Modelling of Biotrickling Filters for Treatment of NOx Analytical Expressions for the NOx Concentration in Both Gas and Biofilm Phases
and Lakshmanan Rajendran 4
Round 1
Reviewer 1 Report
1. The Authors are asked to revise the manuscript and correct spelling and little grammar errors, e.g. too big spacings (e.g. L16, 57, 87 etc.), Latin names should be given in italics, L54 – please write “biofiltration”, and other small mistakes.
2. Fig. 1 is adapted from other paper – it is of poor quality and I suggest to draw the scheme by Yourself.
3. General comment: it is not clear for the reviewer whether the paper deals with modelling of NOx removal based on empirical data by the Authors or the Authors propose and compare the model with some available literature data? This should be clearly stated, e.g. in Materials and Methods section, which is in fact missing.
4. Please consider numeration of sections, there are e.g. two 1st sections.
5. Equations and their numeration must be revised and clearly given. Each equation in one line with a specific number, even if it is an assumption for the model or equation.
6. Authors are asked to slightly enlarge the discussion on modelling of biotrickling filtration, also in general (not only for NOx removal), showing other models and the developments in the field.
7. Authors are asked to revise and shorten the title of the manuscript, it should be more concise in my opinion.
8. The Authors are asked to provide some proposals of future research in the field of biotrickling filtration modelling in the conclusions section.
Author Response
Please see replies to reviewer 1 attached.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The present manuscript deals with the solution of nonlinear differential equations that can be used to estimate/calculate the nitric oxide concentration in both the gas and biofilm phases of biotrickling filter systems.
This is an interesting and original study which merits publication in "electrochem".
However, there are some points that need to be addressed before publication of the manuscript.
Special points (unfortunately, the numbering of pages and lines in the reviewer's copy is quite confusing):
1) It would be better to present Fig.2 before listing the assumptions, and it would be advisable to redraw the figure to make it easier to understand points 1,2 and 5.
2) l.85-88: Assumption number three should be explained here in more detail. (see l. 116-117 later)
3) l.114-115: Please, explain the exact meaning of af. (see also l. 124)
4) the use of [...] marking would be better avoided here.
5) p.7: Eq. 16 should be corrected.
Minor points:
p.5, l.146: T should be italic.
p.9, l.160: v should be italic. 2x
p.7, l.204: NOi and NOf should be italic + 1 space is missing
Nomenclature: the unit of Y should be checked.
Author Response
Please find replies to reviewer 2 attached.
Author Response File: Author Response.docx
Reviewer 3 Report
Major comments
The model presented is compare to another model, rather than being validated with experimental data. This is a major limitation of the work reported. It would great if the authors can use their model to fit experimental data, which are area already available in the literature.
The model assumptions are quite strong and in contrast with the real heterogeneous nature of biofilms, particularly 1. (vertical channels), 2 (no axial diffusion), 3. (homogeneous biofilm). Please discuss the limits of these assumptions in the discussion section.
I may be wrong, but I cannot see any mention to the moving frontier of the biofilm, i.e., the fact that the biofilm thickness Lf = f(t). Instead, the model consider it constant, as if the biofilm is at steady state. Indeed, this is a reasonable assumption for biofilm under oligotrophic conditions. If this is the case, is it really necessary to include the Monod kinetic in the model? Since no cell death is included, this formulation will lead to continuous growth of biofilm, resulting in BTF clogging. Can you please discuss these and other limitations of the model?
Further, there is no mention of how biofilm differs in terms of its internal composition. For example, the amount of EPS can significantly affect the NO diffusion and conversion rate.
What is the point of modeling a single strain in a biotrickling filter? Such system are inherently open and the biofilm observed comprise always multiple microbial species (e.g., bacteria, fungi, and higher organisms). Are the results presented here valid only for the strain C.daeguensis TAD1 or can they be extended to other microbial species?
Please add 2-3 short paragraphs to explain what a biofilm is and describe briefly its chemical composition and microstructure.
Many sections of the text are poorly formulated and require extra effort for the reader. Please revise the manuscript to bring out the key points in term of novelty and technical quality of the work reported.
Examples
"To demonstrate this new approach, graphical data are provided and quantitatively discussed. This theoretical result has good agreement with the numerical data." Do you mean that theoretical results agree with the EXPERIMENTAL results?
"We now discuss how an efficient analytical solution to these coupled non linear differential equations may be solved using the Akbari-Ganji method (AGM)." Do you mean that an analytical solution may be found?
Minor comments
Italicize microorganism names as per standard rules (e.g., mention the full name in the first occurrence and the abbreviated name in the other occurrences)
Change nitric oxide into nitrogen oxide
mention briefly what are the negative effect of nitrogen oxides on environment and human health.
"such as the biotrickling filter (BTF) and the biofilter" please rephrase to carlify the difference between these two technologies.
change "anoxic circumstances" into "anoxic conditions"
"To resolve this concern, a small number of researchers have worked on extracting and exploiting thermophilic microbes to remove NOx". What is the "concern"? What do you mean by "extracting thermophilic microbes"? Maybe "isolate thermophilic microorganisms"?
"save some expenses"? Please clarify.
"propose a new strain"?
"cure nitric oxide"?
"the impacts of BTF modelling on the removal of volatile organic compounds"? Please clarify
"thermophilic circumstances"? Are you referring to process conditions or to the strains adopted?
Correct typos such biofilteration, etc.
"However, to the best of our knowledge, no rigorous and simple analytical expressions of nitric oxide concentration in the gas and biofilm phases have been reported [8]. The aim of this study is to generate approximate analytical results for nitric oxide concentration in both gas and biofilm phases using Akbari- Ganji's method". I am no mathematician, but this sounds contradictive. First you mention a rigorous and simple analytical expression, then you mention approximate analytical results. Please rephrase to improve clarity.
"nitric oxide extraction"? Do you mean conversion into N2 or other useful species? Please clarify.
"No free liquid is recirculated..." Do you mean fresh medium is supplied to the water bath (Fig. 1, right) and not directly to the filter?
"nutritional liquid"? Please use the correct term here.
change "slim interfacial" into "thin interfacial"
Author Response
Please find attached replies to reviewer 3.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The Authors are asked to clearly respond to the reviewer's comments from the first round of review. The reviewer cannot accept the currently obtained responses as well as the changes in the manuscript seem not to be complete, e.g. changes in the introduction section regarding mathematical modelling of biotrickling filtration is are too lilttle (Authors should refer to at least several models and references), also numeration of equations is still not correct for expressions termed eq. 8. Please respond clearly to previously stated comments, depicting what and how the fragments in the manuscript were changed.
Round 3
Reviewer 1 Report
The Reviewer thanks the Authors for the corrections as well as more detailed responses to the comments. I recommend the paper to be published in Electrochem Journal.
