Optimization of Ozonation in Drinking Water Production at Lake Butoniga

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this research the laboratory simulations of pre-ozonation and the main ozonation process were carried out using the Box-Behnken design to determine the optimum conditions. It was found that optimization of ozone dosing in the pretreatment (PT), main ozonation (MO) stages can effectively control bromate formation (<10 µg/L), while maintaining low THM and HAA levels, at THM <100 µg/L, HAA <60 µg/L. Strategy 2 showed effective control of bromate formation and disinfection by-products using constant and lower doses of ozone. To optimize ozonation, strategy 2 was proposed to minimize bromate concentrations while keeping THMs, HAAs and SUVA levels within 80 % of the MCL. The optimal values for ozonation determined by the general model included 0.2 mg/L ozone in pretreatment and 5 minutes of main ozonation with a dose of 0.1 mg/L. This approach minimizes the need for frequent adjustments and increases operational efficiency.

The manuscript could be accepted for publication in the Journal Water with the following minor revisions:

-       Explain the necessity of surveying and simulating for water at 4.0 m above the lake bottom when “the water at 6.7 m above the lake bottom represents the water at the inlet of the plant” (Page 7, line 249)

-       The temperature and the turbidity of the water samples should be added to “Table 1. Characteristics of the inlet water for all experiments performed”. The MCL values ​​of each component in the water sample should be added to Table 2.

-       The results of the survey on the effects of dose of polyaluminium chloride (PAC) should be presented.

-       How are the input variables: Dose PT, Dose OM, Duration MO (Table 2) selected? How is the input data related to the characteristics of the inlet water (Table 1).

      The cause of a significant discrepancy between the model predictions and the actual measurements (Table 5) has not been clearly explained.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

 

This research paper describes the optimization of ozonation in drinking water production.

 

1) Importantly, what are the new findings in this work? The detailed features should be further strengthened.

2) No mechanism or deep discussion is included in the manuscript. Authors should explain the advantages of method. The mechanism in the presence of each component should be highlighted.

3) The abstract should be revised regarding more details and results.

4) Conclusions should be revised to include more results.

 

Comments on the Quality of English Language

Can be improved.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

 

The authors of “Optimization of ozonation in drinking water production at Lake Butoniga” sought to optimize the operation of an ozonation process for a drinking  water treatment plant that treats water from Lake Butoniga.  The optimization process relied on measures of water quality and experimental trials that varied ozonation parameters in a  benchtop ozonation system. Two  optimization approaches  were used to identify ozone dosing (input, duration) that would be high enough to reduce SUVA but low enough to avoid producing unacceptable bromate and haloacetic acid levels. A strength of the research is use of actual surface water samples (rather than laboratory prepared “synthetic” water samples), the conduct of laboratory experiments, recalibration of the model based on discrepancies between predicted and measured ozone levels, and finally, a comparison of the results of the two optimization approaches.  Given the increased use of ozonation processes globally, this approach to balancing over-ozonation and under-ozonation has potential to be useful to other investigators.   One limitation of the study is the laboratory methods did not include the disinfection of bacteria, viruses or protozoan pathogens.  Thus, whether the optimized ozone dosage for SUVA reduction may not be adequate for disinfection.

 

Some comments, and then edits follow.

Line 125-126: “Statistica v.14.0 software (Tibco Statistica, Palo Alto, USA) to obtain optimal conditions….“  Readers willw ant to know more about the statistical analyses that resulted in the optimized treatment parameters. Please provide more information about the optimization process.

 Lines 169-174: This information provides some rationale for this research.  This would be more suitable for the  end of the Introduction section.

 Table 1: The raw data from each sample at each depth is not necessary.  Simply noting the number of samples, the mean and standard deviation for each water parameter would suffice.  This could be done in the text, or as a table with two rows, one for each depth.

 

Figure 3: A color/shading key indicating the meaning of each shade of grade would be useful. 

 Line 438: “ Laboratory simulations of pre-ozonation and main ozonation provided a reliable model for predicting water quality based on the input parameters.”   This was only true after the revisions were made as described in lines 406 (with correction factors of 2.3 for pre-ozonation dose and around 5 for the main ozonation doses).  It would be more accurate to say that “Following an iterative process in which correction factors were identified for ozone dosing), laboratory simulations of pre-ozonation and main ozonation provided a reliable model for predicting water quality based on the input parameters.”  

 

 

Edits:

Introduction

Line 38: The WHO does not set standards- it provides guidelines that can adopted as national standards by countries or countries may adapt (modify) them , depending on local circumstances,  for use standards.  In the addition to the WHO and EU, the US EPA sets drinking water standards. The authors may want to note that, or the be more specific about the area of interest (“Drinking water standards in Europe are set by the EU.”)

Line 40: “… which requires precise knowledge of water resources.”  It’s not clear what this means. Perhaps saying something more specific like “…which requires a detailed knowledge of source water physical, chemical, and microbial water quality parameters.” 

Line 50: Instead of “ …removed by filtration on rapid filters” consider instead,  “…removed by rapid filtration processes.”

Lines 52-53:  Since ozonation does not result in the persistence of ozone residual, it’s not clear what is meant by, “remains active beyond its immediate point of origin…”  Either delete this or clarify what activity persists.

 Lines 83-84: “…raw water taken from a depth of 6.7 m and 4 m 83 from the bottom of the lake.”  Since the sampling depth is provided in reference to the bottom of the lake  please add the depth of the bottom of the lake (relative to the water surface) at the sampling locations.

 

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The introduction should be completely revised. By stating the topic of the research, the introduction should clearly state the relevance of the work by referring to the available information available in the literature.

The reviewer asks the authors to revise this sentence: P1, L31-32 "Drinking water must be free of suspended solids, microorganisms and toxic chemicals". The term toxic chemicals is not appropriate.

Indeed, the toxicity of a product is not intrinsic. It depends on its concentration and the type of organism that absorbs it. In the 16th century, Paracelsus announced the principle “Sola dosis fecit venenum”, much used ever since as the basis of modern toxicology, now expressed as “All things are poison, and nothing is without poison; only the dose makes a thing a poison”.

It would be better to replace the above sentence (P1, L31-32) with this one "Drinking water must be free of pathogenic microorganisms, not have mineral salts in excessive concentration, be colorless, odorless and tasteless.

P1, L35: Please add the word decantation or sedimentation after flocculation.

To justify the need to avoid the formation of trihalomethanes (THMs) and haloacetic acids (HAAs) through Ozonation, the reviewer asks the authors to provide the effects of these chemical compounds contained in water on human health.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

The authors conducted a series of bench and real-scale experiments to develop a model capable of managing the contamination issues affecting Lake Butoniga (Croatia) by ozonation treatment. Ozonation dose and duration were investigated to minimize bromate concentration while ensuring respect of the regulatory limits for trihalomethanes, haloacetic acids and specific ultraviolet absorbance. The impact of sampling point (different depths) and seasonality were considered. Results showed that a 0.2 mg/L ozone dosing in pretreatment and 5 min of main ozonation at 0.1 mg/L ensure minimal bromate formation and respect of the regulatory limits for trihalomethanes, haloacetic acids and specific ultraviolet absorbance. This study is of good interest as it considers a real-scale application. However, the results are barely discussed and most of the referenced articles are old. Also, the methodology used for the lab-scale experiments is unclear. At the current stage, I can not recommend the publication of this study. I will be happy to reconsider my evaluation after major revisions. My comments are reported below.

Major comments:

1.      What is the main goal of this study? Which are the novel aspects compared to the available literature? These aspects should be highlighted at the end of the introduction chapter.

2.      The introduction section should be improved by reporting the potential of the ozonation process for drinking and wastewater treatment.

3.      In the materials and methods section, it is unclear if the authors performed the experiments in triplicates. The experimental results are the input for the Box-Behnken design. Therefore, not performing at least triplicates for Jar tests would result in unreliable conclusions.

4.      Were the ozonation tests conducted in triplicates?

5.      There is some confusion in the definition of the sampling points. At first, the authors state that Lake Butoniga was sampled at 6.7 m depth and 4  m from the bottom of the lake (lines 83-84). Later, the reference for the sampling points is reported as 6.7 m and 4m from the bottom of the lake. This inconsistency is disseminated throughout the entire manuscript, including tables. Please check and revise.

6.      The authors mentioned that Strategy 2 is beneficial in terms of operational costs. This statement should be supported by a proper cost analysis. Please add this to the revised version of this manuscript.

7.      The results and discussion section should be divided into subsections. For instance, a first subsection can describe and discuss the experiments conducted at lab scale, and a new subsection can be dedicated to the real scale tests.

8.      A short subsection should be dedicated to highlighting the limitations of the developed model. These aspects can be placed at the end of results and discussion.

9.      The optimal results should be mentioned in the text and not solely reported in Tables.

10.      In general, the discussion of results is very limited and old references were used to discuss this study. The authors are requested to improve their discussion and compare the achieved results with the available literature focusing on the utilization of ozonation for drinking water treatment.

Minor comments:

1.      The authors are encouraged to provide a catchy graphical abstract to help the dissemination of their work.

2.      Keywords. Please avoid using abbreviations. More specific keywords should be used. For instance, optimization can be replaced.

3.      Line 57. Replace “parameters” with “contaminants”.

4.      Uncommon abbreviations such as MCL and BBD should be removed to enable an easier reading.

5.      Line 80. Remove “the concentration of” before bromates and dissolved organic carbon.

6.      What is the working volume of the experiments conducted at the lab scale?

7.      Line 139. Reference [9] should be embedded in the text, i.e., according to the models used by Chen et al. (2010) [9].

8.      Line 141. What is the reference for these equations?

9.      Line 186. What is the range of the ozone doses tested?

10.   Figure 3. Please define the meaning of the different colors.

11.   Table 3. Are these the optimal results? If that is so, it should be clarified in the caption.

12.   Tables 3 and 4. Why two tables for Strategy 2? The results of Strategy 1 were presented in a single table.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript has been improved. 

Reviewer 5 Report

Comments and Suggestions for Authors

The authors addressed all the comments and the article can be accepted for publication. One last suggestion, that can also be considered at the proofs stage, is to improve the caption of Figure 3 by explaining the meaning of the colors. Also, the authors should consider that some readers may only have access to the black-and-white version. Therefore, different pattern fills could be an effective option for Figure 3.