Techno-Economic Evaluation of Iron and Aluminum Coagulants on Se(IV) Removal

Round 1

Reviewer 1 Report

The authors present a very interesting work about a techno-economic evaluation of the use of different type of coagulants in the removal of tetravalent selenium. The publication of this type of works, that includes an economic assessment, is not as common in literature as simple laboratory assays. For this reason, I consider that the present work could be highly useful for the readers of Water.

Just some suggestions to improve the work:

Specify the source of the following information (lines 175-176): “the energy and water costs required are estimated to be approximately 50+-20 €/m3 treated water” Explain why the tests on Fe(II) addition were carried out only at pH 7.2 The authors state a couple of times (first in line 186 and afterwards in the conclusions section) that “the reagent cost of Fe(II) is comparable to that of pre-hydrolyzed FeCl3 which, however, results in 70 wt.% more sludge production”. I understand that the use of Fe(II) could be beneficial from a point of view of lower sludge production. It would be advisable to explain this more in depth because, in its actual form, a reader may doubt if the best coagulant is FeCl3 or Fe(II).

Author Response

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Reviewer 2 Report

Thanks for your contribution. I have some minor and some major remarks:

Minor

Reference 20 wrong (author sequence):

Mitrakas, M., Mantha, Z., Tzollas, N., Stylianou, S., Katsoyiannis, I., & Zouboulis, A. (2018). Removal of antimony species, Sb (III)/Sb (V), from water by using iron coagulants. Water, 10(10), 1328.

Temperature is indicated sometimes in K and sometimes in °C. I would prefer using always °C.

Table 2, 3, and 5: small letter a,b,d instead of A (table 2), B (table 3), D (table 5)

151 – 153:  the sentence " Since … on Se(IV) removal efficiency” is not clear to understand. I think I know what you mean (better adsorption efficiency for lower pH due to the higher concentration of cationic species of iron), but this is not described well in my point of view.

Major:

You state that you conducted your research with a natural water matrix but it is not in my point of view as you used a tap water means a water after treatment. What kind of treatment process was applied?

Results in terms of coagulant demand and calculated reagent costs will strongly depend on water matrix (e.g. sulphate, phosphate, nitrate, bicarbonate and DOC concentrations) means in my point of view the data cannot be used at all for the design of water works having different water matrizes. By the way: the dependancy of the result with respect to sulphate was actually identified by yourself. Could you please comment on this?

What is actually the additional contribution of your research compared to the study of Hu et al. (ref. 19) justifying another paper? Results are more or less the same. Could you please comment on this?

 

 

 

 

 

 

Author Response

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Reviewer 3 Report

The article in the presented version is not useful for determining the costs and technical possibilities of using coagulants to remove Se (IV) from drinking water and cannot be published in Water. I suggest that the authors become familiar with the use of the coagulation process in drinking water treatment and methods for determining the optimal dose of coagulant. The basic methodological mistake is testing removal of selenium from treated water. The coagulation process is carried out at the beginning of the water treatment technological system, and the coagulant dose is determined based on the turbidity, color or contents of organic substances by e.g. measuring UV absorbance. The removal of heavy metals in this process also occurs, but they do not determine the dose of coagulant. The authors did not state whether it is underground or surface water, nor the technological scheme the water is treated. The turbidity, color and pH of the water mainly  determine the suitability of water for consumption. It is required to name those parameters after the  coagulation process.  However, the Selenium content is an additional parameter. It should be mentioned that each water treatment process changes its properties and physical and chemical parameters. After using traditional (not pre-hydrolyzed) coagulants, coagulation increases the corrosive aggressiveness of water, lowers pH and alkalinity. Therefore, it is necessary to correct the pH after coagulation (this affects the cost of the process), which may be higher than when using higher doses of pre-hydrolyzed coagulants. Water with low turbidity (treated water) usually requires higher doses of coagulants than water with high turbidity and color. The effectiveness of the coagulation process should be graded on the basis not only of the selenium content but also of Fe and Al ions residues, turbidity and color.

Other suggestions:

- the article should also contain raw water characteristics and description of water treatment process

- the drawings should include information about the temperature, it should be given either in ^oC or in K (consistently)

- weak discussion, no comparison with the results of other authors, I recommend to put it in a separate chapter in the future, not together with the results.

Author Response

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Round 2

Reviewer 2 Report

Thanks for your answers and for considering my remarks.

I am however not satisfied with your answer to my statement 1:

The term natural water is somehow misleading as it could have a very different compositon and your research does not cover all of this kind but only special ones.

For me, a natural water could be in the atmosphere, on the ground surface, or under the ground, always contains dissolved minerals and gases as a result of its interaction with the atmosphere, minerals in rocks, organic matter, and living organisms. How much minerals it contains or which Redoxpotential is found depends actually on the location and the kind of the natural water.

Thus your experiments simulate only those "natural waters" consisting of inorganic and organic species in certain concentration. You can of course state that the used water composition is believed to simulate perfectly "thousands of natural waters" in your country which were analyzed by your lab because the found concentration levels of the major ions do not influence the Se(IV) uptake. In this case you have to refer in your paper to the "many studies (e.g. Hu et al., 2015)" "that examined the above mentioned ions at this concentration range". By the way after reading the paper by Hu et al. carefully I found that CO3 (means HCO3 at this pH) in a concentration range between 1 and 10 mmol/L affected the removal efficiency of Fe-coagulation of Selenite considerably.

Your statement "Se(IV) being the dominant one, since the Eh of natural waters at pH range 6-8 is commonly lower than 0.5 V (Fig. S1) [7], while in high oxidizing environments the Se(VI) species dominate [8]." is also misledaing in my point of view. The appearance of the diagram for Selenium speciation as a function of Eh vs pH depends on many water matrix parameters and can, in my point of view, only be used to explain general relationships and not for exact limits. You can find selenate in considerable concentration also in waters with lower Eh than 0.5 V. Further on, a lot of "natural waters" (at least in my country) at pH range from 6 to 8 do have Eh larger than 0.5 V. This depends strongly on the kind of the "natural water". At the end of the day that means that "natural waters" might have a significant part of Selenite.

However, as Selenite is much less well removable by the here used Fe-coagulation you should at least mention somewhere in your paper, that this process and your calculation is not useable for those types of "natural water" which contain Selenite.

I agree with your answer to statment 2 but you should mention that this is only one possible explanation. Hu et al. found that "... precip-itation/coprecipitation also played specific roles at low dosage,especially for Se(IV) removal and with Fe coagulant."

Further, in Tabeli et al. [7] it is stated:

"More recently, the work of Francisco et al. (2018 )illustrated that sorption and co-precipitation re-actions were not the only mechanisms involved in the sequestration of Se species by Fe-oxyhydroxides/oxides from solution. They reported that SeIV could be trapped inside defects or nanopore structures during precipitation-particle aggregation, facilitating the enhanced reaction of SeIV with exposed crystallographic terminations of surrounding crystals, and finally occluding this oxyanion into the mineral along grain bound-aries."

Both statements indicate that the decreased density of cationic species like FeOH2+ might not be the dominant reason.

I agree with your answer to statment 3.

Author Response

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Reviewer 3 Report

The authors' answers are satisfactory. The article has been significantly improved and can be published in WATER.

Author Response

Thank you very much for your contribution to the article's improvement.

Round 3

Reviewer 2 Report

Thanks for your answers and for discussion. Paper is now ready for publication (at least from my side).