Comment 4 — Need for further recommendations for future research and policy implications.

Response: We appreciate this suggestion and expanded the Discussion to include policy-relevant implications and concrete future research directions.

Revision made: In the dedicated Discussion, we added practical implications for HEIs—namely that sanitary waste management should prioritize methane avoidance (aerobic stabilization/diversion where feasible) and, where anaerobic disposal cannot be avoided, effective CH₄ capture and combustion as the principal leverage to reduce the climate indicator. We also added future research recommendations focused on improving cross-campus benchmarking and institutional reporting (e.g., refining site-specific composition/degradability inputs, evaluating time-dependent CH₄ control performance, and harmonizing boundary definitions for consistent integration of SW within institutional GHG inventories).

 

We again thank you for your valuable feedback, which has substantially improved the clarity, transparency, and interpretability of the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents a practical, policy-relevant comparison of GHG emissions for sanitary waste management on a university campus. The scenario framing is clear and the results highlight the large climate penalty associated with uncontrolled anaerobic degradation. The manuscript would be significantly strengthened by improving methodological transparency and by clearly stating assumptions (system boundaries, emission factors, capture efficiencies, and treatment of biogenic carbon). At present, several steps in the emissions calculations are difficult to reproduce from the text.

Major comments

1) Clarify system boundaries and functional unit

Please define explicitly:

• The functional unit (e.g., “per year of sanitary waste generated by the institution”, “per tonne wet waste”, etc.).
• The system boundary: does the assessment include only degradation emissions at the disposal site, or also upstream/downstream processes such as collection, transport, energy use, material handling, and landfill operations?
• Whether the scenarios are meant to represent local, realistic options or “counterfactual” cases.

A simple boundary diagram (or a short table listing included/excluded processes) would improve interpretability.

2) Treatment of biogenic carbon (CO₂ accounting)

Sanitary waste (e.g., toilet paper) is primarily biogenic carbon. Many accounting frameworks (IPCC/typical LCA practice) treat biogenic CO₂ differently from fossil CO₂ (often reported separately or assigned a different convention). The manuscript should explicitly state:

• Whether biogenic CO₂ is counted in CO₂e or reported separately;
• Which accounting convention is used and why.

This is critical because a large share of the calculated CO₂ may be biogenic, while CH₄ is the main climate driver.

3) Provide reproducible stoichiometric derivations and parameter definitions

The manuscript uses empirical formulas and stoichiometric relationships to estimate CO₂ and CH₄ formation under aerobic vs anaerobic pathways. Please provide:

• The exact empirical composition assumed for the sanitary waste (or its dry matter fraction), and how it was obtained (literature, elemental analysis, typical tissue/paper composition, etc.).
• A transparent derivation (step-by-step, or in supplementary material) showing how the reaction coefficients were determined.
• Clear definitions and units for all key parameters (moisture content, volatile solids fraction, degradable organic carbon fraction, etc.), including where each value comes from.

Right now, several coefficients appear “hard-coded” and readers cannot easily replicate the results.

4) Methane capture efficiency and oxidation assumptions (Scenario B)

The paper assumes a capture fraction (e.g., 75%). Capture performance in landfills is time-dependent and site-specific. Please:

• Justify the chosen capture efficiency with a strong reference (or provide a plausible range).
• State whether your account for oxidation of CH₄ in cover soils (if applicable), and whether uncollected CH₄ is assumed to be emitted directly.

At minimum, present a sensitivity analysis for capture efficiency (e.g., 50/75/90%) because it can change the ranking and magnitude of results.

5) Uncertainty/sensitivity analysis

Given uncertainties in waste composition, degradability, CH₄ generation, capture efficiency, and GWP factors, please add at least a simple sensitivity test or uncertainty bounds. Suggested minimum:

• CH₄ capture efficiency range;
• Waste degradable fraction (± reasonable range);
• GWP time horizon (GWP100 vs GWP20) or at least justify the chosen time horizon.

Even a small section showing how robust the conclusions are would substantially improve credibility.

Minor comments (editorial and clarity)

1. Units and normalization: Ensure consistent use of units throughout (e.g., kg CO₂-equivalent (CO₂e, using GWP100) per tonne of wet waste vs per year). Please add a brief conversion check (e.g., mass of waste × emission factor = reported total) so readers can trace how annual totals were obtained.
2. Terminology: Use consistent terminology for “sanitary waste” (and its composition), and clarify whether this includes only toilet paper or also other hygiene materials.
3. Tables and equations: Consider moving long stoichiometric steps to Supplementary Material and keep the main text focused on assumptions and outputs. Ensure equations are numbered and referenced.
4. References: Where possible, cite primary sources for:
• Typical tissue/paper elemental composition;
• Landfill gas generation and capture efficiencies;
• Accounting conventions for biogenic CO₂;
• GWP factors used.
5. Language polishing: A light English edit will improve readability (a few grammatical issues and phrasing inconsistencies remain).

Author Response

Thank you. We have now explicitly defined the “functional unit (FU)” and the “system boundary” in the Methods section. The FU is stated as “1 tonne of sanitary waste (SW), wet basis”, which is used consistently for emission factors and for scaling to annual totals. We also clarify that the assessment focuses on “end-of-life degradation pathways” for SW, and that it is “not a full LCA”: upstream/downstream processes such as collection, transport, operational energy use, and detailed landfill operations are excluded to keep the scenario comparison transparent and policy-relevant.

To improve interpretability, we incorporated a concise “included/excluded process table” (Table 2) that functions as an explicit boundary statement. We also clarified that the scenarios are “stylized end-of-life pathways” designed for comparability in a university setting (i.e., scenario/counterfactual framing for decision support), rather than a detailed representation of site-specific operational practices.

 

Reviewer comment 1:

“Sanitary waste (e.g., toilet paper) is primarily biogenic carbon. Please state whether biogenic CO₂ is counted in CO₂e or reported separately; which convention is used and why.”

Response:

We agree this needed to be explicit. We now state clearly our accounting convention: for inventory-style reporting, “biogenic CO₂ is disclosed separately”, consistent with “IPCC convention”, while “CH₄ is expressed as CO₂e using GWP100”. For scenario comparability, we additionally report a **total climate indicator” defined as: “Total climate indicator = CO₂ (factor 1) + CH₄-derived CO₂e (GWP100)”. This makes clear that methane dominates the climate signal in anaerobic conditions, while CO₂ is reported transparently as biogenic.

 

Reviewer comment 2:

“Please provide the empirical composition assumed; transparent derivation of reaction coefficients; clear definitions and units for key parameters; and avoid ‘hard-coded’ coefficients so results are reproducible.”

Response:

Thank you—this was a key improvement point. We expanded the Methods section to clarify the empirical composition basis and parameter definitions used to construct the stoichiometric framework. We now provide:

1. the empirical formulas used for aerobic and anaerobic pathways (Table 3),
2. the complete stoichiometric equations used (Eqs. 4 and 5), and
3. explicit definitions and units for the relevant proximate/ultimate analysis parameters that support the empirical formula and yield calculations.

To further strengthen reproducibility while keeping the main text readable, we include a “worked, step-by-step reproducibility package” in “Supplementary Information S2”, showing:

• the algebraic derivation of coefficients,
• the numerical substitution using Table 3 inputs, and
• the exact reconstruction of the yields reported in Table 3C (including the wet-to-dry basis used for scaling).

  This ensures that readers can replicate the calculation pathway without ambiguity.

 

Reviewer comment 3:

“Please justify the chosen capture efficiency (e.g., 75%) with a strong reference or a plausible range. State whether oxidation is accounted for, and whether uncaptured methane is assumed emitted. Provide a sensitivity analysis (e.g., 50/75/90%).”

Response:

We appreciate this and have strengthened Scenario B accordingly. We explicitly state that “captured CH₄ is assumed to be combusted (flaring)” and that “uncaptured CH₄ is emitted directly”. We also clarify that “CH₄ oxidation in cover soils is not included” in the baseline boundary statement, for consistency with the scenario framing.

To address variability in capture performance, we added a “capture efficiency sensitivity analysis” at “50%, 75% (baseline), and 90%”. These results are reported in the Results section and summarized in “Supplementary Information S1 (Table S1)”.

Finally, as you requested, we include a “stronger reference” to support the baseline 75% capture assumption and to contextualize it within typical ranges reported in the landfill gas management literature/IPCC guidance.

 

Reviewer comment 4:

“Please add at least a simple sensitivity test or uncertainty bounds: capture efficiency range; degradable fraction range; GWP time horizon justification (GWP100 vs GWP20).”

Response:

We agree, and we have implemented a concise sensitivity/uncertainty check. Specifically, we now include:

“CH₄ capture efficiency sensitivity” (50/75/90%) for Scenario B”; a “±20% envelope” representing uncertainty in stoichiometric yield/degradability inputs; and a brief “GWP horizon sensitivity note”, explaining that shorter horizons increase the weight of methane and thus amplify methane-driven scenarios, while leaving the primarily aerobic scenario largely unchanged.

A consolidated version of these checks is provided in “Supplementary Information S1 (Table S1)”, and the key implications are summarized in the Results/Discussion to demonstrate that the scenario ranking and conclusions remain robust.

 

Reviewer comment 5:

“Ensure consistent use of units and include a brief conversion check so readers can trace annual totals.”

Response:

We standardized the reporting by: expressing yields and emission factors per “1 t SW (wet basis)”, and reporting scenario totals on an “annual basis” using the annual SW generation rate.

We also added traceable calculation statements (e.g., annual mass × EF = annual total), and the worked example in Supplementary S2 provides a transparent mass-balance and unit pathway.

 

Reviewer comment 6:

“Use consistent terminology for ‘sanitary waste’ and clarify composition (toilet paper only or other materials).”

Response:

We revised the wording for consistency throughout and clarified that the sanitary waste considered corresponds to “sanitary paper/toilet paper”, aligning the empirical composition and stoichiometric assumptions with that definition.

 

Reviewer comment 7:

“Consider moving long stoichiometric steps to Supplementary Material; ensure equations are numbered and referenced.”

Response:

We followed this recommendation. The main manuscript now presents the necessary equations and inputs for clarity, while detailed derivations and a numerical worked example are provided in “Supplementary Information S2”, keeping the main text focused on assumptions, outputs, and interpretation.

 

Reviewer comment 8:

“Where possible, cite primary sources for composition, landfill gas generation/capture, biogenic CO₂ conventions, GWP factors.”

Response:

We updated and strengthened citations accordingly, including (i) sources supporting biogenic CO₂ accounting conventions, (ii) the GWP100 factor used for methane, and (iii) references supporting the methane capture assumption and plausible ranges.

 

Reviewer comment 9:

“A light English edit will improve readability.”

Response:

We performed a careful English edit across the manuscript to improve clarity, grammar, and terminology consistency, while preserving the technical meaning. If additional polishing is requested after the second-round review, we will use the journal’s/publisher’s professional language-editing service to ensure the final version fully meets the editorial standard.

 

We again thank you for your detailed review and the opportunity to improve the manuscript. We believe the revised version is now substantially clearer, more transparent, and fully reproducible, and we hope it meets your expectations.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I would propose to delete in the whoe, text LCA - it is 3 times and after I accept as it is.

Author Response

Response: We appreciate the reviewer’s suggestion. We agree that using the term “LCA” may be misleading given our intended scope and boundary. We have therefore removed “LCA” throughout the manuscript and consistently replaced it with “scenario-based GHG emissions assessment”,  which accurately reflects the study design and system boundary.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have responded constructively and thoroughly to the previous round of review comments. The revised manuscript demonstrates substantial improvement, particularly in clarifying the study objective, system boundaries, methodological transparency, and the treatment of emission factors and climate indicators. The inclusion of additional explanation and sensitivity considerations has strengthened the robustness and interpretability of the results.

While minor technical and language issues remain (primarily formatting inconsistencies and numerical presentation), these do not affect the scientific validity of the study and can be addressed during final editorial processing.

In my opinion, the manuscript is now suitable for publication after minor editorial corrections.

Comments on the Quality of English Language

The manuscript is written in generally clear English. Minor grammatical and stylistic improvements would further enhance readability and precision.

Author Response

Response: We sincerely thank you for the careful re-evaluation and for acknowledging the substantial improvements made in the revised manuscript. We appreciate the positive assessment regarding the clarity of the study objective, system boundaries, methodological transparency, and the treatment of emission factors and climate indicators, as well as the added sensitivity considerations.

We agree that minor technical and language issues remain, mainly related to formatting consistency and numerical presentation. We have revised the manuscript accordingly to improve uniformity (units, symbols, significant figures, and separators) and to ensure consistent formatting throughout.

We thank the reviewer again for the constructive guidance, and we are pleased that the manuscript is now considered suitable for publication after minor editorial corrections.