Cost-Effectiveness of Treatment Wetlands for Nitrogen Removal in Tropical and Subtropical Australia

Round 1

Reviewer 1 Report

The article ““Cost-effectiveness of treatment wetlands for nitrogen removal in tropical and subtropical Australia” is interesting and definitely suitable for publication in WATER.

I have several comments to the author. I am sure, that they should have no problems with improving their article before publication.

Abstract

In Europe we use term CW (constructed wetlands instead CWT,

Line 25 What's the difference between CTW and STPW? Do both systems treat wastewater?

Line 31 It should be A$50kg^-1DIN

Keywords

Please add cost-effectiveness and remove Moreton Bay

Introduction

Lines 42

Please describe term “safe levels” or use another term. What level (concentration) is safe?

Line 47 treated sewage instead sewage

Lines 53-67

There are many other applications of CW e.g. for treatment of leachate (reject water, process water) from sewage sludge treatment where very high nitrogen concentrations occur.

In the end of this chapter write clear what was the scientific and practical aim of your research.

Material and methods

The chapter seems to be too broad. Sub-chapters 2.1 and 2.2 should be shortened”

I suggest to create one sub-chapter “Statistical analysis, cost categories and standardisation” instead 2.4 and 2.5

Table 1 is very useful. You can put the data from “Land use …into the column “characteristics”

Do you need Figure 1?

Line 166

mg m^-2 h^-1- this is not clear for me,

Results and discussion

Table 2 and chapter 3.2.

There is no such term as average pH.  When analysing the pH, the range should be given.

So in my opinion you should remove part of figure 4 (pH inflow and outflow)

I like figure 7- clear and visual presentation. Is it possible to add graphical information about cost efficiency to this figure?

Conclusions

The chapter contains only general conclusions. Analysing the article I think it is necessary to give detailed conclusions (selected research results)

Also please consider attaching Fig. S3 (from supplementary data) to the main text.

Author Response

Thank you for your time reviewing this manuscript and your helpful suggestions. Find the response to the revisions below:

Reviewer 1

Abstract

In Europe we use term CW (constructed wetlands instead CWT).

We have changed the acronym CWT to CW throughout the manuscript.

Line 25 What's the difference between CTW and STPW? Do both systems treat wastewater?

We have clarified as follows:

L25: “We assessed the performance of constructed treatment wetlands (CW) and vegetated drains (VD) that treat agricultural runoff, and of sewage treatment plant wetlands (STPW), that polish treated effluent”

Line 31 It should be A$50kg-1DIN

We have changed the units as suggested

Keywords

Please add cost-effectiveness and remove Moreton Bay

We have added “cost-effectiveness metric” as “cost-effectiveness” is already in the title and removed Moreton Bay.

Introduction

Lines 42

Please describe term “safe levels” or use another term. What level (concentration) is safe?

We have rewritten this sentence as follows

L42: “Nitrogen (N) pollution in waterways is one of the most significant environmental challenges of our times, threatening humanity wellbeing [1].”

Line 47 treated sewage instead sewage

We have left the term “sewage”, as unfortunately, in many developing countries sewage is directly disposed in waterways without any treatment.

Lines 53-67

There are many other applications of CW e.g. for treatment of leachate (reject water, process water) from sewage sludge treatment where very high nitrogen concentrations occur.

Thank you for the recommendation, we have added examples of the applications:

L54: “Treatment wetlands are one of the most common treatment systems used to reduce N; they have various applications such as treating leachate during water treatment operations and reducing fertilizer runoff from surface and groundwaters”

In the end of this chapter write clear what was the scientific and practical aim of your research.

We have included the aims as follows:

L90: “. The aims of this research where (1) to investigate if treatment wetlands in Queensland are efficient at removing N, (2) to find environmental factors associated with the N remov-al efficiency, and (3) to determine whether treatment wetlands are cost-effective.”

Material and methods

The chapter seems to be too broad. Sub-chapters 2.1 and 2.2 should be shortened. I suggest to create one sub-chapter “Statistical analysis, cost categories and standardisation” instead 2.4 and 2.5

We have reduced the length of the Methods, dividing them into three main sections: 2.1 Study sites; 2.2 N removal; and 2.3 Cost-effectiveness

Table 1 is very useful. You can put the data from “Land use …into the column “characteristics”

Good idea, we have added the land-use for each site within the characteristics column

Do you need Figure 1?

I understand the Figure is not essential for international readers, but for our local partners it is. However, we are happy to accommodate if the Editor also suggest sending it to Supplementary Material.

Line 166; mg m-2 h-1- this is not clear for me,

We have clarified as follows:

L173: “For the sites with no flow measurements (CW3 and CW4), the areal rate of denitrification (NO₃^- -N removed in mg m^-2 h^-1, [17], and MFA, unpublished data) was used to estimate the annual loads of DIN removal.”

Results and discussion

Table 2 and chapter 3.2.

There is no such term as average pH.  When analysing the pH, the range should be given. So in my opinion you should remove part of figure 4 (pH inflow and outflow)

Thank you for this comment, it gave me the opportunity to learn about the problems of averaging pH values. Averaging is not adequate when the goal is to obtain the final pH of two mixing waters with different pH. In this case, the average should be done after converting pH to hydrogen proton concentrations [H+]. However, for other uses in aquatic sciences when the goal is usually to average the same water at different times, the average is appropriate.

See:

Boyd et al.2011.  Interpretation of pH, Acidity, and Alkalinity in Aquaculture and Fisheries. North American Journal of Aqucaulture. 73: 403-408

I like figure 7- clear and visual presentation. Is it possible to add graphical information about cost efficiency to this figure?

Thank you, we have added the costs range to the Figure.

Conclusions

The chapter contains only general conclusions. Analysing the article I think it is necessary to give detailed conclusions (selected research results).

We have improved the Conclusion section as follows:

L505: “Treatment wetlands in tropical and subtropical Australia can achieve effective DIN (44%) and TN (91%) removal when complying with certain characteristics. N-removal occurs when DIN and TN concentrations in the inflow are > 0.2 mg L−1 for DIN and > 0.7 mg L−1 of TN. Additionally, removal is highest when vegetation cover is > 50%, when the wetland has a length:width ratio of at least 3:1, and when TSS in the inflow are either low, or managed through sedimentation basins.

The costs of designing, constructing and maintaining treatment wetlands in agricul-tural settings of the Great Barrier Reef are lower than previously assumed. These wetlands, when effective, can provide a cost-effective option for N-management, de-livering DIN removal for less than the benchmark cost-effectiveness of US$110 kg-1 DIN at end-of-catchment recommended for Great Barrier Reef catchments.

For STPW in urban settings, costs were higher, but besides their high capacity for N removal, they can provide additional co-benefits, which were not included in these analyses. Cost-effectiveness of agricultural and STP wetlands would be improved further if effective maintenance and repair prolong operational lifetime beyond 20 years and if the value of additional co-benefits is incorporated.

In the future, a standarised monitoring and cost recording methodology is recom-mended to improve cost-effective estimations. Importantly, detailed and long-term measurements of water flows will improve load calculations. Despite uncertainties, our results are conservative and provide clear evidence that treatment wetlands should be included in the mix of options for reducing N pollution in tropical and subtropical Australia.”

Also please consider attaching Fig. S3 (from supplementary data) to the main text.

We have included the two panels of Fig. S3 in the Figure 6 within the main text.

Reviewer 2 Report

The manuscript "Cost-effectiveness of treatment wetlands for nitrogen removal in tropical and subtropical Australia" is an incredibly interesting and worth publishing work that emphasizes the economic feasibility and an environmentally friendly approach to wastewater treatment.

Nonetheless, I have a few observations:

1. Materials and methods:  The brand of the equipment used to measure pH, dissolved oxygen, and so on should be mentioned in this section.

2. Was a plant/macrophyte species evaluation carried out? Because different species in different areas can assimilate different nitrogen concentrations.

Author Response

Thank you very much for the time you took to review this manuscript. Please find a response to your recommendations below:

Reviewer 2

The manuscript "Cost-effectiveness of treatment wetlands for nitrogen removal in tropical and subtropical Australia" is an incredibly interesting and worth publishing work that emphasizes the economic feasibility and an environmentally friendly approach to wastewater treatment.

Nonetheless, I have a few observations:

1. Materials and methods: The brand of the equipment used to measure pH, dissolved oxygen, and so on should be mentioned in this section.

Because this study was a meta-analysis of 11 different datasets of treatment wetlands, we don’t have information on all the equipment that was used for each site.

2. Was a plant/macrophyte species evaluation carried out? Because different species in different areas can assimilate different nitrogen concentrations.

There hasn’t been a formal evaluation on macrophyte composition for all the sites, but we have included the species composition from the sites we are familiar with (see Table 1).

e.g. CW5: “ System draining banana farm. Designed with a retention time of two days, very high length: width ratio. Most reeds and sedges planted did not survive and it became dominated by invasive grasses Urochloa mutica and Hymenachne amplexicaulis.”

CW7: “Treatment train system draining sugarcane with multiple ponds and a high length: width ratio. Vegetation community includes Schoenoplectus sp., Lomandra sp, and U. mutica”

Reviewer 3 Report

The study "Cost-effectiveness of treatment wetlands for nitrogen removal 2

in tropical and subtropical Australia" is interesting and well-written.

It is however has several technical issues and missing some discussions as follow:

Line 21: give a short description of "treatment wetlands"

Line 25, 26, and the rest of the manuscript: Abbreviation should be written in a bracket. For example: "constructed treatment wetlands (CTW)". 

Line 33, 34: monetary values should be presented in USD as readers are international. In the manuscript body (not abstract), the conversion rate or time of conversion should be mentioned. 

Throughout the manuscript: Paragraphs should be forced-justified. Apply the appropriate MDPI style type from the style panes. 

Conclusion is under developed. Start by summarizing what the study has done before listing summary of the findings and then follow with study limitations, implication of the study towards the other regions in the world, and finally end with future works. 

Author Response

Reviewer 3

The study "Cost-effectiveness of treatment wetlands for nitrogen removal 2in tropical and subtropical Australia" is interesting and well-written.It is however has several technical issues and missing some discussions as follow:

Line 21: give a short description of "treatment wetlands"

We have clarified as follows:

L25: “We assessed the performance of constructed treatment wetlands (CW) and vegetated drains (VD) that treat agricultural runoff, and of sewage treatment plant wetlands (STPW), that polish treated effluent”

Line 25, 26, and the rest of the manuscript: Abbreviation should be written in a bracket. For example: "constructed treatment wetlands (CTW)".

We have added brackets when presenting new acronyms

Line 33, 34: monetary values should be presented in USD as readers are international. In the manuscript body (not abstract), the conversion rate or time of conversion should be mentioned.

We have changed the monetary value to USD and clarified conversion rates in the Methods. In the Abstract we wrote both, USD and AUS currency, as this information is important for our local partners.

e.g. L34: “These high performing CWs and VDs removed N for less than US$37 kg-1 DIN (AU$50 kg-1DIN), less than the end-of-catchment benchmark for the Great Barrier Reef of US$110 kg-1 DIN (AU$150 kg-1DIN).”

L220: “Each treatment wetland was constructed at a different time; thus, each cost item was expressed in the current financial year (FY 2020/21) in Australian dollars (A$) using the Consumer Price Index (Australian Bureau of Statistics, 2021), and then converted to US dollars (US$) using an exchange rate of 0.73 on 15 November 2021.”

Throughout the manuscript: Paragraphs should be forced-justified. Apply the appropriate MDPI style type from the style panes.

We have changed the formatting of the paragraphs to be forced-justified

Conclusion is under developed. Start by summarizing what the study has done before listing summary of the findings and then follow with study limitations, implication of the study towards the other regions in the world, and finally end with future works.

We have improved the Conclusion section as follows:

L504: “Treatment wetlands in tropical and subtropical Australia can achieve effective DIN (44%) and TN (91%) removal when complying with certain characteristics. N-removal occurs when DIN and TN concentrations in the inflow are > 0.2 mg L−1 for DIN and > 0.7 mg L−1 of TN. Additionally, removal is highest when vegetation cover is > 50%, when the wetland has a length:width ratio of at least 3:1, and when TSS in the inflow are either low, or managed through sedimentation basins.

The costs of designing, constructing and maintaining treatment wetlands in agricultural settings of the Great Barrier Reef are lower than previously assumed. These wetlands, when effective, can provide a cost-effective option for N-management, delivering DIN removal for less than the benchmark cost-effectiveness of US$110 kg-1 DIN at end-of-catchment recommended for Great Barrier Reef catchments.

For STPW in urban settings, costs were higher, but besides their high capacity for N removal, they can provide additional co-benefits, which were not included in these analyses. Cost-effectiveness of agricultural and STP wetlands would be improved further if effective maintenance and repair prolong operational lifetime beyond 20 years and if the value of additional co-benefits is incorporated.

In the future, a standarised monitoring and cost recording methodology is recommended to improve cost-effective estimations. Importantly, detailed and long-term measurements of water flows will improve load calculations. Despite uncertainties, our results are conservative and provide clear evidence that treatment wetlands should be included in the mix of options for reducing N pollution in tropical and subtropical Australia..”