Life Cycle Analysis of Lab-Scale Constructed Wetlands for the Treatment of Industrial Wastewater and Landfill Leachate from Municipal Solid Waste: A Comparative Assessment
, Md. Al Sadikul Islam 2, Tanveer Saeed 2, Sheikh Mokhlesur Rahman 3 and Nehreen Majed 2,*Round 1
Reviewer 1 Report
Authors have illustrated the environmental impacts using two LCA methods for different CWs. While there are some small improvements and clarifications the authors could make:
1) what is the difference between S3 and S4?
2) Line 195, the author mentioned "similar materials" have been selected as alternatives for those materials not included in the database. would author clarify what criteria have been used to finalize the substitute material in the database? please listed the materials that are not listed in the database and their substitutes.
3) what is data inventory Ecoinvent 3.4? any references?
4) would if possible to clarify the key assumptions for the LCA?
5) there are three sections numbered as "2.5."
6) line 199, why choose "low-voltage electricity"?
7) Table 2 needs to be reorganized.
Author Response
Responses to Independent Review Report from Reviewer 1
What is the difference between S3 and S4?
Response 1: The authors acknowledge the comment from the reviewer. The authors addressed the issue and supplementary document has been uploaded accordingly. To address the comment, authors cited the Supplementary figures in the lines 107, 119, and 121, 125 and 128 of the revise manuscript. As Supplementary figure 3 and Supplementary figure 4 represent, the difference between S3 and S4 is that as media materials, recycled brick was chosen for S3 and Sugarcane bagasse for S4 as treatment media in the respective studies.
Line 195, the author mentioned "similar materials" have been selected as alternatives for those materials not included in the database. would author clarify what criteria have been used to finalize the substitute material in the database? Please list the materials not listed in the database and their substitutes.
Response 2: The constructed wetland units in the studies that have been referred were designed using locally sourced materials, such as Sylhet sand and locally produced gravel. However, no local governing body or institute provides context for life cycle assessment (LCA) databases for local-level materials or processes in Bangladesh. As a result, this study utilized the Ecoinvent database 3.4, which is widely recognized as the largest and most consistent LCI database for impact assessment. Although materials such as Sylhet sand and locally produced gravel are not listed in the Ecoinvent global database, the authors used materials such as gravel and sand from the existing eco-invent database to represent our utilized materials. It was assumed that the locally produced materials have similar life cycles and processes to those in the global scenarios in the ecoinvent database. Further, cupola slag, the byproduct during steel production, is not available in the Ecoinvent database; thus, scraped metal was chosen instead of slag. And this case, we assumed cupola slag and scrapped materials have similar life cycle processes involved. In addition, the authors addressed the comment and revised lines 216-218 in the revised mansucript to mention all the substitutes.
What is data inventory Ecoinvent 3.4? any references?
Response 3: The authors acknowledge the comment from the reviewer. Authors used Ecoinvent 3.4 as inventory data and also updated line 202 in the revised manuscript with updated reference list.
Would if possible to clarify the key assumptions for the LCA?
Response 4: The authors acknowledge the comment from the reviewer. The authors addressed the comment and revised lines 198-202 in the revised manuscript to declare the fundamental assumptions.
There are three sections numbered as "2.5."
Response 5: The authors acknowledge the comment from the reviewer. The comment has been addressed by editing the subsequent section after 2.5 section as 2.6 in the revised manuscript.
- Line 199, why choose "low-voltage electricity"?
Response 6: Taking into account that electricity is generally produced, transported, and consumed at different voltage levels, the ecoinvent database categorizes/distinguishes electricity-related data according to the different voltage levels, namely high voltage (above 24kV), medium voltage (between 1 kV and 24 kV) and low voltage level (below 1 kV) (Electricity - ecoinvent). And in our pilot-scale constructed wetlands, we received a connection from below the 1 kV line. That's why according to Ecoinvent 3.4, authors used low-voltage electricity. Moreover, authors added a reference in line 222 in the revised manuscript to address the comment.
Table 2 needs to be reorganized.
Response 7: The authors acknowledge the comment from the reviewer. The comment has been addressed by reorganizing Table 2.
Reviewer 2 Report
Comments for author File: 
Comments.pdf
Author Response
Responses to Independent Review Report from Reviewer 2
- Clarify the term ‘’wetland train.’’
Response: The authors acknowledge the query from the reviewer. The term 'wetland train' refers to a series of wetland units that are present in a wetland system. Previously, many published reputed articles on constructed wetland used this exact term. The authors are providing reference of two articles below among many published articles before.
- Chavan, P. V., Dennett, K. E., Marchand, E. A., & Spurkland, L. E. (2008). Potential of constructed wetland in reducing total nitrogen loading into the Truckee River. Wetlands Ecology and Management, 16, 189-197.
- Kadlec, R. H., Bays, J. S., Mokry, L. E., Andrews, D., & Ernst, M. R. (2011). Performance analysis of the Richland-Chambers treatment wetlands. Ecological Engineering, 37(2), 176-190.
- Replace the old references such as [5], [10], [12], [19], [22], [26] with new ones.
Response: The authors acknowledge the comment from the reviewer. Accordingly, old references have been replaced with recently published articles in the revised manuscript for mentioned references.
- Photos for pilot-scale CWs are needed at least in supplementary section.
Response: The authors acknowledge the comment from the reviewer. In this revised version, the authors included the schematic diagrams of each of the mentioned CW systems in supplementary sections.
- A statistical data analysis of Table 1 & 2 regarding Influent and effluent characteristics is needed giving std. deviation or std. error.
Response: The authors appreciate the comments from the reviewer. Standard deviations of water quality characteristics of the influents and the effluents have been provided in Table 1. However, standard deviations for inventory materials are not applicable for Table 2 as the inventory data consisted of previously set values and did not have any data variance.
In this case, the authors are referring a previously published article on LCA of constructed wetlands which also did not involve any variation in data for the inventory datasets. Lopsik, K. (2013). Life cycle assessment of small-scale constructed wetland and extended aeration activated sludge wastewater treatment system. International Journal of Environmental Science and Technology, 10, 1295-1308.
- What do you mean about ‘’Damage Categories’’? Justify it. Render it with a proper term
Response: In response to the comment by the reviewer, in the life cycle impact assessment (LCIA), the results are validated into different impact categories as well as to the damage categories at the level of damages to human health, natural environment, and natural resources via damage indicators. The damage categories are end point approaches which goes further by changing the midpoint impact categories into more specific damage categories or indicators. The term 'damage categories' is commonly used in previously published LCIA articles, as many widely used LCA methods such as Impact 2002+, Eco-indicator 99, and Recipe convert impact results into various impact categories and more specific end point indicators, which are referred to as damage categories.
However, for clarification inside the article, two sentences have been included for well understanding of the damage categories in the revised manuscript in the lines 471-475.
- ‘’Effluent’’ was missing in Figure 5.
Response: In response to the comment provided by the reviewer, the effluent emission from the constructed wetlands did not contribute to any significant damage categories, as evidenced by the lack of impact in Figure 5 and its absence from the total score.
- Results of Section 3.3.1 must be discussed with literature. References are missing. The same happens until section 3.3.2.3.
Response: The authors appreciate the comment by the reviewer. The comment has been addressed and references have been included in lines 491, 494 of section 3.3.1 and lines 542, 546 of section 3.3.2.3 in the revised manuscript to discusse with literature with references.
- A comparative table between LCA results and other studies findings will be useful.
Response: The authors acknowledge the comment by the reviewer with due respect. In the present study, the authors quantified the environment impact of laboratory scale constructed wetlands through application of LCA to understand the potential impact of such promising nature based treatment technology. This is very noble in the context of laboratory scales CW and to the authors upmost knowledge, there is scarce amount of published articles on the laboratory scale CWs. Most published results on the LCA of CWs are for full scale CWs treatment systems which have larger treatment capacity compared to our laboratory scale units. It is important to mention, the bigger the scale of a treatment unit, the lesser impact it contributes to the environment. Hence, the authors believes it is not relevant to compare our laboratory scale LCAs to full scale CWs which was also out of scope of this investigation. This has the risk of demonstrating misleading judgements to come into concrete conclusion for this technology and sustenance argument.
- Make conclusion section more quantitative by presenting the results of LCA application.
Response: The authors appreciate the comment by the reviewer. Conclusion section has been modified by including additional quantitative data, such as LCA application outcomes, for presentation in the revised manuscript in the lines 682-686 and 706-709.
Reviewer 3 Report
The paper focuses on a critical issue concerning the environmental impact of wastewater treatment plants. It is rich in scientific findings and well written; I recommend that it be accepted after minor revisions. I would suggest that aurthos address the following point:
To argue for human carcinogenic and non-carcinogenic toxicity, health-risk estimation by total hazard quotient determination should be used.
Author Response
Responses to Independent Review Report from Reviewer 3
- The paper focuses on a critical issue concerning the environmental impact of wastewater treatment plants. It is rich in scientific findings and well-written; I recommend that it be accepted after minor revisions. I would suggest that authors address the following point:
To argue for human carcinogenic and non-carcinogenic toxicity, health-risk estimation by total hazard quotient determination should be used.
Response 1: The authors would like to acknowledge the reviewer for his valuable comment. The main objective of this study was to assess the impacts of the life cycle of constructed wetlands and the materials used in the water treatment process on various environmental and toxicity categories. To quantify these impacts, we used the SimaPro software and the Ecoinvent database 3.4, employing the Recipe 2016 and Impact 2002+ methods. Human carcinogenic and non-carcinogenic toxicity were two categories of results from the chosen impact assessment methods. However, it is essential to note that to measure the health risk by determining the total hazard quotient, a detailed level of data for each compound is required, which is not relevant to the present scope of this research. Our study was primarily designed to explain the life cycle burdens through existing and proven LCA methods. However, this could be potentially addressed in further studies.
Response to suggestions on English language revision
The authors appreciate the suggestion by the reviewers for thorough revision of English language. The authors have employed great effort and have gone vigorous review to make amandments in the revised manuscript.
