Evaluation of Nitrogen and Phosphorus Removal from a Denitrifying Woodchip Bioreactor Treatment System Receiving Silage Bunker Runoff
Round 1
Reviewer 1 Report
The experiment and the topic were indeed interesting. The authors did a good job in the Introduction section to outline the challenges with silage runoff and define the knowledge gaps present. The Result and Discussion sections were however very poorly written. The authors do not seem to understand how to use pairwise comparison results of ANOVA in interpreting their results. I am also unsure if the entire use of ANOVA was appropriate for what was done in their study.
Use of "significant" in the manuscript is confusing as it appears that the authors are merely relying on their p values, disregarding the actual concentrations.
The methodology is somewhat unclear. TKN was not measured. BOD was not measured. NO2/NO3 were not measured. The authors indicate that nitrate is not high in silage runoff, but yet the entire experiment is based on a denitrifying reactor system. Nitrates are only referred to as NOx, and it appears that NO2 and NO3 were not analyzed separately.
The experiment does not provide sufficient information to draw any meaningful conclusions for N transformations. The authors repeatedly indicate that their reactors/tanks were sources of N or P. The authors fail to appropriately explain how the system converted various N species. Many other parameters of interest could be measured/analyzed to help with interpretation of data. This was however not conducted. The authors have a very interesting experiment set up. But I believe the methodology chosen was not scientifically sound enough to make reasonable conclusions.
The way the results are presented is very poor and confusing. I have more detailed information in my comments below. Authors are encourages to read other papers using ANOVA.
Line 12: I’m not sure why you are speculating the effectiveness of this technology for other agricultural wastewaters right in the beginning before even giving the reader a chance to read the paper first. Is this “Featured Application” part of the Journal’s template requirements? I suggest you focus on the scope of your research rather than getting into other applications without proper discussion first.
Abstract: I suggest removing the acronyms and labeling names from the Abstract (EB, WB, System Inflow, etc.). The reader is not familiar with your naming conventions (yet), and the naming should not matter in the Abstract. Using labels unfamiliar to the reader makes it hard to focus on the results.
This last line in the Abstract seems like an afterthought. Is it related to the nitrate issue noted in the previous line? There is really no context given for this (other than the fact that maintenance, to some degree, is required for any system in real life.
Line 43: 3,500 (thousand separator)
I understand breaking down your Introduction in sub sections helps with the flow, but it is typically not required for a short Introduction section like this one in academic manuscripts. I suggest removing the subsection headers.
Line 68: Why is nitrate shown as NOx-N and not NO3-N?
Line 77: evaluated (past), and for the rest of the MS as well. Typically past tense is used to present research results, not present tense.
Line 81/82: Not sure what “like nitrifying bacteria” means here. They sustain biofilm nitrifying bacteria? Looks like this needs to be revised.
Line 86: comma after [11,16]
Line 95/96: Please revise to a more technical language.
Line 101 and Figure 1: When you say “tanks using an MBBR system” and “tanks with MBBR”, it is not clear if you mean these are all MBBR tanks, or a combination of tanks (storage, equalization, etc.) and then MBBR tanks, or these are all MBBR tanks. P.S. I see it now on Figure 2. This figure should be shown earlier in the MS.
Figure 1: Scale is in ft. Are imperial units accepted by the journal? Other units are metric in the MS.
Line 144: Why do you use the term “storm-driven runoff”? The industry term is just “stormwater runoff”.
Line 144-146: No information provided for the mesh/screen sizing.
Line 147: Please indicate the number of tanks. Up to here, it’s always “tanks” without specifying the number of tanks.
Line 149: I am now seeing the configuration of the tanks here. Figure 1 and the previous text should be modified to differentiate between the types of tanks in the system. Figure 2 should be moved up to after Figure 1 and referenced before describing the system in Line 144. Why are you referring to the MBBR as a “pre-treatment” component? To me, it’s one of the main treatment components.
Line 152: 1,514.2 (thousand separator). Tank 2 not shown on Figure 2. What are these dimensions? What is 2.5, and what is 0.4? diameter, width, length, etc.?
Line 154: If you show equivalent volumes in gallons, then the same should be done for all the volumes.
Line 157: Depth, length, width? If you keep separating the reactors as 2 units, why are the dimensions given for the entire system? What is the black line separating the units on Figure 2? What is it made of? What does it do? I am not sure I understand the system here. You noted this is a “paired” system. What do you mean by that?
Line 159: You mean 45 mm?
Line 160: are you really using down to millimeter in your approximate sizing here, or is this just the result of conversion from inch to cm? If approximate, say 5 cm x 5 cm x 0.6 cm.
Line 161 to 162: Where were the wood chips sourced from? What was the reason behind the composition used? How was the composition determined? Done by you? Provided by the supplier?
Line 165: How do you know the wood chip area remained anaerobic? Any DO measurements?
Line 166: You need to look up pipe equivalent sizes and not just multiply by 2.54 to get the equivalent in metric. We don’t have 10.16 cm pipes. If 4”, use 10 cm, or 100 mm.
Figure 2: The “Two side-by-side east/west” line does not show properly on my computer. The font spacing is off.
Line 168: bioreactors (plural)
Line 169: Referencing Figure 2 way too late.
Line 172: This is not a “cross sectional” diagram if you ask a civil engineer. Remove please.
Line 172 to 176: Caption is too long. No need to describe the system in the caption. Move what is not described in the text to the text and shorten the caption.
Line 178: “the” flow rates
Line 178: I believe the purpose of these flow paths is not explained properly here. These flows are not designed to allow for varying degrees of treatment. These are bypass flows designed to protect the treatment system in high storm events. I suggest revising to describe the reason behind the bypass lines.
Line 179/189: awkward grammar?
Line 185: same comment regarding unit conversion. Use 97.0 mm storm event (unless you actually have that number from the rainfall intensity number down to 0.03 mm precision).
Line 183: Any numbers on flow rates (peak flow, average, etc.)?
Same comment about many subsection headings for short paragraphs. Not needed.
Line 226: flow is usually shown as uppercase Q.
Line 236: So, the intervals determined were still fixed, correct? For instance, you would collect x mL every x hours? I’m not sure I understand how this sampling method was adjusted based on the expected flow. Can you please clarify?
Equation 1 and 2: units are not given.
Line 256: due “to”
Eq. 3: Delta t or “dt”?
Eq.4: revise to show in academically acceptable format. Define parameters.
The language needs to be revised here. You need to refer to equations, use commas on Line 262, define parameters, units. Equations are not acceptable in the current format. “mass in” is colloquial term not an academic way of showing the incoming mass in an equation.
Line 262: is this a new line or connected to the previous line. The grammar and punctuation need revision.
Line 269: I don’t seem to understand how the issue about flows skipping the tanks was resolved by accounting for precipitation on the wood chip bed areas. Can you clarify how these are related and revise the text?
Line 280: “were” used.
Line 291: “between” June-November …
Line 297: You need to define NOx. Is this nitrate + nitrite? Why did you not analyse these separately?
So what is the rest of the N from? TKN?
Line 314: why is this a significant increase? System inflow was 21.5 mg/L in Table 1… If you are using “significant” to refer to your statistical analysis, you are not wording it correctly. Same applies to the rest of your manuscript. A significantly different result based on pairwise comparison in ANOVA does not make the result significantly higher or lower. Revision needed.
Line 320: I think I am losing track. Your comparisons don’t make sense. Incorrect way of using the term “significant”. You need a table to show the results and compare. Not in text. Again, why are you using NOx and not NO3 and NO2. Your result presentation needs major revision, unless I am missing something. You have one table, with one row. Why not list all the average concentrations from each sampling point so we can see the trend? I am lost to be honest at this point. You are focusing too much on your p values that are not seeing the trends, perhaps?
Figure 4: so what’s going on here? Ammonia is going up even though it was supposed to decrease in MBBR? Nitrate is not changing… or there was nothing in the samples to begin with… but also not being produced as part of nitrification? …
Line 325: No no no no, this is not how you present the results. What is “marginally significantly lower than the …”… this makes no sense. Please read other reputable papers to understand how to use p values in interpreting results. You are comparing 0.04 mg/L with 21.5 mg/L. This is an increase. Not a marginally significantly …
Figure 4 and 5: you have not described what the bars, mid lines, etc. represent on the graphs.
Line 354 onwards: Very confused on your methodology. Are you using ANOVA to compare effluent vs. influent here? You don’t have different systems to compare with one another. Are you using ANOVA to compare different sampling events? I don’t think this is how ANOVA should be used.
Table 2: 70% of NOx reduction? From 0.14 mg/L to 0.04 mg/L? This is practically nothing.
So what is this 75% TN removal going? What is ammonia converting to?
Line 399: Then why was TKN not measured if NOx was not a big concern? And why was a denitrifying bioreactor used if there was no nitrates to denitrify? Again, I am not sure I can follow the logic here.
Line 421 to 431: Are you saying N was generated and you don’t even know how and where? What was the point of this methodology and study then?
Line 455: “Likely”? There was practically no nitrate going in. Of course it was limited…
Line 458: Did you measure SO4? This is irrelevant. You don’t have complete nitrate reduction here. You have no nitrate reduction because there was no nitrate to be reduced.
Line 472: You can’t just say your system was a source of P, N, etc. Unless your tanks were leaching chemicals. This is not how you interpret biological and chemical reactions in a system, while understanding the laws of conservation of mass.
Line 489: I don’t understand how this is relevant to your study? You didn’t have additives… again, I can’t follow the logic behind your discussion here.
Line 540: What is this based on? Which parameter was effectively removed? Ammonia was removed, which is not even what the reactors were designed for?
Again, there is no table to show the concentrations in influent, effluent, etc.
Author Response
Reviewer 1:
Comments and Suggestions for Authors
The experiment and the topic were indeed interesting. The authors did a good job in the Introduction section to outline the challenges with silage runoff and define the knowledge gaps present. The Result and Discussion sections were however very poorly written. The authors do not seem to understand how to use pairwise comparison results of ANOVA in interpreting their results. I am also unsure if the entire use of ANOVA was appropriate for what was done in their study.
Response: Thank you for this comment. We originally used ANOVA in order to compare mean nutrient concentrations statistically between all sampling points, however we acknowledge there are some issues with this, since different treatments are not being applied and measurements are not independent of on another. We replaced with Wilcoxson Signed Rank tests to compare the mean concentrations of the inflow and outflow of the tanks and inflow and outflow of the bioreactors.
Use of "significant" in the manuscript is confusing as it appears that the authors are merely relying on their p values, disregarding the actual concentrations.
Response: Thank you for noting this. We reviewed the results section and changed to compare between the means and note if the difference was found to be statistically significant, such as: “Compared to the System Inflow, NH4+-N mean concentration in Tank Outflow increased, and that difference was found to be statistically significant (p < 0.001).”
The methodology is somewhat unclear. TKN was not measured. BOD was not measured. NO2/NO3 were not measured. The authors indicate that nitrate is not high in silage runoff, but yet the entire experiment is based on a denitrifying reactor system. Nitrates are only referred to as NOx, and it appears that NO2 and NO3 were not analyzed separately.
Response: TN, NH4, and NO2+NO3 were measured, as a result, TKN can be calculated. By measuring TN, which includes all N contained within TKN (plus NO2+NO3), TKN measurement was not necessary. Since nitrate is not high in silage bunker runoff, the treatment tanks were intended to facilitate ammonification of organic nitrogen and subsequent nitrification transformation prior to runoff entering the bioreactors. While we recognize the importance of BOD and its impact on water quality, resources did not allow for the analysis. Future work on this treatment system should include BOD measurements.
The experiment does not provide sufficient information to draw any meaningful conclusions for N transformations. The authors repeatedly indicate that their reactors/tanks were sources of N or P. The authors fail to appropriately explain how the system converted various N species. Many other parameters of interest could be measured/analyzed to help with interpretation of data. This was however not conducted. The authors have a very interesting experiment set up. But I believe the methodology chosen was not scientifically sound enough to make reasonable conclusions.
Response: We agree that there are many parameters that would be interesting to look at in the system. However, our goal was to describe performance, not to detail transformation mechanisms. Removed the parts about being sources of N and P and described the nutrient transformations that likely occurred instead within the discussion. Detailed investigation of mechanisms was beyond the scope of our study, but we do agree with the reviewer that this work is important. Thus, it should be accomplished in in studies to follow.
The way the results are presented is very poor and confusing. I have more detailed information in my comments below. Authors are encourages to read other papers using ANOVA.
Response: We revised the results section to reflect the change in statistical method noted above.
Line 12: I’m not sure why you are speculating the effectiveness of this technology for other agricultural wastewaters right in the beginning before even giving the reader a chance to read the paper first. Is this “Featured Application” part of the Journal’s template requirements? I suggest you focus on the scope of your research rather than getting into other applications without proper discussion first.
Reponse: Yes, the journal recommends including this section. The description indicates that a potential additional application can be offered too: “authors are encouraged to provide a concise description of the specific application or a potential application of the work” so we would like to keep this sentence as is.
Abstract: I suggest removing the acronyms and labeling names from the Abstract (EB, WB, System Inflow, etc.). The reader is not familiar with your naming conventions (yet), and the naming should not matter in the Abstract. Using labels unfamiliar to the reader makes it hard to focus on the results.
Response: Agreed that this is confusing. Removed the names from the abstract.
This last line in the Abstract seems like an afterthought. Is it related to the nitrate issue noted in the previous line? There is really no context given for this (other than the fact that maintenance, to some degree, is required for any system in real life.
Response: Yes, we agree with this. We removed this sentence from the abstract.
Line 43: 3,500 (thousand separator)
Response: Comma inserted.
I understand breaking down your Introduction in sub sections helps with the flow, but it is typically not required for a short Introduction section like this one in academic manuscripts. I suggest removing the subsection headers.
Response: Subsection headers removed from introduction.
Line 68: Why is nitrate shown as NOx-N and not NO3-N?
Response: The lab method we used measures combined nitrite+nitrate-N.
Line 77: evaluated (past), and for the rest of the MS as well. Typically past tense is used to present research results, not present tense.
Response: Yes, this is true. Thanks for pointing this out. Changed to past tense for describing results.
Line 81/82: Not sure what “like nitrifying bacteria” means here. They sustain biofilm nitrifying bacteria? Looks like this needs to be revised.
Response: Agreed that this is confusing, we changed to: “sustain the growth of microorganisms”
Line 86: comma after [11,16]
Response: inserted comma.
Line 95/96: Please revise to a more technical language.
Response: Yes, thanks this sentence was not worded well. Changed to: ‘This study evaluated the first application of a denitrifying bioreactor used to treat silage leachate or silage bunker runoff.’
Line 101 and Figure 1: When you say “tanks using an MBBR system” and “tanks with MBBR”, it is not clear if you mean these are all MBBR tanks, or a combination of tanks (storage, equalization, etc.) and then MBBR tanks, or these are all MBBR tanks. P.S. I see it now on Figure 2. This figure should be shown earlier in the MS.
Response: We clarified to “tanks with MBBR.” We also moved Figure 2 to be after Figure 1 prior to the description of the system.
Figure 1: Scale is in ft. Are imperial units accepted by the journal? Other units are metric in the MS.
Response: Yes, thanks for pointing this out. Changed scale to metric.
Line 144: Why do you use the term “storm-driven runoff”? The industry term is just “stormwater runoff”.
Response: Authors believe that the broad term “stormwater runoff” used in suburban and urban contexts, is not the best fit for silage bunker runoff in an agricultural context. It is not common parlance in agricultural settings.
Line 144-146: No information provided for the mesh/screen sizing.
Response: Inserted mesh sizing details.
Line 147: Please indicate the number of tanks. Up to here, it’s always “tanks” without specifying the number of tanks.
Response: We added “three” prior to describing the tanks, “After the screen assembly, runoff and leachate flow through a series of three treatment tanks that overflow into each subsequent tank during storms.”
Line 149: I am now seeing the configuration of the tanks here. Figure 1 and the previous text should be modified to differentiate between the types of tanks in the system. Figure 2 should be moved up to after Figure 1 and referenced before describing the system in Line 144. Why are you referring to the MBBR as a “pre-treatment” component? To me, it’s one of the main treatment components.
Response: Figure 2 was moved up to after Figure 1 and referenced in the first sentence at the start of the paragraph in line 124. The tanks are now consistently referred to as “treatment tanks with MBBR.”
Line 152: 1,514.2 (thousand separator). Tank 2 not shown on Figure 2. What are these dimensions? What is 2.5, and what is 0.4? diameter, width, length, etc.?
Response: We added a thousandth separator. For the media description, we changed wording to: “cylindrical media carriers were added to fill 40% of the Tank 2 size by volume (1,514.2 L) which each have a diameter of 2.5 cm, height of 0.4 cm, and a projected surface area of 800 m2/m3.”
Line 154: If you show equivalent volumes in gallons, then the same should be done for all the volumes.
Response: We removed the gallon equivalent.
Line 157: Depth, length, width? If you keep separating the reactors as 2 units, why are the dimensions given for the entire system? What is the black line separating the units on Figure 2? What is it made of? What does it do? I am not sure I understand the system here. You noted this is a “paired” system. What do you mean by that?
Response: Thanks for pointing out these confusing points. We changed the text to: “The woodchip bioreactors are continuously saturated, the bottom, side walls, and center barrier splitting the two bioreactors are lined with 1.1-mm ethylene propylene diene monomer (EPDM)..”
Line 159: You mean 45 mm?
Response: No, the thickness of the EPDM is 45 mil (0.001 inch), or 1.1 mm. Changed to 1.1 mm.
Line 160: are you really using down to millimeter in your approximate sizing here, or is this just the result of conversion from inch to cm? If approximate, say 5 cm x 5 cm x 0.6 cm.
Response: Yes, this was the conversion, so we have changed to acknowledge the approximate size.
Line 161 to 162: Where were the wood chips sourced from? What was the reason behind the composition used? How was the composition determined? Done by you? Provided by the supplier?
Response: Woodchip composition was determined by local supplier, based on available feedstock for their chipping operation. We changed the text to: “determined by a local woodchip supplier, and was based on available feedstock species at the time of construction”
Line 165: How do you know the wood chip area remained anaerobic? Any DO measurements?
Response: Thank you for this observation. While it was highly probable that the bed remained anaerobic, we do not have measurements to support this claim. Removed anaerobic from the text since we only know that the woodchips remained saturated.
Line 166: You need to look up pipe equivalent sizes and not just multiply by 2.54 to get the equivalent in metric. We don’t have 10.16 cm pipes. If 4”, use 10 cm, or 100 mm.
Response: Changed to pipe equivalent size, 100 mm.
Figure 2: The “Two side-by-side east/west” line does not show properly on my computer. The font spacing is off.
Response: Changed the formatting of this file to PNG and should display properly now.
Line 168: bioreactors (plural)
Response: Pluralized to bioreactors.
Line 169: Referencing Figure 2 way too late.
Response: We moved figure 2 up after Figure 1 and referenced earlier in text.
Line 172: This is not a “cross sectional” diagram if you ask a civil engineer. Remove please.
Response: Thank you for pointing this out. We removed and replaced with “conceptual schematic diagram.”
Line 172 to 176: Caption is too long. No need to describe the system in the caption. Move what is not described in the text to the text and shorten the caption.
Removed the second sentence from caption: “The system consists of a particle separating screen assembly, a sequence of three treatment tanks for containment of low flows, settling and aeration, woodchip bioreactors, and an earthen infiltration basin” and replaced at the end of the previous paragraph starting on line 112.
Line 178: “the” flow rates
Response: Inserted “the” flow rate.
Line 178: I believe the purpose of these flow paths is not explained properly here. These flows are not designed to allow for varying degrees of treatment. These are bypass flows designed to protect the treatment system in high storm events. I suggest revising to describe the reason behind the bypass lines.
Response: Authors edited this description to: “The system has three distinct runoff flow paths (Figure 2), depending on the flow rate, which allowed for bypass of some treatment components during larger storm events to protect the system. The three paths are referred to as ‘low flow’, ‘high flow’, and ‘extreme events’ and are designed to accommodate flow up to a certain intensity (Figure 2).”
Line 179/189: awkward grammar?
Response: Original line 179 changed to: “which allowed for bypass of some treatment components during larger storm events to protect the system.” And original line 189 changed to “sent directly into the infiltration basin.”
Line 185: same comment regarding unit conversion. Use 97.0 mm storm event (unless you actually have that number from the rainfall intensity number down to 0.03 mm precision).
Response: We removed this text and now describe the rain event intensities (starting on line 159).
Line 183: Any numbers on flow rates (peak flow, average, etc.)?
Response: We have added the rain event intensities for which the flow paths were designed (line 159). Also added the average flow rates through the bioreactors in the results section under “storm events and flow rates” (line 272).
Same comment about many subsection headings for short paragraphs. Not needed.
Response: Removed subsection headings to consolidate section on storm sampling.
Line 226: flow is usually shown as uppercase Q.
Response: Common convention is uppercase Q for flow volume, here we are referring to q, flow rate.
Line 236: So, the intervals determined were still fixed, correct? For instance, you would collect x mL every x hours? I’m not sure I understand how this sampling method was adjusted based on the expected flow. Can you please clarify?
Response: Clarified this section to be more specific on how the sampling interval was determined. See line 200.
Equation 1 and 2: units are not given.
Response: Added all units in for equations 1-4.
Line 256: due “to”
Response: Added in, thanks for catching this.
Eq. 3: Delta t or “dt”?
Response: Yes, thanks for noting, we changed the form of this equation to Total Mass = Q * EMC.
Eq.4: revise to show in academically acceptable format. Define parameters. The language needs to be revised here. You need to refer to equations, use commas on Line 262, define parameters, units. Equations are not acceptable in the current format. “mass in” is colloquial term not an academic way of showing the incoming mass in an equation.
Response: Referred to equations in text. Changed terms to Mi and Mf and defined in the text and inserted commas. The Applied Sciences format states: “the text following an equation need not be a new paragraph. Please punctuate equations as regular text.”
Line 262: is this a new line or connected to the previous line. The grammar and punctuation need revision.
Response: The journal’s format says that: “the text following an equation need not be a new paragraph. Please punctuate equations as regular text.” So, the text following each equation is not capitalized.
Line 269: I don’t seem to understand how the issue about flows skipping the tanks was resolved by accounting for precipitation on the wood chip bed areas. Can you clarify how these are related and revise the text?
Response: Clarified this sentence to read: “It was determined that summed volumes from bioreactor outflow autosamplers, with a correction for added effective precipitation on the open bioreactors, more accurately accounted for storm event runoff volumes that passed through the entire system.”
Line 280: “were” used.
Response: Changed “was” to “were”.
Line 291: “between” June-November …
Response: Changed to: “from June through November of 2018 and June through October of 2019.”
Line 297: You need to define NOx. Is this nitrate + nitrite? Why did you not analyse these separately?
Response: See comment above about changing to NO2+NO3-N throughout text.
So what is the rest of the N from? TKN?
Response: Measured TN, NO2+NO3, and NH4, so after subtracting nitrate and ammonium from TN the remainder is organic N.
Line 314: why is this a significant increase? System inflow was 21.5 mg/L in Table 1… If you are using “significant” to refer to your statistical analysis, you are not wording it correctly. Same applies to the rest of your manuscript. A significantly different result based on pairwise comparison in ANOVA does not make the result significantly higher or lower. Revision needed.
Response: Thank you for this comment. We edited the wording relating to the results of the statistical analyses.
Line 320: I think I am losing track. Your comparisons don’t make sense. Incorrect way of using the term “significant”. You need a table to show the results and compare. Not in text. Again, why are you using NOx and not NO3 and NO2. Your result presentation needs major revision, unless I am missing something. You have one table, with one row. Why not list all the average concentrations from each sampling point so we can see the trend? I am lost to be honest at this point. You are focusing too much on your p values that are not seeing the trends, perhaps?
Response: Regarding the table, we included Table 1 to highlight average concentrations of silage bunker runoff, for which there are few studies available characterizing concentrations. To show data for all sampling locations, we included boxplots instead of the table, which is more informative than just averages. However, there is a table included in supplementary materials, which lists these average concentrations.
Figure 4: so what’s going on here? Ammonia is going up even though it was supposed to decrease in MBBR? Nitrate is not changing… or there was nothing in the samples to begin with… but also not being produced as part of nitrification? …
Response: Added a description starting on line 281 which describes the overall trends that are shown in figure 4, however this journal requests separate results and discussion sections.
Line 325: No no no no, this is not how you present the results. What is “marginally significantly lower than the …”… this makes no sense. Please read other reputable papers to understand how to use p values in interpreting results. You are comparing 0.04 mg/L with 21.5 mg/L. This is an increase. Not a marginally significantly …
Reponses: This was originally not worded clearly, so we have corrected, and believe it also addresses this comment. Line 294: “Compared to the influent to the WB (i.e., Tank Outflow), mean effluent concentrations of TN and NH4 +-N were both reduced at the effluent, which was found to be statistically significant (p < 0.001, p = 0.002, respectively).”
Figure 4 and 5: you have not described what the bars, mid lines, etc. represent on the graphs.
Response: Edited both figure captions to describe the box and whisker plots.
Line 354 onwards: Very confused on your methodology. Are you using ANOVA to compare effluent vs. influent here? You don’t have different systems to compare with one another. Are you using ANOVA to compare different sampling events? I don’t think this is how ANOVA should be used.
Response: Yes, since we don’t have different treatments, we agree that ANOVA is not the proper way to compare the nutrient concentration means from each sampling location, so like noted above, we changed the statistical analysis to Wilcoxson Signed Rank tests.
Table 2: 70% of NOx reduction? From 0.14 mg/L to 0.04 mg/L? This is practically nothing.
So what is this 75% TN removal going? What is ammonia converting to?
Response: TN is overall going down, so nitrate has to be lost through denitrification This is the only possible pathway for mass reduction. This description added to the discussion section, dictated by the journal. See line 417. Added reference to Table 2 in discussion.
Line 399: Then why was TKN not measured if NOx was not a big concern? And why was a denitrifying bioreactor used if there was no nitrates to denitrify? Again, I am not sure I can follow the logic here.
Response: Please see explanation above for TN, the discussion of this begins on line 417. This is also given in the first paragraph of the discussion section beginning on line 377: “Total N is comprised of dissolved inorganic species, NH4+-N and NO3--N and dissolved and particulate organic N.” In regard to use of the bioreactor, the design of the system included tanks prior to runoff entering the bioreactor, to convert organic N and ammonia to NO3 (through aeration) prior to entering the bioreactor beds. However, as we see in Figure 4, this did not consistently occur. As a result, nitrate concentrations remained low throughout the system (at least when measured during storm events).
Line 421 to 431: Are you saying N was generated and you don’t even know how and where? What was the point of this methodology and study then?
Response: No, there is the unknown of the unmonitored very low-flow silage leachate entering the system, and we believe that this could be a potential reason for the increase in N concentrations between System Inflow and Tank Outflow. This is described in line 410 of discussion: “This could potentially come from unmonitored silage leachate (i.e., not mixed with storm-driven runoff), which flows into Tank 1. Leachate concentration would be diluted within the treatment tanks with MBBR and also contribute to an increase in concentration measured in Tank Outflow during the following storm event.”
Line 455: “Likely”? There was practically no nitrate going in. Of course it was limited…
Response: Removed likely.
Line 458: Did you measure SO4? This is irrelevant. You don’t have complete nitrate reduction here. You have no nitrate reduction because there was no nitrate to be reduced.
Response: We agree that this is speculative, so we have removed this and added a suggestion to measure SO4 in future studies of this system.
Line 472: You can’t just say your system was a source of P, N, etc. Unless your tanks were leaching chemicals. This is not how you interpret biological and chemical reactions in a system, while understanding the laws of conservation of mass.
Response: Changed the wording of “source” to increase in concentration within inflow and outflow of tanks. We hypothesize that this is due to silage leachate, i.e., not runoff, entering the tanks between storms, described in line 460.
Line 489: I don’t understand how this is relevant to your study? You didn’t have additives… again, I can’t follow the logic behind your discussion here.
Response: This section was included to describe how our system had moderate P removal even without a P-binding additive. This was surprising to us, and is unexpected given the existing literature on woodchip bioreactors. We added in a suggestion to look into P-binding additives to improve P removal in future study.
Line 540: What is this based on? Which parameter was effectively removed? Ammonia was removed, which is not even what the reactors were designed for?
Response: We have added in RE% for TN and TP, SRP in the conclusion. For TN, organic N was converted to NH4, nitrified to NO3, and lost through denitrification. This process described starting on line 417 of discussion.
Again, there is no table to show the concentrations in influent, effluent, etc.
Response: These data are contained within the figures. Supplemental table shows mean concentrations and numbers of storm event samples. The authors chose to present data in graphical format so that it is more easily interpreted by the reader.
Reviewer 2 Report
Review of applsci-827891
The article is well written, and the results appear consistent. However, I have a couple of questions and suggestions that the authors are requested to address.
Authors are requested to answer the following questions:
- From figure 1 it seems like the pretreatment tanks have equal diameter, however, the schematic in figure 2 told a different story. How did the capacity vary? Did the tanks have varying depth or Figure 2 tells the actual case?
- Was there any quantitative definition for ‘low flow’, ‘high flow’, and ‘extreme events’?
Additionally, the authors are encouraged to make changes based on the following suggestions:
- Please explicate EB & WB in the abstract (line#20)
- size of the mesh holes could be mentioned (line# 145)
- check for a missing words/prepositions such as in lines 256 and 381
Author Response
Reviewer 2:
Comments and Suggestions for Authors
Review of applsci-827891
The article is well written, and the results appear consistent. However, I have a couple of questions and suggestions that the authors are requested to address.
Response: Thank you!
Authors are requested to answer the following questions:
- From figure 1 it seems like the pretreatment tanks have equal diameter, however, the schematic in figure 2 told a different story. How did the capacity vary? Did the tanks have varying depth or Figure 2 tells the actual case?
Response: Capacity for each tank did vary, and are described in lines 128, 130, 137. Changed Figure 1 to have a larger diameter for Tank 1. Symbols for the tanks are not to scale and this is noted.
- Was there any quantitative definition for ‘low flow’, ‘high flow’, and ‘extreme events’?
Response: Yes, they were designed for different storm event intensities. Clarified this for each flow path. Line 159-166.
Additionally, the authors are encouraged to make changes based on the following suggestions:
- Please explicate EB & WB in the abstract (line#20)
Response: removed these abbreviations from the abstract.
- size of the mesh holes could be mentioned (line# 145)
Response: Included sizes in line 126.
- check for a missing words/prepositions such as in lines 256 and 381
Response: Added due “to” in original line 256, inserted “for” in original line 381.
Reviewer 3 Report
I revised the paper titled: Evaluating nitrogen and phosphorus removal from a denitrifying woodchip bioreactor treatment system receiving silage bunker runoff for a possible publication in APPLIED SCIENCES. Results are interesting but I have some comments. The language should be improved. I suggest major revision before publication. My comments are the following:
- Title should be “Evaluation of…”. It will sound better.
- Line 18: “to reduce influent nutrient concentrations and mass loads”. Please, remove nutrient concentration and refer to load. It is better.
- In the abstract all abbreviations must be declared the first time.
- Line 42: “is a very potent effluent”. Please, use a more scientific language.
- I suggest to not divide the Introduction into different sections. More recent literatures should be inserted.
- Introduction should be better highlight the problem and the contest and not describe in detailed the methods used.
- Line 149: eliminate “-“ in “3.76-m3”. Also in line 166.
- Please, use the third person and not “we”. Please, check all manuscript.
- In all equation (or in the sentence above), please insert units of measures.
- Line 285: Please, cite R project appropriately (as a reference).
- Table 1: Please, insert the unit of measure below every pollutant.
- Why in line 302 and 303 you insert the value in in? I think the value in mm is sufficient.
- In figure 4, 5, …, the black horizontal line represents the mean or the median? Please, specify in the caption.
- Conclusions should be revised. Insert more results (e.g. percentages) for quickly communicate the results of your work
- In the text, there are a very high number of abbreviations. Please, insert a nomenclature at the end of the manuscript.
Author Response
Reviewer 3:
Comments and Suggestions for Authors
I revised the paper titled: Evaluating nitrogen and phosphorus removal from a denitrifying woodchip bioreactor treatment system receiving silage bunker runoff for a possible publication in APPLIED SCIENCES. Results are interesting but I have some comments. The language should be improved. I suggest major revision before publication. My comments are the following:
Title should be “Evaluation of…”. It will sound better.
Response: Title was changed to reflect this suggestion.
Line 18: “to reduce influent nutrient concentrations and mass loads”. Please, remove nutrient concentration and refer to load. It is better.
Response: Changed to only refer to load.
In the abstract all abbreviations must be declared the first time.
Response: Defined all abbreviations in abstract.
Line 42: “is a very potent effluent”. Please, use a more scientific language.
Response: Changed wording here: silage leachate is a byproduct from the ensiling process, which is extremely high in nutrient content”
I suggest to not divide the Introduction into different sections. More recent literatures should be inserted.
Response: Introduction subheadings were removed. There are few studies specifically on bioreactors as well as silage bunker runoff so we feel like relevant references are included. If the reviewer is aware of recent studies that are not included, we will gladly include.
Introduction should be better highlight the problem and the contest and not describe in detailed the methods used.
Response: We understand this reviewer’s comments, however, because options for managing silage bunker runoff are limited, we felt it necessary to explain the design of the treatment system used and include other contexts where MBBR and woodchip bioreactors have been used. The first two paragraphs highlight the problem that we are addressing.
Line 149: eliminate “-“ in “3.76-m3”. Also in line 166.
Response: Including 3.76-m3 is important for clarity and used as a noun to describe the tank.
Please, use the third person and not “we”. Please, check all manuscript.
Response: Made changes throughout manuscript to only use 3rd person.
In all equation (or in the sentence above), please insert units of measures.
Response: Inserted all units for equations 1-4.
Line 285: Please, cite R project appropriately (as a reference).
Response: Yes thanks, inserted a citation for R Core Team, which is reference 33.
Table 1: Please, insert the unit of measure below every pollutant.
Response: Inserted mg/L below each analyte.
Why in line 302 and 303 you insert the value in in? I think the value in mm is sufficient.
Response: We agree, this is not needed. Removed the in. conversion.
In figure 4, 5, …, the black horizontal line represents the mean or the median? Please, specify in the caption.
Response: For both figures, updated the captions to include a description of the box plot.
Conclusions should be revised. Insert more results (e.g. percentages) for quickly communicate the results of your work
Response: We agree with this comment. We have inserted RE for TN and P species in lines 516 and 520.
In the text, there are a very high number of abbreviations. Please, insert a nomenclature at the end of the manuscript.
Response: Yes, this is a good idea. Inserted a nomenclature section after the conflict of interest section in line 549.
Round 2
Reviewer 3 Report
My suggestions have been implemented. I suggest the publication of the paper.
Author Response
Thank you for the review of our manusctipt!
