Review Reports - Comparing the Agronomic and Economic Aspects of Sewage Sludge Composting and Vermicomposting

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General Assessment

The manuscript addresses the comparison between composting and vermicomposting of sewage sludge mixed with plant residues, evaluating both agronomic and economic aspects. While the study is relevant for waste management and circular economy discussions in developing countries, its novelty and impact are somewhat limited due to the restricted experimental design (only two mixtures, leading to four final samples). The paper is well-written and generally clear, but several areas need improvement to enhance clarity, robustness, and scientific contribution.

Strengths:

Clear experimental description and adequate monitoring of key parameters.
Both composting and vermicomposting processes were conducted and compared under the same conditions, ensuring consistency.
Inclusion of economic assessment, which is less common in similar studies.
Results demonstrate compliance with safety and quality standards for agricultural application.

Major Weaknesses:

Limited scope of the study: Only two waste mixtures were tested, resulting in four final products. This restricts the generalizability of the findings and weakens the scientific contribution.
Economic analysis: The cost–benefit evaluation is basic, focusing only on direct costs and nutrient value. Broader aspects (e.g., scalability, market demand, international price comparisons, hidden costs of large-scale implementation) are not discussed.
Literature comparison: The discussion is not sufficiently critical. Results are described, but their novelty compared to previous studies is not fully established.
Environmental implications: The study focuses on agronomic and economic parameters but neglects other important aspects such as greenhouse gas emissions, leachates, or micropollutant removal efficiency.

Specific Comments

Abstract and conclusions: Too descriptive; they should emphasize the main critical message (e.g., composting vs. vermicomposting trade-offs, practical recommendations).
Figures and tables:
- Tables should be reorganized to better highlight the initial composition of raw materials and the differences in final products.
- Figures (e.g., temperature, pH evolution) could be improved with clearer legends and contrasts to facilitate interpretation.
Discussion:
- Needs stronger critical comparison with existing literature (what is really new here?).
- The agronomic implications of low phosphorus levels should be acknowledged.

Author Response

General Assessment

Thank you very much for taking the time to review this manuscript. In the major weaknesses and the specific comments sections, you will find detailed responses and corresponding revisions to the track changes in the resubmitted file.

Strengths:

Clear experimental description and adequate monitoring of key parameters.

Both composting and vermicomposting processes were conducted and compared under the same conditions, ensuring consistency.

Inclusion of economic assessment, which is less common in similar studies.

Results demonstrate compliance with safety and quality standards for agricultural application.

Thank you very much for pointing out the strengths of this work. This review has been very constructive

Major Weaknesses:

Thank you very much for listing the weaknesses of this paper by topic. In this way, we can improve the manuscript by addressing key aspects that we had not initially taken into account.

Limited scope of the study: Only two waste mixtures were tested, resulting in four final products. This restricts the generalizability of the findings and weakens the scientific contribution.

Only two mixtures of sludge with plant residues were made because the plant residues with the greatest availability at the time of the year in which the experiment was conducted were used. Barley is an important crop in the area studied and is harvested between June and July, during the dry season, so a large amount of residue from this crop was available for the preparation of the mixtures. In addition, garden residues are available throughout the year, so they were also selected for the preparation of the mixtures. This has been indicated in the Material and methods section (see page 3; lines 132-138). The authors agree that, having made only two different mixtures, the results obtained cannot be generalized. Therefore, it has been pointed out in the conclusions that more comparative studies of both systems, composting and vermicomposting, are necessary for the treatment of sludge mixed with other plant wastes in order to contribute to the knowledge necessary for the promotion of the most appropriate technique (see page 14; lines 509-511).

Economic analysis: The cost–benefit evaluation is basic, focusing only on direct costs and nutrient value. Broader aspects (e.g., scalability, market demand, international price comparisons, hidden costs of large-scale implementation) are not discussed.

The economic analysis has only focused on direct costs and does not discuss broader aspects because it is an initial economic study in which differences between the two treatment systems have already been found. On the one hand, the cost of vermicomposting was higher than that of composting due to the initial investment in worms. However, the vermicompost obtained had a higher macronutrient content (especially P and K) and, in general, vermicomposts contain a higher concentration of plant growth hormones, which will have an impact on crop production. Therefore, to establish a global economic analysis, the vermicomposts and composts obtained should also be validated agriculturally to see if the increase in crop production compensates or not the higher costs of the vermicomposts. Regarding the study of the scalability of these methods, market demand and hidden costs of large-scale application have not been carried out because, as indicated above, only an initial study has been done to provide data on the costs of both processes under the same conditions and using the same waste mixtures. In addition, in Ecuador there is little data on the scalability of these techniques, since there are not many waste management companies. Most of the wastes are sent to sanitary landfills for final disposal, as mentioned in the Introduction section.

On the other hand, a comparison has been made with international prices, indicating that the costs for the production of compost and vermicomposts in this study were below the range observed in EU countries. (see page 13; lines 472-475)

Literature comparison: The discussion is not sufficiently critical. Results are described, but their novelty compared to previous studies is not fully established.

Throughout the manuscript, the results have been discussed with the few studies found on the comparison of composting and vermicomposting processes in the same waste mixtures, since the novelty of this work lies in the fact that there are no joint studies comparing both techniques using the same waste mixtures to valorize the sewage sludge, where the evolution of both processes, the quality of the materials obtained, and the economic aspects of these processes are studied.

Environmental implications: The study focuses on agronomic and economic parameters but neglects other important aspects such as greenhouse gas emissions, leachates, or micropollutant removal efficiency.

The environmental benefits of composting and vermicomposting techniques with respect to reducing the risks of applying untreated sewage sludge to soil, such as degradation of potentially toxic organic compounds, reduction of antibiotic content and abundance of antibiotic resistance genes and inactivation of pathogenic bacterial load are shown in the Introduction section (see page 2; lines 65-69). Also, a paragraph has been added in this section to indicate that vermicomposting and composting contribute to mitigating of other specific environmental problems, such as reducing greenhouse gases (see page 2; lines 69-74).

Specific Comments

Abstract and conclusions: Too descriptive; they should emphasize the main critical message (e.g., composting vs. vermicomposting trade-offs, practical recommendations).

The abstract has been modified and practical recommendations suggested by the other reviewers have been included in the conclusions section.

Figures and tables:

Tables should be reorganized to better highlight the initial composition of raw materials and the differences in final products.

Done. In all the tables, the raw materials used for the preparation of the composts and vermicomposts studied have been specified in the headings.

Figures (e.g., temperature, pH evolution) could be improved with clearer legends and contrasts to facilitate interpretation.

Done. In all the figures, the raw materials used for the preparation of the composts and vermicomposts studied have been specified in the captions.

Needs stronger critical comparison with existing literature (what is really new here?).

In the introduction section, we have shown the few studies that have been found on the comparison of the composting and vermicomposting processes of sewage sludge, focusing on the technological feasibility of these processes, the capacity of these techniques to reduce the pollutant load of the sludge, and the quality of the final compost and vermicompost. However, no joint studies have been found comparing both techniques using the same waste mixtures to valorize sewage sludge, where the evolution of both processes, the quality of the materials obtained, and the economic aspects of these processes are studied. This would be the novel part of this work.

The agronomic implications of low phosphorus levels should be acknowledged.

The agronomic implications of low phosphorus levels have been indicated how to mitigate them in section 3.4. (see page 10; lines 380-382).

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors, some comments for you:

Although the costs and economic value of nutrients were evaluated, it would be beneficial to include a more detailed analysis of long-term cost-effectiveness and economic impact under different scenarios.
Environmental benefits are mentioned, but the article could be improved by explaining how these techniques contribute to mitigating specific environmental problems, such as reducing greenhouse gases or water pollution.
It would be interesting to include a comparison with other sludge treatment alternatives, such as anaerobic digestion or direct use in soils, to provide a broader context.
The article could include specific recommendations for implementing these techniques in other regions with similar characteristics, considering technical and economic limitations.

With the above recommendations, the conclusions section could be improved, too.

Author Response

Dear Authors, some comments for you:

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in track changes in the re-submitted file.

Although the costs and economic value of nutrients were evaluated, it would be beneficial to include a more detailed analysis of long-term cost-effectiveness and economic impact under different scenarios.

The economic analysis has only focused on direct costs and does not address a more detailed analysis of profitability and long-term economic impact in different scenarios because it is an initial economic study in which differences between the two treatment systems have already been found. On the one hand, the cost of vermicomposting was higher than that of composting due to the initial investment in worms. However, the vermicompost obtained had a higher macronutrient content (especially P and K) and, in general, vermicomposts contain a higher concentration of plant growth hormones, which will have an impact on crop production. Therefore, to establish an overall economic analysis, the vermicompost and compost obtained should also be validated agronomically to see whether or not the increased crop production compensates for the higher costs of vermicompost.

Environmental benefits are mentioned, but the article could be improved by explaining how these techniques contribute to mitigating specific environmental problems, such as reducing greenhouse gases or water pollution.

A paragraph has been included in the Introduction section indicating that vermicomposting and composting contribute to mitigating specific environmental problems, such as reducing greenhouse gases or water pollution (see page 2; lines 66-74).

It would be interesting to include a comparison with other sludge treatment alternatives, such as anaerobic digestion or direct use in soils, to provide a broader context.

Thank you for pointing this out. The authors agree with this comment. Therefore, a comparison between the average costs of composting and vermicomposting of the SS studied with the costs of other more common SS disposal practices, such as land application, thermal drying and incineration has been included in section 3.5 (see page 13; lines 478-482).

The article could include specific recommendations for implementing these techniques in other regions with similar characteristics, considering technical and economic limitations.

The authors agree with this comment. Recommendations for implementing these techniques in developing countries have been included in the conclusions section (see page 13; lines 491-494).

With the above recommendations, the conclusions section could be improved, too.

The conclusions section has been modified with your recommendations.

Reviewer 3 Report

Comments and Suggestions for Authors

The research aims to compare the agronomic and economic aspects of sewage sludge composting and vermicomposting, specifically evaluating these processes using identical sewage sludge mixtures and different plant residues.

The scope encompasses both practical environmental management and the application potential of the end products. The topic aligns with the journal's aims and scope.

The paper offers novel comparative data specific to Ecuador that is of interest to both regional and international readership.

Despite a clear focus and logical structure, some improvements are required to enhance the manuscript’s scientific rigor and clarity. In particular, this Reviewer suggests the following:

1. The experimental design is sound, mainly grounded in standard protocols, but a few issues warrant attention. The authors prepared two mixtures of sewage sludge with garden waste and barley residue, respectively, and subjected both to composting and vermicomposting. While the rationale behind the selected proportions (based on free air space rather than chemical properties) is described, the decision not to adjust for the ideal initial C/N ratio (25–35) should be more fully discussed in light of its recognized influence on process outcomes. Several sentences in the methods section note, “they were not adjusted to the recommended ideal initial CN ratio 25-35,” yet the consequences of this deviation are not critically evaluated when interpreting the results.

2. The temperature monitoring and sanitation protocol during composting are described in adequate detail. However, the paper reports that “none of the piles studied met the requirement established by the EPA… temperatures 55°C for at least 15 consecutive days,” yet the eventual absence of Salmonella and low E. coli levels is attributed to “other factors,” such as “the presence of certain compounds resulting from organic matter degradation.” A more thorough explanation, referencing published data, is recommended to avoid any appearance of speculation.

3. In the vermicomposting experiments, the pre-composting procedure is justified and conforms to accepted practices. However, data presentation is sometimes confusing. The evolution of earthworm biomass is shown, yet its interpretation is vague (“the increase in earthworm population did not correlate with the rise in earthworm biomass”). While possible reasons, such as juvenile stages and population density, are mentioned, a deeper analytical approach (perhaps by referencing previous work in ecological composting or providing additional statistical analysis) would make the discussion more robust.

4. Regarding the presentation of results, tables are generally well laid out; however, there are minor inconsistencies in terminology and abbreviations. For example, various expressions (EC, dS/m, GI, MPN/g, “mature” stage) could benefit from brief explanatory footnotes to improve accessibility for all readers.

5. Statistical treatment appears sufficient, using LSD, one-way ANOVA, and Tukey’s test at p < 0.05. Nevertheless, some results (such as “EC was markedly higher in composts than in vermicomposts, especially in the case of compost 1 EC 5.57 dSm, which exceeded the limit value of the United States guidelines EC 5 dSm”) would benefit from explicit statements about the practical implications of exceeding these standards for agricultural users. Similarly, claims about the “absence of phytotoxicity problems” rely solely on a GI value greater than 50. However, the paper only briefly mentions Zucconi et al. (1981) and could improve confidence by providing supporting references to contemporary compost quality standards.

6. The economic analysis is an important aspect of the manuscript. The pricing structure and methodology for cost estimation are transparent, and the comparison between final product sale prices and production costs is clear. However, the discussion of cost differences, such as “the costs derived from water and energy used during both processes were the lowest,” should point to broader socio-economic implications or constraints, especially considering scaling up in Ecuador’s context. Furthermore, the manuscript should clarify whether the quoted selling prices of compost and vermicompost reflect wholesale or retail figures and mention potential quality differences between imported and locally produced products.

7. The manuscript demonstrates thorough referencing, drawing on a valuable range of contemporary and foundational studies. Nevertheless, some references are incomplete (for example, Table 3 refers to “US guidelinesa, b” without an explicit citation in the table legend). All references should be standardized and thoroughly documented.

8. In terms of writing style and logic, the discussion and conclusions provide a careful synthesis, highlighting both methods’ respective strengths: composting achieves faster OM stabilization and higher nitrogen content, vermicomposting produces amendments with higher potassium and phosphorus content and lower salinity. It is appropriate to suggest “more studies are needed to validate the composts and vermicomposts obtained from crops,” since the work as presented does not include field trials. However, this critical limitation should be emphasized in both the results and conclusions, as it directly impacts the practical applicability of the recommendations.

9. Several specific textual examples highlight areas for improvement. In the results section, the statement, “The transformation of Corg into CO2 during the OM degradation and mineralization processes occurring in composting and vermicomposting contributed to a loss of mass and thus to an increase of Nt in all the mixtures studied,” could be made more explicit by quantifying these effects and explicitly relating them to the input parameters.

10. In discussing the earthworm population, the phrase, “the scarce availability of nutrients for optimal worm development,” should be supported by reference to nutrient levels over time, perhaps drawing from specific figures or tables in the body of the paper.

11. The conclusion, “Therefore, both processes contribute to the circular economy by transforming SS into safe organic fertilizers that meet international standards,” is somewhat overreaching and requires support from field evidence; therefore, it must be rephrased. Recommendations for future research should explicitly involve field trials and assess the long-term environmental fate of possible residual contaminants.

Given these points, the manuscript requires major revisions before it can be considered for publication.

Comments on the Quality of English Language

Vide comments for authors

Author Response

The scope encompasses both practical environmental management and the application potential of the end products. The topic aligns with the journal's aims and scope.

The paper offers novel comparative data specific to Ecuador that is of interest to both regional and international readership.

Despite a clear focus and logical structure, some improvements are required to enhance the manuscript’s scientific rigor and clarity.

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions in track changes in the re-submitted file.

In particular, this Reviewer suggests the following:

The experimental design is sound, mainly grounded in standard protocols, but a few issues warrant attention. The authors prepared two mixtures of sewage sludge with garden waste and barley residue, respectively, and subjected both to composting and vermicomposting. While the rationale behind the selected proportions (based on free air space rather than chemical properties) is described, the decision not to adjust for the ideal initial C/N ratio (25–35) should be more fully discussed in light of its recognized influence on process outcomes. Several sentences in the methods section note, “they were not adjusted to the recommended ideal initial CN ratio 25-35,” yet the consequences of this deviation are not critically evaluated when interpreting the results.

In this study the initial waste mixtures were not adjusted to the ideal initial C/N ratio (25-35), but on the basis of optimal gas diffusion, to ensure that the composting process would develop properly. The ventilation system used was aeration by turnings and with this system it is important that the gas diffusion is adequate because the oxygen supply is not as effective as in the case of forced ventilation.

The temperature monitoring and sanitation protocol during composting are described in adequate detail. However, the paper reports that “none of the piles studied met the requirement established by the EPA… temperatures 55°C for at least 15 consecutive days,” yet the eventual absence of Salmonella and low E. coli levels is attributed to “other factors,” such as “the presence of certain compounds resulting from organic matter degradation.” A more thorough explanation, referencing published data, is recommended to avoid any appearance of speculation.

Thank you for pointing this out. The authors agree with this comment. A bibliographic reference has been included where other authors have found sanitization in sludge compost with the maintenance of lower temperatures than indicated by the EPA, to corroborate the result found in this study. These authors indicate that the determination of sanitization, based on a minimum required temperature and exposure time profile, is an indirect way of determining sanitization and that the direct way of determining sanitization should be by microbiological testing of the product (see page 6; lines 267-269).

In the vermicomposting experiments, the pre-composting procedure is justified and conforms to accepted practices. However, data presentation is sometimes confusing. The evolution of earthworm biomass is shown, yet its interpretation is vague (“the increase in earthworm population did not correlate with the rise in earthworm biomass”). While possible reasons, such as juvenile stages and population density, are mentioned, a deeper analytical approach (perhaps by referencing previous work in ecological composting or providing additional statistical analysis) would make the discussion more robust.

The authors regret that they cannot provide more data to corroborate the hypothesis that juvenile stages and population density could explain why the increase in earthworm population did not correlate with the rise in earthworm biomass. In this study, only the number of earthworms was determined without separating them by growth stages.

Regarding the presentation of results, tables are generally well laid out; however, there are minor inconsistencies in terminology and abbreviations. For example, various expressions (EC, dS/m, GI, MPN/g, “mature” stage) could benefit from brief explanatory footnotes to improve accessibility for all readers.

In all tables the abbreviations used have been specified, indicating them at the foot of the table or redirecting the reader to other tables where they have been specified.

Statistical treatment appears sufficient, using LSD, one-way ANOVA, and Tukey’s test at p < 0.05. Nevertheless, some results (such as “EC was markedly higher in composts than in vermicomposts, especially in the case of compost 1 EC 5.57 dSm, which exceeded the limit value of the United States guidelines EC 5 dSm”) would benefit from explicit statements about the practical implications of exceeding these standards for agricultural users. Similarly, claims about the “absence of phytotoxicity problems” rely solely on a GI value greater than 50. However, the paper only briefly mentions Zucconi et al. (1981) and could improve confidence by providing supporting references to contemporary compost quality standards.

The authors consider that compost 1 exceeded the EC limit value indicated by United States guidelines (EC < 5 dS/m) by very little. Therefore, no phytotoxic effects were observed when the GI test was performed. Furthermore, the limit established by Zucconi et al. (1981) of GI > 50% to indicate absence of phytotoxicity is of current use, because this index is performed with highly concentrated compost extracts and Lepidium sativum seeds, whose germination and root growth are very sensitive to the presence of phytotoxic compounds.

The economic analysis is an important aspect of the manuscript. The pricing structure and methodology for cost estimation are transparent, and the comparison between final product sale prices and production costs is clear. However, the discussion of cost differences, such as “the costs derived from water and energy used during both processes were the lowest,” should point to broader socio-economic implications or constraints, especially considering scaling up in Ecuador’s context. Furthermore, the manuscript should clarify whether the quoted selling prices of compost and vermicompost reflect wholesale or retail figures and mention potential quality differences between imported and locally produced products.

The economic analysis has only been focused on the Ecuadorian context, because it is an initial study that has been carried out to see if the composting and vermicomposting techniques were cost-effective for the treatment of sludge in the study area. The costs indicated are those of the process for obtaining one ton of final compost and vermicompost and would be the direct prices of these materials in this quantity (1 ton). Regarding the differences in quality between imported and locally produced products, we cannot make this comparison because we only have data on the quality of the composts and vermicomposts obtained in this study.

The manuscript demonstrates thorough referencing, drawing on a valuable range of contemporary and foundational studies. Nevertheless, some references are incomplete (for example, Table 3 refers to “US guidelinesa, b” without an explicit citation in the table legend). All references should be standardized and thoroughly documented.

The US guidelines^{a, b} references in Table3 are referenced in the foot of this table (see page 11; line 397).

In terms of writing style and logic, the discussion and conclusions provide a careful synthesis, highlighting both methods’ respective strengths: composting achieves faster OM stabilization and higher nitrogen content, vermicomposting produces amendments with higher potassium and phosphorus content and lower salinity. It is appropriate to suggest “more studies are needed to validate the composts and vermicomposts obtained from crops,” since the work as presented does not include field trials. However, this critical limitation should be emphasized in both the results and conclusions, as it directly impacts the practical applicability of the recommendations.

The authors agree with this suggestion and have included in the conclusions section that further studies are required to validate the compost and vermicompost obtained on crops (see page 14; lines 509-513).

Several specific textual examples highlight areas for improvement. In the results section, the statement, “The transformation of Corg into CO₂ during the OM degradation and mineralization processes occurring in composting and vermicomposting contributed to a loss of mass and thus to an increase of Nt in all the mixtures studied,” could be made more explicit by quantifying these effects and explicitly relating them to the input parameters.

The manuscript indicates how throughout the composting and vermicomposting processes both OM and Corg are reduced and this reflects the degradation of the organic compounds present, part of which is converted into CO₂, contributing to a loss of mass. In addition, the loss of OM that occurs in each process is quantified. This is indicated with the following text: “OM content decreased over time in all the mixtures studied in the composting and vermicomposting processes, reaching a total loss at the end of these processes of 28%, 29%, 27%, and 28% for composting mixes 1 and 2 and vermicomposting mixes 1 and 2, respectively (Table 2). Analogous to OM, the Corg content also decreased in all the mixtures during composting and vermicomposting” (see page 8; lines 303-307).

In discussing the earthworm population, the phrase, “the scarce availability of nutrients for optimal worm development,” should be supported by reference to nutrient levels over time, perhaps drawing from specific figures or tables in the body of the paper.

The authors regret not being able to corroborate with their own data the hypothesis of "low nutrient availability for optimal worm development" as an explanation for the fact that the increase in worm population did not correlate with the increase in worm biomass, because the nutrient content was only determined in the final materials and not throughout the whole process.

The conclusion, “Therefore, both processes contribute to the circular economy by transforming SS into safe organic fertilizers that meet international standards,” is somewhat overreaching and requires support from field evidence; therefore, it must be rephrased. Recommendations for future research should explicitly involve field trials and assess the long-term environmental fate of possible residual contaminants.

The authors agree with this suggestion and have included in the conclusions section that more studies are required to assess the long-term environmental fate of possible residual contaminants (see page 14; lines 509-513).

Given these points, the manuscript requires major revisions before it can be considered for publication.

The authors hope to have adequately addressed all the suggested modifications, which we consider important for improving the manuscript.

Comments on the Quality of English Language

Vide comments for authors

The manuscript has been reviewed by a native English speaker prior to submission to the journal. In addition, the other two reviewers have marked in the Quality of English Language that The English is fine and does not require any improvement, within their Review Report Forms.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have thoroughly revised the manuscript and implemented the suggested changes, which have considerably enhanced the clarity, structure, and overall quality of the work. The paper is now well-organized and suitable for publication

Author Response

Thank you very much for taking the time to review this manuscript again. The authors are pleased that all the suggested modifications have been well addressed.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors:

Your MS has been improved significantly, and the new information added contributes to clarifying several aspects of your results section.

Therefore, I have no further questions.

Author Response

Dear Authors:

Your MS has been improved significantly, and the new information added contributes to clarifying several aspects of your results section.

Therefore, I have no further questions.

Thank you very much for taking the time to review this manuscript again. The authors are pleased that all the suggested modifications have been well addressed.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript has been substantially improved, addressing most of the reviewers' concerns. However, further refinements are required.

In particular, please note the following points:

The rationale for using free air space instead of adjusting the initial C/N ratio is described. However, the scientific consequences of deviating from the recommended C/N ratio (25–35) are not discussed and should be evaluated. The authors should integrate a comparison with studies demonstrating the impact of C/N on compost stabilization, nutrient retention, and emission profiles, as this factor may significantly influence the interpretation of agronomic data.

While the addition of supporting references is a positive revision, the explanation regarding pathogen reduction mechanisms (chemical factors, microbial antagonism) would benefit from more in-depth referencing and cautious language to avoid speculation.

Despite statistical rigor, some interpretive claims (such as the practical implications of EC, GI thresholds, nutrient dynamics) remain underdeveloped. The implications of exceeding EC standards and reliance on GI as the sole phytotoxicity marker should be explicitly discussed with comprehensive referencing to international and local compost standards.

The evolution of earthworm biomass is quantitatively presented; however, the lack of stage-specific population data limits interpretation. The authors acknowledge these limitations, though the discussion of nutrient availability and population dynamics would benefit from more explicit referencing to published ecological studies.

Laboratory data support the claimed benefits of the circular economy and safe fertilizer standards; however, field validation is not included, a fact the authors now acknowledge. This key limitation must be further emphasized in conclusions and results, as policy and practical recommendations hinge on field evidence.

The manuscript now clearly defines cost and pricing structures; direct and indirect costs are well presented, and average market prices for Ecuador are compared to production costs. However, a brief note on quality differences between imported and local products is absent due to data limitations. This is reasonable, but a broader discussion of market constraints, socio-economic impacts, and future scalability is encouraged.

References and footnotes have been improved; however, a final check for consistency, completeness (e.g., “US guidelinesa, b”), and cross-referencing across tables is necessary to ensure scientific rigor.

Overall, in this Reviewer's opinion, the paper is not yet suitable for acceptance as-is; thus, major revisions are still required.

Author Response

The manuscript has been substantially improved, addressing most of the reviewers' concerns. However, further refinements are required.

Thank you very much for taking the time to review this manuscript again. Please find the detailed responses below and the corresponding revisions in track changes in the re-submitted file.

In particular, please note the following points:

The rationale for using free air space instead of adjusting the initial C/N ratio is described. However, the scientific consequences of deviating from the recommended C/N ratio (25–35) are not discussed and should be evaluated. The authors should integrate a comparison with studies demonstrating the impact of C/N on compost stabilization, nutrient retention, and emission profiles, as this factor may significantly influence the interpretation of agronomic data.

The authors have discussed the scientific consequences of not preparing waste mixtures in accordance with the recommended initial C/N ratio (25-35). In the case of temperature, it is indicated that mixture 1 reached thermophilic values (temperature > 40°C) during the first 2 days of composting, while in mixture 2, these values were not achieved until day 8 of composting. These results could be due to the higher initial value of the Corg/Nt ratio of mixture 2 compared to mixture 1, which slows decomposition due to an excess of organic carbon in relation to nitrogen for the microorganisms. In these cases, the composting microbiota must go through numerous life cycles using the only available nitrogen, which is that of the microorganisms that have died, oxidizing part of the excess C, and converting it into recalcitrant residues that degrade more slowly (see page 6, lines 243-251).

The authors have also included a comment on the lower final Nt content of the waste mixtures with very low initial C/N ratio (compost 1 and vermicompost 1). These amendments presented lower Nt contents compared to compost 2 and vermicompost 2, probably due to N loss by NH₃ volatilization during the high temperature period and also in the form of N₂O and N₂ emissions. When the initial Corg/Nt ratio is low, there is an excess of organic N mineralization and the inorganic N produced exceeds the metabolic needs of the microorganisms, favoring the loss of N in the form of different emissions. These losses reduce the agronomic value of the final materials and increase the environmental impact of the composting and vermicomposting processes (see page 8, lines 323-331).

While the addition of supporting references is a positive revision, the explanation regarding pathogen reduction mechanisms (chemical factors, microbial antagonism) would benefit from more in-depth referencing and cautious language to avoid speculation.

Done. Cautious language has been used to avoid speculation (see 6, lines 270-271).

Despite statistical rigor, some interpretive claims (such as the practical implications of EC, GI thresholds, nutrient dynamics) remain underdeveloped. The implications of exceeding EC standards and reliance on GI as the sole phytotoxicity marker should be explicitly discussed with comprehensive referencing to international and local compost standards.

The authors have included a comment indicating that more studies are needed to validate the compost and vermicomposts obtained in crops to confirm that their agricultural use will not cause damage to plant development, since the continued agricultural use of compost 1 has limitations in saline soils and plants with low tolerance to salinity due to its higher salt content (see page 10, lines 399-403).

The evolution of earthworm biomass is quantitatively presented; however, the lack of stage-specific population data limits interpretation. The authors acknowledge these limitations, though the discussion of nutrient availability and population dynamics would benefit from more explicit referencing to published ecological studies.

The authors have included a comment to point out that when there is a low amount of carbon, as in the case of waste mixtures with a low Corg/Nt ratio, this element becomes the limiting factor for earthworms. The Corg/Nt ratio was low during most of the vermicomposting processes for both waste mixtures 1 and 2, which could have led to less earthworm development. Also, a reference supporting this comment has been included (Xing et al., 2015) (see page 9, lines 354-359)

Laboratory data support the claimed benefits of the circular economy and safe fertilizer standards; however, field validation is not included, a fact the authors now acknowledge. This key limitation must be further emphasized in conclusions and results, as policy and practical recommendations hinge on field evidence.

The need for agricultural validation in crops of the composts and vermicomposts obtained has been indicated in the conclusions and in the abstract, as well as in section 3.4. Agronomic value of the final materials (see page 1, line 30; page 10, lines 399-403 and page 14, lines 533-534)

The manuscript now clearly defines cost and pricing structures; direct and indirect costs are well presented, and average market prices for Ecuador are compared to production costs. However, a brief note on quality differences between imported and local products is absent due to data limitations. This is reasonable, but a broader discussion of market constraints, socio-economic impacts, and future scalability is encouraged.

The authors do not have sufficient data to be able to indicate market limitations, socio-economic impacts and future scalability. These aspects are important and therefore, a comment has been included indicating the need to study them before selecting one of the techniques studied for the treatment of sewage sludge (see page 13, lines 507-509).

References and footnotes have been improved; however, a final check for consistency, completeness (e.g., “US guidelinesa, b”), and cross-referencing across tables is necessary to ensure scientific rigor.

Table 3 specifies the corresponding reference in each of the limits established by the US guidelines for compost (^aUS Composting Council (2001) and ^bEPA (2003)). Likewise, abbreviations have been specified in all tables so that there are no cross-references between them.

Overall, in this Reviewer's opinion, the paper is not yet suitable for acceptance as-is; thus, major revisions are still required.

The authors hope that all suggested modifications have been adequately addressed in the current version of the manuscript. We consider your comments important for the improvement of this manuscript.