Carbon Monoxide Formation during Aerobic Biostabilization of the Organic Fraction of Municipal Solid Waste: The Influence of Technical Parameters in a Full-Scale Treatment System

Round 1

Reviewer 1 Report

In "Carbon monoxide formation during aerobic biostabilization of the organic fraction of municipal solid waste: the influence of technical parameters in a full-scale treatment system", Stegenta-Dabrowska et al. describe a series of experiments measuring CO, CO2, and CH4 concentrations in six pile waste reactors under three different sets of conditions.  They perform several model analyses (a general additive model, some non-linear correlation analysis, and principal component analysis) on their results to derive insight into CO production during aerobic biostabilization of the organic fraction of municipal solid wastes.

There are several issues with this paper that the authors need to address.  One major issue is the inconsistency of the figures.  Figures 2 and 3 are identical, but one claims to show oxygen content and the other claims to show CO2 content.  There are also inconsistencies between Figures 8 and 9, some of which should be nearly identical (e.g. 8a and 9b), but are not (and not even scaled the same way!).  There are also issues with the axes labels on several figures (CO2 in % versus ppm).  The inconsistencies in the figures, as well as inconsistencies between the discussion in the text and what is shown in the figures referred to, makes it especially challenging to fully evaluate the authors' interpretations of the data and the resulting conclusions.  Some of the plots even show data points that the authors claim to exclude (e.g. CO > 1000 ppm) due to it being outside the range of the measurements.  The authors need to address and / or reconcile these inconsistencies.

Unsurprisingly, there is a large degree of uncertainty due to spatial heterogeneity in the waste piles.  However, the authors largely avoid discussing the differences in terms of the spatial heterogeneity.  They also do not provide much justification for how their sampling scheme is representative (or even whether or not it is) given the large spatial heterogeneities.  It is not expected for the authors to know all of the answers to these questions, but they should at least address the issue and provide some thoughts as to whether their sampling and conclusions are representative or not.

I recommend that the authors revise the manuscript and resubmit a revised version for evaluation prior to publication.

 

Comments:

This paper would be much easier to follow if the authors made a table with the conditions (setup conditions and measurements) for each of the six piles (A1, A2, B1, B2, C1, C2).  They have part of this in Table 1, but it should be expanded to include all the relevant parameters for each pile (e.g. aeration rate, waste mass load, presence of sidewall, duration of cycle, etc.). 

The authors use both "content" (e.g. CO content) and "concentration" (e.g. CO concentration) to mean the same thing throughout the paper.  However, the CO "content" is the integrated CO across the entire pile (e.g. total mass of CO), whereas the CO concentration is a measure of the proportion of the gas that is CO at a particular point (e.g. partial pressure per total pressure).  They are not the same and should not be used in the same way.

Figure 1: This figure should be reformatted to be consistent with the format of Figures 2 - 5.  In general, all of the figures should have the same format to make them easier to follow.  It was especially confusing to have the pile number (A1, etc.) below the respective figure in Figure 1 and above the respective figure in Figures 2 - 5.

Figure 3: Caption says "Carbon dioxide content" but the axis is labeled "O2 (%)".  In fact, Figures 2 and 3 are identical in the current manuscript.  I recommend that this be corrected.

Line 42: "the-composting" should be "the composting"

Line 46: Should have ")" between "[7]" and ", research"

Lines 49-53: Authors begin by discussing greenhouse gases, but then conclude that the lack of CO research limits understanding of worker exposure.  As written it is confusing - perhaps discuss CO2, CH4, and N2O emissions without referring to GHG's, since you are currently setting it up to discuss CO as a greenhouse gas, then switch on the reader and instead talk about it as a health hazard.

Lines 56-57: It isn't clear what you mean by physical-chemical or thermo-chemical, since I would expect moisture content and material type to impact both physical-chemical and thermo-chemical production of CO.  The "However," in line 56 suggests that these observations are inconsistent with a physical-chemical or thermo-chemical origin.  It isn't clear the distinction you are trying to make between the different processes here.

Lines 60-61: You claim that the research "does not confirm" the role of microorganisms.  Does it suggest that microorganisms are not important?  Does it refute that?  It's not clear what the conclusions of Hellebrand and Schade's study is, as written, or how that conclusion relates to the rest of the paragraph.

Lines 68-71: This sentence is confusing as written - you are implying that previous research has shown a link between aeration of waste and CO formation.  However, that is what the current study is trying to look at, so perhaps it is not already known?

Lines 175-178: Does excluding the high datapoints (values above 1000 ppm for CO, for instance) affect the results at all?  I would expect that excluding the high data points would skew all of the data lower and might result in incorrect conclusions.  Make sure this is addressed in the discussion.

Lines 187 - 190: Why is this particular model chosen for this system?  You provide a reference, but you should provide additional discussion as to why this particular functional form makes the most sense here.

Line 336: Authors mention "As in the case of CO and CO2" but haven't presented the cases of CO and CO2 yet.  This is confusing - it requires me to jump forward, then return back to the CH4 part.

Section 3.4.1: Authors should expand their description of the general additive model setup, parameterization, and results.  Even referring to the GAM reference, it is difficult to understand exactly what form the model took.  Was it parameterized by each individual measurement point, inclusive of all the data?  It may also make sense to parameterize the model using a subset of the data (e.g. A1, B1, C1), and then test the model on the remaining data (e.g. A2, B2, C2) to provide better external validation for the model results.  Overall, I am not sure the GAM section adds anything to the paper in terms of conclusions - it is possible that the authors could omit this section altogether.  If the authors choose to retain it, they should improve the discussion of the model and its results, especially in terms of any new insight gained from the model.

Section 3.4.2: The authors show non-linear relationships between different parameters (temperature, CO2, O2) and discuss the correlations.  Some of these relationships do not seem to fit the data very well, at least based on visual inspection. 

 

Line 405: Authors mention a "very high correlation between carbon monoxide concentration and oxygen (Fig 9).".  However, this correlation is shown in Figure 8(c), not Figure 9, and only has a R2 of 0.5451 (which is not considered a "very high correlation"). A visual inspection of Figure 8(c) suggests that the data seems to group into several different groupings. 

Figures 8 and 9: There are lots of discrepancies between the plots shown in Figure 8 and Figure 9.  For example, CO ranges from 0 - 1000 ppm (approximately) in Figure 8(a),(b),(c), but goes up to 2000 ppm in the Figure 9 subplots.  Similarly, Temperature goes from 35 - 75 degrees C in Figure 8a but goes down to 20 degrees C in Figure 9 B,E,H.  It is not clear why there are differences between Figure 8(a) and Figure 9(b), between Figure 8(b) and Figure 9(a), and between Figure 8(c) and Figure 9(D).

Line 431 and Figure 9 suggest that CO was measured up to 2000 ppm; however, authors state (Line 176) that CO concentrations above 1000 ppm were excluded from the dataset and statistical analysis.  It is not clear what the CO values above 1000 ppm are.  Figure 7 also shows CO > 1000 ppm.

 

Author Response

Dear Reviewer,

 

Thank you very much for all your comments. Please find our replies in attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Dear Authors,

Thank you very much for this interesting article. I am very honor of review it.

This paper to investigate the formation of carbon monoxide (CO) during aerobic biostabilization (AB) of the organic fraction of municipal solid waste (OFMSW)
 in forced aerated piles. Understanding the factors influencing CO formation may be important not only for safety but also for environmental and technical reasons.

I suggest improving the following points:

• on the materials and methods: in row 101, maybe descript the term "OFMSW" and I suggested of write of the "list of terms" or nomenclature.
• on the row 153 describe the technical law (EN or ISO 11346???). I don't understand what the core and the methodology used.
• Improve the conclusions, It is very poor for the several data on this article.
• The figure 6-9-11 is low-quality image, maybe improve them?

Bets regards

 

 

 

Author Response

Dear Reviewer,

 

Thank you very much for all your comments. Please find our replies in attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

This study investigates the effect of several parameters on the level of CO generation during aerobic biostabilization of the organic fraction of municipal solid waste. I believe the most valuable aspect of this study is designing experiment for a full-scale system and analyzing data using a statistical model. In my opinion the introduction provides enough information about knowledge gap and the way this study is going to address. Also the results are very well presented and discussed. Abstract and experimental parts are also well-written but could be a little bit shortened.

Author Response

Dear Reviewer

 

Thank you very much for all your comments. Please find our replies in attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I appreciate the effort the authors made to address the concerns raised by myself and the other reviewers with the initial manuscript.  I believe that the resulting revised manuscript is significant improved as a result of these changes.  The authors addressed most of the issues, inconsistencies, and errors in the original manuscript and the result is a much stronger manuscript.  I think that this revised manuscript is acceptable for publication at this point.