Assessing Urban Metabolism through MSW Carbon Footprint and Conceptualizing Municipal-Industrial Symbiosis—The Case of Zaragoza City, Spain

Round 1

Reviewer 1 Report

Currently, the entire world is witnessing several environmental issues. One such issue is climate change. Mitigating climate change is therefore necessary using suitable and effective strategies. In this context, urban metabolism, municipal solid waste management and industrial symbiosis are the topics of considerable interest to the researchers.

The authors of this paper have made an excellent attempt to address the issue by way of analyzing the urban metabolism and conceptualizing municipal-industrial symbiosis. The paper is comprehensive and does talk something new or original. In my view, this manuscript can be accepted for publication in the journal 'Sustainability'. However, before acceptance the authors must carry out major revisions with reference to the comments given below:

1. The title of the paper too long. It should be shortened and reframed. The title could be something like this - "Assessing urban metabolism through MSW carbon footprint and conceptualizing municipal-industrial symbiosis - the case of Zaragoza city, Spain" or something similar.
2. In general, the mathematic data presented in the manuscript is fine. 
3. The authors need to elaborate "a little more" on the theoretical background of industrial symbiosis in the Introduction Section. Or a separate section can be added. Currently, it is given in a distorted way.
4.  Figure 1: What is MBT is not clear throughout the manuscript (Stage 2). In Stage 3, one of the important losses from Biomethanation is leakages of methane gas. So, alongside electric generation and organic decomposition, 'methane gas leakage' should also be shown. 
5. Mention the distance (in miles or km) between the stage 2 and stage 3 i.e. recyclable industries at appropriate place. 
6. Lines 100-102: Here the authors should mention which are those recyclable industries and what recycled products (names) are being produced.
7. Line 120: What is foreign landfill? It is unclear. Replace 'foreign landfill' with simple "landfill".
8. What is the specific weight or density (in Kg/m³) of MSW generated from Zaragoza city, Spain?
9. Table 1: What does asterisk means above Briks.
10. Table 2: In the title of the table, please provide 'suitable reference' for the assumptions made for briks.
11. Table 6: From the data it is not clear whether the MSW generated from Zaragoza city is per day or per month or per year? Similarly, what is amount of MSW generated per capita per day?
12. Line 342: What is TMB plant? Please ensure full forms of the acronyms used for the first time in the manuscript.
13. Figure 2: It is not clear why the 68% of MSW is transported in mixed form and another 38% in separated waste. Please clarify this in the text at appropriate place.
14. Figure 4: Some part of text written in this figure is in Spanish language. Please convert it to English. 
15. Line 633: Please also mention the euros in USD as well (in parenthesis).
16. Line 703: The last phrase in this sentence is unclear. Rephrase it.
17. Many references mentioned in the Reference Section are not written scientifically. Also, for net references, the retrieval dates are not mentioned. Please strictly follow the guidelines of the journal. 
18. As regard English, please use past tense at appropriate place throughout the manuscript. Instead of 'will be' or 'is', the authors should use "was". Also, get the manuscript checked from the English expert.

Author Response

We would like to thank the reviewer for taking the time in checking our paper. Please find attached a document with point-by-point answers.

Author Response File: Author Response.docx

Reviewer 2 Report

The topic of Urban Metabolism of Municipal Waste is very important especially extending the study to include the manufacturing processes. Overall, the manuscript addresses the issue adequately and the results are coherent.

Some comments are shown below:

A common note across the manuscript: Please expand on any abbreviation the first time it is mentioned. For example DRS (page 2 L94) WEEE (L296), TMB plant at the CTRUZ (L342), SAICA (L372)

Introduction:

P2

L63-66

UM studies from source to sink so from source to upstream production then demand then downstream processes then sink. Many scholars of UM have applied this method even for waste, where municipal waste is seen as a product of the demand phase within the city. You have only highlighted studies that start with municipal waste in their LCA accounting. Please also refer to other studies that utilize UM for the whole IE. For example:

Moffatt, S., & Kohler, N. (2008). Conceptualizing the built environment as a social–ecological system. Building Research & Information, 36(3), 248–268.

Zhang, Y. (2013). Urban metabolism: A review of research methodologies. Environmental Pollution, 178, 463–473.

Khalil, H. & Alahwal, A., Reunderstanding Cairo through urban metabolism: Formal versus informal districts resource flow performance in fast urbanizing cities. J Ind Ecol. 2021; 25:176–192. https://doi.org/10.1111/jiec.13056 

L83-86

Currently there is EPR Extended producer responsibility. Please refer to this concept.

P4

L129-135

Recommendations shouldn't be in the introduction.

L148-150

These sentences should be removed from the introduction as here the research statement should be mentioned and not the results.

2. Materials and Methods

L159-162

Please revise the grammar and spelling of this part.

3. MSW Analysis Results

P8 L303-305, L313-317, P9 L352-354, P16 L484, P18 L518-521

Any discussion of results should be in the results section and not within the results.

Author Response

We would like to thank the reviewer for taking the time in checking our paper. Please find attached a document with point-by-point answers.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

In my view, the authors have addressed almost all the comments given by both the reviewers. The quality of the paper has been significantly enhanced. Without doubt the paper should be accepted for publication.