Comparison of Different Enhanced Coagulation Methods for Azo Dye Removal from Wastewater

Round 1

Reviewer 1 Report

The paper is concerned with the chemical decolorization process of Congo Red using response surface methodology. The effect of different coagulation pretreatment processes (ECPP) on the microbial community structure of PDW was investigated. Paper covers some important issues, so I recommend it for publication in Sustainability journal after revisions.
Some comments are listed below:
1. Keywords and highlights need to be corrected.
2. The authors should add a list of all abbreviations used in the manuscript.
3. Why the authors choose Congo Red from the group of azo dyes?
4. Have the authors done reproducible studies? If so, they should show the error bars in figures.
5. Scales on figures are too small, therefore it is difficult to analyze them. Figures are of poor quality.
1. The discussion of obtained results with reference to be published so far paper is poor, so the manuscript needs to be improved.
2. The paper should be corrected very carefully especially according to editorial and language style.

Author Response

Point 1: Keywords and highlights need to be corrected.

Response 1: Thank you very much for your valuable comment. Keywords and highlights have been revised. Please refer to the keywords: Azo dye, Decolorization, Coagulation and flocculation, bacterial community, Response Surface Methodology (RSM).

Please refer to the highlights:

Parameters of enhanced coagulation pretreatment processes(ECPPs) were optimized by Response Surface Methodology (RSM). Different chemical decolorization processes can be selected according to the initial pH and concentration of Congo Red. It provided a comprehend for the relationship between physicochemical and biochemical treatment of printing and dyeing wastewater (PDW).

Point 2: The authors should add a list of all abbreviations used in the manuscript.

Response 2: Thank you very much for your valuable comment. We have added a list of all abbreviations used in the manuscript. Please refer to Table 4 in the manuscript.

Table 4

a list of abbreviations

Point 3: Why the authors choose Congo Red from the group of azo dyes?

Response 3: Thank you very much for your valuable comment. Congo red is a typical azo dye. Congo red is used to simulate azo dye wastewater for this study.

Point 4: Have the authors done reproducible studies? If so, they should show the error bars in figures.

Response 4: Thank you very much for your valuable comment. We did repetitive experiments. We took the average value to draw the response surface curve. We could not make the error bars in the 3D Response surfaces and contour plots.

Point 5: Scales on figures are too small, therefore it is difficult to analyze them.

Response 5: Thank you very much for your valuable comment. We have modified the scale of the drawings in the manuscript. Please refer to Fig.1, 2, 3, 4and 5 in the manuscript.

Point 6: The discussion of obtained results with reference to be published so far paper is poor, so the manuscript needs to be improved.

Response 6: Thank you very much for your valuable comment. We have improved the discussion section of the manuscript. Please refer to section 3.1, 3.2, 3.3 and 3.4.

Point 7: The paper should be corrected very carefully especially according to editorial and language style.

Response 7: Thank you very much for your valuable comment. The manuscript has been polished by a professional company. Please refer to Certificate of English Editing.

Reviewer 2 Report

Dear Authors,

I'm sorry, but after some thought, I have negative opinions on your article. The main disadvantage is a poorly planned experiment as well as the another minor weaknesses (presented below).

In the title and in the manuscript text you assumed that used coagulation methods were enhanced. I think, used coagulants were typical, just like an experiment. Perhaps the carrying out of the process was enhanced, but you did not describe it. The enhancement was unexplained.

line 14: I suggest to replace the word “chroma” witch “color”

lines 21,22: You wrote in Abstract “the microbial community of the wastewater after three different ECPP treatments was similar to the raw water and the oxidation ditch wastewater from a treatment plant”, but in results you presented only microbial community of raw wastewater from the regulating tank and coagulated wastewater (Fig. 4 and 5), not of wastewater from oxidation ditch. So it is a faulty conclusion.

lines 37,38: I suggest to rearrange the sentence “Any dyes left in the water after the dyeing process (printing and dyeing wastewater; PDW) are discharged into the water environment, and can cause surface water pollution” It is not permitted to discharge wastewater to environment before treatment, in this reason the sentence is not precise.

line 42: incorrect citation – the information (2.8x105 tones) does not come from Jin et al. (2007), but from Robert Maas and Sanjeev Chaudhari article (Adsorption and biological decolourization of azo dye Reactive Red 2 in semicontinuous anaerobic reactors)

line 52: incorrect citation – Lucyna et. al., 2019. Should be – Bilińska et al., 2019

lines 68,69: You wrote: “These results provide reference for further research studying the relationship between physical, chemical, and biological PDW treatment methods” I do not agree. You did not investigate any physical process (for example sedimentation). The coagulation is a physico-chemical process.

line 75: the actual wastewater come from regulating tank – was it an averaging/retention tank? Was the wastewater mechanically pretreated by a grid?

line 80: probably Al₂(SO₄)₃ x 18 H₂O

line 81: In 2.2 paragraph is presented polyacrylamide as a used reagent. You did not inform about ionic form of this substance as well as about its function in experiment (flocullant?).

line 100: The measurement methods should be given, not only instruments.

lines 102: In this lines and in all text you sometimes substitute word “wastewater” by “water”

lines 245 and 247: is Fimicutes, should be Firmicutes

Comments to methodology:

1. You did not describe the run of coagulation experiments – conditions of coagulation, flocculation and sedimentation phases. As we know, the conditions significantly affect the results.

2. You used the Ca(OH)2 for pH adjustment (that's my guess, no information in text). This hydroxide is used for coagulation and complexation too. So, I think, you should have done parallel series with NaOH, to eliminate the synergy effect.

3. In my opinion the RSM experiment designed by Box-Behnken is not adequate for investigation of coagulation process. It is need to remember, the treatment effectiveness dependence on pH and coagulant dose is not proportional. Your experiments witch only 3 levels of this factors can be insufficient to show the changes of obtained effect.

example:

4. The experiments #1, #2, #3 were planned in different ways. The selection of the range of parameters seems to be random. They differ in range of values of factors and in my opinion, for a better comparison, the pH and the CR concentration should have the same ranges in subsequent experiments #1-#3.

5. You used in experiment the actual wastewater from regulating tank. Was the sewage from one factory or many? You did not give characteristic of used wastewater? What kind of dye the wastewater contained? This information is missing. If you didn’t know the composition of actual wastewater, did you transfer the results from model experiment?

6. You didn’t present in methodology chapter the information about the parameters of coagulation of the “actual wastewater”. Only perfunctory information about used coagulant. This insufficient.

Results

The results form coagulation experiment are foreseeable. The polymeric coagulants are usually more effective in the wide range of pH.

In results no information about effects of coagulation of wastewater subjected to microbiological analysis.

In general, the results do not allow drawing general conclusions.

Comments for author File: Comments.pdf

Author Response

Point 1: line 14: I suggest to replace the word “chroma” witch “color”

Response 1: Thank you very much for your valuable comment. We have replaced the word “chroma” with “color” in the manuscript. Please refer to abstract:

Printing and dyeing wastewater (PDW) is considered to be one of the most difficult industrial wastewaters to treat because of its large quantities, high pH values, and high color and toxicity, which may endanger the lives of animals and humans.

Point 2:lines 21,22: You wrote in Abstract “the microbial community of the wastewater after three different ECPP treatments was similar to the raw water and the oxidation ditch wastewater from a treatment plant”, but in results you presented only microbial community of raw wastewater from the regulating tank and coagulated wastewater (Fig. 4 and 5), not of wastewater from oxidation ditch. So it is a faulty conclusion.

Response 2: Thank you very much for your valuable comment. It can be seen from the sentence " the microbial community of L0, L1, L2, L3 and the oxidation ditch (Fig. S1) was nearly the same " in section 3.4.1 that the microbial community in the result section also comes from the oxidation ditch sewage. Fig S1 is in Appendix A.

Point 3:lines 37,38: I suggest to rearrange the sentence “Any dyes left in the water after the dyeing process (printing and dyeing wastewater; PDW) are discharged into the water environment, and can cause surface water pollution” It is not permitted to discharge wastewater to environment before treatment, in this reason the sentence is not precise.

Response 3: Thank you very much for your valuable comment. We have rearranged this sentence in the manuscript. Please refer to introduction:

Coloring agents and dyes are widely used in the modern textile industry as well as other manufacturing industries. Any dyes left in the effluent after treatment (printing and dyeing wastewater; PDW) are discharged into the water environment, and can cause surface water pollution (Li et.al., 2018; Hussain et.al., 2018).

Point 4: line 42: incorrect citation – the information (2.8x105 tones) does not come from Jin et al. (2007), but from Robert Maas and Sanjeev Chaudhari article (Adsorption and biological decolourization of azo dye Reactive Red 2 in semicontinuous anaerobic reactors).

Response 4: Thank you very much for your valuable comment. We have revised the incorrect citation. Please refer to introduction:

Mass et al. (2005) estimated that 2.8×10⁵ tons of textile dyes are discharged into the water with textile industry wastewater every year around the world.

Point 5: line 52: incorrect citation – Lucyna et. al., 2019. Should be – Bilińska et al., 2019.

Response 5: Thank you very much for your valuable comment. We have revised the incorrect citation. Please refer to introduction:

Physical, chemical and biological treatment technologies should, therefore, be combined for its treatment (Bilińska et. al., 2019; Su et. al., 2016; Vanitha et. al., 2018).

Point 6: lines 68,69: You wrote: “These results provide reference for further research studying the relationship between physical, chemical, and biological PDW treatment methods” I do not agree. You did not investigate any physical process (for example sedimentation). The coagulation is a physico-chemical process.

Response 6: Thank you very much for your valuable comment. We have revised it in the manuscript. Please refer to introduction:

These results provide reference for further research studying the relationship between physico-chemical and biological PDW treatment methods.

Point 7: line 75: the actual wastewater come from regulating tank – was it an averaging/retention tank? Was the wastewater mechanically pretreated by a grid?

Response 7: Thank you very much for your valuable comment. It was an averaging/retention tank. The wastewater was mechanically pretreated by a grid.

Point 8: line 80: probably Al2(SO4)3 x 18 H2O

Response 8: Thank you very much for your valuable comment. We have changed Al₂SO₄·18H2O to Al₂(SO₄)₃·18H₂O.Please refer to section 2.2.

Point 9: line 81: In 2.2 paragraph is presented polyacrylamide as a used reagent. You did not inform about ionic form of this substance as well as about its function in experiment (flocullant?).

Response 9: Thank you very much for your valuable comment. The polyacrylamide is non-ionic. Please refer to section 2.2:

The following chemical agents were selected to use in the coagulation process: Calcium Hydroxide (Ca(OH)₂, analytical pure), Ferrous Sulfate (FeSO₄·7H₂O, analytical pure), Aluminum Sulfate (Al₂(SO₄)₃·18H₂O, analytical pure), Polymeric Iron Sulfate ([Fe₂(OH)n(SO₄)₃-n/2]_m (where n<2, m=f(n)), analytical pure) used as for coagulant, and non-ionic Polyacrylamide used as for flocculant.

Point 10: line 100: The measurement methods should be given, not only instruments.

Response 10: Thank you very much for your valuable comment. We have revised them in the manuscript. Please refer to section 2.4:

pH was measured directly using a digital pH meter (PHS-3C, China) and Congo Red concentrations were measured by a method with a wavelength equal to 496nm using an HACH instrument (DR5000, USA).

Point 11: lines 102: In this lines and in all text you sometimes substitute word “wastewater” by “water”.

Response 11: Thank you very much for your valuable comment. We have revised it in the manuscript.

Point 12: lines 245 and 247: is Fimicutes, should be Firmicutes

Response 12: Thank you very much for your valuable comment. We have revised them in the manuscript. Please refer to section 3.4.1.

Reviewer 3 Report

This manuscript is well written and informative. Below are few revision suggestions for the authors' considerations:

Line 42: Clarify the statement to indicate whether the estimate of dye waste discharge is at global level or region specific.

Line 61: Provide some examples of past studies indicating different effects on microbial coomunities.

Line 73: Describe how the simulated wastewater was prepared.

Line 99: Provide more details on the Hach method for dye measurement, such as method number and procedurer. Also at what temperature were the experiments conducted?

Line 104: Reference to Biomarkers Technologies, should include, location information for this provider.

Line 279: In the conclusion section, discuss the pH of the raw wastewater and how it might influence the chemical and biological treatment processes at a perscribed coagulant dosage. Also, discuss how temperature variations might influence the treatment processes.

Author Response

Point 1: Line 42: Clarify the statement to indicate whether the estimate of dye waste discharge is at global level or region specific.

Response 1: Thank you very much for your valuable comment. The estimate of dye waste discharge is at global level. Please refer to introduction:

Mass et al. (2005) estimated that 2.8×10⁵ tons of textile dyes are discharged into the water with textile industry wastewater every year around the world.

Point 2: Line 61: Provide some examples of past studies indicating different effects on microbial communities.

Response 2: Thank you very much for your valuable comment. We have provided some examples of past studies indicating different effects on microbial communities in the manuscript. Please refer to introduction:

There are a large number of microorganisms indigenous to the wastewater regulating pools of printing and dyeing industrial parks. Different enhanced coagulation pretreatment processes (ECPPs) will have different effects on these microorganisms(Juliana et. al., 2019; Zhu et. al., 2018).

Point 3: Line 73: Describe how the simulated wastewater was prepared.

Response 3: Thank you very much for your valuable comment. Please refer to Appendix A. Simulated wastewater of Congo Red of 50mg/L, 100mg/L and 150mg/L were prepared in Experiments 1#and 3# respectively. Simulated wastewater of Congo Red of 100mg/L, 200mg/L and 300mg/L were prepared in Experiment 2#. Deionized water was used as solvent in all three simulated wastewater.

Point 4: Line 99: Provide more details on the Hach method for dye measurement, such as method number and procedurer. Also at what temperature were the experiments conducted?

Response 4: Thank you very much for your valuable comment. Congo Red concentrations were measured by a method with a wavelength equal to 496nm using an HACH instrument (DR5000,USA).

Please refer to section 2.3:

In this study, three surface response experiments were conducted at indoor temperature.

Please refer to section 2.4:

pH was measured directly using a digital pH meter(PHS-3C,China) and Congo Red concentrations were measured by a method with a wavelength equal to 496nm using an HACH instrument (DR5000,USA).

Point 5: Line 104: Reference to Biomarkers Technologies, should include, location information for this provider.

Response 5: Thank you very much for your valuable comment. We have added location information for Biomarkers Technologies. Please refer to section 2.5:

The microbial community of raw wastewater, water samples pretreated by enhanced coagulation, and water samples from the oxidation ditch of the wastewater treatment plant were analyzed. Two mL samples were extracted from each water sample and sent to Biomarker Technologies CO., LTD (Beijing, China) for high throughput sequencing.

Point 6: Line 279: In the conclusion section, discuss the pH of the raw wastewater and how it might influence the chemical and biological treatment processes at a perscribed coagulant dosage. Also, discuss how temperature variations might influence the treatment processes.

Response 6: Thank you very much for your valuable comment. Three primary influencing factors（reaction pH, dosage of coagulants and initial Congo Red concentration）were studied in the three response surface experiments. The effect of temperature is not considered in the actual treatment process. Therefore, the effect of temperature on the treatment process is not discussed in the conclusion.

Round 2

Reviewer 2 Report

Dear Authors,

you have corrected the simplest comments. You did not answer the most important comments regarding the methodology and results of the research. If this is not possible I suggest you do new tests that are better planned.

This applies to my comments, but also to Reviewer 1 “point 6 – insufficient discussion of obtained results with reference”. The text of manuscript still lacks the most important corrections. 

Yours sincerely,

Reviewer 

Author Response

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

Dear Authors,

please take into consideration the following deficiencies:

page 3, paragraph 1, red text - please, change "PH" to "pH"

table S4 - the unit of color is usually mg Pt/L, if was measured at 455 or 465 nm; but the scale range is 0-500 and the value 1900; maybe it is the value of an absorbance, than the name "color" in wrong; please complete the unit 

In 2.4. you wrote "adding Ferrous Sulfate until a pH of 8.5 was achieved". I don't understand. Was the pH after coagulant addition always 8.5 ? In RSM methodology you wrote about pH adjustment (e.g 5-9) and in results you presented at figures 1-3 the changing values of "the pH of the reaction". Please, write in the text correctly.Current information is mutually exclusive.

Best regards

Author Response

Dear reviewer,

Thank you very much for your careful review. We have revised the manuscript. We hope to get your positive evaluation. The point-by-point response is as follows:

Point 1: page 3, paragraph 1, red text - please, change "PH" to "pH".

Response 1: Thank you very much for your valuable comment. Please refer to page 3, paragraph 1.

Point 2: table S4 - the unit of color is usually mg Pt/L, if was measured at 455 or 465 nm; but the scale range is 0-500 and the value 1900; maybe it is the value of an absorbance, than the name "color" in wrong; please complete the unit.

Response 2: Thank you very much for your valuable comment. The concentration of raw water was beyond the test limit. We tested it after dilution and then multiplied it by the dilution factor. We have added the unit of color. Please refer to table S4. 

Point 3: In 2.4. you wrote "adding Ferrous Sulfate until a pH of 8.5 was achieved". I don't understand. Was the pH after coagulant addition always 8.5 ? In RSM methodology you wrote about pH adjustment (e.g 5-9) and in results you presented at figures 1-3 the changing values of "the pH of the reaction". Please, write in the text correctly.Current information is mutually exclusive.

Response 3: Thank you very much for your valuable comment. Please refer to section 2.4.