Simultaneous Recovery of Struvite and Irrigation Water for Agricultural Purposes Obtained from Dewatering Liquor through Electrodialysis
, Katrin Calábková, Silvie Heviánková, Jan Halfar, Iva Kotalová and Barbora ValováRound 1
Reviewer 1 Report
Resource recovery is an important topic that gets widespread attention. The authors present data on treatment of real wastewater, which is always interesting to read. I should also note the great attention that was paid to review of published sources in the introduction.
However, I currently have one big problem with methodology and concept - or with the presentation of the result that prove the necessity of such concept. The authors don't present the data on phosphate content in concentrate and diulate or their comparison with the concentration in the input solution. What if the concentrations are comparable? Can one add the magnesium sulphate and NaOH directly to the input solution, filtrate it to remove precipitate and decrease TDS and then send the filtrate to irrigation (not considering the hygienic factors now), since the heavy metals content in the input solution fits the standards as well? What is the evidence that the electrodialysis is needed here? I guess this can be listed as "control is missing". Please find the smaller comments that mostly address the typos below. 1) Since the introduction discusses the molar ratios of magnesium, ammonia and phosphates, the table 1 may give molar concentrations instead of weight ones. 2) Not a native speaker here - what is DS105 (table 1 onwards)? Is it total dissolved solids? 3) Check the heading line in table 3 - I guess one of "sample 2"s should be "sample 3". 4) The authors may consider moving table 3 to results and discussion section. 5) Guess "table 3 below" in line 170 was intended as "table 4 below". 6) I am not a native speaker and it took me some time to understand what "The final percentages of the precipitated struvite" in line 169 means. Got easier at "The obtained precipitate contained at least 93 % of struvite, while the remaining percentage was represented by halite (NaCl).", where I understood that the percentage means content of substance in precipitate. Can you please check again the phrasing there with a person fluent in English to ensure it's just me who gets the problem with understanding there? 7) Regarding the table 4 - can the authors please present the absolute weights along with purity there? Because while the 1:1.3 content leads to superior purity, it might theoretically result in smaller quantity.Author Response
Reviewer´s comments are included in document.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments
to the article
“Zero Liquid Discharge: Synergetic recovery of struvite and irrigation water for agricultural purposes obtained from dewatering liquor through electrodialysis”
submitted by Petra MalÃková et al for publication in the “Water”
The article is devoted to the problem of processing phosphate-containing municipal wastewater by electrodialysis, followed by the precipitation of struvite (as a fertilizer) from the ED concentrate and the simultaneous use of diluate as water for irrigation.
The topic of the work is relevant and corresponds to the subject of the Journal. There are several comments on the article.
1. To line 152. It is necessary to clarify what ratios (molar, weight) are in question.
2. To tables 5-7. An explanation should be given to the following contradiction: phosphate anions pass through the membrane into the concentrate, but the cations of copper, nickel, zinc, and iron predominantly remain in the diluate. On the other hand, a significant part of the lead leaves the diluate and goes into the concentrate.
3. From simple balance estimates and taking into account the content of components in the input DL samples, it follows that after the precipitation of struvite, a large amounts of ammonium salts, sulfates, and sodium chloride (judging by the composition of the precipitates) remain in the ED concentrate It should be clarified: a) where does the concentrate go and whether the described process corresponds to Zero Liquid Discharge; b) is the increased complexity of the subsequent disposal or processing of ED concentrate a serious drawback of the described approach?
4. The content of heavy metals in the obtained fertilizer samples - struvite should be given in one of the tables
After insignificant revision of the article, connected with the required explanations, it can be published in the Journal.
Reviewer
Comments
to the article
“Zero Liquid Discharge: Synergetic recovery of struvite and irrigation water for agricultural purposes obtained from dewatering liquor through electrodialysis”
submitted by Petra MalÃková et al for publication in the “Water”
The article is devoted to the problem of processing phosphate-containing municipal wastewater by electrodialysis, followed by the precipitation of struvite (as a fertilizer) from the ED concentrate and the simultaneous use of diluate as water for irrigation.
The topic of the work is relevant and corresponds to the subject of the Journal. There are several comments on the article.
1. To line 152. It is necessary to clarify what ratios (molar, weight) are in question.
2. To tables 5-7. An explanation should be given to the following contradiction: phosphate anions pass through the membrane into the concentrate, but the cations of copper, nickel, zinc, and iron predominantly remain in the diluate. On the other hand, a significant part of the lead leaves the diluate and goes into the concentrate.
3. From simple balance estimates and taking into account the content of components in the input DL samples, it follows that after the precipitation of struvite, a large amounts of ammonium salts, sulfates, and sodium chloride (judging by the composition of the precipitates) remain in the ED concentrate It should be clarified: a) where does the concentrate go and whether the described process corresponds to Zero Liquid Discharge; b) is the increased complexity of the subsequent disposal or processing of ED concentrate a serious drawback of the described approach?
4. The content of heavy metals in the obtained fertilizer samples - struvite should be given in one of the tables
After insignificant revision of the article, connected with the required explanations, it can be published in the Journal.
Reviewer
Author Response
Reviewer´s comments are included in document.
Author Response File: Author Response.pdf
Reviewer 3 Report
The present study has a significant interest in the problem of wastewater recovery and reuse of water resources; however, in my opinion, the study was performed at a level significantly lower than required for publication in the highly-rated journal Water (Q2).
In summary, the presented study is devoted to the desalination and purification of wastewater by the widely used method of electrodialysis followed by the removal of phosphate ions in the form of struvite. Electrodialysis is a well-known commercially available technique of water desalination. The work does not investigate approaches to optimizing this method or any improvements associated with it. The approach focused on the precipitation of phosphate anions in the struvite form also does not have any new information, since it was repeatedly investigated in the 2000-2010s [Le Corre K. S. et al. Phosphorus recovery from wastewater by struvite crystallization: A review //Critical Reviews in Environmental Science and Technology. – 2009. – Т. 39. – â„–. 6. – С. 433-477.].
Special questions and recommendations for text:
- For all tables: errors are required. It is necessary to present only the significant digits in numbers.
- What parameters do not allow using the initial water for irrigation? Why is electrodialysis necessary, or could the first category for water be achieved using more simple and more suitable ways?
- Is it necessary to remove the phosphate ions or is the water with them suitable for irrigation?
- What do the authors mean by «synergistic recovery»? In the paper, separate additive effects of electrodialysis and precipitation are observed.
- The residual concentration of phosphate ions is unknown, and therefore it is impossible to evaluate the completeness of precipitation and the efficiency of phosphate removal.
- It is unclear how the fraction of struvite was determined. How could the precipitate additionally include soluble sodium chloride? Was the precipitate washed?
- It is necessary to complete paper by the initial X-ray diffraction data with the obtained phases interpretation and a description of the phase fraction determination. The error of this method is several per cent. Thus, the accuracy of the values given in Table 4 is overestimated.
- The mention of ZLD technique in the title of the article is not confirmed by the results of the study.
Concluding, in my opinion, the scientific value and significance of results obtained in the work are insufficient for publication in this journal.
Nevertheless, this study can be improved either by considering the optimization of both electrodialysis and phosphates precipitation approaches in the view of physics or chemistry, or by statistical analysis and the social and economic problems of these approaches. Unfortunately, now the article has insignificant contribution to water treatment knowledge, being a special case of using well-known techniques.
Author Response
Reviewer´s comments are included in document.
Author Response File: Author Response.pdf
Reviewer 4 Report
The authors proposed an method to process the wastewater, which may attract interest, but some issues should be emphasized.
Please clarify the what the difference between the author’s work and the published work.
Please visualize the paper instead of bunch of text and tables as readers have little interest of paper without figures. If the values in tables are not critical for the paper providing quantitative information, please transform them into figures or delete them.
Please provide the XRD spectra as you mentioned in the content.
Please consider adding short paragraph to introduce electrodialysis as the background knowledge.
Please explain why you only chose the two ratios of and Mg2+.
Please clarify the meaning of improving 3% of struvite after changing the ratio.
Please analyze the economical cost of this method.
It seems that in the paper the authors did not give the solution of heavy metal in the ED product.
Author Response
Reviewer´s comments are included in document.
Author Response File: Author Response.pdf
Reviewer 5 Report
Minor comment:
Lines 169-170: "The final percentages of the precipitated struvite obtained under two different doses of magnesium sulphate are given in Table 3 below." should be Table 4?
Author Response
Reviewer´s comments are included in document.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Dear authors, the reviewed version reads better and now more clearly presents why the input water is unsuitable for irrigation. I also thank you for consulting again with the translator, even when it turned out I was wrong with several moments I've highlighted as suspicious.
However, the present version is still not persuasive in necessity of electrodialysis for this process. If the main goal was to obtain a concentrate with a high content of phosphate ions in order to precipitate struvite, it might be shown by the experiment with the dewatering liquor that without the ED concentration the results of sedimentation become inferior (lesser amount precipitates, or the purity decreases). I guess it is partly done by "In comparison with Sýkorová et al. in [19] when at best, they obtained a precipitate in the amount of 0.54 g/L of “raw“ dewatering liquor (without concentration), of which 40% were struvite crystals. In our proposed procedure, i.e. concentration of dewatering liquor and precipitation of struvite in concentrate, the amount of precipitate in the dry state is 10 g with a struvite yield of 94 % (molar ratio 1:1) and 12 g with a struvite yield of 97 % (molar ratio 1:1.3).", but it would be better to compare the results of sedimentation experiments conducted by the same protocol. You may also list the DS content of solution post cellulose membrane filter but pre-ED to show that it's precisely ED which is responsible for DS values reaching threshold of suitability for irrigation.
I also have a problem with this moment: lines 234-235 say that "The average yield per liter of concentrate was 10 g of dry precipitate from both molar ratios." while table 6 says that the amount of phosphate in concentrate was about 720 mg/L. The precipitate is not a 100% struvite, and the molar mass of struvite is about 2.5 times of that of phosphates, but the masses still don't add up for me. Please clarify.
Best regards to authors.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Despite the authors have corrected a number of comments, there are some answers related to the insufficient interpretation of the X-ray diffractometry data, based on which conclusions of the work are formed.
- For table 1 “Measurement uncertainty – 5 %” is not correct because, for example, pH value has significantly lower measurement error. Moreover, excessive accuracy is written for most data. Replace, for example 255.00 with 255.
- “XRD diffractogram” is not correct. Please, use XRD pattern.
- On the XRD patterns (fig 3 and fig 4) corresponding to the Hazenite phase peaks absent. Fitting of the XRD pattern in the case of Struvite phase is of poor quality. Generally, for quantitively phase analysis is require to show the Rp and Rwp factors, which are define the fitting quality.
- It is necessary to describe the procedure for calculating the quantitative phase composition. Which RIR or Rietveld method were used?
- For all phase fractions calculated written in the Table 4 values have excessive accuracy.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 4 Report
The authors have given thorough response to the reviewer, but some issues should still be emphasized.
Please combine figure 1 and figure 2 into one figure and just highlight the area of novelty of this paperwork using color or other annotations.
Please increase the qualities of all figures. The figure 1 and figure 2 can be vector graphics so that there will be no blurring.
Please provide the setting parameters of XRD test.
Please give the details how you used the XRD to obtain the values in table 4.
Please redraw the figure 3 and figure 4 using the raw data instead of images directly generated by the software. The label in x axis is not right. The font size is too small and peak annotation should be added. Please discuss the information you want to demonstrate by the XRD figures.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
Dear authors, I liked the improvements made for improvement of clarity of presentation. I guess the rest is the question for long (and possibly fruitless), so I guess all the major points are solved.
Author Response
Dear reviewer, thank you very much for your feedback.
Best regards
Reviewer 3 Report
Table 4. Indicate the actual errors that TOPAS calculated for each XRD pattern. Taking into account the Rp and Rwp factors and the integral intensity of the hazenite peaks, less than the differential curve, I assume the errors are quite high. Also, in this way two digits after decimal point are redundant.
Please, plot 3 and 4 figures (and insert graphs) in the same ranges.
Author Response
Dear reviewer,
for the molar ratio 1:1, the interpolation errors are ±0.88, and for molar ratio 1:1.3 the interpolation errors are ±0.33, both for struvite and hazenite. Therefore, we agree the digits after decimal point are redundant.
Figures were modified to be in the same ranges.
Best regards
