Electrochemical Oxidation Treatment of Organic Matter in Wastewater from Wet Fermentation of Yunnan Arabica Coffee

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

 

 

MDPI-WATER-REVIEW REPORT

 

Manuscript Title:

Electrochemical Oxidation Treatment of Organic Matter in Wastewater from Wet Fermentation of Yunnan Arabica Coffee  

Authors:

Liang Fanfan, Wang Yan, Liu Yungen, Yang Silin, Yin Fajin, and Peng Liping

1-General Comments:

In this study, the electrochemical treatment of  wastewater from the wet fermentation of Yunnan arabica coffee in the presence of the oxidants of O₃, H₂O₂ and FeCl₂ was investigated. For this purpose, the effect of the selected oxidants on treatment efficiency, the binding capacities of these oxidants to proteins present in the wastewater, degradation of organic pollutants/COD parameter found in the wastewater, and the formation of degradation products were examined in more detail. The  obtained findings indicated that (as expected…) electrochemical oxidation in the presence of ozone (the strongest oxidant among the studied ones) performed better than electrochemical treatment alone or in the presence of the other two oxidants. A raise in the pH is also reported for the ozone/electrochemical treatment combination reducing the NaOH concentration required for pH-adjustment of the slightly acidic wastewater. Organic matter was followed collectively as the COD parameter (originally being above 30,000 mg/L, which is an extremely high value for chemical oxidation processes!). Organic matter was also followed spectrophotometrically and by fluorescence. Some interesting results were obtained but need to be presented in a clearer way. Generally speaking, this is a well-organized manuscript with an interesting subject fitting to the journal’s scope. However, some parts have not been very clearly written and therefore these parts are rather difficult to follow. Some major improvements could be made in the description of the methodology, the presentation of experimental results in the text, figures and tables as well as in the discussion section to comparatively evaluate major findings and come to a general conclusion.

More details in the form of section-based comments are given right below in the 2-specific comments section.  

2-Specific Comments:

·       Abstract and elsewhere: How can FeCl₂ be an “oxidant” (electron acceptor)? Fe(II) is a typical reducing agent that is quickly oxidized to Fe(III)… If its reduction to elemental Fe is meant due to another electron transfer (redox) reaction, this should be explained in the manuscript text.

·       Abstract and elsewhere in the manuscript: When a chemical (here for instance the oxidant H2O2) is added once and at the beginning of the reaction, then the term “concentration” should be used instead of “dose”. Please consider revision.

·       Abstract: It is important to give numerical results (the most important/critical ones) in this section about the selected reaction conditions (pH, ozone dose, oxidant concentrations, applied current density, etc.) to give the reader more specific and interesting information. Comparison of treatment performance and removal rates / yields in the presence of different oxidants is also some critical information in this paper.   

·       Introduction, typical properties/characterization of the model wastewater should start with organic matter (measured as COD, BOD5, TOC), followed by nutrients (ammona, nitrite, nitrate, orthophsophate), solids (TDS, SS, MLVSS, DS, etc.), cations and anions, color, turbidity, electrical conductivity, color (peak absorbance), other specific parameters and finally pH. Please consider this order of measured parameters for presentation of the envionmental characterization of the wastewater.

·       Introduction: What is meant with chromticity (color?absorbance in the visible light region)?

·       Introduction, final part: This section immdiately turns to electrochemical treatment and how advantageous it is compared to conventional, alternative treatment methods. No general information about this treatment type and what electrochemical method was employed in this work  (together with advantages disadvantages, reaction mechanism, process parameters, etc). is generally missing.

·       Introduction, final paragraph:  This section should give more information about the major purpose of this study (scope of work). The scope of this study (how the purpose of the present study was envisioned, how/which procedures and protocols were  used) in the present work is missing. It has been mentioned very briefly and only at the end of this section.  

·       Introduction: Almost no word / technical details about the electrochemical unit (process) has been mentioned in this section.

·       Experimental, materials: From where were the chemicals obtained / purchased (chemicals of which company?) and what are their CAS numbers? Are the % purities given in w/w (mass-weight) units? Some purities/grades of chemicals are missing.

·       Experimental: The electrochemical “combustion”? reactor…what does this mean? What was the material of the electrode plates? Are the plates home-made or purchased (Company? Model?) Please provide more technical details together with the related reference citations.

·       Experimental: The ozone contactor (ozonation set-up) and the electrochemical treatment procedure (conditions) have not been described in detail. Important is to indicate the applied ozone dose (in mass per time unit-sometimes mass -or concentration- also divided to the initial organic concent-mass/volume), the mass transfer coefficient, how the off-gas and residual (bulk phase) ozone were measured (procedures, citations of procedures, related references, etc). Also, the calibration of the applied ozone dose and current densities are critical since these serve as major design parameters and eare xpected to primarily affect the removal (treatment) performance (the degree of oranic matter oxidation). Units of concentrations, expression of applied doses should be unified with and used as is being typical for the related scientifc literature and similar applications to industrial effluents. For example, current densities are typically given in “mA/cm2” or “A/m2” which is the major design parameter of electrochemical treatment. The applied ozone dose is typically given in “mass of ozone per mass of initial pollutant or collective environmental parameter (per volume)” In this manuscript, when presenting experimental results, different units were given for electrochemical treatment, oxidant concentrations (mass or mole/ volume)  and applied ozone doses.  

·       Experimental, analysis of COD: Please correct the name of the company and model of the COD apparatus: It should be Hach model, not Hasch (?), and the procedure (analytical detail) of the COD measurement is not complete; (i) what standard method was used to determine COD in the samples, describe the sample preparation stage (since it is a strong wastewater, dilution would be necessary), and also indicate the reference of each analytical method; (ii) how was the interference of residual/unreacted O3/H2O2/Fe(II) oxidants prevented (these oxidants will positively interfere with the COD test procedure by reacting with the excess reagents added into the reaction solution)?? Thereafter, results in the figures displaying temporal changes in COD should be corrected, if necessary.

·       Experimental: The entire experimental set-up, ozone generation/ozone production, also the determination of inlet (feed)-, off-gas, absorbed and bulk ozone concentrations has not been described (analytical procedures + related reference. For example, calibration of the mass transfer rate, ozonation kinetics, the applied and absorbed ozone doses after/before the electric current, etc. have not been described. Please clarify and give more details about the analytical and instrumental procedures. 

·       Experimental: How were the oxidants added before the electrochemical treatment step, that is not very clear. Moreover, ozone is very unstable and has to be introduced into the reaction solution continuously. Were the oxidants added initially, at the initial stage of the reaction or continuously? Please explain the experimental procedures to clarify the reaction conditions.

·       Experimental: It should be noted that not the treatment time but the applied ozone dose and charge flow should be indicated to compare reaction conditions and results with previous, related work. Indicating the time selected/used for oxidative treatment is not so meaningful (it is a relative parameter). More important is the applied ozone dose (in ozonation)  or charge (in electrochemical treatment), both of which increase during treatment time together (parallel) with the electrical energy required to run the ozone generator and the electric current applied to obtain a specific voltage during electrochemical treatment.

·       Figures and Tables: Please indicate all reaction conditions (initial concentrations, pH, ozone dose, charge density, etc.) in the figure captions and table headings.

·       Experimental Results and elsewhere: The term “COD” or “COD content” is more appropriate than “COD concentration” (rarely used in this way, since it is a collective environmental parameter representing the total organic carbon content of the wastewater in dissolved, molecular oxygen equivalents).

·       Results, Figure 2 is difficult ot follow. COD (%) and DCOD (absolute COD removals, in mg/L, why not % relative COD removal efficiencies) on both “y” axes could be shown for O3, H2O2 and O3/electrochemical treatments as well as electrochemical treatment alone (in the absence of oxidants) as a control experiment. Not the ozonation/electrochemical treatment time, but the applied ozone&charge densities are important and affect treatment costs (operating costs, due to electricity and chemicals consumption). Are these systems introduced separately or in combination? The type of experiments that are shown in Figure 2 are not clear.

·       Results, Figure 3: Since removal mainly occurs within the first 2 hours of treatment, it would be more meaningful to display total phosphate (TP) removals in “minute” intervals, showing initialchanges in TP versus treatment time (0-10-20-40-60-90-120 min, for example). Please modify this figure, since it is shown in an unusual way and rection conditions are not very clear. Time is presented on the “y” axis and it is really diffcult to examine/compare in ths figure for different processes (parameter changes for different treatment systems). On  the other hand, the figure may not be necessary anyway if samples were not taken throughout the first 120 min, since there is no kinetic trend for TP removal (it does not change with treatment time, so why showing TP versus time?). Instead, the results can be mentioned directly in the manuscript text (TP removals).

·       Line 194: “….oxidized radicals (??) ® please consider to edit to “oxidizing radicals”. Further in this paragraph, the Fenton reaction is explained in the beginning and end of the paragraph. The contribution of oxidants should be presented comparatively in one single figure.

·       Figure 8, Y axis showing Absorbance = what s the unit of absorbance? It is “cm-1”?

·       Results; Line 322: What is meant with “municipal biological wastewater”, may be “biologically treated municipal effluent”? Please clarify. The authors state in this paragraph that combined ozone/electrochemical treatment provides an effective method for COD and TP removal; however, the O₃/electrochemical reaction mechanism and what basic/control reactions contribute to COD removal (HOW?) is missing.

·       Where is the Discussion Section? Why is it not combined with the Results Section?

·       Conclusions: A list of major findigs including numerical results of most important experimental results should given in this summarizing section (removal rates/kinetics and per cent removal efficiencies for specific applied ozone doses, oxidant concentrations and/or the specific charge density).

·       Conclusions: Estimation of the total (chemicals, catalyst regeneration/replacement, electricity, etc.) operating costs for the electrochemical treatment / ozone combined treatment systems should be mentioned in brief tojudge the feasibilty of the proposed treatment processes.

Comments on the Quality of English Language

As also has been mentioned in the review report, there are some language/English/terminology and  typographical errors that the authors should consider prior to re-submission.

Details are given in the review report.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

My comments:

1. The authors did not compare with O2 oxidation, which is mainly the source of oxidation. It would have been useful to compare the effectiveness of electrooxidation to the more conventional O2 approach.  

2. O3 is very expensive, so the suggested work using O3 may also be very expensive to implement. The cost-effectiveness would need to be analyzed.

3. What analytical methods were used to identify the chemical composition before and after combustion? It would be useful to report the results and show how composition changed. 

4. Discuss the advantages of using electrooxidation/O3 processes for degrading organic compounds compared to alternative approaches. Faster reaction rates? More complete degradation? Less waste products? 

5. Compare the findings of this new work to previous experiments on this topic. What novel insights or advantages does this research provide over past studies?

6. Include photos of the actual Electrochemical combustion reactor setup used in this experiment for readers to better visualize the methods.  

7. There appear to be some typos and errors in grammar/vocabulary usage. A proofreading pass could help improve clarity.

Comments on the Quality of English Language

It will be revise.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study utilized electrochemical oxidation with O₃, H₂O₂, and FeCl₂ to treat Yunnan arabica coffee wet fermentation wastewater. It systematically examined the impact of oxidants on treatment efficiency, protein binding capacities, organic pollutant degradation, and formation of oxidized organic components. O₃-combined electrochemical oxidation outperformed H₂O₂ and FeCl₂, effectively elevating wastewater pH, reducing NaOH dosage, and degrading proteins, lipids, hydrocarbons, and carbohydrates.

However, when a hybrid process results are described, it is a standard procedure to compare with individual processes. The results of electrochemical alone and oxidation alone should be presented and compared with the hybrid electrochemical oxidation studies. This can enhance the quality of this detailed study.

what is the procedure and settings used for simulation software

 

Line 363: wrong suffix, O₃

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have unfortunately not addressed all comments properly. Some details are given below:

1-Experimental conditions are missing in Figure Captions and Table Headings.

2-References need update (more recent research and rewiew papers should be cited in the References Section)

3- Some terminology revision has not been considered, such as "concentration" instead of "dose" for chemicals (reagents) added once and at the beginning of the reaction.

4-The following questions (numbers are listed below) hae not been fully and properly answered:

Question/Query Nr. 2, 3,4,9,11,12,13,15,25.

Comments on the Quality of English Language

The English is acceptable and will only need light to moderate revision.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

It is ok

Comments on the Quality of English Language

It is ok

Author Response

Thanks to the journal editorial department and reviewing experts for their valuable comments and suggestions on the manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

Manuscript quality improved 

Author Response

Thanks to the journal editorial department and reviewing experts for their valuable comments and suggestions on the manuscript.