sustainability
Concept Paper
Making Fe0-Based Filters a Universal Solution for
Safe Drinking Water Provision
Elham Naseri 1, Arnaud Igor Ndé-Tchoupé 2, Hezron T. Mwakabona 3,4,5,
Charles Péguy Nanseu-Njiki 6, Chicgoua Noubactep 7,8,9,*, Karoli N. Njau 5 and
Kerstin D. Wydra 10
1 Department of Soil Science, Tarbiat Modares University, Tehran 009821, Iran; e_naseri@modares.ac.ir
2 Department of Chemistry, Faculty of Sciences, University of Douala, Douala P.O. Box 24157, Cameroon;
ndetchoupe@gmail.com
3 Department of Physical Sciences, Sokoine University of Agriculture, Morogoro P.O. Box 3038 67115,
Tanzania; hezronmwakabona@suanet.ac.tz
4 Department of Chemical Engineering, Faculty of Engineering Sciences, Katholieke Universiteit Leuven,
3000 Leuven, Belgium
5 Department of Water and Environmental Science and Engineering, Nelson Mandela African Institution of
Science and Technology, Arusha P.O. Box 447 23311, Tanzania; karoli.njau@nm-aist.ac.tz
6 Laboratory of Analytical Chemistry, Faculty of Sciences, University of Yaoundé I, Yaoundé B.P. 812,
Cameroon; nanseu@yahoo.fr
7 Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany
8 Kultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D-37005 Göttingen, Germany
9 Comité Afro-européen—Avenue Léopold II, 41-5000 Namur, Belgium
10 Plant Production and Climate Change, Erfurt University of Applied Sciences, Leipziger Straße 77,
99085 Erfurt, Germany; kerstin.wydra@fh-erfurt.de
* Correspondence: cnoubac@gwdg.de; Tel.: +49-551-393-3191
Received: 16 May 2017; Accepted: 5 July 2017; Published: 12 July 2017
Abstract: Metallic iron (Fe0)-based filtration systems have the potential to significantly contribute to
the achievement of the United Nations (UN) Sustainable Development Goals (SDGs) of substantially
improving the human condition by 2030 through the provision of clean water. Recent knowledge
on Fe0-based safe drinking water filters is addressed herein. They are categorized into two types:
Household and community filters. Design criteria are recalled and operational details are given.
Scientists are invited to co-develop knowledge enabling the exploitation of the great potential of Fe0
filters for sustainable safe drinking water provision (and sanitation).
Keywords: design criteria; permeability loss; reactive filtration; revolving purifier; sponge iron;
zero-valent iron
1. Introduction
Water pollution caused by chemical, microbial, and physical contamination is a worldwide
health issue [1–14]. While microbes cause acute diseases (e.g., cholera, diarrhea, typhoid fever),
chemicals mainly cause chronic diseases including cancer [4,9,11,14–19]. Physical contamination (e.g.,
color, suspended solids) is generally easy to remove. There are many classifications for chemical
contaminants (e.g., (i) organic, inorganic, heavy metal, radioactive, (ii) conventional micro-pollutants
vs. emerging contaminants like pesticides, pharmaceuticals, and personal care products (PPCPs)),
among which water-soluble species can be collectively classified into three main groups: anionic,
cationic, and neutral (nonionic). The next universal classification criterion is the size of the soluble
species (small/medium/large). The presence of any contaminant in drinking water is a potential
cause for concern as it might be toxic or be transformed into toxic species [4,19,20]. Therefore, the
Sustainability 2017, 9, 1224; doi:10.3390/su9071224 www.mdpi.com/journal/sustainability
Sustainability 2017, 9, 1224 2 of 32
development of efficient and affordable technologies for water treatment in developing countries, and
specifically under remote and marginal living conditions, is urgently required.
Providing universal access to reliable, chemical- and pathogen-free water supplies is the ideal
solution to water-borne illness [1,2,4,6,8,9,17,18,20–25]. This objective has not been achieved by
previous efforts, including the Millennium Development Goals (MDGs; 2000–2015) [20]. In September
2015, the countries of the world identified goals and set targets to substantially improve the human
condition by 2030. This was done by adopting the United Nations (UN) Sustainable Development
Goals (SDGs). Goal 6 (one of the 17 SDGs) focuses explicitly on freshwater: “Ensure availability and
sustainable management of water and sanitation for all” [26]. Accordingly, Goal 6 calls for improving water
quality as well as protecting and restoring water-related ecosystems [10]. However, the goal does not
explicitly include universal access to safe drinking water. This view gains importance, considering
that in 13 years (by 2030) the countries of the world will evaluate the extent to which the UN SDGs
have been achieved and set new goals/targets. This communication reiterates that universal access to
safe drinking water is possible and feasible within one or two decades [27]. Thus, existing knowledge
from the science of aqueous iron corrosion (Corrosion Science) needs to be effectively translated into
practical solutions [27–30] by designing efficient filtration systems based on metallic iron (Fe0 filters)
for safe drinking water provision. This includes the use of established and recommended efficient
slow sand filters (SSFs) and biosand filters (BSFs) [31,32], which can be optimized by amendment
with Fe0 [30,33]. Based on these studies, the long-lasting need for an appropriate, demand-based,
affordable, efficient, and sustainable water treatment technology, which is additionally centered on
local communities (not only in the developing world) has been scientifically resolved [26–30,33]. Means
of universal, practical implementation are presented herein. An overview of recent achievements in
using Fe0 for decentralized safe drinking water provision is given first.
2. Current Knowledge on Using Fe0 Filters for Decentralized Safe Drinking Water
2.1. General Aspects
Mwakabona et al. [33] recently discovered that using Fe0 for safe drinking water provision
both at a household level and at a larger scale is a technology more than 130 years old [33,34].
The presentation herein is focused on the named technology as derived from Fe0 reactive walls for
groundwater remediation [35–40]. The Fe0 reactive wall technology was introduced in Canada during
the early 1990s [35,37,38,40,41] as a reductive tool for the degradation of halocarbons from polluted
plumes [42–48]. Fe0 materials were then tested following a pragmatic case-by-case approach and their
suitability for the removal of several classes of aqueous contaminants established [36,37,39,40,49,50].
Ex situ applications of Fe0 filtration systems for water treatment were then introduced. Such systems
could have considered two main subsurface characteristics prompt at introduction: (i) the prevalence
of darkness and (ii) the prevalence of anoxic conditions (low oxygen levels; <2 mg/L) [51]. A profound
understanding of the Fe0/H2O system revealed that subsurface Fe0 walls are sustainable mostly by
virtue of the prevailing anoxic conditions keeping a reductive environment and favoring the generation
of less voluminous Fe oxides (Fe3O4) [26–30]. Fe0 ex situ applications include both domestic use
(household level) [16,52,53], and middle–large size units (community level) [54–56].
Ex situ applications of Fe0 filters for safe drinking water were mainly tested and applied in the
context of arsenic (As) removal in South Asia, Southeast Asia, and Latin America [57–65]. Selected
aspects of the corresponding literature have been reviewed [26,30,66] and actualized [27–29,50,67].
However, a complete review is still missing and information regarding the efficiency of Fe0 filters for
As removal is still conflicting [52,65].
2.2. Fundamental Aspects
High arsenite (AsIII) levels and low iron concentrations make As removal from natural waters
challenging. The issue is exacerbated by high phosphate and silicate concentrations in natural
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waters [52]. Accordingly, oxidizing arsenite (AsIII—not charged) to arsenate (AsV—negatively charged)
and increasing the iron concentration are two common tools to improve As removal from natural
waters. The second tool (increasing iron level) was the rationale that guided the first use of Fe0 for As
removal in filters [48,67] some 17 years ago. Despite the successful introduction of SONO filters [24,51]
using solely a porous iron composite matrix (no oxidizing agent), it is still correctly reported that Fe0
filters are not “as efficient for AsIII removal as for AsV removal” [52]. Preliminary chemical oxidation
of AsIII is still suggested, tested and used [65–70]. However, the use of any chemical undermines the
frugality of the technology. Tepong-Tsindé et al. [29] argued that quantitative removal of AsIII and
AsV by conventional Fe0 filters is a pure design issue as properly selected materials would produce
enough iron corrosion products to remove all available As species by adsorption, co-precipitation, and
adsorptive size-exclusion. According to [29], the variability of experimental/operational conditions
and the lack of a systematic designing approach are the main barriers to progress in Fe0 research.
2.3. The Variability of Operational Conditions
The operational conditions of 12 representative studies testing Fe0 for As removal in column
experiments or pilot studies are presented herein (Table 1) to underline the crucial significance of the
design issue. Considered studies were published in various journals between 2000 and 2016 [56,57,71–79].
Tables 2 and 3 show that system design differs in terms of the size of the columns, Fe0 type, Fe0 size,
Fe0 elemental composition, Fe0 mass, Fe0 ratio in the reactive layer (RZ, reactive zone), initial As
concentration, initial pH value, flow rate, and duration of the experiments. Each parameter has been
shown to be of great importance for the long-term efficiency of Fe0 filters. However, the high degree of
diversity among operational parameters renders inter-system comparability challenging [27–30,80,81].
Material selection should be the first step in designing a Fe0 filter. As a rule of thumb, only readily
corroded materials are used (e.g., no stainless steel). However, for the comparison of independent
results, each material should be primarily characterized for its intrinsic reactivity. It has been clearly
demonstrated that conventional parameters for the characterization of solid materials (e.g., specific
surface area, particle size, elemental composition, adsorption capacity, surface structure) do not give
a full picture of the intrinsic reactivity [82–85]. While the intrinsic reactivity has not been determined,
Table 2 clearly shows that from the 12 selected studies, only one [77] specified the elemental composition
of the used Fe0. In the broad Fe0 literature, the specific surface area is being determined to define
a system-independent descriptor [86,87]. This concept (kSA concept) introduced in 1996 [86] was not
successful but is still widely used [88,89]. Given that characterizing the intrinsic reactivity of Fe0
materials is a prerequisite for discussing Fe0 selection, the few available characterization tools [90–93]
should be routinely used (Section 5.2).
Table 1. Summary of type and origin of Fe0 materials used for As removal in column experiments in the
12 studies utilized herein for the discussion of the impact of design criteria and operational parameters
on the performance of an Fe0 household filter. “Nr” is the number referencing individual articles in
Tables 2 and 3. EMPA refers to the Swiss Federal Laboratories for Materials Testing and Research.
Nr. Anno Ref. Fe0 Type Origin
1 2016 [71] Granular and powder
Shandong Kaitai Group Co., Ltd. (Shangong, China)
and Sinopharm Chemical Shanghai Reagent Co., Ltd.
(Shanghai, China)
2 2015 [72] Nanoparticles Nanoiron, s.r.o. (Czech Republic)
3 2015 [56] Filings n.s.
4 2014 [57] Spiral coils n.s.
5 2013 [73] Steel wool n.s.
6 2013 [74] Iron-oxide-coated HBC Synthesized from FeSO4.7H2O
7 2013 [75] Iron spikes and stainless steel n.s.
8 2013 [76] Filings Fischer Scientific Co.
9 2005 [77] Iron filings U.S Metals Inc. (Mentune, IN, USA)
10 2005 [78] Iron filings EMPA (dubendorf, Switzerland)
11 2003 [79] Iron filings n.s.
12 2000 [70] Iron chips (filings) Renwick Ironworks, Kushtia (Bangladesh)
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Table 2. Summary of iron characteristics used in the 12 selected studies. “SSA” is the specific surface
area and “PD” the particle density, while HBC = Honeycomb Briquette Cinders. It is seen that the SSA,
PD, particle size, and the elemental composition were seldom specified.
Nr. Fe0 Type Size SSA (m2/g) PD (g/cm3)
Elemental Composition
Fe C
1 Granular and powder 1 mm; 4.44–5.56 mm n.s. n.s. n.s. n.s.
2 Nanoparticles 50–60 nm 20–25 n.s. n.s. n.s.
3 Filings n.s. n.s. n.s. n.s. n.s.
4 Spiral coils n.s. (3–30 mm length) n.s. 0.95 n.s. n.s.
5 Steel wool n.s. n.s. n.s. n.s. n.s.
6 Iron-oxide-coated HBC n.s. - - - -
7 Iron spikes and stainless steel n.s. n.s. n.s. n.s. n.s.
8 Filings 40 mesh n.s. n.s. n.s. n.s.
9 Iron filings 100 mesh 0.55 n.s. 95 1.2
10 Iron filings n.s. n.s. n.s. n.s. ≤0.17
11 Iron fillings n.s. 1 n.s. n.s. n.s.
12 Iron chips (filings) n.s. n.s. n.s. n.s. low
Besides the Fe0 intrinsic reactivity, there is a need to consider the solution chemistry of the water
to be treated. Luepin and Hug [77] investigated the effects of O2 and the pH on As removal in Fe0
columns and reported a decrease of effluent As level with increasing initial O2 level for short empty
bed contact time (EBCT). For long EBCT, similar results were achieved, only at comparatively lower
initial pH values (pH 5.0). Given that pH value and O2 level are not independent parameters, the
results of [77] suggest that some literature discrepancies are related to the insufficient consideration
of the importance of some design parameters (here the thickness of the reactive zone or Fe0-based
layer). The water flow velocity (residence time) co-influences such systems as well. Actually, the
water flow velocity depends on several parameters including Fe0 reactivity (rate of production of solid
corrosion products), Fe0 size and shape, the Fe0 proportion in the reactive zone, and the thickness of
the reactive zone.
Table 3 evidences a high variability in all considered operational parameters. For example, the
initial As level varied from 0.02 to 100 mg/L and the Fe0 varied mass from 0.4 to more than 3000 g.
The column dimensions and the tested water flow velocity also varied widely, but the most obvious
variability was observed in the duration of the experiments (18 min to 224 h). Given that the kinetics
of Fe0 corrosion is never linear, there is no way to extrapolate results from short-term experiments
to real-world situations (months and years). The large diversity in system design coupled with
the variability in Fe0 characteristics render comparison of independent results challenging or even
impossible. This evidence justifies the frequency of discrepancy in the literature [92,94] and underlines
the need for a more systematic approach [26–30,69]. Figure 1 presents the breakthrough curve for pure
adsorbents (e.g., activated carbons) and raises questions regarding the predictability of the efficiency
of Fe0 filters.
In summary, the development of efficient ex situ Fe0-based systems for safe drinking water
provision has been impaired by an exceedingly pragmatic research approach [26–30]. The huge
potential of Fe0 filters as a reliable, affordable, and efficient technology is yet to be exploited for
households and small communities. The effort engaged herein aims to redirect research for Fe0 filters
and establish a common base for pilot testing.
Sustainability 2017, 9, 1224 5 of 32
Table 3. Summary of some experimental conditions used for the column experiments in the 12 selected articles. X stands for the used contaminant and [X] for its
concentration. ID is the inner diameter of the column, L its length, and RZ the thickness of the reactive zone. The numbers (Nr.) are related to relevant references as
specified in Table 1.
Nr. X [X] (mg/L) Fe0 (g)
Dimension
Porosity (%) PH (−) Duration Flow Rate
ID (cm) L (cm) RZ (cm)
1 Sb(V), Cd(II), Hg(II) and As(V) 0.2 for each 100.0 1.8 40.0 0.1 n.s. 7.5 224 d 2300 BV/8.0 min EBCT
2 Iopromide 200.0 0.4 2.5 15.0 n.s. n.s. 6.6–7.3 75 min 8.5 m/d
3 As(III) and As( V) 0.5 for each 2.5 n.s. n.s. 3–5 n.s. 7.0 n.s. 0.75–1.0 L/h
4 As 0.02 n.s. 31.0 n.s. 120.0 86.0 7.1–7.5 45 m 10,000 L/h
5 As( V) 0.3 47.0 4.0 0.1 n.s. n.s. n.s. 90 h <600 mL/h
6 As(III) and As( V) 0.2 for each n.s. 7.0 65.0 n.s. n.s. 7.1 24 d 7502.2 mL/h
7 As 0.04–0.26 2200 and 1813 14.0 n.s. n.s. n.s. 6.5–8.5 105 d 0.189 m/h
8 As 70.0 17.2 n.s. 30.0 n.s. n.s. 4–7 7 h 1.8 L/h
9 As(III) 0.09 150 2.5 17.8 11.6 n.s. 6.0 9 h 30 mL/min
- As( V) 85 600 5.1 17.8 11.3 n.s. 5.0 56 d 200 mL/d
- As(III) and As( V) 50 and 100 400 3.8 17.8 13.6 n.s. 6.0 200 d 700 mL/d
10 As(III) 0.5 6 1 n.s. 1 3.5–4 7.0 n.s. 1 L/h
11 As(III) 0.35 73,000–74,300 30 n.s. 127 n.s. 6.0–6.3
Sustainability 2017, 9, 1224  5 of 31 
Table 3. Summary of some experimental conditions used for the column experiments in the 12 selected articles. X stands for the used contaminant and [X] for its 
concentration. ID is the inner diameter of the column, L its length, and RZ the thickness of the reactive zone. The numbers (Nr.) are related to relevant references as specified 
in Table 1.  
Nr. X [X] (mg/L) Fe0 (g) 
Dimension
Porosity (%) PH (−)ID (cm) L (cm) RZ (cm) Duration Flow rate
1 Sb(V), Cd(II), Hg(II) and As(V) 0.2 for each 100.0 1.8 40.0 0.1 n.s. 7.5 224 d 2300 BV/8.0 min EBCT 
2 Iopromide 200.0 0.4 2.5 15.0 n.s. n.s. 6.6–7.3 75 min 8.5 m/d 
3 As(III) and As( V) 0.5 for each 2.5 n.s. n.s. 3–5 n.s. 7.0 n.s. 0.75–1.0 L/h 
4 As 0.02 n.s. 31.0 n.s. 120.0 86.0 7.1 5 45 m 10,000 L/h 
5 As( V) 0.3 47.0 4.0 0.1 n.s. n.s. n.s. 90 h ˂ 600 mL/h 
6 As(III) and As( V) 0.2 for each n.s. 7.0 65.0 n.s. n.s. 7.1 24 d 7502.2 mL/h 
7 As 0.04–0.26 2200 and 1813 14.0 n.s. n.s. n.s. 6.5–8.5 105 d 0.189 m/h 
8 As 70.0 17.2 n.s. 30.0 n.s. n.s. 4 7 7 h 1.8 L/h 
9 As(III) 0.09  150 2.5 17.8 11.6 n.s. 6.0 9 h 30 mL/min 
- As( V) 85 600 5.1 17.8 11.3 n.s. 5.0 56 d 200 mL/d 
- As(III) and As( V) 50 and 100 400 3.8 17.8 13.6 n.s. 6.0 200 d 700 mL/d 
10 As(III) 0.5 6 1 n.s. 1 3.5–4 7.0 n.s. 1 L/h 
11 As(III) 0.35 73, 0–74,300 30 n.s. 127 n.s. 6. ˃ 18 m 2722 L/d 
12 As(III) and As( V) 0.8 and 1.1 3000 n.s. n.s. n.s. n.s. 6.8 n.s. 6.2 L/h 
18 m 2722 L/d
12 As(III) and As( V) 0.8 and 1.1 3000 n.s. n.s. n.s. n.s. 6.8 n.s. 6.2 L/h
Sustainability 2017, 9, 1224 6 of 32
Sustainability 2017, 9, 1224  6 of 31 
 
 
Figure 1. Concentration and breakthrough profiles in packed beds of pure adsorbents and Fe0 filters. The 
adsorption breakthrough behavior (top) is well understood and the unusual effects widely characterized. 
For Fe0 filters (bottom) this knowledge is yet to be confirmed. 
3. The Affordability of Fe0-Amended Sand Filters 
One major argument against universal access to safe drinking water in the short term is the high capital 
cost of piped supply systems, which are still regarded as the default option [4,8,9,95]. Current cost 
estimations are based on universal safe piped water for many developing regions. Accordingly, household 
water treatment and safe storage (HWTS) practices like boiling, chlorination, or filtration are collectively 
regarded as an interim solution [95–97]. Ojomo et al. [8] reported on controversies surrounding the question 
of whether HWTS practices yield improvements in drinking water quality and reductions in diarrheal 
disease [97–99]. In particular, it was argued that studies claiming the efficiency of HWTS practices were 
assessed over too short a duration [99]. However, these reports are not based on the instrumental analysis 
of treated water, making this discussion questionable, as only biological and chemical water analyses 
should be used to determine the water quality: the presence, level, and nature of contamination [11,27]. 
Figure 1. Concentration and breakthrough profiles in packed beds of pure adsorbents and Fe0
filters. The adsorption breakthrough behavior (top) is well understood and the unusual effects widely
characterized. For Fe0 filters (bottom) this knowledge is yet to be confirmed.
3. The Affordability of Fe0-Amended Sand Filters
One major argument against universal access to safe drinking water in the short term is the high
capital cost of piped supply systems, which are still regarded as the default option [4,8,9,95]. Current
cost estimations are based on universal safe piped water for many developing regions. Accordingly,
household water treatment and safe storage (HWTS) practices like boiling, chlorinatio , or filtr tion
are collectively regarded as an interim solution [95–97]. Ojomo et al. [8] reported on ontroversies
surrounding the question of whether HWTS practices yield improvements in drinking water quality
and reductions in diarrheal disease [97–99]. In particular, it was argued that studies claiming the
efficiency of HWTS practices were assessed over too short a duration [99]. However, these reports are
not based on the instrumental analysis of treated water, making this discussion questionable, as only
biological and chemical water analyses should be used to determine the water quality: the presence,
level, and nature of contamination [11,27].
The success of HWTS practices in treating water (e.g., eliminating pollution) has been randomly
interchanged with “the success in preventing diarrheal disease,” mostly for children under five. This
oversimplification is no longer acceptable [28]. Diseases are potentially caused by many other factors
Sustainability 2017, 9, 1224 7 of 32
including sudden changes in the diet or the natural growth process [100]. On the other hand, the
most common HWTS practice (disinfection by boiling or chlorination) addresses only biological
contamination [33]. In other words, a child drinking water polluted with As, F, or U (the three ‘natural
inorganic killers’) will not suffer from any diarrheal disease, but the water s/he is drinking is not safe.
Therefore, relating the efficacy of HWTS methods to the frequency of diarrheal diseases is misleading.
Additionally, HWTS definitively has the potential to improve water safety, but does not address
water accessibility [101–103]. Nevertheless, because affordable methods for water accessibility are
increasingly available (rainwater harvesting, solar pump), HWTS is not just a “partial and interim
solution to unsafe water” [8], but a potentially reliable stand-alone solution for sustainable, safe
drinking water [17,26–28,30,103].
The affordability of Fe0 filters results from the evidence that they rely on two universally available
materials: Fe0 and sand. As stated in Section 1, amending conventional BSFs with Fe0 reactive
layers will make them efficient at removing (i) pathogens in the BSF part and (ii) ‘excess’ pathogens
and micro-pollutants in the reactive layers. Moreover, the BFS should precede Fe0 layers and acts
as an O2-scavenger to enable the operation of Fe0 layers under subsurface-like anoxic conditions
(Section 2). Conventionally, the Fe0 reactive layer is made up of sand and Fe0, wherein the volumetric
proportion of sand should exceed 50% [80,81]. In practice, sand can be partly or totally replaced by
other available and affordable natural minerals like anthracite, MnO2, or pumice [104–108]. These inert
(e.g., anthracite, pumice) or reactive, but non-expansive (MnO2) materials primarily serve as a storage
surface for in situ generated corrosion products (in situ coating) [109], and thus as an adsorptive
surface for inflowing contaminants. This is the fundamental mechanism of contaminant removal
in Fe0/H2O systems [33]. The role of MnO2 in sustaining the efficiency of Fe0 filters was explained
as follows [105–110]: MnO2 uses Fe2+ from Fe0 oxidative dissolution by water (Equation (1)) for its
reductive dissolution (Equation (2)):
Fe0 + 2 H+ ⇒ Fe2+ + H2 (1)
MnO2 + Fe2+ + 2 H2O⇒MnOOH + FeOOH + 2 H+. (2)
MnO2 works as an Fe2+ scavenger and thus a sustaining reaction after Equation (1) according
to the Lechatelier Principle. Despite stoichiometric disadvantage, small amounts of MnO2 will act as
a catalyst because MnOOH is permanently recycled into MnO2 [110]. This catalytic aspect has been
put forward to rationalize the sustainability of SONO arsenic filters [17,25,53].
Some existing efficient Fe0 filters have been built by local populations without any particular
skills and are maintained by them [56,111–113]. As an example, the community-scale Fe0 arsenic filter
developed at the Indian Institute of Technology Bombay (IITB filter) [56,113] uses commercial iron
nails. Each community filter contains some 10 kg of iron nails and should work for about five years.
Fe0 filters use the same construction materials as any other filtration systems (e.g., biochar, biosand,
activated carbon). As an example, Kearns [114] used commercially available 200-liter high-density
polyethylene (HDPE) drums to build biochar-based filtration systems in Thailand. The same drums
could be used for Fe0 filters. For IITB-like filters, 10 kg of iron nails are needed; 10 kg of iron nails
every five years is certainly affordable. However, ‘iron nails’ is not a well-characterized class of Fe0
materials [92,115,116]. This makes the transferability of results achieved with IITB filters difficult.
4. The Efficiency of Fe0 Filters
The removal of many contaminants from natural water to meet drinking water standards
is difficult as a result of their high solubility in water. Common decentralized water treatment
technologies such as adsorptive filtration, boiling (pasteurizing), chlorination, ceramic filtration,
membrane filtration (e.g., reverse osmosis (RO), biosand filtration (BSF), and solar water disinfection
(SODIS)) are either energy-intensive (RO) or restrictive under marginal living conditions due to high
complexity (chlorination), the possible production of toxic byproducts (chlorination), and the small
Sustainability 2017, 9, 1224 8 of 32
spectrum of contaminants addressed (boiling, BSF, chlorination, SODIS) [1,117]. Adsorptive filtration
is considered the most affordable, reliable, and effective means for decentralized safe drinking water
production [17,118]. Further arguments for adsorptive filtration include its simplicity, ease of operation,
economic feasibility, recyclability of adsorbents, and the availability of a wide range of adsorbents such
as activated carbon, metal oxides, and zeolites. Despite the large spectrum of available adsorbents, it is
still challenging to find efficient, readily available, economically feasible, and high-adsorption capacity
materials for field applications. In recent years, Fe0 has been established as an in situ generator of
hydroxides and oxides for water treatment (Section 2) [3,21,26,119,120].
The in situ generation of metal oxides for the removal of aqueous biological and chemical
contaminants was known prior to the era of Fe0 filters [121–130]. Despite their inherently engineered
nature (fabrication costs), Fe0-based materials are abundantly available and are sometimes low
in cost [130,131]. Fe0 in the forms of granulated iron (chips, fillings, nails, plates), iron powder,
nanoscale iron (nano-Fe0), sponge iron, steel wool, etc. has been widely used to remove aqueous
contaminants. At pH values of natural waters (6.0–9.0), Fe0 filters are basically ion-selective
as the surface of iron (hydr)oxides (iron corrosion products, FeCPs) shielding Fe0 is positively
charged [132–134]. Accordingly, Fe0 filters are more suitable for the removal of negatively
charged species (e.g., fluoride and arsenates/AsV). However, regardless of their surface charge and
molecular size, contaminants are (i) physically sequestrated or enmeshed during the precipitation
of FeCPs (co-precipitation) [125–127,135–137], and/or (ii) removed from the aqueous phase by size
exclusion. Size exclusion is mediated by the volumetric expansive nature of iron corrosion (Voxide >
Viron) [80,81,138,139]. This implies that contaminant removal by size exclusion is constantly improved
during the filter lifespan (provided the system does not get clogged). Therefore, well-designed Fe0
filters efficiently remove all classes of aqueous contaminants in a single-stage process by a synergy
between adsorption, co-precipitation, and size-exclusion [26–30,33,140].
A major challenge of the Fe0 filtration technology is to select the appropriate material capable
of efficiently treating polluted water within an appropriate packed-bed. None of the tested/used
Fe0 material classes (e.g., iron fillings, iron nails, steel wool) is homogeneous in term of intrinsic
reactivity or efficiency for contaminant removal. Accordingly, despite 27 years of extensive research,
information is lacking to confidently select Fe0 materials to be used in filters for safe drinking water
provision [44,92–94]. This evidence makes the design of Fe0 filters challenging.
5. Designing Efficient Fe0 Filters
A critical understanding of past design efforts made in the arena of Fe0 filters is a fundamental
prerequisite for future research. Considerable information on contaminant removal by Fe0 filters is
available (Sections 2 and 3), but randomly scattered in the literature [3,141–146]. Three concise review
articles have been recently presented to catalyze further advancements [26,27,52]. Hence, this section
focuses on conceptual aspects.
5.1. General Aspects
Fe0 fixed beds are a reactive filtration technology. Like adsorptive filtration, it is expected to be
an extremely versatile technology. Adsorptive filtration has proved to be the least expensive treatment
option for many water treatment applications [16,52,147,148]. Research over the past three decades has
demonstrated the suitability of Fe0 filters to quantitatively remove a wide variety of pathogens and
toxic chemicals [17,25,53,71,116,119,142,144–146,149]. Its suitability on a specific application depends
on costs as they relate to the amount of Fe0 consumed.
5.2. Fe0 Characteristics: Form, Size, and Intrinsic Reactivity
Determining the Fe0 amount to be used in an application is a challenging task for at least two
reasons: (i) Fe0 is not the contaminant-removing agent but rather the generator of contaminant
collectors, (ii) contaminant collectors are in situ generated and further transformed in a highly
Sustainability 2017, 9, 1224 9 of 32
dynamic process. Reason 1 implies that, unlike inert adsorbents (e.g., granular activated carbons),
the determination of the “specific adsorption capacity” for Fe0 materials is not easy and cannot be
derived from short-term adsorption isotherms [52,147]. The ‘adsorption capacity’ gives the maximum
amount of each contaminant that can be removed per unit weight (usually in grams) of adsorbing
material (Fe0 is not one such). When additionally considering that the initial reaction products (FeII
and H/H2 species) are further transformed to a variety of hydroxides and oxides (Reason 2) and that
each Fe0 reacts with its own reaction kinetics (intrinsic reactivity), it becomes evident that selecting
the appropriate Fe0 for a specific application is a challenging task [52,86,93,150–152]. Tested and used
reactive Fe0 exists in several sizes and forms, including iron fillings, iron nails, scrap iron, sponge iron,
and steel wool. Materials of each class have been positively tested for contaminant removal without
any effort to link individual materials to specific applications. Moreover, a common tool to characterize
the intrinsic reactivity of Fe0 materials is still missing [45,92,93].
5.3. Characteristics of the Admixing Aggregates
The next important feature for the design of Fe0 filters arises from the evidence that pure Fe0
systems (100% Fe0) are not sustainable [28,29,80–82]. In fact, iron corrosion is a volumetric expansive
process as each iron corrosion product (hydroxide, oxide) is at least 2.1 times larger than the parent atom
(Voxide > Viron) [80,81,138,139,153]. Therefore, Fe0 should be mixed with at least one non-expansive
aggregate (e.g., gravel, pumice, sand) [80,81,152]. The nature and proportion of the appropriate
aggregate is an important operational parameter for the design of Fe0 filters. Each aggregate has
adsorptive affinity to individual contaminants but another criterion for their selection is the high
surface area to volume ratio (porosity) as the accessible porous system may enable the accumulation of
in situ generated corrosion products and thus delay permeability loss [80,81,150].
To summarize, from a purely material perspective, the efficiency of a Fe0 filter depends on
(i) the intrinsic reactivity of the used Fe0, (ii) the form and size of used Fe0, (iii) the nature of the
admixing aggregate, and (iv) the Fe0:aggregate ratio. It appears that material selection is crucial for
the functionality and sustainability of Fe0 filters. Regardless of the Fe0 type used, it is applied for
the mitigation of the extent of contamination from polluted water. Accordingly, besides the nature of
the contaminant, the characteristics of the polluted water (solution chemistry) should be considered
as well.
5.4. Impact of Solution Chemistry
As a polluted stream passes through a Fe0 filter, a dynamic condition develops that should produce
a decrease in the contaminant concentration from the initial to the final level, ideally lower than the
maximum contamination level (MCL). When the contamination level starts off higher than MCL, there
is a “breakthrough.” In adsorptive filtration, the breakthrough corresponds to the exhaustion of the
capacity of used adsorbent (breakthrough capacity) [147,148]. For Fe0 filters, however, breakthrough
may/should be observed before Fe0 is exhausted. This is because there is no real ‘removal front’ and
Fe0 is oxidized in all parts of the filter, even in the absence of contaminants and dissolved O2. For
this reason, all parameters influencing aqueous water corrosion must be considered and their impact
of the decontamination process discussed. In essence, such parameters influence the dissolution of
Fe0, the solubility of iron, the formation of the oxide scale on Fe0, and the permeability of the oxide
scale [154,155].
The decontamination with Fe0 is affected by various interrelated chemical and physical
characteristics of the water. These parameters include: (i) pH value, (ii) nature and extent of
contamination, (iii) nature and extent of co-solutes (e.g., NO3−, PO43−), (iv) presence of organic
matter, (v) availability of dissolved O2, and (vi) availability of CO2. From the relevant parameters,
the pH value and the availability of dissolved O2 will be commented on in some detail. The nature
of the contamination is a site-specific issue. However, for a concept paper, the four most common
contaminants can be considered: arsenic, bacteria, fluoride, and uranium.
Sustainability 2017, 9, 1224 10 of 32
5.4.1. Impact of pH Value
Fe0 oxidative dissolution rates are faster at lower pH values (abundance of H+—Equation (1))
than at higher pH values. For safe drinking water provision, especially in the developing country,
the water source should have a pH > 5.0. Because Fe0 is the source of contaminant collectors, the
real contaminant scavengers are low-soluble iron corrosion products and their solubility is low at
pH > 4.5 [156]. The surface of iron oxides and hydroxides is positively charged in the pH range of
interest, suggesting that negatively charged contaminants will be more efficiently removed. In other
words, because of the ion-selective nature of the Fe0/H2O system at pH > 5.0, the most important
impact of the pH value is its impact on the contaminant speciation. The large majority of available
studies have not properly discussed this aspect (Sections 2 and 3).
5.4.2. Impact of O2 Level
O2 accelerates the initial kinetics of Fe0 oxidative dissolution but its most important impact is
indirect as it cannot quantitatively reach the shielded O2 surface. The O2 level might favors the
formation of a thick oxide scale on Fe0 (lowering the oxidation kinetics) as well as the cementation of
Fe0 particles and other aggregates (yielding permeability loss). It appears that the effect of O2 level is
situation-dependent, even within the same system. Accordingly, it is not surprising that controversial
reports have been published (Section 2).
A certain impact of O2 is on the nature of in situ generated iron hydroxides and oxides. When O2
is abundantly available, more voluminous compounds are generated (FeIII oxides/hydroxides). The
net result is a rapid porosity loss yielding system clogging (permeability loss). When the system is
clogged, it become useless despite the remaining Fe0 amount. For this reason, a sure way to sustain
Fe0 filters is to operate at low O2 levels (e.g., [O2] < 2.0 mg/L) [33]. Fortunately, such conditions are
achieved in biosand filters (BSF). This means that using a BSF as pre-treatment unit for a Fe0 filters is
advantageous [26–30].
5.5. Design Considerations
The oxidative dissolution of Fe0 and the associated reactions are not instantaneous. In other
words, the generation of contaminant collectors needs time. Fe0 filters should be designed such that
contaminant removal is completed within the bed for the selected flow velocity [26–30,122–126]. As the
rate of aqueous Fe0 corrosion is not a constant function of the time, and each Fe0 has its own intrinsic
reactivity, the prediction of the efficiency of each filter goes through pilot testing. Another key factor is
the expected change of the hydraulic properties (permeability) of Fe0 filters during their operation.
The question arises: which testing approach would enable the achievement of reliable results that may
be regarded as system-independent?
The efficiency of a Fe0 filter for As removal has illustrated the complexity of the issue (Section 2).
The efficiency depends on the following five parameters: (i) the pH value determining the ion selectivity
of the filter, (ii) the redox speciation of As (AsIII, AsV or AsIII/AsV ratio), (iii) the As concentration and
the concentrations of all other species interfering with As adsorption by competing for adsorption sites,
modifying surface charges, or modifying As and Fe solubility, (iv) porosity and pore size distribution
of iron precipitates (and aggregates), and (v) the hydraulic conductivity of the filter as influenced by in
situ generated contaminant collectors.
5.6. Approach for the RSM Modeling
The presentation herein has reiterated the availability of limited guidance for the Fe0 selection
for designing efficient Fe0 filters. However, Fe0 is the heart of the system and the efficiency of each
system depends on the (i) initial kinetics of Fe0 corrosion (corrosion rate) and (ii) its time-dependent
change, which is not a linear function ([28] and refs. cited therein). Accordingly, the determination
of the corrosion rate of various Fe0 materials under relevant field conditions is urgently needed [89].
Sustainability 2017, 9, 1224 11 of 32
The next section (Section 6) will present a protocol for the characterization of steel wool as Fe0
material for drinking water production at a community level. Steel wool is selected for its worldwide
availability [25,131] and the rapid kinetics of its corrosion [127,128].
The illustrative RSM model herein will discuss the effects of Fe0 type (four samples), the nature of
the contaminants (As, bacteria, F, U) (four species), flow rate (0.2 to 2.4 mL/min) (five values), bed
height (10 to 70 cm) (5 values), and initial fluoride concentration (2 to 15 mg/L) (five values) on the
efficiency of Fe0 filters.
6. Testing Steel Wool as a Starting Fe0 Material
To illustrate the design practice of Fe0 filters, a procedure to systematically test a system based on
steel wool is discussed in this section. Three cylindrical columns (Column 1 through 3) with a diameter
(D) of 15 cm and a height (H) of 100 cm are used (Figure 2). The bed volume of this filter is 17.7 L
(V = pi × D2 × H/4). The first and third columns are filled with fine sand (d < 0.4 mm). The first
(Column 1) is basically a conventional BSF, whereas the second (Column 3) fixes dissolved Fe from
the Fe0-based column (in situ coating) to optimize decontamination. Column 2 contains a 70 cm thick
reactive layer sandwiched between two layers of fine sand. Accordingly, the volume of the reactive
layer is 12.4 L. Table 4 summarizes the volumes of sand and Fe0 to be taken to build six different
reactive layers. In each case the corresponding masses should be documented as well as the effective
depth of the reactive zone. The pure Fe0 (100%) reactive zone is only to be tested for porous (e.g.,
foam/sponge) and filamentous (e.g., steel wool) materials.
Sustainability 2017, 9, 1224  11 of 31 
at a community level. Steel wool is selected for its worldwide availability [25,131] and the rapid kinetics of 
its corrosion [127,128]. 
The illustrative RSM model herein will discuss the effects of Fe0 type (four samples), the nature of the 
contaminants (As, bacteria, F, U) (four species), flow rate (0.2 to 2.4 mL/min) (five values), bed height (10 to 
70 cm) (5 values), and initial fluoride concentration (2 to 15 mg/L) (five values) on the efficiency of Fe0 filters. 
6. Testing Steel Wool as a Starting Fe0 Material 
To illustrate the design practice of Fe0 filters, a procedure to systematically test a system based on steel 
wool is discussed in this section. Three cylindrical columns (Column 1 through 3) with a diameter (D) of 15 
cm a d a height (H) of 100 cm are used (Figure 2). The bed volume of this filter is 17.7 L (V = π × D2 × H/4). 
The first and third columns are f lled with f ne san  (d < 0.4 mm). The first (Colum  1) i  basically a 
conventional BSF, whereas the second (Column 3) fixes dissolved Fe from the Fe0-based column (in situ 
coating) to optimize decontamination. Column 2 contains a 70 cm thick reactive layer sandwiched between 
two layers of fine sand. Accordingly, the volume of the reactive layer is 12.4 L. Table 4 summarizes the 
volumes of sand and Fe0 to be taken to build six different reactive layers. In each case the corresponding 
masses should be documented as well as the effective depth of the reactive zone. The pure Fe0 (100%) 
reactive zone is only to be tested for porous (e.g., foam/sponge) and filamentous (e.g., steel wool) materials. 
 
Figure 2. Concept of water treatment train based on filtration on granular Fe0 (Fe0/sand) and including at 
least one sand filter. The sand filter is either a roughing filter or a biosand filter. Treated water is stored for 
distribution.  
Table 4. Guide for the implementation of a 70 cm thick reactive zone. The apparent volume occupied by the 
Fe0 material is used as reference and the actual volume of the reactive zone should be documented. The 
corresponding masses of sand and Fe0 should be documented as well. 
Fe0 Viron Vsand miron msand Fe0 
(v. %) (L) (L) (g) (g) (w. %) 
100.0 12.4 0.0 x0 0.0 100.0 
75.0 9.3 3.1 x1 y1 z1 
50.0 6.2 6.2 x2 y2 z2 
25.0 3.1 9.3 x3 y3 z3 
10.0 1.2 11.1 x4 y4 z4 
5.0 0.6 11.8 x5 y5 z5 
6.1. Natural Water as a Complex Design Parameter 
Natural waters are contaminated and eventually polluted due to three main inherent processes [5,20]: 
(i) atmospheric particle dissolution, (ii) rock weathering, and (iii) soil leaching. Increasing human 
population, industrialization, and the use of fertilizers and manufactured materials (including metal-based 
Figure 2. Concept of water treatment train based on filtration on granular Fe0 (Fe0/sand) and including
at least one sand filter. The sand filter is either a roughing filter or a biosand filter. Treated water is
stored for distribution.
Table 4. Guide for the implementation of a 70 cm thick reactive zone. The apparent volume occupied by
the Fe0 material is used as reference and the actual volume of the reactive zone should be documented.
The corresponding masses of sand and Fe0 should be documented as well.
Fe0 Viron Vsand miron msand Fe0
(v. %) (L) (L) (g) (g) (w. %)
100.0 12.4 0.0 x0 0.0 100.0
75.0 9.3 3.1 1 y1 z1
50.0 6.2 6.2 x2 y2 z2
25.0 3.1 9.3 x3 y3 z3
10.0 1.2 11.1 x4 y4 z4
5.0 0.6 11.8 x5 y5 z5
6.1. Natural Water as a Complex Design Parameter
Natural waters are contaminated and eventually polluted due to three main inherent processes [5,20]:
(i) atmospheric particle dissolution, (ii) rock weathering, and (iii) soil leaching. Increasing human
Sustainability 2017, 9, 1224 12 of 32
population, industrialization, and the use of fertilizers and manufactured materials (including
metal-based ones) have worsened water pollution due to weathering and leaching. It is necessary
that the quality of any water source (e.g., lake, river, well) is checked to establish its (drinking) quality.
Irrespective of the presence of any toxic contamination (pollution), details about conventional
physico-chemical parameters should be documented. These parameters include acidity (pH value),
alkalinity (HCO3−), chloride (Cl−), color, dissolved organic carbon (DOC), dissolved oxygen (O2),
electrical conductivity (EC), hardness (Ca2+, Mg2+), nitrate (NO3−), phosphate (PO43−), potassium
(K+), sodium (Na+), sulfate (SO42−), temperature, and turbidity. Although the enumerated parameters
are not mutually exclusive, it is seen that up to 14 parameters are necessary to characterize a water
source. The relevance of the measurement of individual parameters arises from the evidence that
measured EC values give an idea of the salinity of the water but no idea about the actual nature of
dissolved ions (co-solutes).
This sub-section recalls that each water body is unique in its nature [157,158] and that the water
quality is not limited to the nature and extent of contamination. In the context of using Fe0 for water
treatment, a pollutant (e.g., As, F, U) is just one of the operational parameters capable of influencing
iron corrosion by (i) enhancing/inhibiting the kinetics of Fe0 oxidative dissolution, (ii) influencing the
process of oxide scale formation, (iii) influencing the ionic conductivity of the oxide scale, and (iv)
influencing the porosity/permeability of the oxide scale [159–165].
Apart from the pH value, it is not yet established whether selected parameters (including the
nature and extent of contamination) are more significant than others for the process of contaminant
removal in Fe0/H2O systems. Accordingly, designing Fe0 filters based solely on the extent of
contaminant removal at selected initial pH values is a highly qualitative task. Natural waters do contain
different types of dissolved, suspended, and microbiological contaminants. Suspended contaminants
are successfully removed in roughing filters [22]. To obtain reliable design criteria for Fe0 filters, the
extent to which inherent water contents influence iron corrosion should be established. Only once this
task is accomplished can rules of thumb for site-specific design be developed.
6.2. Design Criteria
The principal design criteria for Fe0 filters are (alphabetically): (i) depth of the reactive zone,
(ii) extent (and nature) of water contamination (including dissolved O2 and co-solutes) (Section 2),
(iii) Fe0 intrinsic reactivity, (iv) media depth and size (all aggregates including Fe0; number of columns),
(v) proportion of Fe0 within the reactive zone, (vi) required treatment level, and (vii) water flow
velocity (residence time). The performance of the filter is assessed by monitoring changes in the
(i) concentrations of relevant species including contaminants and iron, (ii) hydraulic conductivity
(permeability), (iii) pH value, and (vi) electrical conductivity.
6.3. Preparing and Implementing Media in Fe0 Filters
Sand is first sieved through a series of sieves to be separated into its different grain sizes. This
operation is essential because the filtration rate is influenced by grain size and the grain size distribution.
Only the fraction <2.0 mm is used. The fraction 0.2 to 0.4 mm can be used for the BSFs (columns
1 and 3, Figure 1), the fraction 1.0 to 2.0 to support the reactive zone (column 2, Figure 1), and the
fraction 0.4 to 1.0 mm used in the reactive zone. Sieved sand is then washed to remove fine silt, clay,
and other impurities that the media may contain. Store the washed sand in a protected dry area away
from possible human or animal contamination.
Fe0 is also sieved and the fraction lower than 2.0 mm used. Filamentous steel wool is chopped into
pieces less than 2 cm in length. It should be ensured that the Fe0 does not contain grease and/or toxic
species (perform appropriate cleaning/washing, if applicable). Table 5 summarizes the characteristics
of commercial steel wool that should be systematically tested in long-term experiments. It is obvious
that homogeneously mixing fine grade steel wool with sand will be a difficult task in rural conditions.
Sustainability 2017, 9, 1224 13 of 32
Accordingly, only coarser materials, for instance with widths >50 µm, will be suggested for testing
(four materials: 60, 75, 90, and 100 µm).
Table 5. The eight typical levels of abrasiveness (grade) of steel wool and their width. The grade
is determined by the thickness of the wire of which the wool is made of. Stainless steel wool is not
considered herein. (www.steelwooldirect.com).
Grade 0000 000 00 0 1 2 3 4
Name Finest Extra Fine Fine Medium Fine Medium Medium Coarse Coarse Extra Coarse
d (Inch) 0.0010 0.0015 0.0018 0.0020 0.0025 0.0030 0.0035 0.0047
d (mm) 0.03 0.04 0.04 0.05 0.06 0.08 0.09 0.10
d (µm) 25 35 40 50 60 75 90 100
6.4. Simulating Testing Practice: RSM Model
In this section, the statistical framework of experiments is simulated to understand the
methodology of designing a filter. As the effective parameters influencing filter design according to
concerning dilemma may vary from place to place and from one investigator to another, the parameters
are simplified according to the following considerations. Section 6.1 listed 14 parameters relevant for
water quality (Table 6); Section 6.2 listed seven principal design criteria (see also Table 7) and three
major monitoring parameters. Section 6.3 suggested four steel wool materials to be tested. From the
design criteria, the intrinsic reactivity of Fe0 (steel wool in Table 5 or kEDTA value in Table 6) [94], the
media depth and size, the proportion of Fe0 within the reactive zone (Table 4), the required treatment
level (e.g., WHO guidelines), and the initial water flow velocity (mediated by ∆H in Figure 1) can be
considered as fixed. The nature and extent of water contamination are also fixed for any specific case
(Section 6.1).
As shown in Table 6, many variables as well as their interactions might influence the design
of an efficient Fe0 filter. For this reason, the conditions need to be optimized. Unfortunately, the
conventional methods for optimization are “one factor at a time” approaches that frequently fail to
identify the variables that give rise to the optimum response because the effects of factor interactions are
not taken into account in such procedures. These procedures are time-consuming and require a large
number of experiments. They are also incapable of reaching truly optimal conditions due to ignoring
such interactions among variables [166–168]. In order to overcome these problems, a multivariate
statistical design approach could be adopted through RSM, which is a multivariate statistical tool that
uses quantitative data from appropriate experiments to solve multivariate equations.
Table 6. Initial variables for the design of Fe0 filters. Temperature and turbidity are not considered.
Based on a single Fe0 material (kEDTA value), it is seen that more than 45 variables should be considered
to design a filter.
No. Parameter (x) Unit Variables Comments
1 Acidity (pH value) - 6.0, 7.0, 8.0, 9.0 pH of natural waters
2 Akalinity (HCO3−) mg/L 5, 10, 20, 35, 50 no
3 Dissolved oxygen (O2) mg/L 1.0, 2.0, 4.0, 6.0, 8.0 Relevant natural waters
4 Hardness (Ca2+, Mg2+) mg/L 5.0, 7.5, 10.0, 15.0, 20.0 Relevant natural waters
5 Electrical conductivity (EC) µSv/cm 50, 150, 300, 450, 200 Relevant natural waters
6 Depth of the reactive zone (RZ) cm 10, 20, 30, 50, 70 For a 100 cm length column
7 Extent and nature of contamination µM water quality Including co-solutes
8 Initial concentration of fluoride mg/L 1.0, 4.0, 8.0, 12.0, 20.0 As example for the RSM model
9 Fe0 intrinsic reactivity kEDTA n.a. A standard approach is missing
10 Fe0 proportion within the RZ % 5, 10, 25, 40, 60 Higher values not to be tested
11 Nature of aggregates (e.g., Fe0, sand) n.a. n.a. Inert or reactive; non expansive
12 Required treatment level MCL Requested water quality Not considered in RSM model
13 Size of Fe0 µm 250, 500, 1000, 2000 The diameter is meant
14 Size of other aggregates (e.g., gravel, sand) mm 0.25, 0.50, 1.0, 2.0, 4.0 The diameter is meant
15 Water flow velocity mL/min 0.5, 1.0, 2.5, 5.0, 10.0 Fixed by the daily need
Sustainability 2017, 9, 1224 14 of 32
Table 7. Summary of the operational factors retained for the RSM modeling. The parameters selected
are the most globally monitored for the quality of natural waters.
No. Parameter Minimum Maximum
1 Nature of contaminant (e.g., fluoride) 1 20
2 (%) Fe0 volumetric proportion 5 50
3 Hydraulic conductivity (mL/min) 0.5 10
4 (−) pH value 6 9
5 Electrical conductivity (µS/cm) 50 200
Compared to conventional optimization methods, RSM is introduced as an economic and
time-saving instrument because it can provide more information from fewer experiments [169]. In
addition, the estimates of the effects of each factor are more precise, the interaction between factors
can be estimated systematically, and there is experimental information in a larger region of the factor
space, which improves the prediction of the response in the factor space by reducing the variability of
the estimates of the response [170]. In addition, the main aim of RSM is to find the optimal response,
and it has been widely used to describe the interactive and synergistic effects among experimental
variables as well as to work on the optimization of operation conditions [171–174].
Over the last few decades, many researchers have been using various design of experiment
(DoE) techniques in RSM including two-level full factorial design (FFD) [175], Box–Behnken design
(BBD) [176], and Central Composite Design (CCD) [177] to predict the ultimate response. CCD was
first studied by Box and Wilson in 1951 and is still the most popular design for experiments. CCD has
three groups of design points that are codified to summarize the data and ease of statistical calculations:
(i) two-level factorial design points (2k), consists of all possible combinations of the +1 and −1 codified
levels of the factors, (ii) axial or star points (2k) codified as (α = (2k)1/4) have all of the factors set to 0,
the midpoint, except one factor, which has the value ± α., and (iii) center points are points with all
levels set to coded level 0 [178].
The total number of experiments is calculated by the following equation:
N = 2k + 2k + nc, (3)
where k is the number of factors and nc the number of central points, which is calculated by the
following equation:
nc = (
√
nF + 2)
2 − nF − 2k, (4)
where nF is the number of factorial points and k is calculated by the following equation:
k =
(
(k + 3) +
(
9k2 + 14k− 7
) 1
2
)
/(4(k + 2)) (5)
The methodology mainly involves a quadratic model to explain the behavior of the system. This
model is flexible and covers all linear, non-linear, and interaction effects between the factors [179]. The
quadratic polynomial equation is as follows:
y = β0 +∑ki=1 βixi +∑
k
i=1 βiix
2
i +∑
k
i=1∑
k
i 6=j 6=1 βijxixj + ε, (6)
where y is the predicted response, β0 is the offset term, βi is the ith linear coefficient, βii is the quadratic
coefficient, βij is the ijth coefficient, and ε is the error or residual value. Solving this equation and
calculating the coefficients are done by using the least squares method. The fitness of the given
models can be tested by ANOVA statistics (R2, adjusted R2, F-test and t-test) and residuals analysis.
As Bezera et al. [180] introduced RSM as a tool for optimization in analytical chemistry, this kind of design
experiment has been widely used by different researchers in order to optimization of water and soil
remediation [166,173,181–183] or designing different types of water filters [184,185]. The experimental
design and statistical analysis of the data were done by Design-expert10 statistical software.
Sustainability 2017, 9, 1224 15 of 32
In this study, the design of experiments is simulated by considering five factors; the total number
of experiments will be 32, which is more economic and time-saving compared to 50 for a full-factorial
design (Table 8).
Table 8. Central composite design experiments. Φ = Hydraulic conductivity.
StdOrder RunOrder PtType Blocks
[F−] [Fe0] Φ pH EC
(mg/L) (vol. %) (%) (−) (µS/cm)
1 1 1 1 5.75 16.25 2.875 6.75 162.5
2 2 1 1 15.25 16.25 2.875 6.75 87.5
3 3 1 1 5.75 38.75 2.875 6.75 87.5
4 4 1 1 15.25 38.75 2.875 6.75 162.5
5 5 1 1 5.75 16.25 7.625 6.75 87.5
6 6 1 1 15.25 16.25 7.625 6.75 162.5
7 7 1 1 5.75 38.75 7.625 6.75 162.5
8 8 1 1 15.25 38.75 7.625 6.75 87.5
9 9 1 1 5.75 16.25 2.875 8.25 87.5
10 10 1 1 15.25 16.25 2.875 8.25 162.5
11 11 1 1 5.75 38.75 2.875 8.25 162.5
12 12 1 1 15.25 38.75 2.875 8.25 87.5
13 13 1 1 5.75 16.25 7.625 8.25 162.5
14 14 1 1 15.25 16.25 7.625 8.25 87.5
15 15 1 1 5.75 38.75 7.625 8.25 87.5
16 16 1 1 15.25 38.75 7.625 8.25 162.5
17 17 −1 1 1.00 27.50 5.250 7.50 125.0
18 18 −1 1 20.00 27.50 5.250 7.50 125.0
19 19 −1 1 10.50 5.00 5.250 7.50 125.0
20 20 −1 1 10.50 50.00 5.250 7.50 125.0
21 21 −1 1 10.50 27.50 0.500 7.50 125.0
22 22 −1 1 10.50 27.50 10.000 7.50 125.0
23 23 −1 1 10.50 27.50 5.250 6.00 125.0
24 24 −1 1 10.50 27.50 5.250 9.00 125.0
25 25 −1 1 10.50 27.50 5.250 7.50 50.0
26 26 −1 1 10.50 27.50 5.250 7.50 200.0
27 27 0 1 10.50 27.50 5.250 7.50 125.0
28 28 0 1 10.50 27.50 5.250 7.50 125.0
29 29 0 1 10.50 27.50 5.250 7.50 125.0
30 30 0 1 10.50 27.50 5.250 7.50 125.0
31 31 0 1 10.50 27.50 5.250 7.50 125.0
32 32 0 1 10.50 27.50 5.250 7.50 125.0
7. Discussion
7.1. Calling for Co-Development
Slow sand filters (SSFs), as first built in Scotland by John Gibb of Paisley in 1804, are the
conventional form of filtration for safe drinking water provision [98,157]. Biosand filters (BSFs)
are an adaptation of traditional SSFs for water treatment at the household and small community levels.
BSFs mostly remove pathogens and unpleasant taste from water by virtue of biological processes taking
place in a sand column ‘covered’ with a biofilm (termed as Schmutzdecke). These filtration systems
have no specific vocation for the removal of chemical contaminations (e.g., As, F, U) [22,26,30,186].
A particular feature of BSFs is that dissolved oxygen is consumed in the upper layer of the filter,
some 10 cm under the resting water (Schmutzdecke). The BSF is recommended by the World Health
Organization (WHO) for decentralized water treatment in the developing world. Efforts to optimize
the efficiency of biosand filters are undergoing [30,57,186–192].
An established way to improve the efficiency of a BSF is to amend it with metallic elements
(Al0, Fe0, Zn0) [126,130,131,189,193–195]. Depending on the intrinsic reactivity of the used Fe0, the
extent of water contamination, the size of the filter, and water flow velocity, a relative small volumetric
Fe0 ratio (e.g., <10%) could yield efficient long-term water treatment. Even in the case of larger Fe0
ratios, the sustainability of the system is proven by the fact that the Fe0 layer works under anoxic
Sustainability 2017, 9, 1224 16 of 32
conditions as O2 is consumed in the BSF [30,80,81]. In other words, the combination BSF/Fe0 filter
promises sustainability. Given that (i) Fe0 characterization [92], (ii) Fe0 design [29], and (iii) BSF
construction [187,190] are all frugal in nature, the science of universal safe drinking water provision
using Fe0 materials is established. Thus, systematic material characterization and site-specific design
would complete the studies needed for implementation.
Arguably, this work can be performed anywhere in the world. The originality of the approach is
that a global concept is presented that can be applied to specific contexts. The scientific or technical
experts need to be informed on local priorities, needs, and opportunities [10]. These local conditions
are perfectly known to water professionals who are now collaborating with (international) researchers,
for example in the framework of an established consortium. There are several effective ways for
individuals to coordinate their special skills and expertise to meet the challenge posed by the UN
SDGs. Two examples from Germany include (i) the short- and long-term appointments from German
Academic Exchange Service (DAAD) and (ii) support by programs of the German Federal Ministry of
Education and Research (BMBF) that focus on sustainable development.
Herewith scientists are called to collaborate with water professionals in the field to fill the pressing
gaps to achieve universal safe drinking water provision. The same technology will be modified for
sanitation for a healthier world.
7.2. The Science of the Universality of Fe0 Filters
Methods to tackle water pollution are based on biological, chemical, physical, and thermal
principles. Methods based on thermal principles include boiling, distillation, and pasteurization.
The most important are [17]: adsorption, boiling, coagulation, crystallization, distillation, electrolysis,
evaporation, filtration, flotation, ion exchange, oxidation, pasteurization, precipitation, reduction,
reverse osmosis, sedimentation, and solvent extraction. Relevant oxidation and reduction processes are
biological, chemical, or electrochemical in nature. From the listed technologies, filtration on packed-beds
of adsorbents (adsorptive filtration) and reverse osmosis are considered as the best [16,52,147]. Among
them, adsorptive filtration is the most applicable due to its ease of operation and is also considered
a universal water treatment technology [16].
The scientific rationale for the universality of adsorptive filtration is that chemical methods (e.g.,
oxidation, precipitation, reduction) are collectively limited by the equilibrium constant. Reasoning
on the case of a simple salt (AB), which is dissolved to An+ and Bn−, the equilibrium constant (Ks) is
given by Equation (7):
Ks = [An+] × [Bn−]. (7)
If An+ is the pollutant, the water is satisfactorily treated by pure chemical precipitation (not
co-precipitation) only if [An+] = [Ks]1/2 is less than the maximum contamination level (MCL) for the
species A. It is well documented that this is rarely the case [27,196,197]. For this reason, as far as safe
drinking water is concerned, chemical methods alone are not satisfactory. Their performance should
be enhanced by at least one filtration step. The suitability of each filtration technology depends on
the adsorbing material and its affinity for the pollutant. This makes adsorptive filtration necessarily
selective and thus multi-barriers are applied to treat multi-contaminated water sources [18,198]. Note
that, in coagulation and flotation, adsorbents are formed in situ to accumulate contaminants (e.g.,
An+) and the contaminant-laden flocs (precipitates) are eliminated during subsequent filtration by
size exclusion.
The suitability of Fe0 filters to treat multi-contaminated waters (as a stand-alone solution) arises
from the fact that two non-selective mechanisms are involved in the removal process: co-precipitation
and size-exclusion (beside selective adsorption) [26,52,132]. Noubactep and colleagues [199–201] have
demonstrated that the dynamic nature of the process of in situ generation and precipitation of iron
oxides implies that colloidal forms remove species without affinity to iron oxides to an unpredictable
extent. For this reason, upon proper design, Fe0 filters will be efficient at removing even very small
species that would have needed high pressure (electricity) to be eliminated by reverse osmosis. Thus,
Sustainability 2017, 9, 1224 17 of 32
well-designed Fe0 filters certainly ameliorate the biological and chemical quality of natural water,
which makes Fe0 filters a technology of choice in contexts where analytical instrumentation is lacking.
However, universal analytical quality control should remain the goal [28].
7.3. Fe0 Filters for Pathogen Removal
The biocidal property of Fe0 was discovered and applied in Western Europe as early as the 19th
century, when the outbreak of disease was connected to water quality [33,122,123]. Fe0 was shown
effective at turning even sewage water potable within an appreciably short contact time ([33] and
refs. cited therein). Interestingly, research over the past 15 years has demonstrated that Fe0 filters are
a cost-effective option helping utilities meet their multiple water treatment goals [142–146,202–205].
Pilot studies have confirmed the potential achievement of this goal [206]. Accordingly, the question of
whether Fe0 filters are efficient at removing pathogenic contamination from water is clearly answered
in the affirmative. However, the design question remains as for chemical contamination (Section 4):
how to evaluate the long-term performance of Fe0 filters while taking into account the effects of relevant
operational variables? Operational variables include the presence of natural organic matter (NOM)
and the pH value. Answering these two questions would enable researchers to (i) design sustainable
Fe0 filters for actual water treatment plants and (ii) understand changes in filter performance over
time, as affected by the water chemistry.
The discussion on the operating mode of Fe0 filters for pathogen removal is ongoing despite
two important facts [207–211]: (i) Bojic et al. [130] have demonstrated that producing metal oxides
from a Fe-micro-allowed Al0 is an efficient tool to remove bacteria from water, and (ii) You et al. [194]
have demonstrated that Fe0 filters remove viruses from water. Because Fe0 filters also remove NOM,
they automatically help prevent the formation of disinfection byproducts (DBPs). This knowledge
is an independent proof of the efficiency of Fe0 filters as a powerful, stand-alone, non-oxidant-based
technology to control the microbial contamination of water. Using Fe0 filters would additionally avoid
the challenges of (i) financing oxidant-based technologies and (ii) controlling DBPs. In other words,
Fe0 filters are compact devices helping us solve a daunting challenge facing water utilities: controlling
microorganisms without dealing with DBPs or residual disinfectant level. In addition, they are more
affordable and easy to install and operate.
7.4. Spent Media: Dispose or Recycle?
The presentation herein implies that spent materials from Fe0 filters are a matrix of (i) residual
Fe0 and (ii) iron oxyhydroxides entrapping contaminants. The first piece of good news is that
quantitative contaminant release from spent material under environmental conditions is not likely as
iron oxyhydroxides are not dissolved by natural waters [54,212,213]. The second piece of good news is
that iron oxyhydroxides are the raw material for iron making [214]. These key characteristics of the
element Fe and its water chemistry make Fe0 filters a green technology. Additionally, contaminant
recovery from the spent material is possible and should be investigated on a case-by-case basis. Thus,
it is essential to take the aspect of ‘contaminant recovery’ into consideration in the design equation. In
essence, this recovery aspect is not new, as Tseng et al. [127] used steel wool [Fe0] to ‘adsorb’ cobalt-60
(60Co) from sea water some 35 years ago [28]. 60Co was then recovered from corrosion products and
analytically determined.
Properly disposing of spent materials is always possible, as well as for toxic pollutants, including
radioactive ones. However, recycling is basically only universally possible for organic pollutants
as they will be burnt off during heating. For inorganic pollutants the decision will be taken on
a case-by-case basis but the recovery of the pollution should make the residual material recyclable.
Intensive research is needed on all involved aspects. However, given the urgency of meeting the
UN SDGs, efficient filters are needed and spent materials can be either disposed of or stocked until
a recovering technology is made available.
Sustainability 2017, 9, 1224 18 of 32
7.5. Limitations of Fe0 Filters
Fe0 has a high potential to induce the removal of various aqueous contaminants by adsorption,
co-precipitation, and size exclusion; as recalled herein, a systematic investigation will enable the
successful implementation of Fe0 filters, which should include recycling filters’ wastes and recovering
pollutants. The most critical factors are discussed in Section 6. It appears that Fe0 filtration technology
has no real limitations. No chemical additives are needed (chemistry free), no sophisticated accessories
are needed (simplicity), no electrical power is needed (energy-free), installation and maintenance can
be performed by the owner (household or community), and the design is demand-oriented (e.g., the
size of the community). The evidence that Fe0 filters are prone to fouling will be tackled by increasing
the frequency of unit replacement or using short-living Fe0-materials. All these reasons make Fe0
filters the ideal solution for developing countries as there is no constraint for implementation other
than locally available materials and manpower.
The culture of filter maintenance and Fe0 regeneration will be established in an intern dynamic.
The technical viability (proof of concept) [26,121–126,215–221] and economic feasibility are already
demonstrated [54,201]. The environmental and social acceptance of the technology in the community
can be considered a given as the technology is developed locally. However, advice and knowledge
transfer can be acquired in collaboration with other groups, including scientific ones. Considering the
significant advancements made in research on ‘Fe0 for environmental remediation and water treatment’
over the past three decades [26–30,52,76,116,146,222–229] as the cornerstone for a universal sustainable
solution to a long-lasting crisis, the universal provision of safe drinking water is no longer an ‘elusive
goal’ [7]; Fe0 filtration provides a solution, specifically for the developing world [229,230].
8. Design Your Fe0 Filter
Fe0 filters for households and small communities are presented as a further development of the
biosand filter (BSF) [26–30,56,111–113,229]. BSF is neither standardized nor patented [22,111,230]. The
BSF is considered one of the most promising household water treatment methods but the demonstration
of its suitability and efficiency is impaired by two key factors: (i) The quality of treated water is not
routinely analytically assessed and (ii) overarching designing or manufacturing and quality control
guidelines do not exist. The challenge of accessing equipment for water analysis as a cornerstone for
achieving the UN SDGs is obvious [11,28]. This section summarizes the efforts achieved in designing
consistently high-quality Fe0 filters. If an open collaboration in designing Fe0 starts now, it will be easy
to identify areas where manufacturing and quality control guidelines are needed. The presentation is
limited to three key issues: (i) The nature of used Fe0 materials, (ii) the design of the treatment units,
and (iii) quality control practices.
8.1. Nature of Fe0 Materials
Available studies suggest that improper Fe0 selection can affect the performance of locally
designed Fe0 filters to the point where their ability to produce safe drinking water is compromised.
The historical example of the Kanchan arsenic filter [111,112] is well documented [30]. The variability
in the efficiency of designed filters implies that the iron nails used were of different intrinsic reactivity.
Smith et al. [229] recently presented a comparison of two different iron-nail-based Fe0 filters for
arsenic removal in China: (i) the original Kanchan arsenic filter (SONO-style) and (ii) a modification
in which the nails are embedded in the upper sand layer (before the fine sand layer). The filter with
embedded nails was found to be more efficient. However, this result is only qualitative as different
iron nails would show different results and there is no tool to correlate the obtained results to those
of Smith et al. [229]. During the last 20 years, sporadic efforts to characterize the intrinsic reactivity
of Fe0 materials have been presented [83,84,90,91,93,94,115,116,224]. However, only the methods of
Reardon [90] (H2 evolution) and Noubactep (Fe0 dissolution in EDTA) [84] are really simple and
affordable. Fe0 dissolution in EDTA further presents the advantage that it used very small amounts of
Sustainability 2017, 9, 1224 19 of 32
materials and lasted for just four days. Recently, Btatkeu et al. [45,92] further developed the EDTA
method and demonstrated an excellent correlation of kEDTA values and the extent of methylene blue
discoloration from aqueous solutions. Accordingly, the routine determination of kEDTA values is
proposed as a tool to characterize the intrinsic reactivity of Fe0 materials and ease discussion of results
from various sources.
8.2. Design of Fe0 Treatment Units
A starting rule of thumb for designing Fe0 could be: “Design an efficient BSF and add a Fe0-based
unit (new filter or a reactive zone in the BSF) [30]. Then increase the water flow velocity and control
the biological and chemical quality of the filtered water.” Accordingly the reference system is a BSF
operating under the same conditions. By altering the current filter design, it will be possible to identify
the range of parameters that can be changed while still having an improved BSF filter. It is essential to
recall that the experiments should last for at least six months. Results obtained at lab scale demonstrate
the viability of the system and could be directly used to design household Fe0 filters. These results are
the cornerstone for the design of pilot tests for scalable community treatment plants. However, lab
and current pilot tests are lasting just for a few weeks or months [67,231–233]. This is not acceptable
given the non-linearity of the kinetic of iron corrosion [89].
8.3. Assessing the Efficiency of Fe0 Filters
Designing an efficient Fe0 filter could be regarded as ascertaining whether the flow rate of a BSF
could be increased after the addition of a Fe0 unit without sacrificing filter effectiveness in terms of
pathogen removal while inducing satisfactory chemical decontamination. Thus, once the specific
design variables are identified, the task remaining is to monitor the quality of filtered water as result of
the performed alteration.
The research method can be summarized as: “Take a BSF as reference system and create new
filter designs by changing one of relevant design variables (Section 5). Then produce the new designs
(e.g., in triplicate along with three control BSF). Finally, pass the model solution or the local natural
water through the filters daily, and test the filtrates once a week (or twice a month) for at least six
months.” Original and filtrated natural waters should be tested for electrical conductivity, flow rate,
iron concentration, major ions (Ca2+, K+, Mg2+, Na+, Cl−, HCO3−, PO43−, SO42−) pH value, total
coliforms (TC), and turbidity. Additionally, natural waters should be monitored for arsenic, fluoride,
and uranium, which have been revealed as global killers.
The large variability in water sources, Fe0 materials, and design options raises concerns about the
consistency and quality of locally produced Fe0 filters, especially in the absence of standardized
quality control procedures. If the approach presented herein is adopted widely, areas where
designing/manufacturing guidelines are needed would soon be identified and this would contribute
to consistently high-quality Fe0 filters. The next logical step is to identify areas where further research
is needed to refine design recommendations. A significant goal can be to guide the development
of a best-practice manual that describes science-based recommendations for quality-controlled Fe0
filters worldwide.
8.4. Local Candle Fe0 Filters
8.4.1. General Aspects
Candle-style Fe0 filters (Figure 3) can be locally manufactured, based on local knowledge. Using
locally affordable materials would ensure the affordability of the resulted filters. One key benefit is
the ability to adjust efficiency by manipulating the number of candles in each filtration system. It is
understood that such candle filters may require plastic parts and some type of adhesive that are not
yet locally available. Nevertheless, the candle Fe0 filter would still be an appropriate solution because
resulting problems are related to leakage and/or replacing broken parts and not to water treatment.
Sustainability 2017, 9, 1224 20 of 32
Moreover, the introduction of such candles would galvanize the development of the market for such
additional materials or even their local production in the short/long term. Once locally designed and
produced candle Fe0 filters are established, their efficiency would be improved in a locally induced
development dynamic. Clearly, the comparison of first-generation local Fe0 candles with commercial
filters does not give a fair sense of their suitability. All that is needed is (i) the readiness to improve
own systems with a systematic approach, (ii) an initially efficient system, and (iii) a clear indication for
users as to when to replace the candle.
It is essential to recall that ‘old’ systems like BSF or ceramic filters are still in
development [186,190–192,234,235]. Accordingly, being a ‘complication’ of BSF, many design-related
topics are yet to be addressed in further research efforts. Understanding some of the specific causes
of Fe0 filter failures will help to optimize the cost-effectiveness of the Fe0 filtration technology as
a whole [26]. Long-term studies are needed to warrant sustainable Fe0 filters. Finally, locally
manufactured candle Fe0 filters could encourage the recycling of filter wastes.
Sustainability 2017, 9, 1224  20 of 31 
local production in the short/long ter . Once locally designed and produced candle Fe0 filters are 
establish d, their efficiency would be improved in a locally induc d development dynamic. Clearly, the 
comparison of first-generation local Fe0 candles with commer ial filters does ot give a fai  sense of their 
uitabili y. All that is need d is (i) the readiness to improve own systems with a systematic approach, (ii) 
an initially efficient system, and (iii) a clear indication for users as to when to replace the candle.  
It is essential to recall that ‘old’ systems like BSF or ceramic filters are still in development [186,190–
192,234,235]. Accordingly, being a ‘complication’ of BSF, many design-related topics are yet to be addressed 
in further research efforts. Understanding some of the specific causes of Fe0 filter failures will help to 
optimize the cost-effectiveness of the Fe0 filtration technology as a whole [26]. Long-term studies are needed 
to warrant sustainable Fe0 filters. Finally, locally manufactured candle Fe0 filters could encourage the 
recycling of filter wastes. 
 
Figure 3. Schematic representation of three candle-type filters. A candle is either a container like in a Brita 
filter, a bucket like in a n-Kolshi filter (a,c) or simply the hollow portion of a domestic candle filter (filled 
with adsorbent materials) (b). 
8.4.2. Design Aspects 
Using Fe0 materials in domestic candle-type filters has been experimentally tested but not used in 
practice for the reasons elucidated herein (Section 2). In essence, the n-Kolshi systems (n = 2 or 3), largely 
tested in Bangladesh and Nepal, are candle Fe0 filters as they can be regarded as “candles” whose hollow 
parts (here a Kolshi) are filled with Fe0. For example, [73] tested cast-iron filings and steel wool (SW) in 
domestic candle-type filters. Due to the large difference in density, the tested candle could contain 176 g of 
fillings and just 47 g of SW. As breakthrough (initial concentration: 300 μg/L) was observed after 6.0 L in 
the fillings filter and only 1.5 L in the SW filters (per run of intermittent filtration). The authors speculated 
that using three candles in series would enable the treatment of 18.0 and 4.5 L of water per filtration run, 
respectively. As demonstrated herein (see also [56]), this speculation is not acceptable as the kinetics of Fe0 
corrosion is not linear. Moreover, the candles were completely filled with Fe0 (100%), meaning that even 
with a supposedly linear rate of iron corrosion, permeability loss will soon occur and at different extents 
for individual materials [80,81]. This example demonstrates that only long-term systematic experiments will 
determine the feasibility of Fe0 candle filters for water treatment at the household level. While it is certain 
that candle filters will be efficient, the main issue to address is the Fe0 selection and its proportion in the 
candle. The service life of the candle is to be determined experimentally until enough data is available for 
the establishment of some correlations (e.g., long-term reactivity). The next section specifies some key issue 
for sustainable candle Fe0 filters. 
8.4.3. Sustainability Aspects 
It is frustrating to observe that even recent reports are not unanimous as to whether candle-type Fe0 
filters are viable or not [56,69]. In essence, a newly designed Fe0 filter can be compared to a highly permeable 
sand filter, which is characterized by a high level of interconnectivity between the inter-granular voids 
(pores). Once the corrosion process starts, the pores are progressively filled by iron corrosion products 
Figure 3. Schematic representation of three candle-type filters. A candle is either a container like in
a Brita filter, a bucket like in a n-Kolshi filter (a,c) or simply the hollow portion of a domestic candle
filter (filled with adsorbent materials) (b).
8.4.2. Design Aspects
Using Fe0 m terials in omestic candle-type filter has been experimentally tested but not used
in practice for the reasons elucidated herein (Section 2). In essence, the n-Kolshi systems (n = 2 or 3),
largely tested in Bangladesh and Nepal, are candle Fe0 filters as they can be regarded as “candles”
whose hollow parts (here a Kolshi) are filled with Fe0. For example, [73] tested cast-iron filings and
steel wool (SW) in domestic candle-type filters. Due to the large difference in density, the tested candle
could contain 176 g of fillings and just 47 g of SW. As breakthrough (initial concentration: 300 µg/L)
was observed after 6.0 L in the fillings filter and only 1.5 L in the SW filters (per run of intermittent
filtration). The authors speculated that using three candles in series would enable the treatment of
18.0 and 4.5 L of water per filtration run, respectively. As demonstrated herein (see also [56]), this
speculation is not acceptable as the kinetics of Fe0 corrosion is not linear. Moreover, the candles ere
completely filled with Fe0 (100%), meaning that even with a supposedly linear rate of iron corrosion,
perme bility loss wil soon o cur and at different extents for individual materi ls [80,81]. This example
demonstrates that only long-term systematic experiments will determine the feasibility of Fe0 candle
filters for water treatment at the household level. While it is certain that candle filters will be efficient,
the main issue to address is the Fe0 selection and its proportion in the candle. The service life of the
candle is to be determined experimentally until enough data is available for the establishment of some
correlations (e.g., long-term reactivity). The next section specifies some key issue for sustainable candle
Fe0 filters.
Sustainability 2017, 9, 1224 21 of 32
8.4.3. Sustainability Aspects
It is frustrating to observe that even recent reports are not unanimous as to whether candle-type
Fe0 filters are viable or not [56,69]. In essence, a newly designed Fe0 filter can be compared to
a highly permeable sand filter, which is characterized by a high level of interconnectivity between the
inter-granular voids (pores). Once the corrosion process starts, the pores are progressively filled by
iron corrosion products [80,81]. Thus, the effective porosity decreases because the proportion of voids
in which water flows decreases. In other words, permeability loss in Fe0 filters is fundamentally due to
reduced interconnectivity. Unconnected pores are often called dead-end pores. The non-sustainability
of conventional Fe0 filters is thus due to increased dead-end pores [28,30,80,81].
Material particle size and shape, Fe0 ratio, Fe0 intrinsic reactivity, the height of the reactive zone
(Fe0 layer), and packing arrangements are among the factors that determine the occurrence of dead-end
pores (Section 2). In addition, the porosity of the admixing aggregates (e.g., gravel, MnO2, pumice) also
impacts the water flow. The large array of significant operational parameters impacting the efficiency
and sustainability of Fe0 filters and the absence of a systematic approach to assess the efficiency of such
filters explain why, despite 17 years (from 2000 onwards) of intensive research, controversial reports
are still published [56,69,236]. To end this frustration, systematic investigations with well-characterized
materials are needed [237,238].
9. Concluding Remarks
To gain acceptance, each stand-alone water treatment technology should quantitatively remove
a broad range of contaminants at an economical cost and in an environmentally benign manner.
Reports on Fe0 filters over the last two decades have shown encouraging results as Fe0 filters removes
a wide variety of aqueous chemical and microbial contaminants. However, the major factor that
limits the acceptance of Fe0 filters is the pragmatic (practical rather than theoretical considerations
of the reactivity of Fe0) approach with which related research has been performed. The presentation
herein has reiterated the reasons for the efficiency of Fe0 filters and presented ways to make them
a universal solution for decentralized safe drinking water provision. The experiment could start by
testing commercial steel wool at the community level.
Acknowledgments: Thanks are due to Sabine Caré (Université ParisEst/France); Emmanuel Chimi (University of
Douala/Cameroon); Seteno K.O. Ntwampe (Cape Peninsula University of Technology/South Africa); Mohammad
A. Rahman (Leibniz Universität Hannover/Germany) and Paul Woafo (University of Yaoundé 1/Cameroon)
for fruitful collaboration on investigating iron filtration systems. Glaydson Simões dos Reis (Department of
Metallurgy, Federal University of Rio Grande do Sul, Porto Alegre/Brazil) is thanked for his valuable advice.
The Universitätsbund Göttingen e.V. has supported the attendance of CN at the 6th World Sustainability Forum.
The manuscript was improved by the insightful comments of anonymous reviewers from Sustainability. We
acknowledge support by the German Research Foundation and the Open Access Publication Funds of the
Göttingen University.
Author Contributions: The text of this article was written by all co-authors. Elham Naseri performed the
RSM modeling.
Conflicts of Interest: The authors declare no conflict of interest.
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