Characterizing a Newly Designed Steel-Wool-Based Household Filter for Safe Drinking Water Provision: Hydraulic Conductivity and Efficiency for Pathogen Removal

Round 1

Reviewer 1 Report

This is an interesting work trying to utilize the easily obtained steel wool and river sand to construct a decentralized water purification system for household use. The experimental setup is reasonable and the results indicate satisfactory performance and particularly the permanence of this system for practical application. I recommend this article to be published after the following comments are issued:

1. I am still not quite understanding the composition of the biological sand filter (BSF). From the experimental section, this column is just packed with sand that is precleaned and boiled, thus no microbe is present in the sand filter. Then why this column is named biosand filter? and the authors mentioned the role of biofilm in the removal of pollutants, which confused me. Will the microbe in the feeding well water cause biofilm in the first BSF? and when will this biofilm form? if the removal of pollutants were related to the biofilm, then how to explain the good performance of this system at the initial stage of the test, before the formation of biofilm?

2. line 356-359: the statement here is not clear, do the authors mean that the dissolution of Fe hydroxides in the pH range of 4.9-8.6 is pretty low? But since the Fe0 was not completely consumed, the dissolution of Fe0 should be considered, and the dissolution of Fe0 at the above pH range should be significant, in that case, the low level of iron in the effluent should be ascribed to the removal capacity of the second BSF. Then I am very interested in why the BSF could remove dissolved iron? According to the experimental setup and the results, the dissolved oxygen content after the steel wool column should be minor, therefore the oxidation of Fe2+ and the subsequent precipitation should not be essential. Was it due to the adsorption of Fe2+ by the BSF?

3. line 361-368: the removal of the coli by the Fe0 column should not only due to the adsorption of the bacteria by the Fe corrosion products, but also related with the intrinsic bacterial inactivation capacity of Fe⁰, as reported by the following articles which should be referred to and cited in this work: Environ Sci Technol, 2019, 53, 3707-3717; Environ Sci-Nano, 2019, 6, 2061-2073; Environ. Sci. Technol., 2008, 42, 4927-4933.

Author Response

Reviewer 1

Are the results clearly presented? Can be improved.

This is an interesting work trying to utilize the easily obtained steel wool and river sand to construct a decentralized water purification system for household use. The experimental setup is reasonable and the results indicate satisfactory performance and particularly the permanence of this system for practical application.

Many thanks for this evaluation!

I recommend this article to be published after the following comments are issued:

1. I am still not quite understanding the composition of the biological sand filter (BSF). From the experimental section, this column is just packed with sand that is pre-cleaned and boiled, thus no microbe is present in the sand filter. Then why this column is named biosand filter? (The biofilm is formed after some 6 weeks!) and the authors mentioned the role of biofilm in the removal of pollutants, which confused me (bacteria are killed in the biofilm, mostly by predation). Will the microbe in the feeding well water cause biofilm in the first BSF? (Yes) and when will this biofilm form? (some 6 weeks after the start of the filtration) If the removal of pollutants were related to the biofilm, then how to explain the good performance of this system at the initial stage of the test, before the formation of biofilm? (The Fe⁰/sand unit)

Lines 59 to 63 (not modified):

“BSFs preceding the Fe⁰/sand filter(s) are designed to remove suspended solids by straining (size-exclusion) and pathogens by metabolic breakdown, natural death and predation [18,19]. However, because specific biological processes may also generate metal oxides and organic matter, quantitative scavenging chemical pollutants in BSF has also been reported [20].”

The biofilm (Schmutzdecke) is formed after some 4 to 6 weeks. Initially decontamination is secured by the Fe⁰/sand unit (second column). After the 6 weeks contaminant removal is a synergy of the two systems in the three columns.

2. line 356-359: the statement here is not clear, do the authors mean that the dissolution of Fe hydroxides in the pH range of 4.9-8.6 is pretty low? But since the Fe⁰ was not completely consumed, the dissolution of Fe⁰ should be considered, and the dissolution of Fe⁰ at the above pH range should be significant, in that case, the low level of iron in the effluent should be ascribed to the removal capacity of the second BSF (Indeed the O₂ level after a BSF is never zero. Often it is close to 2 mg/L at that is sufficient to oxidize Fe²⁺ in the second BSF). Then I am very interested in why the BSF could remove dissolved iron? According to the experimental setup and the results, the dissolved oxygen content after the steel wool column should be minor, therefore the oxidation of Fe²⁺ and the subsequent precipitation should not be essential (A key issue is that iron dissolution is not quantitative – all processes are slow due to the time-dependent kinetics of iron corrosion). Was it due to the adsorption of Fe²⁺ by the BSF?

The idea of the reviewer was presented in Tepong-Tsinde et al. (2015): Using BSF as O₂ scavenger such that the iron/sand unit is used to quantitatively produce Fe²⁺ that will be oxidized in a cascade system for “co-precipitation”. Obviously, under the experimental conditions (used SW, the corresponding amount and proportion), escaped Fe²⁺ could not achieve breakthrough in the second BSF. The surface of sand is negatively charged and attracts Fe²⁺ and Fe³⁺ (mechanism of sand coating). We have added the following:

The fact that no iron breakthrough is observed herein suggests that the amount of iron (mainly Fe²⁺) escaping column 2 could not saturate the amount of sand in the second BSF. Fe²⁺ is adsorbed onto sand by pure electrostatic interactions [46,59].

River sand could contain traces of MnO₂ acting as Fe²⁺ scavengers. In his Fe⁰ filters, Bischof used pyrulosite to fix Fe²⁺ from the sponge iron layer. [not discussed herein as the electrostatic argument alone is convincing – no iron breakthrough]

3. line 361-368: the removal of the coli by the Fe⁰ column should not only due to the adsorption of the bacteria by the Fe corrosion products, but also related with the intrinsic bacterial inactivation capacity of Fe⁰, as reported by the following articles which should be referred to and cited in this work: Environ Sci Technol, 2019, 53, 3707-3717; Environ Sci-Nano, 2019, 6, 2061-2073; Environ. Sci. Technol., 2008, 42, 4927-4933.

Many thanks, this is correct and the reason why the system was performing before the formation of the Schmutzdecke (first comments). The three references were cited.

Lee, C; Kim, J.H.; Lee, W.I.; Nelson K.L.; Yoon, J.; Sedlak, D.L. Bactericidal effect of zero-valent iron nanoparticles on escherichia coli. Environ. Sci. Technol. 2008, 42, 4927–4933. Ref. [65]

Hu, Y. Wang, J.; Sun, H.; Wang, S.; Liao, X.; Wang, J.; An, T. Roles of extracellular polymeric substances in the bactericidal effect of nanoscale zero-valent iron: trade-offs between physical disruption and oxidative damage. Environ. Sci. Nano 2019, 6, 2061–2073. Ref. [67]

Sun, H.; Wang, J.; Jiang, Y.; Shen, W.; Jia, F.; Wang, S.; Liao, X.; Zhang, L. Rapid Aerobic Inactivation and Facile Removal of Escherichia coli with Amorphous Zero-Valent Iron Microspheres: Indispensable Roles of Reactive Oxygen Species and Iron Corrosion Products. Environ. Sci. Technol. 2019, 53, 3707–3717. Ref. [68]

Addition:

It is essential to recall that the intrinsic bacterial inactivation capacity of Fe⁰ was already reported in the 19^th century [60-63] and has been independently demonstrated in the “Fe⁰ remediation” research [64-68].

Reviewer 2 Report

The paper is interesting and well written. The topic is of interest to a wide broad of researchers. My suggestion is to publish the paper. 

The only remark is that the authors may include some comments on other filtration methods, maybe starting from:

Mattia, Davide, Kah Peng Lee, and Francesco Calabrò. "Water permeation in carbon nanotube membranes." Current opinion in chemical engineering 4 (2014): 32-37.

Author Response

Reviewer 2

Is the research design appropriate? Can be improved.

The paper is interesting and well written. The topic is of interest to a wide broad of researchers. My suggestion is to publish the paper.

Many thanks for this evaluation!

The only remark is that the authors may include some comments on other filtration methods, maybe starting from:

Mattia, D.; Lee K.P.; Calabrò F. Water permeation in carbon nanotube membranes. Current opinion in chemical engineering 2014, 4, 32-37.

The paper is focused on remediation using metallic iron and has been introduced to discuss using steel wool. It is out of scope to consider other techniques especially the sophisticated ones. Please also consider that the is part of a Special Issue, within a bloc on Fe⁰ comprising a review article and an article on Fe⁰ characterization.

Many thanks for your evaluation!