Microplastic Concentrations in Raw and Drinking Water in the Sinos River, Southern Brazil
Round 1
Reviewer 1 Report
- The treatment units of the water purification plants should be introduced.
- Some important reference should be cited in the review papers “Microplastics in freshwaters and drinking water: Critical review and assessment of data quality” and “Microplastics in freshwater systems: A review on occurrence, environmental effects, and methods for microplastics detection” published on water research.
- The MP should be removed significantly by the sand filtration process (> 90%). The removal of this study, however, was only 68%. Why?
- The fragment is the most frequently reported shape reported by the references. Why did not the authors find this shape in the samples?
- Line 89, the data “1x10-2 p m-3” should be “1x10-2 p m-3”. Moreover, the unit of MP concnetrations should be unified p/L or p m-3.
Author Response
Referee 1: We copied your questions and our answers in the following text:
1. The treatment units of the water purification plants should be introduced.
Answer of the authors: We do not completely understand this suggestion. Do you mean to show the locations of treatment plants on the map? If this suggestion is related to the remaval rate of MP by the water treatment plants, please see the comments on this subject below. We removed the calculation of the removal rate, following the suggestion of the second referee.
2. Some important reference should be cited in the review papers “Microplastics in freshwaters and drinking water: Critical review and assessment of data quality” and “Microplastics in freshwater systems: A review on occurrence, environmental effects, and methods for microplastics detection” published on water research.
Answers of the authors: The study of Koelmans et al. 2019 is already cited. Please see reference 24. The review of Li et al. 2018 was included. Please see reference number 50.
3. The MP should be removed significantly by the sand filtration process (> 90%). The removal of this study, however, was only 68%. Why?
Answer of the authors: In the revised version of the ms we followed the suggestion of the second referee and excluded the calculation of the removal rate. This was necessary, because we could not exclude erosion processes of plastic pipes and water reservoirs, commonly used in Brazilian residences, which could have microplastic particles on the way between the treatment plant and the consumer.
4. The fragment is the most frequently reported shape reported by the references. Why did not the authors find this shape in the samples?
Answers of the authors: The cited references show the dominance of fibers: Hendrickson et al. 2018, page 1791: Particle Morphology and Spatial Distributions: “The most frequent particles observed were fibers (228 particles), followed by fragments (200), and then films (121). Beads/spheres (9), foams (3), and others (21) were observed in lesser quantities.”
Zhao et al. 2014: Table 3 - shows 79.1% fibers in the Yangtze estuary; 83.2% fibers in coastal waters.
Vermaire et al. 2017, fig. 2: Plastic microfibers as shown in photos (c) and (d) were the most common plastic particles observed in the samples.
5. Line 89, the data “1x10-2 p m-3” should be “1x10-2 p m-3”. Moreover, the unit of MP concentrations should be unified p/L or p m-3.
Answer of the authors: We converted the units to particles (p) L-1, except the values given for areas, like the study of Free et al. in a remote lake of Mongolia.
Reviewer 2 Report
Review of Ferraz et al 2020 - Water
In this paper, the authors took samples from river water and drinking water to look at trends in MP contamination along the Sinos River basin. While the lab methodology appears robust, unfortunately, due to the mentioned issues of contamination between the treatment plant and the tap, I don’t believe any of the drinking water sample numbers can be used, especially as indicators of removal by the plant. If the authors want to salvage some of this data, I suggest focusing on the river samples and the story about their gradient. It is also key that they authors discuss the timing of the samples and whether there had been any rain in December 2018. The drinking water numbers might potentially be used to estimate consumption of plastics, but can’t be used to discuss the treatment process.
Abstract
The graphical abstract is mostly good, although I am not entirely sure what I am meant to get out of the 4 images at the bottom.
Introduction
The paragraph structure in the introduction is very imbalanced. You shouldn’t have a paragraph with two sentences. This could definitely be condensed to one page and refined so that it flows more naturally and emphasises the synthesis of the literature. Some specific comments below:
Line 32: This history lesson in plastics isn’t really necessary.
Line 46: Again, this is a bit extraneous information. I think it is general knowledge that microplastics have come to the forefront of water research in recent years.
Line 61, Line 66, Line 68: These numbers from other papers really mean nothing without context. I have no clue if these are big numbers are small numbers. I realise it is because these papers report plastics in different metrics, but if you are just reporting your results in particles/L, I would suggest you stick with only reporting numbers from papers that also report in this way.
Line 89: Convert these to p/L
Materials and Methods
Line 98: Fix number
Line 106: Were the samples all collected on the same day? If not, what was the precipitation patterns around the data collection?
Line 111: Were these shoreline samples?
Results
Line 155: I don’t know if it makes sense to calculate the removal rate this way. It seems to do that you would need to sample near the intake of each drinking water plant and then compare with the tap water on that distribution network.
Discussion
Line 185: See comment above.
Line 199: This really throws a wrench into your sampling plan and makes it impossible to draw any conclusions about removal rates.
Line 240-248: This isn’t really relevant.
Line 249-254: Did you look at nano plastics?
Line 255-262: Again, this isn’t really relevant in a discussion of your results. A couple of sentences about the impact of ingestion is sufficient, but not multiple paragraphs.
References
This is far too many references for an 8 page paper. You need to be more targeted and concise when addressing previous literature.
Author Response
Referee 2 - We copied your questions and our answers in the following text
Abstract
The graphical abstract is mostly good, although I am not entirely sure what I am meant to get out of the 4 images at the bottom.
Answer of the authors: The four images are related to the use of drinking water. In the current version we included some large, more visible fibers to show their presence in drinking water.
Introduction
The paragraph structure in the introduction is very imbalanced. You shouldn’t have a paragraph with two sentences. This could definitely be condensed to one page and refined so that it flows more naturally and emphasises the synthesis of the literature. Some specific comments below:
Line 32: This history lesson in plastics isn’t really necessary.
Answers of the authors: History lesson excluded.
Line 46: Again, this is a bit extraneous information. I think it is general knowledge that microplastics have come to the forefront of water research in recent years.
Answers of the authors: We excluded the first three paragraphs with generic information.
Line 61, Line 66, Line 68: These numbers from other papers really mean nothing without context. I have no clue if these are big numbers are small numbers. I realise it is because these papers report plastics in different metrics, but if you are just reporting your results in particles/L, I would suggest you stick with only reporting numbers from papers that also report in this way.
Answer of the authors: We converted the units to particles (p) L-1, except the values given for areas, like the study of Free et al. in a remote lake of Mongolia. The intention of this paragraph is to show the wide range of microplastic concentration.
Line 89: Convert these to p/L
Answer of the authors: Values converted
Materials and Methods
Line 98: Fix number
Answer of the authors: Number fixed. The correct number is 1.3*106 habitants.
Line 106: Were the samples all collected on the same day? If not, what was the precipitation patterns around the data collection?
Answer of the authors: We rephrased: “All samples were collected during three days in December 2018. No rain fell during the sampling”.
Line 111: Were these shoreline samples?
Answer of the authors: Yes, all river water samples were collected close do the riverbanks. We used waders to enter the water, which limited total water depth to about 1.2 m. We included the sentence “All samples were taken close to the riverbanks at a total water depth of approximately 1.2 m” in the text.
Results
Line 155: I don’t know if it makes sense to calculate the removal rate this way. It seems to do that you would need to sample near the intake of each drinking water plant and then compare with the tap water on that distribution network.
Answer of the authors: We removed the calculation of the removal rate.
Discussion
Line 185: See comment above.
Answers of the authors: We deleted the results and the discussion of the removal rate (heavyheartedly). It is not 100% possible to exclude the possiblity that at least part the counted particles in drinking water are the result of erosion of pipes or water reservoirs. However, erosion of pipe and water reservoir material would most probably produce fragments, not fibers. The fact that we found the fibers in the same proportions in raw and in drinking water may support the idea that the fibers in drinking water were not removed by the treatment process.
Line 199: This really throws a wrench into your sampling plan and makes it impossible to draw any conclusions about removal rates.
Answers of the authors: We deleted the discussion of the removal rate. Please see the comment above.
Line 240-248: This isn’t really relevant.
Answer of the authors: We could not locate the paragraph you are referring to. In our version of the manuscript line numbers are different from your version. If an additional round of revision is necessary, please copy the text in question.
Line 249-254: Did you look at nano plastics?
Answer of the authors: No. The detection limit in our study was 200 µm. The optical differentiation between very small plastic particles and any kind of organic or inorganic debris is extremely difficult and, unfortunately during this project stage, we had no access to FTIR or Raman spectroscopy. The question is addressed in the discussion (lines 206-209).
Line 255-262: Again, this isn’t really relevant in a discussion of your results. A couple of sentences about the impact of ingestion is sufficient, but not multiple paragraphs.
Answer of the authors: We deleted the last paragraph but maintained the discussion of the possible health effects of microplastics and its function as vectors for chemical substances. Most probably future research will focus on these questions.
References
This is far too many references for an 8 page paper. You need to be more targeted and concise when addressing previous literature.
Answers of the authors: We deleted 16 references
Round 2
Reviewer 2 Report
In response to my previous comments, the authors have removed many of the inconclusive results that were presented in the first version of the manuscript. While it is hard to make generalised conclusions from the results, they do add another useful datapoint in our study of microplastics in freshwaters.
Introduction: This has been appropriately shortened but still needs editing for flow.
Methods: Details have been added per my comments.
Discussion
Line 200: I think here is where you can mention that you think the type of fiber profile that would result from piping systems is different than that in the raw water. I still think it is way too speculative to calculate a removal rate based on the mean raw water and drinking water values, as I believe you did in the first version. You have sort of paired your sites, but it looks like the drinking water samples are upstream of the raw water samples, so computing removal rates by paired sites doesn’t really work either.
The paragraph that seemed largely unnecessary before is the one that starts “many studies analysed microplastic particles in food”. This is part of the reason I asked if you studied nanoparticles, since it talks a lot about those. I think this can be shortened and just lead into a discussion of why these microplastic exposures might be dangerous.
Author Response
MICROPLASTIC CONCENTRATIONS IN RAW AND DRINKING WATER IN THE SINOS RIVER, SOUTHERN BRAZIL
This text includes the copied suggestions of the referees and our answers
Referee:
Introduction: This has been appropriately shortened but still needs editing for flow.
Answer of the authors: We revised the text thoroughly, removed misspellings and doubtful use of past tense in a paragraph of the introduction. The discussion was partly restructured (please see the comment below).
Referee:
Methods: Details have been added per my comments.
Referee:
Discussion
Line 200: I think here is where you can mention that you think the type of fiber profile that would result from piping systems is different than that in the raw water. I still think it is way too speculative to calculate a removal rate based on the mean raw water and drinking water values, as I believe you did in the first version. You have sort of paired your sites, but it looks like the drinking water samples are upstream of the raw water samples, so computing removal rates by paired sites doesn’t really work either.
Answer of the authors: We changed the text (line 198 – 204), discussing the 70% decrease in drinking water: “The comparison of mean MP counts in raw and drinking water from the Sinos River as well shows a decrease of approximately 70%. Further studies have to show, if this value corresponds to the MP removal rate by the water treatment plants. Corrosion of plastic pipes and reservoirs may have occurred between the treatment plants and the faucets, adding MPs to the treated water. However, corrosion of PVC pipes and reservoirs should produce predominantly fragments [37] and not fibers, which were the dominant shape in both groups.”
We included the reference [37] on corrosion of PVC water pipes:
Brossia, S. Corrosion of Pipes in Drinking Water Systems. In Handbook of Environmental Degradation of Materials; Elsevier, 2018; pp. 489–505 ISBN 978-0-323-52472-8.
We hope that these alterations address the concerns of the referee adequately.
Referee
The paragraph that seemed largely unnecessary before is the one that starts “many studies analysed microplastic particles in food”. This is part of the reason I asked if you studied nanoparticles, since it talks a lot about those. I think this can be shortened and just lead into a discussion of why these microplastic exposures might be dangerous.
Answer of the authors: We restructured part of the discussion. Line 206-208 of the former version were deleted because of the inclusion of the text mentioned above. Lines 209-212 we also deleted and partially transferred to the paragraph “Many studies analysed MPs in food…” (line 249 of the new version). The rest of the paragraph we did not change. We are sorry to contradict the advice of the referee, but we consider the presence of microplastics as proxies for nanoplastics, which in most studies are not analysed, due to the methodological restrictions. Most probably ingested small particles like nanoplastics are transferred to the blood stream of animals at a much higher probability than the larger MPs. The cited lab study of Mattson et al. 2017 proves the presence of nano polystyrene particles in fish brains. If we accept fish as simplified models for other vertebrates like humans, these findings indicate, that nanoplastics and health will be a very important upcoming issue in the future. The objective of this paragraph is to draw the attention of the reader to this still underestimated relation.
