Does Methane Contribute to Growth of Invertebrate Communities in Drinking Water?
Round 1
Reviewer 1 Report
General comments
The article is very interesting in that methane concentration may be added to media age as a hypothesis for variation of the presence of invertebrate communities. Note that the general comments below do not necessarily require changes.
11) ‘The article would be stronger if there were 2 waterworks with methane and 2 without. This would give an indication of the variation between waterworks in each category. However, the data is still a valuable contribution to knowledge.
22) The methods section is somewhat unclear and is lacking some detail, especially since no set-up figure was included. Several specific comments below will improve this.
33) A little assistance in interpreting the photos in the supplementary material would be helpful.
44) There are 3 or 4 references in the Danish language, possibly in non-peer reviewed literature. This may be necessary if the information is only found there. Therefore, no change is required here.
Specific Comments (line numbers)
46 ”parametric value” could be replaced with a more descriptive term like ”limit value”
55 Present progressive form ”is lacking” is needed rather than ”lack”
74, 84 Superscript for the number 3 in ”m3”
80 ”aeration cassettes” should be more fully described, such as ”using compressed air to inject bubbles at the bottom”. I assume that ”closed” means NO exhaust fromthe unit.
80 Basic info is needed on filter media (type, size distribution), backwash (frequency and steps), and reaction chamber (present or not) to understand the waterworks
88 Perhaps add the word ”medium” after ”age of the filter”
89 Why is GAC used? If large amounts of organics are removed, this would be important to understand the microbial community.
101 How was the acrylic tube used? Hammered or augured into the filter medium?
121 Change to “as with the sand samples,”
152 Spelling: March
154 Perhaps add “outlet” to the “from the top” location to remind less-aware readers of the upflow
156,189 What kind of sample is the the one with visible growth? Water, swab, piece of hose for later swabbing or later ultrasounding?
149 Please state if the reactor is open to the atmosphere at the top.
162 If it is to be mentioned, Sulzer needs a reference.
166 Size & form of the packing material should be stated.
169 Is the filter best described as “submerged” or “trickiling” with and without nitrogen addition?
170 How is the aeration done? Is this really an “aeration/stripping”?
175 What is the water/solid ratio when shaking?
211 Do you mean “throughout”?
219 The AOC outlier on Figure 1 should be commented somewhere. From the iron results in Figure S5, it appears there were many particles in the sample. Perhaps this is due to turbulence during operation or disturbing the chamber during sampling?
234 The sentence is confusing: are there few samples, few eukaryotes and many protists? If so, then what is in the other (many) samples?
360 “in throughout” should perhaps just be “throughout”?
362 The relative share of Eukaryotes increases, but this is likely due to a drop in the absolute number of primary producers. The unaware reader may be mislead by the current sentence and not catch this distinction.
382 contributes
383 “as” should perhaps be “as observed at”.
393 “dominated with” would be better if written “is dominated by”.
396 “will contribute” is a quite strong statement. Perhaps it should be “may”. I can imagine that sediment is stagnant and does not contribute to the organics that continue in the treatment train in the water phase.
411 should either be “the methane concentration” or “concentrations”
428 Growth “of” methane. “of” should be added.
423 Table 2, Column 1 should include the words “pellet softener” to assist the reader, just as vacuum stripping is mentioned
430 “within the aeraion step” should perhaps be reformulated to “on the aeration steps” or “in connection with aeration”. Also, add “the” in front of “methane concentration”.
442 Perhaps “oxygen” would be more specific than “air” in this context.
447 Remember to complete the link with “xx”
S1 Spelling errors in table: 2 x nitrite, 2 x dioxide, sulphide. It would have been nice to have sulphate and nitrate values at Hvidovre as well.
S5 Figure: At the top of the page it mentions Hvidovre and pilot, yet the figure seems only to have Slangerup.
Author Response
Dear editor and Reviewer
Thank you very much for the review. Since the review was recieved after we revised the manuscript according to the reviews of the first two reviewers the comments have not been incoorporated into the revised manuscript. We have attached the changes already made. Please let us know if additional time for extra revisions are given - then we will make the revisions according to the latest review. We have attached the current revisions.
Author Response File: 
Author Response.docx
Reviewer 2 Report
This manuscript presents the results of pilot and full scale studies investigating the role of methane concentration of the source groundwater on the microbial growth in drinking water treatment systems. The research is important and has global applications where biological treatment is utilized for maintaining water quality in drinking water treatment and distribution systems. Following are some suggestions for clarification of presented content:
Line 44: Clarify what range of temperatures is considered low for the given bacterial species.
Lines 64-70: This last paragraph would be better placed in another section of introduction where past results are communicated. The study objectives and aims are most appropriate for the ending paragraph.
Line 73: Is there a specific reasoning for selecting the 2 water works out of the 13 for this study?
Line 100: How were the three different sampling location were selected?
Line 114: The title indicates water sampling and analysis; however, mainly sample collections is discussed in under this section.
Line 184: Define abbreviation, HOFOR's accredited laboratory
Line 189: By "Samples of organisms", do the authors mean sample of the biofilm?
Line 294 and figure 4: Clarify the organisms' classification of prokaryotes and eukaryotes.
Line 353: Clarify what levels are regarded as "generally low carbon content".
Line 421: How were the durations of the two studies different?
Discussion (Lines 436-444) : Based on the study results, what recommendation is provided for waterworks operators monitoring the distribution system for "unwanted microbial growth"? Are the authors recommending methane as an indicator for monitoring "unwanted microbial growth" in the water systems?
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
Reviewed paper deals with the investigation of the question does methane contribute to growth of invertebrate communities in drinking water? The subject of paper well fits the scope of Water. The paper could be recommended for publication in Water after revision taking into account the following comments:
1. Monitoring the content of methane in water at water treatment facilities, its binding by hydrobionts, is important not only from the viewpoint of ensuring the normative quality of the supplied drinking water, as it is considered in this paper. The estimates of methane emissions are also very important from the viewpoint of climate stabilization. Despite the fact that methane takes only the third position in the formation of a greenhouse effect, after water steam and carbon dioxide, as the calculations are shown, 1 % of the increase in methane in the atmosphere provides 25 times larger effect than an increase in CO2 content also by 1 %. Moreover, if over the past 200 years of the industrial era, the carbon dioxide content in the atmosphere has increased only 25%, then the methane content has increased by 2 times. It is important that the characteristic time of the stay of methane molecules in the atmosphere is 8-12 years, that is much less than the characteristic cycle of carbon dioxide in the atmosphere. Anthropogenic factors currently control the issue of methane in the atmosphere at 2/3. Therefore, complete and objective information about anthropogenic sources of methane emission, including those related to water supply systems, is very important. In order to draw conclusions on the importance of these sources of methane emission, their intensity is necessary both in terms of gross indicators of the T/year and in specific indicators - the release of methane per cubic meter of water supplied - mg/m^3. In the considered work, on the basis of carefully set and performed experiments, the role of various hydrobionts present at water treatment facilities is shown, in connecting methane, however, for the efficient use of experiments, both for utilitarian water supply tasks and to evaluate the emission of methane, it is necessary to highlight the scale and role in the work abiotic and biotic factors. For further analysis, it would be very convenient to present the values of controlled physical and chemical factors in the form of individual tables, primarily the most important ones that determine the methane content in water: the initial methane content, the temperature and mineralization of water, its oxidative and renewal potential-EH, it is also necessary to indicate whether chlorine -containing reagents are used during water treatment, which interacts with methane very actively.
2. When analyzing the role and scale of biotic factors in the binding of methane, it is necessary to show the rate of binding of methane per unit mass of hydrobionts in mg/kg*day. and units of the working surface of the water treatment structure, i.e. mg/m^2*day. Currently, a fairly large number of studies on producing methane have been performed with various surface water bodies. Thus, according to experimental estimates, in sand -bottom deposits of running zones, the intensity of methanogenesis does not exceed 0.001–0.2 ml CH4/(dm^3 day), and in other soils is an average of 0.5 ml CH4/(dm^3 day). In general, the emission of methane by reservoirs varies from ~ 5-10 mg CH4/m^2*day in reservoirs located in the temperate climate zone, up to 140 mg CH4/m^2*day in the reservoirs of the tropical zone. Therefore, it would be very useful to conduct their comparative analysis. Such estimates could play a significant role in the analysis of the processes of the emission of methane and the role of biotic factors in its binding.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
