Evaluating the Performance of a Lab-Scale Water Treatment Plant Using Non-Thermal Plasma Technology

Round 1

Reviewer 1 Report

In this work, the authors test a lab-scale water treatment plant based on non-thermal plasma for degradation of three model pollutants: indigo, p-chlorobenzoic acid and phenol. The first set of experiments aim at calculating the power consumption of the plant and at studying the dependence of the power on the electrical parameters of the plasma. The second set of experiments studies the dependence of the degradation of pollutants on the power and on the addition of oxygen, synthetic air or hydrogen peroxide to the systems. The third and fourth sets of experiments investigate the influence of the liquid flow and the propertied of the solution (conductivity, pH and temperature).

The topic of this work is of high interest for the scientific community and the authors addressed it in a very complete and detailed way. The analysis of the state of the art is fine but could be improved, the aim of the work is clearly presented, the methods are adequately described and the results are clear and pertinent to the scope. The discussion of the results can be improved as well (see later).

Here a list of requests/suggestions that should be implemented in the manuscript before publication.

Major comments:

• A comparison of the energy efficiency of this treatment plant with other alternatives in the literature (plasma-based and not) is necessary to evaluate properly its performance. I suggest to choose the sets of working parameters that corresponds to better performances and to calculate the energy efficiency as G₅₀ (Malik M. A. 2010 Plasma Chem. Plasma Process., 30, 21-31) or any other equivalent parameters. A comparison of efficiencies of plasma systems in treatment of phenol, for example, can be found in Bosi F. J. et al. 2018 Plasma Process. Polym., 15, 1700130.
• Can the author comment on the average residence time of the pollutants in the reactor chamber? Is there recirculation of the solution? I don’t understand the how the experiments as function of time were performed (Fig.s 2 right, and 3 right), this should be clarified.
• Lines 270-271. Ozone is known to be one of the major species that decomposes indigo by attacking the carbon-carbon double bond. Authors should put some references in support of their interpretation here.
• Lines 274-277. Ozone is continuously produced by plasma. The concentration measured is the balance of production and consumption mechanisms. I do not think it is correct to compare stoichiometrically ozone and indigo to infer information about other reactive species.
• Error bars should be added in all the figures.
• Figure 7 left. Why only one point for pH 8 and 8.3 was measured. More points for these pH values should be added to the graph.

Minor comments:

• Consider rewriting the first sentence of the abstract.
• Figures numeration and references in the text should be fixed.
• Lines 125-126. By what criterion were the concentrations of indigo, p-chlorobenzoic acid and phenol chosen?
• Line 202. Phenol index should be defined.
• How were the empirical equations 1a and 1b obtained? Add units of measure to all the empirical equations in the text (1a, 1b, Fig. 2 left, Fig. 3 left, Fig. 4 left, Fig. 6).
• Fig.s 2 left, 3 left, 4 left, 5 and 8. Large parts of these graphs are empty and sometimes it’s difficult to distinguish between different experimental points and to notice trends. Please adjust the vertical scales. Fig 2 left can be plotted from 1.0E-5 M to 5.0E-4 M for example.
• Section 2.3. Was the quantification of phenol and its oxidation products by HPLC/UV done using standards? Please clarify.
• Section 3.4. Why the pH values 8 and 8.3 were chosen? Why a larger range was not studied?
• Section 3.4. Concentration of phosphate buffers used in the work should be added.

Author Response

Dear reviewer,
we thank you for the critical review of our manuscript. As a result of your comments we have thoroughly revised it.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Authors,

This is an interesting work, which has great importance for the improvement the ecological situation, in particular the removal of organic pollutants from the waste water. The advantage of this work is the application of low-temperature plasma. This process has great prospects.

 Despite the undoubted merits of the work, the manuscript in its present form cannot be published in the Water journal.

 My comments is in attached pdf-file. Also, I would like to emphasize the following:

1. Probably, article title should be corrected.
2. An introduction on the volume and information provided (in particular, various small details) is excessive.
3. Line 42: Perhaps it is not worth mentioning neutrons, since under these conditions their generation is negligible and generally does not relate to the subject of the article.
4. Line 97: The term “swelling energy” is not well understood. If this is not a common phrase in this field, its meaning should be clarified.
5. Lines 124-126: What determines the choice of these concentrations?
6. Lines 303-306: Explanation is not clear. How it can be connected with a different degree of degradation of the Indigo carmine and para-Chlorobenzoic acid.
7. Lines 311-312: “A further increase in the addition of H2O2 by a factor of two or three had no measurable effect on this finding” - corresponding curves are absent in the figure Figure 3, right.
8. Lines 319-320: UV-Vis spectra is absent.
9. Line 379: “molar addition of a phosphate buffer” – is the conductivity also changed (with pH) upon addition of phosphate buffer. Specify this.
10. Figure 3: There is a mismatch between the experimental graphs and the approximation formula.
11. In the case of studying the dependence of the efficiency of degradation on pH, despite the corresponding difficulties, more experiments should be performed to confirm the existing dependence.
12. Lines 461-463: We couldn’t find in the text how the formation of the species listed was confirmed.
13. Lines 463-466 (last sentence of Conclusions): this phenomenon can be due to the design features of your unit, namely an extremely high power plasma generator in relation to the volume of water supplied.
14. There are no conclusions, which is based on the three reference substances degradation, what type of species is responsible for degradation.

Comments for author File: Comments.pdf

Author Response

Dear reviewer,
we thank you for the critical review of our manuscript. As a result of your comments we have thoroughly revised it.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this article, non-thermal plasma technology based on high-voltage (HV) pulsed-barrier discharge (PBD) is employed for treating organic wastewater containing indigo carmine, para-chlorobenzoic acid or phenol. The experiment conditions are investigated, and its degradation mechanism is also analyzed and discussed. The methodology and results are well presented, and the data and conclusions are reliable and convincing. However, revisions are needed prior to a possible publication in this journal. 1. The abstract must be rewritten. Non-essential description is presented in Line 15-21, it should be reduced. Besides, increase more introduction about the experimental results, and the suggestion of this manuscript can also be described at the end of Abstract. 2. In Keywords, delete “radical; ozone; AOP”, and the “indigo carmine; para-chlorobenzoic acid; phenol” can be added. 3. In the first paragraph of “1. Introduction” part, too many descriptions about HV-PBD, just like popularizing knowledge to the public, please condense this paragraph into a few sentences. Besides, add the descriptions of the differences between this research and the others, because there are many literatures about the use of HV-PBD to control water pollution. 4. The generation, pollution and treatment methods of organic wastewater also need to be briefly described in the Introduction part. And the following papers will be helpful in some aspects: International Journal of Environmental Research and Public Health, 2019, 16(24), 5129; Journal of Environmental Engineering, 2018, 144(6), 04018041; Journal of Environmental Sciences, 2017, 57, 211-220. 5. Many “Error….” (such as in Line 115, 297) are presented in this manuscript, the authors should edit this manuscript more carefully. 6. The serial number of figures is disorder, please revise. Besides, if there are two or more figures in one picture, please mark “a, b, c…”. 7. Put 3.1 section in 2 part. 8. In “3. Results and Discussion”, all the titles should be simplified, and the content is also verbose in the 3. part, please condense this part.

Author Response

Dear reviewer,
we thank you for the critical review of our manuscript. As a result of your comments we have thoroughly revised it.

Author Response File: Author Response.pdf

Reviewer 4 Report

Article entitled Evaluating the Performance of a Lab-Scale Water Treatment Plant With Non-Thermal Plasma Technology Based on the Degradation Behavior of Indigo Carmine, Para-Chlorobenzoic Acid, and Phenol, written by Stefan Schönekerl, Stephan Uhlig, Katharina Wellner, Richard Pörschke, Christel Pfefferkorn, Karsten Backhaus, Astrid Weigert, and André Lerch and submitted to Water journal as a draft no 822657, deals with an important issue pollutants removal from water.

The article is interesting and could be considered for publication in Water journal. However, while reading I found some statements missing, confusing or unclear. Below I enclose list of my comments. As English is not my native language, I am not able to assess its correctness.

Abstract – “Against the background of increasing contamination of raw waters for potable or industrial process water treatment by natural and anthropogenic components, simultaneously rising demands on the quality of pure water and increasing cost pressure in municipal water management, there is currently a great interest in new, innovative and versatile water treatment technologies. Over the past decades, worldwide research has been particularly focused on various advanced oxidation processes, in order to find methods to effectively remove pollutants with high chemical stability and low biodegradability.” It is just general information. That should be in introduction.

Literature review on Non-Thermal Plasma Technology should be added.

Line 115, 279, 311 and others, part of the text is missing – I assume it should be fig 1 and 3, respectively.

Line 160 was ozone sensor working in water or gaseous phase? Under such conditions, there are usually many problems with the calibration of the ozone sensor and its reliable indications. Have the authors encountered such difficulties? Later in the article some problems were mentioned.

Equations 1a and 1b how the Authors found the formula?

Figure 1. left panel. Values are hard – to – read. It should be rearranged.

General remark: why the Authors use mol/L instead of mg/L? Usually in wastewater discharge regulations mg/L are used as a units. As far as I know, in technology doses are given in mg/L or even in terms of working fluids in mL/L. On the other hand, mol/L are used when chemistry, mechanisms/reactions are considered. While here I can not see any deeper chemical considerations. It is 100% technological article, mainly focused on energy consumption and physical parameters influence on process efficiency.

Line 309 reaction should be in separate line as equation.

Figure numbering is incorrect (1, then 3, then 2, then 5, etc).

Why conditions for experiment on fig 1 and 3 are different (oxygen, hydrogen peroxide, air ventilation)?

Figure 2, left panel. It is not clear. Especially the enlarged chart. Crosses are almost not visible. It should be rearranged and maybe divided in two figures.

No radicals detection tests were employed, so reflections and comments on radicals are credible but only speculation.

Technology costs are important for patents and practical implementation. How much does it cost to use this installation. There is no chapter on the economics of this solution.

Based on my comments and general impression I suggest major revision.

Author Response

Dear reviewer,
we thank you for the critical review of our manuscript. As a result of your comments we have thoroughly revised it.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Dear Authors,

Thank you for the detailed answers and the work done to improve the manuscript.

Some minor corrections are required (see attached pdf-file).

I also have some comments:

1. The abstract is excessive (must be no more than 200 words).
2. Fig. 5b: Products formed during phenol removal are less or more harmful to the environment and humans? Please, note this in the text.

Kind regards!

Comments for author File: Comments.pdf

Author Response

Dear reviewer, thank you very much for your time and the critical review of our manuscript. We really appreciate that

Reviewer 3 Report

All the comments and suggestions have been addressed carefully by authors. I have no other comments about this paper.

Author Response

Dear reviewer, thank you very much for your time and suggestions that have helped to improve the current version of the paper.

Reviewer 4 Report

This is my second review of this article. The Authors responded to my comments. Suggested corrections have been applied. My doubts have been dispelled. I suggest accepting this article in its current form.

Author Response

Dear reviewer, thank you very much for your time and suggestions that have helped to improve the current version of the paper.