Field and Laboratory Assessment of a New Electrolytic Point-of-Use Water Treatment Technology

Round 1

Reviewer 1 Report

Research on a POU device that is easy to handle and can be easily sterilized is of practical value. The fact that the research included field studies as well as laboratory studies is a good evaluation. However, the description of the experimental method and the results and discussion need to be revised in many areas. It is also strange that there is no conclusion. I hope you will check the details and correct them.

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Author Response

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Reviewer 2 Report

1. Summary of Manuscript

 

The submitted manuscript involves lab scale evaluation of Silver and copper ion generation through electrolysis for specific purpose of ground water disinfection. Certain operational parameters such as the length of the metal wires, depth of immersion, applied DC voltage and ionic strength were varied to study the effect on the ion release efficiency and disinfection efficacy of the POW unit. The lab study was then replicated in the field in household setting in Limpopo, South Africa with real ground water. The point of use water disinfection units that are affordable and efficient are an absolute necessity in resource scarce geographical areas such as Africa. This study, if modified and significantly improved may contribute towards the objective in the long run.

 

 

1. General and specific Comments:
   1. Electrolytic ion generation using Silver/Copper metals for disinfection is not new at all and there are several publications that have already explored several variables of the process and achieved similar or better disinfection rates. The current manuscript does not offer any novelty or any advancement to the current developments in disinfection. The authors have not established a strong correlation between the lab results and the field data.

2. What is the basis for the SGW composition chosen for the lab study? Is it based on the ground water analysis in South Africa?
3. Ground water quality changes significantly with in the same geographical area depending upon proximity to other water bodies or polluting factors. Have the authors made an attempt to evaluate the effect of other contaminants in the ground water on the disinfection rates or on the ion release rates?
4. Even though it was mentioned several times (e.g. Line 119) that “..results from other experiments indicated that copper should not be used as the sole disinfection..”. However, it was never mentioned how this was concluded about. Also, due to this reason, copper was excluded from the experiments studying effect of distance between the electrodes and the longevity experiments. However, data from copper testing has been included in all other tests. It is not clear why. I request the authors to either include copper all the way or delete it completely or provide a sound justification why it must be excluded in certain experiments.
5. Some data is collected from 3 trials while others were reported for two trials (Longevity, Figure 11,etc.). Is there a reason why 3 data points could not be collected for Figures 10, and 11?
6. In general, the data point size needs to be smaller so that the error bars are visible.
7. It is understandable that the voltages of 4.5 and 9 V were selected because POU application becomes easier with commercially available batteries. However, have the authors attempted using a DC power supply and running the experiments at variable voltage (constant current mode) to see the broader effect of voltage on the ion release rate?
8. Besides the ion concentrations and log removal rates, have the authors evaluated other characteristics such as pH, alkalinity etc? Particularly in the longevity experiment? Did they observe any increase in pH or alkalinity?
9. Figure 5 a. The Silver ion concentration (1 mm length) at 20 min time point has a significant variability and the error bar encompasses all other data points. From a statistical point of view, all data points in the figure are same. It could be an outlier that is causing the wide error bar. Suggest that the experiment is repeated to ensure reproducibility.
10. Figure 5a. it appears that the ionic strength of SGW used in this experiment is different from any of the 3 values tested in Figure 6. Since the ionic strength is constant in Fig 5 experiments, this value should be part of the Fig 6 experiments. This is because ionic strength is the only other main effect (from a DOE point of view) causing a significant change in ion release rate other than the voltage and different baseline ionic strengths cannot be used for different experiments.
11. Figure 7, Figure 8, and Figure 9… which ionic strength SGW was used in these experiments?
12. Figure 10. This data is very important in translating the lab work to field validation. However, adequate explanation for the variability in the data over time and the graph in general has not been provided. There is no explanation for the sudden increase in Ag+ concentration from 100 tests to 150 tests, then gradual decline over the next 200 tests and then sudden rise again. Is the Ion concentration decreasing due to some fouling? Is there any periodic cleaning (e.g. after 100 tests or 300 tests) that is causing the concentration to rise suddenly and significantly? This data I used as the basis for the high log removal and it has been claimed that the “near Zero slope of the trendline” supports the conclusion that the sufficient silver ions are released over time, which is not accurate. The concentration drops well below 50 ug/L for most of the time. The trendline is not linear and cannot be interpreted this way. Ideally, the variability plot is to be looked at to see how far each of the data points are from the group mean.
13. There is no ground water characterization data provided for the field testing. What are the other constituents in the ground water? How do they interfere with the electrolysis? How does the pH of the treated water change over 8 hours?

Author Response

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Reviewer 3 Report

General comment, the data on the graphs should be proportional. Some of the points/data are too big compared to graph size.

Tech comments: 1. Need some details on the QA/QC of the analysis performed.

2. Cost analysis would help in determining the conventional treatment/this technology.

3. Kinetics data/ rate constants would help understand the mechanisms and also, 4hrs of treatment may be too long. Can the process be improved further?

 

Author Response

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Round 2

Reviewer 1 Report

The points I pointed out were corrected approximately appropriately. However, 2 points remain that require further consideration.

Comments for author File: Comments.pdf

Author Response

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Reviewer 2 Report

Authors have made a reasonable attempt at improving the manuscript by addressing all the issues raised. The scientific soundness has certainly improved after all corrections. I recommend that this manuscript be accepted. 

Author Response

Authors have made a reasonable attempt at improving the manuscript by addressing all the issues raised. The scientific soundness has certainly improved after all corrections. I recommend that this manuscript be accepted. 

We thank you for your helpful suggestions.