Fe⁰/H₂O Filtration Systems for Decentralized Safe Drinking Water: Where to from Here?

Round 1

Reviewer 1 Report

The aim of this communication was to highlight the possibility of using the Fe0-based water treatment systems for decentralized safe drinking water provision. In addition, several future directions for the design of the next generation Fe0-based systems have also been discussed.  The manuscript is written in a well-organized and systematic way, with a necessary introductory overview on the history of Fe0-based water treatment systems. It is certainly the continuity of previous papers published by the research group headed by Dr. Noubactep. Therefore, I would recommend publishing this paper after considering the following suggestion:

 

 

Lines 57-58: ”The clogging problem was solved by using Fe0 as coagulant generator in a revolving purifier (batch system).”

 

I am not sure if the Anderson revolving purifier can be considered a batch system. As you correctly stated at lines 101-103, in batch experiments, the same volume of solution remain in contact with a given amount of Fe0 for a given experimental duration. In my opinion, the apparatus presented in the Medlock patent is a typical example of batch system. But, in the Anderson purifier, water has a continuous flow, entering at the inlet end of the apparatus and exiting at the outlet end. This is exactly the definition given at lines 108-110 for the column system: ”contrary to the pseudo-equilibrium in batch systems, column experiments are dynamic, and solution continuously enters and leaves the column”. The only resemblance of the Anderson revolving purifier with a batch experimental system is the fact that Fe0 grains are continuously mixed with the water due to the rotation of the revolver.

Author Response

Reviewer 1

 

Comments and Suggestions for Authors

 

The aim of this communication was to highlight the possibility of using the Fe0-based water treatment systems for decentralized safe drinking water provision. In addition, several future directions for the design of the next generation Fe⁰-based systems have also been discussed. The manuscript is written in a well-organized and systematic way, with a necessary introductory overview on the history of Fe⁰-based water treatment systems. It is certainly the continuity of previous papers published by the research group headed by Dr. Noubactep. Therefore, I would recommend publishing this paper after considering the following suggestion:

We thank the reviewer for the overal positive comments. It is indeed a great pleasure to read that our reserach efforts for universal safe drinking water are being followed by colleagues.

Lines 57-58: ”The clogging problem was solved by using Fe⁰ as coagulant generator in a revolving purifier (batch system).”

I am not sure if the Anderson revolving purifier can be considered a batch system. As you correctly stated at lines 101-103, in batch experiments, the same volume of solution remain in contact with a given amount of Fe0 for a given experimental duration. In my opinion, the apparatus presented in the Medlock patent is a typical example of batch system. But, in the Anderson purifier, water has a continuous flow, entering at the inlet end of the apparatus and exiting at the outlet end. This is exactly the definition given at lines 108-110 for the column system: ”contrary to the pseudo-equilibrium in batch systems, column experiments are dynamic, and solution continuously enters and leaves the column”. The only resemblance of the Anderson revolving purifier with a batch experimental system is the fact that Fe⁰ grains are continuously mixed with the water due to the rotation of the revolver.

We agree and thank the reviewer for drawing our attention to this point. We have consulted literature and revised acordingly.

To addres the reviewer’s comment, we rephrased Lines 57-60 as follows:

‘The clogging problem was solved by using Fe⁰ as coagulant generator in a revolving purifier, which can be considered as a continuous flow reactor [11-13,15]. However, the fact that Fe⁰ grains are continuously mixed with the water due to the rotation of the revolver suggests some semblance of a batch system.‘

 

To reflect this correction, Lines 100-103 have also been corrected:

 

Ln 100-103:  ‘Although batch laboratory studies have some limited practical applications (e.g., in the Anderson Process (revolving purifiers), and provide useful information in treatability studies for column experiments, continuous column studies provide more practical information for the design of Fe⁰ filters’

 

Revised to:

‘Although batch laboratory studies provide useful information in treatability studies for column experiments, continuous column studies provide more practical information for the design of Fe⁰ filters’

Author Response File: Author Response.doc

Reviewer 2 Report

In my opinion the communication paper entitled "Fe0/H2O filtration systems for decentralized safe 2 drinking water: Where to from here?" is a particular interest work. 

It is an article which critically evaluate the recent literature on  using Fe0 for safe drinking water provision in order to identify flaws in system design and recommend ways for better investigations.  The present communication is not a comprehensive review article tailored on using Fe0-based systems for small  communities. Its objective is to demonstrate how better systems can be designed.

In the Indroduction section detailed background information are provided and article's objectives are crearly stated. 

Follows a detailed presentation of the existing technologies and a critical discussion on the use of specific potable water treatment filters. Their advantages and disadvantages are clearly outlined and the technological gaps are highlighted.

Suggestions are made to couple Fe0 and other affordable materials such as biochars and conventional bio-sand filters in order to develop more robust and efficient water treatment systems.

Finally, specific future research directions are given by the authors. 

I think that the manuscript should be published in the present form. 

Author Response

Reviewer 2

 

Comments and Suggestions for Authors

 

In my opinion the communication paper entitled "Fe⁰/H₂O filtration systems for decentralized safe  drinking water: Where to from here?" is a particular interest work.

It is an article which critically evaluate the recent literature on using Fe⁰ for safe drinking water provision in order to identify flaws in system design and recommend ways for better investigations. The present communication is not a comprehensive review article tailored on using Fe⁰-based systems for small communities. Its objective is to demonstrate how better systems can be designed.

In the Indroduction section detailed background information are provided and article's objectives are crearly stated.

Follows a detailed presentation of the existing technologies and a critical discussion on the use of specific potable water treatment filters. Their advantages and disadvantages are clearly outlined and the technological gaps are highlighted.

Suggestions are made to couple Fe⁰ and other affordable materials such as biochars and conventional bio-sand filters in order to develop more robust and efficient water treatment systems.

Finally, specific future research directions are given by the authors.

I think that the manuscript should be published in the present form.

We thank the reviewer for the positive comments.

 

Since all the comments were positive, made no corrections were made.

 

Author Response File: Author Response.doc

Reviewer 3 Report

Attached

Comments for author File: Comments.pdf

Author Response

Water - 415426R2 Revision; Response to Reviewer #3

 

Fe/H₂O filtration systems for decentralized safe drinking water: Where to from here?

 

The manuscript reviewed Fe⁰-based technology for decentralized water treatment system and provided recommendations. The topic of the manuscript should be of interest to the readers of Water. Overall, the manuscript covered diverse literatures of Fe⁰-based filtration with urgent need of affordable and sustainable decentralized drinking water treatment system as a review article, identified scientific research and practical application gaps for the technologies and provided recommendation of (near) future work.

 

Authors’ Response:

 

We are thankful for this evaluation. However, the submission is not a comprehensive review of past efforts but a corrective overview meant to shape future research. Available review articles are properly referenced. While preparing this article, we realized that there is actually no comprehensive review on decentralized water treatment using Fe⁰. Even the authors of the ITTB filters have already compared their system to existing systems for arsenic removal in an overview articles, which are properly referenced in the manuscript. In our view, what is currently lacking in a critical review on Fe⁰ for decentralized drinking water supply. This is an immense amount of work we have started, but which cannot be included in this communication. In summary, the purpose of the current paper is to communicate key knowledge gaps and how research on Fe⁰/H₂O systems should move forward, rather than provide a comprehensive review of the subject. Accordingly, a detailed discussion of issues that have been the subject of earlier reviews were avoided for brevity.

 

However, to address the reviewer’s comments, we have corrected as follows:

(1)   Briefly discussed the capacity of Fe⁰/H₂0 and biochars to remove contaminants, including pathogens and indicator organisms, and pointed the reader to detailed reviews on the subjects.

(2)   We included Table 1 summarising some of the studies using Fe⁰/H₂O systems for contaminant removal.

(3)   Further elaborated on the novelty of developing Fe⁰-biochar dual water filters

(4)   Briefly explained by widespread use and adoption of both Fe⁰/H₂0 and biochar water filters is still lacking.

(5)   Indicated that the history of Fe⁰/H₂O systems has been discussed in an earlier paper and cited the study.

(6)   Highlighted the mistakes and misconceptions on Fe⁰/H₂O systems, and that this has been discussed in detail in some of our earlier papers.

(7)   We have restructured and re-arranged some sections to enhance logical flow

 

However, the breadth and depth of discussion on the topics are general and relatively shallow partly due to lack of detailed information, comparisons, and critical analysis. Additionally, some sections were not well organized to carry out the content. I believe that this manuscript can be improved with addition of information and reorganization. Thus, I recommend it to be reconsidered with major revision for publication. Specific comments are below:

 

Authors’ Response:

We considered the comments and would like to respond that the comments emanate from the fact that the reviewer considers this communication as a comprehensive review. This submission is a communication meant to draw attention to the identified gaps in the literature, thus prompting researchers to initiate work to address these gaps. Here we have intentionally focus on some of the recent work covering about 2000 to 2018, but we are currently considering a detailed critical review to be expected in about 5 to six months.

 

In summary, the purpose of the current paper is to communicate key knowledge gaps and how research on Fe⁰/H₂O systems should move forward, rather than provide a comprehensive review of the subject. Accordingly, a detailed discussion of issues that have been the subject of earlier reviews were avoided for brevity. However, we have corrected as follows:

(1)   Briefly discussed the capacity of Fe⁰/H₂0 and biochars to remove contaminants, including pathogens and indicator organisms, and pointed the reader to detailed reviews on the subjects.

(2)   We included Table 1 summarising some of the studies using Fe⁰/H₂O systems for contaminant removal.

(3)   Further elaborated on the novelty of developing Fe⁰-biochar dual water filters

(4)   Briefly explained by widespread use and adoption of both Fe⁰/H₂0 and biochar water filters is still lacking.

(5)   Indicated that the history of Fe⁰/H₂O systems has been discussed in an earlier paper and cited the study.

(6)   Highlighted the mistakes and misconceptions on Fe⁰/H₂O systems, and that this has been discussed in detail in some of our earlier papers.

(7)   We have restructured and re-arranged some sections to enhance logical flow

 

Detailed corrections made are highlighted in responses to specific comments and in the manuscript with track changes.

 

Specific comments are:

P3L82-86. It is needed to define the term “universal safe drinking water”. Is it based on WHO’s Guidelines for drinking-water quality?

 

Authors’ Response:

We agree and revised ‘Universal’ which means 'for all'. We can achieved universal safe drinking water within a country, meaning that available water is treated to meet the accepted standard such as the WHO guidelines for drinking water. Therefore, we revised as follows;

‘The term ‘universal’ entails the provision of clean drinking water meeting the acceptable standards such as the WHO guidelines to all people, including those in low-income countries.’

 

P4L111-112 It is true the column system is dynamic and transient condition. The statement is rather simplified and overstated. In some cases, it may reach pseudo-equilibrium condition when reaction kinetics of Fe⁰ with contaminant are rapid. During active treatment period, the filtration system can be considered as steady state.

 

Authors’ Response:

            We have considered the reviewer comments, but we totally disagree with the reviewer. We would like to clarify as follows: There are no chemical reactions between Fe⁰ and the contaminants, instead, Fe⁰ 'just' generates; (i) contaminant scavengers (iron hydroxides and oxides), and (ii) reducing species including Fe^II, Fe₃O₄ and H/H₂. Our research group and some few others have been insisting on this key aspect for the past decades. Unfortunately, the majority of scientists is still regarding Fe⁰ as a reducing agent as can be read in the introductions (and abstracts) of almost all recent overview articles. It is our perception that this reviewer is still thinking in that paradigm, hence the comments. While he has recognized the importance of the comments, we propose to bring the discussion to a larger audience by publishing the article 'as is', and further highlight this misconception.

            Please allow us to further mention that the confusion introduced by the electrochemical nature of aqueous iron corrosion (corrosion by water) and the chemical nature of contaminant reduction (no electrons from Fe⁰) is a general issue in environmental research. In a recent publication Daniel J. Blackwood (Corros. Mater. Degrad. 2018, 7, 59-76) demonstrated that the electrochemical nature of microbial influenced corrosion (MIC), repeated in dozens of scientific publications over the years has never been established. Moreover, as an electro-chemist he thinks that it is just a mirage (own words). The Fe⁰ remediation community is facing the same confusion. This aspect has been discussed in some of our earlier papers, thus here, we summarize as follows:

 

‘Some studies predominantly attribute contaminant removal by Fe⁰/H₂0 systems to direct reduction by Fe⁰, while overlooking the role of indirect reduction (Kumar et al., 2017; Fu et al., 2014; Jiao et al., 2009). In fact, the confusion introduced by the electrochemical nature of aqueous iron corrosion and the chemical nature of contaminant reduction (no electrons from Fe⁰) appears to be a common source of confusion in environmental research. In a recent publication, Blackwood et al., (2018) highlighted that the electrochemical nature of microbial influenced corrosion (MIC) widely reported in scientific publications over the years has never been established.’

 

It is also important to state that Fe⁰ as filter material keep evolving/changing as it reacts with contaminants. Additionally, it is mostly surface reaction.

Authors’ Response:

We agree with the reviewer that it is a surface reaction (iron corrosion) but again, Fe⁰ react with water to produce contaminant collectors not directly with contaminants. Contaminants can be reduced (or even oxidized) but are removed by adsorption, co-precipitation and size-exclusion. This is the reason why even living microorganisms including pathogens and species without redox properties are removed in Fe⁰/H₂O systems.

 

We revised as follows to indicate the continuous evolution of Fe0 and that the pseudo-equilibrium mentioned is transient:

 

The removal process ideally continues, because Fe⁰ as a filter material keeps evolving, until a dynamic or transient pseudo-equilibrium is reached between the dissolved and removed contaminants.

 

L154-155 I suggest to categorize contaminants: heavy metals (zinc, cadmium, lead), organics (halogenated hydrocarbons, nitrosamines), and nutrient (nitrate, phosphate).

 

Authors’ Response:

We agree and revised as follows:

 

‘Relevant contaminants include widespread ones such toxic metals and metalloids (e.g., arsenic, zinc, cadmium, lead), organics (e.g., halogenated aromatics, nitrosoamines), nutrients (e.g., nitrates, phosphates) and radionuclides (e.g., uranium).’

 

L168 I am not sure “ wrong impression” is right word choice here. It may be not connected with ion strength Treatment design and operation procedures are evolving. If the author intended to address mixed results on the performance of Fe0-based filtration systems or to address gaps between scientific performance, I suggest to revise the sentence.

 

Authors’ Response:

We considered the reviewer’s comments and rephrased the statement as follows:

 

‘Consequently, findings on Fe0 systems have been mixed, resulting in some studies drawing the generalized and often misleading conclusion that Fe⁰/H₂O systems are not reliable for water treatment [62-65].’

 

I do not think that section titles represent contents of the sections. Likewise, the section 3.2 is titled as Optimizing treatability studies but the contents rarely discussed the topic. The contents are more related to materials and design aspects. Generally speaking, treatability studies tells us how the water might be treated based on problem identification (type of contaminants, volumn, time), regulations, and even sometimes cultures. In other words, treatability studies shows proper treatment options for certain case. It is not right terminology here . Moreover, optimization is the last step. Materials, design, and operatiation parameters were not fully discussed yet at this point of the manuscript. If the author keeps the section for treatability, the section should have type of contaminants, duration, ion strength, type of water. Some of which were discussed in the section 3.3.

 

Authors’ Response:

We considered the reviewer’s comments and rephrased the titles/subtitles to reflect the content.

 

I suggest the authors use a table or figure to summarize studies and/or cases of Fe⁰-based filtration systems discussed in the manuscript. The table includes process type, materials, contaminants treated, lab vs field scale, duration, reference etc.

 

Authors’ Response:

We agree and would like to thank the reviewer for the suggestion. We revised by including Table 1 summarizing type and concentration of contaminant, nature and amount of Fe⁰, scale (laboratory, pilot or full-scale), filter dimensions, capacity, remarks and references. We also include text citing and summarizing the table. We revised as follows:

 

‘Table 1 presents the large variability of operational conditions used in a few selected independent investigations. It is seen for example that the Fe⁰: aggregate ratio varies from 1: 1 to 1:3 while used materials also varied in form and type. The variation of the experimental durations and contact times also considerable, from a few seconds to months.’

 

L177-182. I wonder if this should be addressed in the beginning.

 

Authors’ Response:

 

We agree and highlighted this knowledge gap in the Introduction.

 

 

Section 3.1 and 3.2 The section division within SONO filter and IITB filter may not be necessary since the breakdown does not help the organization. Moreover, I suggest to reorganize the contents as background/overview, process/fundamentals and advantage/limitation (. For example, background/overview is L198-L202, process/fundamentals (L202-L210 and L225-L233) and advantage/limitation (L210-L219 and L234-236) for SONO filter. What volume of water are treated for SONO filter and IITB filer? What were water characteristics for both system?

 

Authors’ Response:

 

We considered the reviewer’s comments and would like to respond that the structure was used to illustrate that the SONO is the success story for household, while the IITB is the larger scale. Details of the operating conditions are provided in the original papers, which are cited and avoided here for brevity. There, we made no substantial corrections except to rephrase the sub-titles. We revised as follows:

‘Details of operating conditions and performance of the SONO filter are given in literature [40,53,54].’

 

L 289-230. Provide the values of removal rate, permeability range etc.

 

Authors’ Response:

We agree and revised provided removal efficiency data for the SONO filter:

 

‘According to Munir et al. (2001), at an optimum flow rate is 8.4 L/hr, a SONO filter a can reduce arsenic concentration in groundwater from an initial value of 1600 ppb to about 20 ppb. This corresponds to an arsenic removal efficiency of 98.8%.’

 

Munir, A.K.M., Rasul, S.B., Habibuddowla, M., Alauddin, M., Hussam, A. and Khan, A.H., 2001, May. Evaluation of performance of Sono 3-Kolshi filter for arsenic removal from groundwater using zero valent iron through laboratory and field studies. In Proceedings International Workshop on Technology for Arsenic Removal from Drinking Water, Bangladesh University of Engineering and Technology and United Nations University, Japan (pp. 171-189).

 

L 234-235 List a few examples of other contaminants.

 

Authors’ Response:

We agree and revised by qualifying the statements as follows:

 

‘Moreover, SONO filters have been found to remove a myriad of other contaminants of which some are not redox active [74,76]. For example, Tuladhar and Smith [74] reported on the removal of up to 24 different species including metals, nitrates and pesticides.’

 

Tuladhar S., Smith L.S. (2009): SONO filter: An excellent technology for save water in Nepal. SOPHEN 7 (1), 18–24.

 

L234-236. It is rather emotional opinion in my viewpoint.

Authors’ Response:

We agree and revised as follows:

 

‘It is rather surprising that, despite its potential to contribute to the provision of clean water, the SONO design has not achieved widespread use even in developing countries.’

 

Section 3.2 As I suggested for section 3.1.

Authors’ Response:

We considered the comment and rephrased titles to reflect context.

 

 

 

L262-263 I do not think it is needed to mention Ph.D student as published work.

Authors’ Response:

We considered the comment, but the point was to illustrate how creativity of a students, a PhD in this can provide a solution to a global problem. This is contrary to the widely held belief that solutions are developed only by large multi-national companies or scientists in developed countries. In view of this context, we made no correction.  

 

L286-289 Background information is needed for timeline of the filter development and operation.

 

Authors’ Response:

 

“Therefore, rooting the design on the scientific understanding of the Fe⁰/H₂O system could lead to the development of the next generation IITB filters. In particular, current IITB filters were developed between 2008 and 2014 (6 years) [5,67]. This means that the remaining 12 years to 2030 are enough to harness existing scientific knowledge to optimize such systems for a universal safe drinking water supply.”

We considered the reviewer’s comments and would like to respond that Refs. [5] and [67] report on the development of the IITB filters by the authors themselves. Our main point here is that the remaining 12 years should focus on translating this technology into practical solutions. Moreover, the history of Fe0 filters is covered elsewhere and for brevity we avoided repeating it here. To indicate this, we revised as follows:

 

 ‘A history of Fe⁰/H₂0 systems, highlighting the key chronological events is presented in our earlier paper (Hu et al., 2018), thus is not repeated here.’

 

L289-291. The connection to biochar or alternative improvement for Fe⁰-based filtration appears to be weak. Based on the concise summary of current status and pro/cons of Fe⁰-based filtration, it is needed to discuss why alternatives or improvements are needed (e.g. is it for wild range of or specific contaminants, simplified operation/maintenance), potential options (e.g. more for biofiltration using activated carbon, biochar, advanced oxidation) and why the authors focused on biochar in the manuscript.

 

Authors’ Response:

We considered the reviewer’s comment. As indicated earlier, the purpose of the current communication was not to provide a comprehensive review of Fe0 systems, but highlight how this field of research should move forward. Therefore, a detailed discussion of the pros/cons Fe⁰-based systems is beyond the scope of the current study, but we referred the reader to some earlier reviews where these issues are discussed. In summary, the use of both biochars and Fe0 systems, which are proven to be efficient, thus their performance should be optimize, is to accelerate the achievement of the UN SDGs and improve the performance of the filters by combining Fe⁰ and biochar. The rationale is given and good references cited. For clarity, we rephrased is as follows:

 

‘In summary, the novelty for developing filters based on both Fe⁰ and biochars, includes: (1) to develop ‘hybrid’ systems harnessing the contaminant removal capacities of the two materials, (2) biochar is highly porous and stable [99-101], thus can prevent loss of porosity and hydraulic conductivity, and enhance sustainability of Fe⁰/H₂O systems.’

 

 

L305-309. This general statement should have been addressed at the beginning of the manuscript. Additionally, microbiological water quality (fecal indicators, pathogens) is key water criteria for safe drinking water but it has not addressed for Fe⁰ technology until now.

 

We have considered the reviewer’s comment and would like to respond that the statement that removal of pathogens and microorganisms by Fe⁰ technology has not be addressed is not entirely correct. A number of studies have investigated this aspect (e.g., You et al., 2005; Bojic, 2001, 2004).  

We revised and cited references addressing the subject and included table 1 with some data on removal of virus.

 

 We revised and included the statement below in Introduction:

 

‘Fe⁰- and Fe⁰/biosand - filters are reported to effectively remove organic, inorganic, and indicator and pathogenic organisms, including bacteria and viruses in aqueous systems (You et al., 2005; Tellen et al., 2010; Ingram et al., 2012; Shi et al., 2012; Lefevre et al., 2016; Sizirici, 2018; Sizirici et al., 2019). Table 1 presents a summary of some of the inorganic and microbiological contaminants removed by Fe⁰. According to literature, microbiological organisms are removed via irreversible adsorption and inactivation iron (You et al., 2005). Fe⁰/H₂O systems also remove toxic metals and their ions (e.g., Zn, Pb, chromate),   radionuclides (e.g., U) and metalloids (e.g., As, Mo) in aqueous systems (Wilkin and Mcneil, 2003, Rangsivek and Jekel, 2005, Bartzas et al., 2006, Zhang et al., 2010; Kishimoto et al., 2011, Kim et al., 2013). Several detailed reviews exist on the application of Fe⁰ and its composites for environmental remediation (Henderson and Demond, 2007; Fu et al., 2014; Tosco et al., 2014; Stefaniuk et al., 2016; Zou et al., 2016), including several papers by one of the current authors (Noubactep et al., 2011; 2013, 2015, 2016).’

 

See also Table 1 including examples on removal of viruses.

 

The manuscript can be improved by reviewing Fe0-, biochar-, and Fe⁰/biochar- technologies by development stages (lab-scale, pilot-scale, field-scale), which can show the current status of such technologies for decentralized drinking water, gaps for scientific research and practical application, and the author’s recommendation/future work. That can be great contribution to scientific community in this area.

 

Authors’ Response:

We have considered the reviewer’s comments and would like to respond that a number of reviews exists on the biochar, including the developmental stages to be followed. For example, excellent reviews on biochar and nano-scale Fe⁰ and its composites are available, while the combination Fe⁰/biochar is not yet investigated to the point of warranting a stand-alone review. The details of the developmental stages of Fe⁰/H₂O systems is a broader topic than cannot be addressed in this communication, and is the subject of a different paper in preparation and limited to Fe⁰. Therefore, we avoided an extensive review because several earlier reviews exists on the subject, and we referred the reader to this reviews.

 

To address the reviewer’ comments we revised as follows:

‘The capacity of biochars and their activated derivatives to remove organic, inorganic and microbiological contaminants in aqueous systems and removal mechanisms have been discussed in several reviews (Mohan et al., 2014; Ahmad et al., 2014; Inyang et al., 2015, 2016; Tan et al., 2016; Gwenzi et al., 2017).’

 

‘Fe⁰- and Fe⁰/biosand - filters are reported to effectively remove organic, inorganic, and indicator and pathogenic organisms, including bacteria and viruses in aqueous systems (You et al., 2005; Tellen et al., 2010; Ingram et al., 2012; Shi et al., 2012; Lefevre et al., 2016; Sizirici, 2018; Sizirici et al., 2019). Table 1 presents a summary of some of the inorganic and microbiological contaminants removed by Fe⁰. According to literature, microbiological organisms are removed via irreversible adsorption and inactivation iron (You et al., 2005). Fe⁰/H₂O systems also remove toxic metals and their ions (e.g., Zn, Pb, chromate),   radionuclides (e.g., U) and metalloids (e.g., As, Mo) in aqueous systems (Wilkin and Mcneil, 2003, Rangsivek and Jekel, 2005, Bartzas et al., 2006, Zhang et al., 2010; Kishimoto et al., 2011, Kim et al., 2013). Several detailed reviews exist on the application of Fe⁰ and its composites for environmental remediation (Henderson and Demond, 2007; Fu et al., 2014; Tosco et al., 2014; Stefaniuk et al., 2016; Zou et al., 2016), including several papers by one of the current authors (Noubactep et al., 2011; 2013, 2015, 2016).’

 

‘The capacity of biochars and Fe0/H₂O systems to remove potential toxic contaminants highlights their potential application for drinking water treatment, particularly in low-income countries. ’

 

‘Surprisingly, widespread use of Fe⁰/H₂0 and biochar-based water filters is still lacking even in low-income countries, which are expected to benefit immensely from such low-cost technologies. This trend could be attributed to two reasons: (1) lack of detailed design data and procedures for both Fe⁰/H₂O) and biochar water filters, and (2) in the case of Fe⁰/H₂0 systems, inconsistencies in scientific data reported in literature. Therefore, addressing these issues is critical in advancing the scientific knowledge, and enhancing the uptake and adoption of both technologies.’ 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

In the revision of the manuscript, the authors addressed this reviewer's comments. They re-organized some sections, clarified the scope of this communication and generated a summary table of Fe0/H2O systems. I am happy the way the authors addressed the reviewers comments. Therefore, I recommend to accept the manuscript with a few minor corrections for the publication. Specific comments are:

L78. Be specific on flaws or provide examples for flaws.

L88. Remove ‘ after solutions.

L188 I want to note that unlike chloride salts (conservative electrolyte), sulphate and nitrate induce microbiological activities (e.g. biofilm formation, hydrogen sulfide generation) in Fe0/sand filtration system and can be consumed.

Author Response

Water-415426R2 Minor Revision

 

Fe⁰/H₂O filtration systems for decentralized safe drinking water: Where to from here?

 

In the revision of the manuscript, the authors addressed this reviewer's comments. They re-organized some sections, clarified the scope of this communication and generated a summary table of Fe⁰/H₂O systems. I am happy the way the authors addressed the reviewers comments. Therefore, I recommend to accept the manuscript with a few minor corrections for the publication. Specific comments are:

 

Authors‘ response:

Many thanks for this evaluation. We are very grateful to the comments that greatly improved the quality and clarity of our work.

 

L78. Be specific on flaws or provide examples for flaws.

 

Authors‘ response:

We totally agree and provided details as follows (Pg 3 Ln 80-83):

 

‘The fundamental flaws include: (i) the misconception that contaminant removal is a reductive process occurring via the cathodic reaction of electrochemical oxidative dissolution of Fe⁰ [54], and (ii) failure to consider the volumetric expansive nature of iron corrosion during the design of Fe⁰/H₂O filtration systems [74,75].‘

 

L88. Remove ‘ after solutions.

Author s‘ reponse:

We agree and corrected by deleting ‘ (Pg 3 Ln 92)

 

L188 I want to note that unlike chloride salts (conservative electrolyte), sulphate and nitrate induce microbiological activities (e.g. biofilm formation, hydrogen sulfide generation) in Fe⁰/sand filtration system and can be consumed.

Author responses:

We agree and thank the reviewer for alerting us of that important point. We revised as follows (Pg 6 Ln 193-196):

 

‘However, unlike chlorides, which are conservative, sulphates and nitrates may induce microbial activities in the Fe⁰/sand filter systems, including biofilm formation and hydrogen sulphide generation, thus can be consumed in the process.‘

Author Response File: Author Response.doc