Impact of Combined Sewer Systems on the Quality of Urban Streams: Frequency and Duration of Elevated Micropollutant Concentrations
Round 1
Reviewer 1 Report
I propose the following:
line 58: should read ....is limiting the quantification... or ...is limited by the quantification....
line 65: a sentence cannot start with [24]. Rephrase
line 134: ensures that ......is kept constant
line 156: if bottom width is 4 m, top width cannot be 2 m
line 215:...August 2004. This does not agree with the values of the horizontal axis in Fig. 2
line 221: no mention of Fig. 3 in text
line 231: .....on September
line 237: a sentence should not start with 50%. Rephrase.
line 270:...is also not degraded. Delete "as diclofenac"
line 271:instead of "contrary" write "quantitatively inverse from"
line 279: On the contrary
line 286:similar to observation for line 237
line 301: ...of figure 8.
line 330: the phrase "when the occur during high flow" does not make sense
line 373: ...that have been reached...
line 383 ..they are generated...
line 385: a comma after In the simulated year
line 388: a comma after mecoprop
line 390: ...concentrations and(?)..
line 400: ...show how strongly...
Author Response
Thank you for the detailed comments. We followed all of your suggestions. In the following we do not address the comments where you provided word-for-word suggestions.
line 58: should read ....is limiting the quantification... or ...is limited by the quantification....
line 65: a sentence cannot start with [24]. Rephrase
Rephrased to “In [24] an integrated model …”
line 134: ensures that ......is kept constant
line 156: if bottom width is 4 m, top width cannot be 2 m.
The width is 11.6 m at a water level of 1.9 m. We corrected that in the text.
line 215:...August 2004. This does not agree with the values of the horizontal axis in Fig. 2
We corrected it to December 2004.
line 221: no mention of Fig. 3 in text
line 231: .....on September
line 237: a sentence should not start with 50%. Rephrase.
Rephrased to “A proportion of …”
line 270:...is also not degraded. Delete "as diclofenac"
line 271:instead of "contrary" write "quantitatively inverse from"
line 279: On the contrary
line 286: similar to observation for line 237
line 301: ...of figure 8.
line 330: the phrase "when the occur during high flow" does not make sense
Rephrased to “… when flow is high.”
line 373: ...that have been reached...
line 383 ..they are generated...
line 385: a comma after In the simulated year
line 388: a comma after mecoprop
line 390: ...concentrations and(?)..
line 400: ...show how strongly...
Reviewer 2 Report
The manuscript represents a study on the influence of CSO/WWTP effluence to river water quality. The writing is generally good with the following suggestions for revisions:
Figure 2: The reviewer suggests tilting the date tags so each date tag takes less horizontal space.
Figure 10: The legend and caption do not provide clear information for readers to understand the figure.
General criticism: 1) Please provide the setup parameters of the SWMM model so readers can replicate your study with confidence. Please also provide a detailed explanation about how the overland pollutant concentration. 2) Overland runoff appears to be totally neglected in this study. Since this is a long-term simulation, neglecting such an important element will cause serious inaccuracy of results. 3) The SWMM model appears to simulate the sewer part only, but how do you simulate river routing? 4) Since this study is designed to investigate “general patterns and phenomena that are typical for wet weather conditions in urban streams affected by combined sewer systems” (lines 80-82), this study should experiment with different scenarios to signal how potential variations at different parts of the system can affect the overall river water quality.
Comments for author File: 
Comments.pdf
Author Response
Thank you for your valuable comments that helped us improve the quality of the paper.
Figure 2: The reviewer suggests tilting the date tags each date tag takes less horizontal space.
Done
Figure 10: The legend and caption do not provide clear information for readers to understand the figure.
We extended the caption to “Concentration-duration-frequency curves in the river immediately upstream and downstream of the WWTP. The curves indicate the concentration that is exceeded continuously over the time D with the frequency n.”
General criticism:
1) Please provide the setup parameters of the SWMM model so readers can replicate your study with confidence. Please also provide a detailed explanation about how the overland pollutant concentration.
We included the SWMM-parameters in the Supplementary Materials and referred to them in the text (line 125) and added information on how subcatchment parameters were determined in line 122f. The entire model input data is subject to confidentiality due to the operator of the sewer system. However, we can provide the model input file upon request for scientific purposes.
Pollutant concentration in surface runoff was considered as constant, like in the other components wastewater and infiltration water (see line 200 ff). We are aware of the fact that in reality concentrations vary. However, these variations appear to be random. Knowledge on the underlying processes is too scarce to justify amore detailed approach. We added a reference to this problem in the last paragraph of the conclusion section (line 445 ff).
2) Overland runoff appears to be totally neglected in this study. Since this is a long-term simulation, neglecting such an important element will cause serious inaccuracy of results.
The description of the model was obviously misleading as we only referred to the sewer system and did not mention the hydrological model of the urban sub-catchments. We added a reference to the hydrological model in line 113: “The hydrological parameters of the sub-catchments were calibrated in a previous study.”
We also changed caption 2.1 to “Simulation of the Artificial System.”. This refers to line 63, where we defined the artificial system in our context as “the combination of paved surfaces and underground pipe network”. We hope that these modifications make clear that we did consider the overland runoff in the urban catchment.
3) The SWMM model appears to simulate the sewer part only, but how do you simulate river routing?
The river itself was not modelled as we used measured discharge data (see lines 139 ff and 149 f). To account for the mixing in the river we represented the river section that receives the discharge from CSO structures and WWTP as described in lines 160 ff.
To avoid misunderstandings, we replaced the term “river” in line 60by “The river section that receives discharge from the artificial system”. We hope that with this specification the concept is clear.
4) Since this study is designed to investigate “general patterns and phenomena that are typical for wet weather conditions in urban streams affected by combined sewer systems” (lines 80-82), this study should experiment with different scenarios to signal how potential variations at different parts of the system can affect the overall river water quality.
Of course, you are right. Our long-term goal is to assess the potential impact of specific management and treatment scenarios. However, that was beyond the scope of this study. We did not want to show results for single scenarios that are chosen somewhat arbitrarily. In future studies we will develop realistic scenarios systematically. We added a reference on that in line 445ff.
Round 2
Reviewer 2 Report
Thanks for providing a revised manuscript. The reviewer would like to provide the following comments:
- Please indicate in the manuscript that details of the model cannot be provided due to confidentiality reasons.
- Please provide a clear reference to the "previous study" related to the overland runoff.
- The reviewer can accept that overland runoff pollutant concentration was assumed constant in this study. However, the fact that these concentrations vary should be clearly addressed in the manuscript by citing the following publications: https://0-link-springer-com.brum.beds.ac.uk/article/10.1007%2Fs11270-018-3777-2 ; https://0-www-sciencedirect-com.brum.beds.ac.uk/science/article/pii/S0022098197000105 ; https://0-www-sciencedirect-com.brum.beds.ac.uk/science/article/abs/pii/S0025326X03004053
- Continued from the previous point, the reviewer suggests trying several different concentration scenarios to see the influence of concentration variation on the results. This kind of scenario testing should still be in the scope of this study.
Author Response
Thank you again for your valuable comments. We have adopted all of the proposed changes to further improve the quality of the paper.
- Please indicate in the manuscript that the details of the model cannot be provided due to confidentiality reasons.
"The full model input file cannot be provided due to confidentiality reasons." was added in line 125f.
- Please provide a clear reference to the “previous study” related to the overland runoff.
The reference was added in line 114.
- The reviewer can accept that overland runoff pollutant concentration was assumed constant in this study. However, the fact that these concentrations vary should be clearly addressed in the manuscript by citing the following publications: https://0-link-springer-com.brum.beds.ac.uk/article/10.1007%2Fs11270-018-3777-2 ; https://0-www-sciencedirect-com.brum.beds.ac.uk/science/article/pii/S0022098197000105 ; https://0-www-sciencedirect-com.brum.beds.ac.uk/science/article/abs/pii/S0025326X03004053
The sentence “Despite the fact that MP concentrations in rainfall runoff are not constant during rain events,…” citing the suggested references has been added in line 206f.
- Continued from the previous point, the reviewer suggests trying several different concentration scenarios to see the influence of concentration variation on the results. This kind of scenario testing should still be in the scope pf this study.
The aim of the study was not to do an accurate concentration modeling of the specific micropollutants shown in the study, but to assess the general behavior of groups of micropollutants and their dominant emission pathways. The four shown substances were chosen because they represent groups of micropollutants with different origin and removal efficiencies in the wastewater treatment plant. Therefore, we refrained from performing a general model sensitivity analysis with varying input concentrations of micropollutants in the different flow components. However, we understand that a variation of concentration and the impact of these variations is of interest to the readers.
A broad variation of the concentration in all flow components may be misleading and bring the reader to the conclusion that we tried to do a detailed concentration modeling. We e decided to vary only the concentration in the rainwater runoff for fluoranthene and mecoprop because these concentrations are most uncertain and have the highest impact on overall simulation results. We added a section on the variation of fluoranthene and mecoprop concentration in rainwater runoff from line 311 to 327 including a graphic of the results (figure 8).
Furthermore, we added a section on the influence of rainwater runoff concentration for fluoranthene and mecoprop on the concentration-duration-frequency relation from line 430 to 444 supported by a graphic in the Supplementary Materials (figure 14).
Author Response File: 
Author Response.docx
Round 3
Reviewer 2 Report
The reviewer thanks the authors' efforts in improving the manuscript. It should be able to be published in the current form if the editor and other reviewers do not have comments otherwise.
