Degradation of Trimethoprim Using the UV/Free Chlorine Process: Influencing Factors and Optimal Operating Conditions

Round 1

Reviewer 1 Report

Line 42 - Typo - 'antibitios'?

Line 53 - There are volumes of published work on the degradation of antibiotics in wastewater treatment. . . why cite only one work? Provide more context since this is the basis of researching advanced oxidation treatment.

Line 54- Sentence is poorly worded

Line 60 - Sun et al 2014 - This is the same work that described bioavailability of ABs? Need to expand the literature review for this work.

Section 2.1.4 - Provide your method detection limit for TMP

Line 143- Where did your NaOCl solutions come from?

Line 177 - What is meant by "the gear's"?

Section 3.1.1 - Looks like free chlorine alone can oxidize TMP in a time frame that is relevant to wastewater disinfection with chlorine. Would be good to see some discussion of what your results mean for TMP degradation in chlorine disinfection processes at WWTPs. Following that discussion. . . why do we need UV/AOP for TMP destruction? Faster? less toxic degradation products? 

Line 264 - "HOCl had higher quantum yield. . " of what? OH? Cl? Also, it seems like an oversight to not reference the two part work of Hoigne on the quantum yields from OCl- and HOCl. 

Figure 4 Caption - pH of these solutions?

Line 372 - "more effective. . " how? Faster? 

Line 381 - your work quantifies the contribution of OCl radical to destruction of TMP. seems like this is the biggest contribution of your work to this field of study, but a discussion of this observation as compared to what other studies have found (that OH radical is the dominant treatment mechanism) seems to be missing or glossed over. would recommend adding some more context with review of some of the important kinetic studies that have been published on water treatment with UV/HOCl.

Author Response

请参阅附件

Author Response File: Author Response.docx

Reviewer 2 Report

Authors present an interesting work on the degradation of TMP by UV/chlorine. Prior to publishing in Water journal, the following issues should be addressed. 

1. Please revise the English, specially the use of "the" which seems excessive. 
2. Introduction, please write the full name of TMP the first time it appears in the manuscript. It is in the abstract, but it should be also included in the introduction. 
3. Line 42: antibiotics, not antibios
4. Line 51, do you mean absorption or adsorption? 
5. Analytical methods, please clarify the mobile phase employed for HPLC, you only mention 20% ACN, 0.3% acetic acid, there is a 79.7% missing. 
6. Table 1, TMP structure image is cut
7. It would be interesting to see the evolution of free chlorine along the reaction to have a better understanding of the process
8. Line 292. the correct nomenclature for hydroxyl radicals, according to IUPAC is HO·, not ·OH; please revise throughout the text. Same for Cl· in line 292
9. Please uniformize the format of Figures. Figs 1-2 have a different format than Fig 4. 
10. No results are shown for the UV/H2O2 process or chlorine (dark) oxidation. Yet, the abstract and conclusions talk about a higher efficiency, these data should be included in a Figure in the main manuscript. 

Author Response

Please see the attachment.

Reviewer 3 Report

See file attached.

Comments for author File: Comments.pdf

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 4 Report

Abbreviations have to be defined at its first instance even if it is already defined in the abstract. Language has to be checked thoroughly throughout the manuscript as there are obvious grammatical errors that should be avoided.

Lines 55-57: The authors mention that “The AOPs are effective to destroy the refractory organic contaminants (such as antibiotics) due to the generation of the various reactive radicals at high temperature and high pressure.”  This is misleading because, OH radicals can be produced via AOPs at room temperature and atmospheric pressures as well and not restricted to only extreme operating conditions. Therefore, this line must be revised.

Line 136: What was the volume of phosphate buffer added to the tube?

Table 1 is not necessary

Section 2.3: What is the model equation? Give the derivation.

Figure 2: All the 3 sub figures need to be overlaid for better comparison and interpretation. What are the error bars for the experimental data for this figure and all the other relevant figures?

Line 281: The authors have mentioned ‘V’ as the reactor volume, should ‘V’ not be working volume?

Lines 309-310: The authors have mentioned that for a 1-5 mM concentration of Cl- the degradation rate was slightly reduced. Looking at Figure 4a, without error bars they do not seem different. Even with error bars, I presume they would comfortably overlap. The conclusion for this figure should rather be ‘no effect of Cl- concentration (in the investigated range) on TMP degradation’. Similar conclusion would apply for Figure 4b too.

Lines 315-316: The authors further mention that Fang et al’s study on benzoic acid degradation was little impacted by Cl-. A comparison cannot be drawn here because TMP has 2 pka values and rather a large molecule compared to benzoic acid with just an aromatic ring and an acid side chain with a single pka of 4.2.

Overall, the authors have presented experimental work and kinetic modelling of the degradation of TMP using UV/free chlorine process. The manuscript however is quite similar to the previously published work by Wu et al, 2016 (https://doi.org/10.1016/j.watres.2016.08.011). The model equation used to fit the experimental data is not clearly mentioned. Error bars between replicates for experimental data is also missing. The authors would need to differentiate how their work is different from Wu et al’s work and what is novel in their work. An additional EE/O calculation does not make the work exclusive. Unless the novelty is presented (which I do not see), this work cannot be published. I therefore recommend against the publication of this manuscript in Water.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

I didn't receive the response to my questions. Still I only have one issue to point out prior to publication. I don't understand at all what is going on from line 191 to line 245. I suddenly thought the authors were mistaken when introducing that information. Please simplify that part. 

Author Response

非常抱歉之前未能将修改意见的回复发给您。这次一并发给你，见附件

Author Response File: Author Response.docx

Reviewer 4 Report

Table 1 can be moved to the supplementary information.

The explicit mention of which equation or set of equations were used to model the degradation of TMP is still not clear. Instead, the authors have expanded on the algorithm by comparing it with a genetic population/trait determination which is confusing and unnecessary.

Figure 2 is missing.

The authors have failed to address how their manuscript is different to the previously published work by Wu et al, 2016 (https://doi.org/10.1016/j.watres.2016.08.011). The authors would need to differentiate how their work is different from Wu et al’s work and what is novel in their work. An additional EE/O calculation does not make the work exclusive. Unless the novelty is presented (which I still do not see), this work cannot be published. I therefore recommend against the publication of this manuscript in Water.

Author Response

Thank you for your review comments once again. The following is my reply to the review comments，Please see the attachment.

Author Response File: Author Response.docx

Round 3

Reviewer 4 Report

The authors have justified the novelty in the comments document but failed to incorporate it in the manuscript. The use of river water is not a novelty and need not be included. 

The following two points mentioned in the comments document (highlighted below) can be rephrased and included in the manuscript where appropriate to strengthen the novelty. The gap, hence addressed should also be clearly mentioned in the introduction section. 

(1) We studied not only the influencing factors,but also the optimal operational conditions for the degradation of TMP by the UV/chlorine process.The best operational conditions resulted in the lowest use of the energy and electrical energy per order (EE/O), which were (1) for the ultrapure water, the optimum intensity of the UV light and the free chlorine dosage were 2.56 Einstein/L-s and 0.064 mM, respectively, with the minimum EE/O of 0.136 kWh/m3 and (2) for the water matrix containing 3-mg/L NOM, the optimum intensity of the UV light and the free chlorine dosage were 3.45 Einstein/L-s and 0.172 mM, respectively, with the minimum EE/O of 0.311 kWh/m3.

(3) The genetic algorithm was implemented to solve the ODEs system in our work. This method is more accurate and advanced in the process of fitting and modeling, so it is adopted by us. This is not involved in the results of Wu et al. in 2016.

Upon making these changes, the manuscript may be considered for publication.