Ozone and Photocatalytic Processes for Pathogens Removal from Water: A Review

Round  1

Reviewer 1 Report

This mini review is correctly written and it will be of interest to the scientific community. I suggest minor revision.

I believe that the word "review" should always appear in title and abstract to help localizing a paper. Let the authors make this change.

Even though, this is not absolutely necessary, it would help to introduce some diagrams. Something to attract the attention of the reader. I wonder what is  the graphical abstract? Have the authors submitted  any?

Author Response

REPLY TO REVIEWER COMMENTS

REVIEWER #1_____________________________________________________________

This mini review is correctly written and it will be of interest to the scientific community. I suggest minor revision.

We truly acknowledge the reviewer opinion and the suggestions. The modifications performed truly improved the manuscript.

I believe that the word "review" should always appear in title and abstract to help localizing a paper. Let the authors make this change.

In accordance with the reviewer opinion, the title was changed as follow:

“Ozone and Photocatalytic Processes for Pathogens Removal from Water: A Review”

Also, the word review was added to the abstract as follows (Page 1, Line 23):

“This review paper gives a critical overview on the application of ozone and photo-based disinfection systems bearing in mind their advantages and disadvantages when applied to water and municipal wastewater. Also, the possibility of integrated disinfection systems is considered.”

Even though, this is not absolutely necessary, it would help to introduce some diagrams. Something to attract the attention of the reader. I wonder what is  the graphical abstract? Have the authors submitted  any?

Thank you for your suggestion, a Graphical Abstract resuming the objectives of the paper is now given.

Graphical Abstract:

Author Response File: Author Response.pdf

Reviewer 2 Report

The subject matter is adequate for Catalysis Journal but from a scientific point of view, the paper needs a revision as follows:

- In the manuscript would be interesting, if the authors introduced some figures.

- In the review, a distinction should be made between homogeneous and heterogeneous processes of ozone and photocatalysis in wastewater disinfection.

Author Response

REPLY TO REVIEWER COMMENTS

REVIEWER #2_____________________________________________________________

The subject matter is adequate for Catalysis Journal but from a scientific point of view, the paper needs a revision as follows:

Thank you for your opinion and suggestions that truly improved the quality of our paper.

- In the manuscript would be interesting, if the authors introduced some figures.

We agree with the reviewer opinion. Thus, a Graphical Abstract resuming the objectives of the paper is now given.

Graphical Abstract:

Also, some Figures were added to the manuscript to make it easier to read. (Page 9, Lines 287-289.)

“Disinfection by light is one of the most widely used method around the world [25,26]. The conventional method is based on UV light. However, solar light is also efficient for the deactivation of biohazards {Formatting Citation} and research regarding its application is increasing using several combinations (Figure 1).

Fig.1. The schematic range of electromagnetic radiation used in photochemical disinfection processes and its effects on patogens (based on [35,51,69–75])  “

Page 12, Lines 396-399.

 “The authors compared this process with single ozonation and verified it was able to totally remove E. coli in 30 s corresponding to a transferred ozone dose of 0.16 mgO₃/L (Figure 2). Still, the use of Pd-TiO₂ and Ag-TiO₂ seems more suitable since no energy was required for disinfection.

 Fig. 2. E. coli in synthetic wastewater as function of TOD during single ozonation (based on [126])”

 Page 16-17, Lines 487-490

 “The combination of ozone with catalysts/light/H₂O₂ may be a suitable solution for disinfection (Figure 3). Still the operating costs associated may be prohibitive specially for low incoming countries.

 Fig. 3. Ozone-aided processes for water disinfection.”

- In the review, a distinction should be made between homogeneous and heterogeneous processes of ozone and photocatalysis in wastewater disinfection.

We agree with the reviewer. In what regards ozone processes the only studies involving heterogeneous processes are referred in section 3.4. Photocatalytic ozonation. Section 2. Ozone based disinfection systems is solely devoted to homogeneous processes since no solid catalysts were tested. As for photo-driven disinfection systems we decided to highlight works dealing with heterogeneous catalysts from those occurring in homogeneous phase. Thus, the comparison between several heterogeneous photocatalytic processes is also now given focusing the effect of different water sources and characteristics (Pages 13-16, Lines 426-460):

 “Table 3 resumes some results obtained from literature regarding bacteria and ARG removal from water (from different sources) through heterogeneous photocatalysts.

Dunlop et al. [135] tested P25 efficiency under UVA light for the inactivation of E. coli. It was verified that the reduction of ARG genes was lower for real wastewater which was attributed to ROS scavenging by the organic and inorganic components of the effluent. It was highlighted that in addition to bacterial re-growth, ARG transfer may increase if treatment is not continued to the point of complete pathogen inactivation prior to discharge.

TiO₂ activity under simulated sunlight was enhanced when it was doped with N which led to higher E.coli inactivation efficiency [136]. The process did not significantly affect the resistance of E. coli strain to TET and VAN genes as irradiation time increased, but a decreasing trend in the resistance to CIP and sensitivity to CEF was observed. The higher efficiency of N-TiO₂ is related with the semi-conductor band-gap decrease when N is incorporated. This leads to a higher amount of electrogenerated electron-holes. Also, metal (Mn/Co) doped-TiO₂ exhibited better activity compared to the commercially available TiO₂ on the removal of Klebsiella pneumoniae [137]. Karaolia et al.[11] shown complete and irreversible removal of E. coli after 3h of photocatalytic oxidation using simulated sunlight. Specific genes were degraded, while others, such as sul1, ermB resistance genes, and enterococci detected via the 23S rRNA gene sequences, were persistent throughout the treatment (suggesting that their removal is more challenging compared to the removal of the other sequences examined). TiO₂ photocatalytic treatment displayed no reduction of ecfX, but the photocatalytic treatment with TiO₂-rGO-PH and TiO₂-rGO-HD successfully reduced it.

The effect of the water matrix was evaluated by Guo et al. [138] on the removal of Staphylococcus aureus and Pseudomonas aeruginosa through UVC/TiO₂ and concluded that disinfection efficiency was independent from the aqueous matrix. Similar conclusion was withdrawn by Guo et al. [138] on the damaging of ARGs during photocatalytic oxidation aided by H₂O₂. Moreover, the removal of mecA and ampC ARG was significantly improved by 2.7–3.4 and 2.7–3.2 log units in comparison to H₂O₂, H₂O₂/UV. In fact, Moreira et al. [139] verified that P25/H₂O₂ and solar/H₂O₂ are suitable processes for ARGs removal from an effluent coming from an urban wastewater treatment plant.

Table 3. Application of heterogeneous photocatalytic processes for water disinfection

ASE- autoclaved secondary effluents, DW- distilled water, PBS-phosphate-buffered saline, MBR - membrane bioreactor, NW- natural water, CAS- Conventional Activated sludge process; UWWTP – Urban wastewater treatment plant.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper summarizes some of the approaches for use of ozone and UV or viible (solar) ligth for wastewater disinfection. The overall results are presented and discussed clearly. The topic of this paper is of wide and general interest, and many publications have appeared on this matter in the past years. It is not clear to me the real need of this review, considering a number of several other works (some also cited by authors) on the same subject. 

The bibliography is somhow adequate and bibliographic references are accurate. 

Therefore, I do not reccomend the publication of this paper unless the authors can provide solid arguments to sustain the need of another review on the topic.

Author Response

REPLY TO REVIEWER COMMENTS

REVIEWER #3_____________________________________________________________

The paper summarizes some of the approaches for use of ozone and UV or viible (solar) ligth for wastewater disinfection. The overall results are presented and discussed clearly. The topic of this paper is of wide and general interest, and many publications have appeared on this matter in the past years. It is not clear to me the real need of this review, considering a number of several other works (some also cited by authors) on the same subject.

The bibliography is somhow adequate and bibliographic references are accurate.

Therefore, I do not reccomend the publication of this paper unless the authors can provide solid arguments to sustain the need of another review on the topic.

We acknowledge the reviewer opinion and the suggestions that truly improved our work. Water scarcity is one of the major challenges being faced by humanity nowadays. In this context, water reclamation from treated municipal wastewater is a new paradigm. Moreover, these streams constitute a secondary source of potable water since can be discharged into the aquifers. In this context, the treatment processes must guarantee safe water in what regards both chemical and biological characteristics so that ecosystems and public health are safeguarded.  Although bacteria are the microorganisms group usually reported and evaluated when assessing water quality, other pathogens such as viruses and protozoa must be assessed. In fact, for example viruses constitute an environmental and public health danger due to their high infectivity and resistance to disinfection technologies. More recently the concern with antibiotic resistant bacteria spreading is increasing due to the scaling consume of antibiotics. In this context, bearing in mind the relevance of reaching safe reusable water and ensuring ecosystems protection as well as public health, the seek for suitable disinfection technologies is a hot topic nowadays. Ozonation and photo-based processes appear as interesting alternatives since the formation of disinfection by-products can be avoided by controlling the operating conditions. Thus, there have been several studies involving such processes for the disinfection of water and wastewater. Specially in the last years the focus has been given to solar photocatalytic oxidation and photo-ozonation. To the best of our knowledge there is no recent review involving the analysis of disinfection through ozonation and photo-aided processes. In fact, the most recent review regarding any of these technologies is from 2013 (McGuigan, K.G.; Conroy, R.M.; Mosler, H.J.; du Preez, M.; Ubomba-Jaswa, E.; Fernandez-Ibañez, P. Solar water disinfection (SODIS): A review from bench-top to roof-top. J. Hazard. Mater. 2012, 235–236, 29–46, doi:10.1016/j.jhazmat.2012.07.053. ) and it is only concerning solar water disinfection.

In this context, we believe that our review is timely and novel. In fact, the novelty is related with the fact that this work critically revises water and wastewater disinfection comparing two key technologies: ozonation and photo-aided processes that present advantages regarding the traditional chlorination method since it is possible to control the by-products formation. Besides, our approach bears in mind not only bacteria but also other pathogens such as protozoa and virus. These pathogens may present even greater infectivity than bacteria. Therefore, their presence in treated water may not be neglected. The aims and novelty of this review is now highlighted in this new version of the manuscript (Page 4, Lines 114-123):

 “Bearing in mind the growing interest in innovative disinfection technologies and the especial focus on ozone and photo-based systems, this paper presents a critical review concerning ozone and photocatalytic processes for pathogens disinfection. Special emphasis was placed on recent research on ozone-based technologies as well as light driven technologies including UVC, solar and photocatalytic disinfection. In fact, these processes show interesting features since undesired disinfection by-products formation can be avoided if the operating conditions are carefully selected. To the best of our knowledge there is no recent review comparing these two disinfection approaches. Moreover, the focus is not only bacteria removal but also protozoa and viruses. In fact, these pathogens may present even higher infectivity than bacteria, therefore, their presence in treated water may not be neglected.”

Author Response File: Author Response.pdf

Reviewer 4 Report

The paper review approaches to water purification through ozone and photocatalyic technologies, while the paper includes a large number of example studies more care should be taken to compare and contrast the studies to give an overall picture of the state of the art.

More information about the scale and possible applications in small delocalised water sources would be beneficial in addition to the examples of which technology would suit which application.

 More specific examples should be give taking into account water quality and source which compare and contrast technologies rather than listing the results of a range of studies. 

The addition of figures to the manuscript could help to make the manuscript easier to read.  

Author Response

REPLY TO REVIEWER COMMENTS

REVIEWER #4_____________________________________________________________

The paper review approaches to water purification through ozone and photocatalyic technologies, while the paper includes a large number of example studies more care should be taken to compare and contrast the studies to give an overall picture of the state of the art.

More information about the scale and possible applications in small delocalised water sources would be beneficial in addition to the examples of which technology would suit which application.

Thank you for your opinion. In fact, in section 4. Application to real water and integrated disinfection schemes some discussion regarding the benefits of each technology (ozone-based and light driven disinfection processes) was missing with special focus to small delocalised water sources and low-incoming countries where the lack of safe water is alarming. Thus, that section was completed as follows (Pages 18-19, Lines 542-569):

“The selection of the most suitable technology must bear in mind the costs associated to the process and also the specificities of the area where the treatment/disinfection plant will be installed. For example, in remote areas it is also critical that the unit requires low maintenance, and low specialized intervention [156]. In this context, ozone-based systems may be an interesting solution due to the high degree of automation and low amount of chemicals needed for the process. The main disadvantage is related with the need of complex equipment such as air treatment (to concentrate oxygen), ozone generator and off-gas ozone destroyer. All this implies energy costs during the operation. The main advantage of these processes is related with their capability of removing organic compounds as well as disinfecting which may increase the final water quality. Also, UV systems can be easily automatized. However, in that case, as discussed before, a secondary oxidant will be required to maintain a residual disinfectant effect. Although the high interest in photocatalytic oxidation/ozonation disinfection, the use of catalysts in their powder form is an important disadvantage since a solid/liquid separation step must be designed [157]. Thus, research in that field should focus on the development of structured catalysts that may be used in fixed beds without losing much of their activity.

Solar water disinfection processes are of special interest since Sun can be used as a renewable and low-cost radiation source. The simple method of storing water in PET bottles that are left under Sun irradiation has shown important results in the disinfection of water in low-incoming countries [99]. In fact, results show that this methodology already reduced significantly the incidence of dysentery and diarrhea [158,159]. The wide spreading of such simple technology seems, however, difficult in developed countries where it will be hardly seen as suitable for water disinfection [104]. Besides, the use of plastic bottles leads to some scepticism regarding the possibility of some chemicals leaching to the water. Research in this field is focused on the development of reactors able to enhance solar disinfection such as the one developed by Polo-López et al. [160] that would allow the process automatization. Still, the activity of such methods is not very much explored on the reduction of viral activity in water and it is verified that usually these pathogens are more resistant to solar disinfection [161]. Thus, aiming to reach safe reusable water, such problematic must be also addressed in future works. ”.   

In this context, the following new references were added:

 156. Zhang, J.; Knight, A.; Duke, M.; Northcott, K.; Packer, M.; Scales, P.; Gray, S. A new integrated potable reuse process for a small remote community in Antarctica. Proc. Saf. Environ. Prot. 2016, 104, 196-208, doi:10.1016/j.psep.2016.08.017.

157. Gomes, J.; Costa, R.; Quinta-Ferreira, R.M.; Martins, R.C. Application of ozonation for pharmaceuticals and personal care products removal from water. Sci. Total Environ. 2017, 586, 265-283.doi: 10.1016/j.scitotenv.2017.01.216

158. Preez, M.; Conroy, R.; Ligondo, S.; Hennessy, J.; Elmore-Meegan, M.; Soita, A.; McGuigan, K. Solar disifection of drinking waetr (SODIS) in the prevention of dysentery in Kenyan children aged under 5 years. Environ. Sci.Technol. 2011, 45, 9315-9323.

159. McGuigan, K.; Samaiyar, P.; Preez, M.; Conroy, R. A high compliance randomised controlled field trial of solar disifection (SODIS) of drinking water and its impact on childhood diarrhoea in rural Cambodia. Environ. Sci. Technol. 2011, 45, 7862-7867.

160. Polo-López, M.; Fernández-Ibáñez, P.; Ubomba-Jaswa, E.; Navntoft, C.; McGuigan, K.; Dunlop, P.; Byerne, J. Elimination of water pathogens with solar radiation using an automated sequential batch CPC reactor. J. Hazard. Mater. 2011, 196, 16-21.

161. Harding, A. ; Schwab, K. Using limes and synthetic psoralens to enhance solar disinfection of water (SODIS): a laboratiry evaluation with norovirusm Escherichia coli, and MAS2. Am. J. Trop. Med. Hyg. 2012, 86, 566-572.

More specific examples should be give taking into account water quality and source which compare and contrast technologies rather than listing the results of a range of studies.

Following the reviewer suggestion, Tables (1, 2 and 3) comparing some results found in literature were introduced in this new version of the manuscript. Their objective is to compare and contrast the discussed technologies efficiency bearing in mind the water characteristics (Page 4, Lines 132-148):

“Ozonation was able to remove up 2 log units of cultivable fungi, 16SrRNA and intl1 genes, about 4 log units of heterotrophs, enterobacteria and enterococci (Table 1). Moreover, the antibiotic resistance genes (ARG) can be depleted below the detection limit. The presence of dissolved organic matter in real municipal wastewater reduces the disinfection ability of the process when compared with its performance when applied to a synthetic effluent. In fact, organic compounds may quench some of the oxidant species which will not be available for the microbial cells [46]. Even if the synthetic water presented higher COD than the real effluent, disinfection was less efficient when applied to the actual stream. Thus, the lower efficiency of the disinfection process was attributed to other species presented in the actual secondary wastewater such as suspended solids that may consume oxidants. Besides, the higher complexity of the biota in real streams may be another explanation. Although the high efficiency of the process, regrowth tests showed that, in general, there was the prevalence of antibiotic resistance and integrase genes amongst the survival microorganisms. Thus, some care must be taken on the selection and optimization of tertiary treatments to avoid ARG in reused water [8,10,46]. Table 1 compares some works dealing with water disinfection through ozone. In all the reported results, real wastewater was tested with different characteristics. The required ozone dose for an efficient disinfection is highly depended upon the water characteristics. For example, Xu et al. [15] verified that the optimal ozone dose for disinfection could range from 2 – 15 mgO₃/L depending upon the effluent organic matter load.

Table 1. Transferred ozone dose (TOD) required for effective disinfection for different water qualities and sources.

*COD – Chemical oxygen demand; TSS – Total suspended solids

 Moreover, the processes ability to remove different types of pathogens is now also discussed (Pages 6-7, Lines 203-212):

“CT , i. e., the disinfectant residual concentration in water times the contact time can be used to check pathogens susceptibility to inactivation by a certain reactant. Table 2 summarizes some CT values for 2 log removal of bacteria (E. coli), protozoa and virus by ozone disinfection.

Table 2. CT values required by ozonation for 2 log inactivation of selected pathogens from water.

 All results from the literature were obtained for synthetic wastewaters. Bacteria and virus are much more susceptible to ozone oxidation than protozoa since lower ozone doses are required to achieve bacteria and virus removal. Thus, ozone is an interesting disinfectant for both bacteria and virus inactivation. This means that disinfection systems based on ozone must be designed bearing in mind protozoa removal since these will be the limiting pathogen in the process. “

 The comparison between several heterogeneous photocatalytic processes is also now given, mainly focusing the effect of different water sources and characteristics (Pages 13-16, Lines 426-456):

 “Table 3 resumes some results obtained from literature regarding bacteria and ARG removal from water (from different sources) through heterogeneous photocatalysts.

Dunlop et al. [135] tested P25 efficiency under UVA light for the inactivation of E. coli. It was verified that the reduction of ARG genes was lower for real wastewater which was attributed to ROS scavenging by the organic and inorganic components of the effluent. It was highlighted that in addition to bacterial re-growth, ARG transfer may increase if treatment is not continued to the point of complete pathogen inactivation prior to discharge.

TiO₂ activity under simulated sunlight was enhanced when it was doped with N which led to higher E.coli inactivation efficiency [136]. The process did not significantly affect the resistance of E. coli strain to TET and VAN genes as irradiation time increased, but a decreasing trend in the resistance to CIP and sensitivity to CEF was observed. The higher efficiency of N-TiO₂ is related with the semi-conductor band-gap decrease when N is incorporated. This leads to a higher amount of electrogenerated electron-holes. Also, metal (Mn/Co) doped-TiO₂ exhibited better activity compared to the commercially available TiO₂ on the removal of Klebsiella pneumoniae [137]. Karaolia et al.[11] shown complete and irreversible removal of E. coli after 3h of photocatalytic oxidation using simulated sunlight. Specific genes were degraded, while others, such as sul1, ermB resistance genes, and enterococci detected via the 23S rRNA gene sequences, were persistent throughout the treatment (suggesting that their removal is more challenging compared to the removal of the other sequences examined). TiO₂ photocatalytic treatment displayed no reduction of ecfX, but the photocatalytic treatment with TiO₂-rGO-PH and TiO₂-rGO-HD successfully reduced it.

The effect of the water matrix was evaluated by Guo et al. [138] on the removal of Staphylococcus aureus and Pseudomonas aeruginosa through UVC/TiO₂ and concluded that disinfection efficiency was independent from the aqueous matrix. Similar conclusion was withdrawn by Guo et al. [138] on the damaging of ARGs during photocatalytic oxidation aided by H₂O₂. Moreover, the removal of mecA and ampC ARG was significantly improved by 2.7–3.4 and 2.7–3.2 log units in comparison to H₂O₂, H₂O₂/UV. In fact, Moreira et al. [139] verified that P25/H₂O₂ and solar/H₂O₂ are suitable processes for ARGs removal from an effluent coming from an urban wastewater treatment plant.

Table 3. Application of heterogeneous photocatalytic processes for water disinfection

ASE- autoclaved secondary effluents, DW- distilled water, PBS-phosphate-buffered saline, MBR - membrane bioreactor, NW- natural water, CAS- Conventional Activated sludge process; UWWTP – Urban wastewater treatment plant.

The addition of figures to the manuscript could help to make the manuscript easier to read. 

We agree with the reviewer opinion. Thus, a Graphical Abstract resuming the objectives of the paper is now given.

Graphical Abstract:

Also, some Figures were added to the manuscript to make it easier to read. (Page 9, Lines 287-289.)

“Disinfection by light is one of the most widely used method around the world [25,26]. The conventional method is based on UV light. However, solar light is also efficient for the deactivation of biohazards {Formatting Citation} and research regarding its application is increasing using several combinations (Figure 1).

Fig.1. The schematic range of electromagnetic radiation used in photochemical disinfection processes and its effects on patogens (based on [35,51,69–75])  “

 Page 12, Lines 396-399.

 “The authors compared this process with single ozonation and verified it was able to totally remove E. coli in 30 s corresponding to a transferred ozone dose of 0.16 mgO₃/L (Figure 2). Still, the use of Pd-TiO₂ and Ag-TiO₂ seems more suitable since no energy was required for disinfection.

 Fig. 2. E. coli in synthetic wastewater as function of TOD during single ozonation (based on [126])”

 Page 16-17, Lines 487-490

 “The combination of ozone with catalysts/light/H₂O₂ may be a suitable solution for disinfection (Figure 3). Still the operating costs associated may be prohibitive specially for low incoming countries.

 Fig. 3. Ozone-aided processes for water disinfection.”

Author Response File: Author Response.pdf

Round  2

Reviewer 3 Report

I wish to thank the authors for their efforts to reply to Reviewers comments.

Is Fig. 2 correct? could the authors comment this figure?

Considering the evidences provided, I can understand the motivation for an additional review on the matter, although I am still afraid that some considerations are missing (e.g. the concentration of insulin, antibiotics and anti-inflammatory drugs in municipal wastewater).

In conclusion, I feel that this work is not suitable for the journal Catalysis.

My recommendation is to consider to publish the work, but in another journal such as Water, whose scope might be a better fit for this manuscript.

Author Response

REPLY TO REVIEWER COMMENTS

REVIEWER #3_____________________________________________________________

Comments and Suggestions for Authors

I wish to thank the authors for their efforts to reply to Reviewers comments.

We acknowledge the reviewer opinion. The reviewer suggestions truly improved our manuscript.

Is Fig. 2 correct? could the authors comment this figure?

Figure 2 is correct. It is based on our previous work: 126.- Gomes, J.F.; Lopes, A.; Gonçalves, D.; Luxo, C.; Gmurek, M.; Costa, R.; Quinta-Ferreira, R.M.; Martins, R.C.; Matos, A. Biofiltration using C. fluminea for E.coli removal from water: Comparison with ozonation and photocatalytic oxidation. Chemosphere 2018, 208, doi:10.1016/j.chemosphere.2018.06.045.

In that work water spiked with E. coli (10³-10⁴ CFU/mL) was subjected to single ozonation. Samples were withdrawn along time and the remaining bacteria were counted through the membrane filtration method. While low bacteria removal was obtained when 0.05 mgO₃/L were used, total depletion was observed when a TOD of 0.16 mgO₃/L were applied (corresponding to 30 s of reaction). Moreover, no regrowth was detected which confirms the efficiency of ozone to degrade E. coli.

This is now discussed in this new version of the manuscript:

“…E. coli from water (10³-10⁴ CFU/mL). Pd-TiO₂ and Ag-TiO₂ did not required light to totally remove bacteria. Moreover, no regrowth was observed. This behaviour was related with the bactericide role of Ag and Pd. The authors compared this process with single ozonation and verified it was able to totally remove E. coli in 30 s corresponding to a transferred ozone dose of 0.16 mgO₃/L (Figure 2). While low bacteria removal was obtained when 0.05 mgO₃/L were used, total depletion was observed when a TOD of 0.16 mgO₃/L were applied. Moreover, no regrowth was detected which confirms the efficiency of ozone to degrade E. coli.”

Considering the evidences provided, I can understand the motivation for an additional review on the matter, although I am still afraid that some considerations are missing (e.g. the concentration of insulin, antibiotics and anti-inflammatory drugs in municipal wastewater).

We acknowledge the reviewer opinion. We understand the relevance of the presence of the presence of emerging concern chemical contaminants such as pharmaceutical and personal care products in municipal wastewater. In fact, we revised that issue and the role of ozonation and biofiltration on the removal of such dangerous pollutants in treated municipal wastewaters bearing in mind their potential impact over ecosystems and human health (14. Gomes, J.; Matos, A.; Quinta-Ferreira, R.M.; Martins, R.C. Environmentally applications of invasive bivalves for water and wastewater decontamination. Sci. Total Environ. 2018, 630, 1016–1027, doi:10.1016/j.scitotenv.2018.02.292. and 157. Gomes, J.; Costa, R.; Quinta-Ferreira, R.M.; Martins, R.C. Application of ozonation for pharmaceuticals and personal care products removal from water. Sci. Total Environ. 2017, 586, 265–283, doi:10.1016/J.SCITOTENV.2017.01.216.).

Now, our focus in this paper is the application of ozonation and light-driven processes (with special emphasis to heterogeneous catalytic processes) on the removal of enteric pathogens (bacteria, virus and protozoa) from water. These biological contaminants constitute another type of pollution sometimes not very well addressed by legislation that may lead to serious ecological and public health issues. Ozonation and light-driven systems seem to be suitable systems to remove those contaminants from water reaching safe reusable water.

In conclusion, I feel that this work is not suitable for the journal Catalysis.

My recommendation is to consider to publish the work, but in another journal such as Water, whose scope might be a better fit for this manuscript.

We dare to disagree with the reviewer. This paper is submitted to Catalysts special issue Environmental Catalysis. This section has as goals “… review articles on the topics of the catalytic elimination of gas, liquid, and solid-phase pollutants. “ and “Advance knowledge of recent catalytic processes (photo-catalysis, plasma-catalysis, electro-catalysis, etc.) applied to environmental protection is also addressed, as well as an understanding of the key challenges to face to promote their application to full scale.”.

Our focus in this paper is the application of ozonation and light-driven processes (with special emphasis to heterogeneous catalytic processes) on the removal of enteric pathogens (bacteria, virus and protozoa) from water. Moreover, in section 4. The key challenges to be faced by catalytic processes in this field are addressed as follows:

 “The selection of the most suitable technology must bear in mind the costs associated to the process and also the specificities of the area where the treatment/disinfection plant will be installed. For example, in remote areas it is also critical that the unit requires low maintenance, and low specialized intervention [156]. In this context, ozone-based systems may be an interesting solution due to the high degree of automation and low amount of chemicals needed for the process. The main disadvantage is related with the need of complex equipment such as air treatment (to concentrate oxygen), ozone generator and off-gas ozone destroyer. All this implies energy costs during the operation. The main advantage of these processes is related with their capability of removing organic compounds as well as disinfecting which may increase the final water quality. Also, UV systems can be easily automatized. However, in that case, as discussed before, a secondary oxidant will be required to maintain a residual disinfectant effect. Although the high interest in photocatalytic oxidation/ozonation disinfection, the use of catalysts in their powder form is an important disadvantage since a solid/liquid separation step must be designed [157]. Thus, research in that field should focus on the development of structured catalysts that may be used in fixed beds without losing much of their activity.

Solar water disinfection processes are of special interest since Sun can be used as a renewable and low-cost radiation source. The simple method of storing water in PET bottles that are left under Sun irradiation has shown important results in the disinfection of water in low-incoming countries [99]. In fact, results show that this methodology already reduced significantly the incidence of dysentery and diarrhea [158,159]. The wide spreading of such simple technology seems, however, difficult in developed countries where it will be hardly seen as suitable for water disinfection [104]. Besides, the use of plastic bottles leads to some scepticism regarding the possibility of some chemicals leaching to the water. Research in this field is focused on the development of reactors able to enhance solar disinfection such as the one developed by Polo-López et al. [160] that would allow the process automatization. Still, the activity of such methods is not very much explored on the reduction of viral activity in water and it is verified that usually these pathogens are more resistant to solar disinfection [161]. Thus, aiming to reach safe reusable water, such problematic must be also addressed in future works.”

In this context, we believe that the paper is suitable for Catalysts.

Author Response File: Author Response.pdf

Reviewer 4 Report

The changes made significantly improve the submission, careful attention to english language is needed during the editorial/proof stage. 

Author Response

REPLY TO REVIEWER COMMENTS

 REVIEWER #4_____________________________________________________________

The changes made significantly improve the submission, careful attention to english language is needed during the editorial/proof stage.

We acknowledge the reviewer opinion. English language was improved in this new version of the manuscript.

Author Response File: Author Response.pdf