Designing iron-amended biosand filters for decentralized safe drinking water provision 1 
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Noubactep C.a,c (*), Temgoua E.b, Rahman M.A.a,d 
(a) Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D-37077, Göttingen, Germany. 
(b) University of Dschang, Faculty of Agronomy and Agricultural Science, Dschang, Cameroon. 
(c) Institute of Water Modelling, Mohakhali, Dhaka - 1206 , Bangladesh 
(d) Kultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D-37005 Göttingen, Germany 
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(*) e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax: +49 551 399379 
 
Abstract 
There are ongoing efforts to render conventional biosand filters (BSF) more efficient for safe 
drinking water provision. One promising option is to amend BSF with a reactive layer 
containing metallic iron (Fe0). The present communication presents some conceptual options 
for efficient Fe0-amended BSF in its fourth generation. It is shown that a second fine-sand 
layer should be placed downwards from the Fe0-reactive layer to capture dissolved Fe. This 
second fine-sand layer could advantageously contain adsorbing materials (e.g. activated 
carbons, wooden charcoals). An approach for sizing the Fe0-reactive layer is suggested based 
on 3 kg Fe0 per filter. Working with the same Fe0 load will ease comparison of results with 
different materials and the scaling up of household BSF to large scale community slow sand 
filters (SSF). 
Key words: Biosand filter, Iron/sand filter; Point of use, Drinking water, Zerovalent iron. 
Acronym List 
BSF  Biosand Filter 
RZ  Reactive Zone 
SSF  Slow Sand Filtration 
WHO  World Health Organization 
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1 Introduction 26 
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Safe drinking water may derive from surface water or groundwater. Surface water is often 
polluted with pathogens (e.g. bacteria, viruses). Groundwater is mainly contaminated/polluted 
by inorganic species (e.g. arsenic, iron, nitrate, uranium). Both surface water and groundwater 
may be turbid (physical pollution) and contain organic and inorganic contaminants from both 
natural and anthropogenic origins [1-5]. Accordingly, at any location available water may 
contain biological, chemical and physical contamination. The World Health Organization 
(WHO) guidelines for drinking water quality [6] outline a preventive management framework 
for safe drinking water [2,7-9]. The WHO guidelines [6] entail: (i) health based targets, (ii) 
system assessment from source through treatment to the point of consumption, (iii) 
operational monitoring of the control measures in the drinking water production, (iv) 
management plans documenting the system assessment and monitoring plans and (v) a system 
of independent surveillance that verifies that the above are operating properly.  
Whenever a water is polluted, e.g. after the WHO Guidelines [6], it should be rendered safe 
before consumption. Universal appropriate treatment technologies should be able to 
efficiently remove all three classes of contamination. For rural and peri-urban areas in the 
developing world, the treatment system should ideally occur in a single-stage filtration 
process at household or small community level [2,7,10-23]. From the available technologies, 
household biosand filters (intermittent slow sand filters) have been tested the most [2,24-36]. 
Currently, at least 500,000 people are using biosand filters (BSF) to provide safe drinking 
water [28,30,32]. However, it has been traceably shown that BSF do not remove all of the 
pathogens from water. Accordingly, several attempts are tested to improve the efficiency of 
conventional BSF [14,16,20,27,28,37-45]. For example, Baig et al. [16] used locally available 
biomass in their innovative biosand filter and reported on significant efficiency enhancement. 
On the other hand, amending BSF with metallic iron (Fe0) has been proven beneficial for 
water treatment [27,28,38,39.40,45,46]. However, as with most innovations, the early 
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development of Fe0-amended BSF is marked by empirical designs [32]. Therefore, the Fe0-
amended BSF technology should now be translated into rational engineering design criteria. 
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The present communication is a part of a series of theoretical works based on the aqueous 
chemistry of iron corrosion and aiming at easing research on water treatment using Fe0 [47-
51]. The expansive nature of Fe0 oxidative dissolution followed by precipitation of FeII/FeIII 
species (oxides and hydroxides) at pH > 4.5 [52,53] is properly considered. Much of the 
impetus for this work has come from the work of Noubactep et al. [12], who have proposed (i) 
Fe0 as a universal agent for save drinking water provision, and (ii) to amend BSF with a 
reactive zone made up of Fe0 and inert materials (e.g. sand). Subsequent works have 
rationalized the mixture of Fe0 with (i) inert materials [47,54], and (ii) reactive but non 
expansive materials [51] for long-term water treatment. A key question that remains is how to 
avoid that dissolved iron is present in the treated water? An analysis of published data from 
Khan et al. [37] to Ingram et al. [46] suggests that a fine sand layer must be placed 
downstream from the reactive zone containing Fe0. Based on this observation several design 
options are discussed. For the sake of clarity a conventional BSF will be first presented. 
2 Biosand filter (BSF), 
The BSF was developed at the University of Calgary in the early 1990s by Dr. Manz [27,28]. 
The BSF is a downscaling of the slow sand filtration (SSF) technology [55] for intermittent 
water filtration at household level. The BSF has been reported to be efficient at removing 
chemicals (e.g. iron, manganese, and sulphur), pathogens (e.g. bacteria, viruses) and turbidity 
from low turbid water [27-30]. Unlike a ‘simple’ slow sand filter, the BSF has the ability to 
perform multiple functions as a single unit [4]. In a BSF, settlement, straining and filtration 
act in synergy to remove biological, chemical and physical contamination/pollution. Ideally, 
safe drinking water is produced.  
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The most important process in a BSF occurs in a biological layer (biofilm) called the 
Shmutzdecke [4,12,33,34,57]. The Shmutzdecke develops in the presence of atmospheric 
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oxygen on the surface of the uppermost layer of sand that is responsible for the removal of 
microorganisms. Ideally, the fine sand layer downwards from the Shmutzdecke is almost 
anoxic (O
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2 free) as O2 is consumed by microbial activity within the Shmutzdecke. It should be 
anticipated here that water exiting such an anoxic system is ideal for a Fe0 filter (reactive 
zone) as volumetric expansion is mostly limited to the formation of Fe3O4 with an expansion 
coefficient (η) of 2.08 [52,53]. Under oxic conditions more volumetric expansive iron oxides 
and hydroxides formed (η ≤ 6.40). As a rule, the larger the η value, the more rapid the loss of 
hydraulic conductibility (permeability loss) of the reactive zone.  
2.1 Design of a BFS 
The engineering principles of household BSF is described in several recent communications 
[2,4,27-30,32,56-59]. All household BSF share five basic design components [56]: (i) a good 
source of sand and gravel, (ii) a bucket filled with 40 to 75 cm of fine sand (sand bed), (iii) a 
layer of static standing water (supernatant water), (iv) a maturation time of 14 to 21 days for 
formation of the Shmutzdecke, and (v) a flow control system. Ideally, water should flow freely 
from the filter (not tap).  
Intensive research is still targeted at designing smaller, lighter (portable), less expensive but 
efficient BSF [4,60]. Alternative BSF designs for better efficiency in aqueous contaminant 
removal have been tested [2,4,45,46,58,59]. Tested design parameters include: (i) sand type, 
sand size and sand depth [31,32,61], (ii) maturation time or time to formation of the 
Shmutzdecke, (iii) elevation head, (iv) standing water depth, (v) filter pause time or daily 
water throughput, as well as (vi) amendment of conventional BSF with reactive layers 
containing metallic iron (Fe0). The present communication is focused on the design of Fe0-
amended BSF.  
Each of the six named operational parameters enumerated above has been reported to 
significantly impact the performance of the BSF for water treatment [2,4,33,45,59]. In 
particular, despite amendment with Fe0, resulted BSF are still tested for their capacity for 
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microbial attenuation and the removal mechanism controversially discussed [33,62,63]. This 
controversial discussion is not in accordance with the contemporary knowledge on the 
mechanism of contaminant removal in Fe
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0 beds [14,15,64-70] as iron corrosion products 
should be regarded as ‘collectors’ in BSF [71]. 
2.2 Operating mode of a BSF 
The supernatant water layer provides a head of water that is sufficient to drive the water 
through the filter bed, whilst creating a retention period of several hours for the water 
[2,55,72]. A BSF is doted with a underdrain system which provides an unobstructed passage 
for treated water from the filter bed and supports the sand bed. The outlet flow control 
maintains submergence of the medium during operation to minimise potential air-binding 
problems. 
Water percolates slowly through the porous sand medium. Thereby, inert particles and 
micoorganisms are removed from the aqueous phase. An algal mat forms on the surface of the 
sand bed and this is termed Schmutzdecke (a biolayer or biofilm of living organisms). After 
several months of operation, the surface of a BSF becomes clogged due to the deposition of 
suspended solids. At this time, cleaning of the filter bed is required. The BSF is cleaned by 
scapping off the top 2-3 cm of the sand bed including the Schmutzdecke layer. After a 
scraping of the sand bed, a re-sanding is necessary with the accompanying maturation time of 
the formation of a new Schmutzdecke. 
2.3 Limitations of a conventional BSF 
Discounting any design limitations, the BSF is not destined to treat chemical contaminants in 
general and inorganic contaminants in particular. In fact, the affinity of sand for metal 
adsorptive removal is very low [73,74]. Accordingly, whenever chemical contamination is 
suspected, alternative to conventional BSF should be sought. One such an alternative is the 
amendment of conventional BSF by a reactive layer containing reactive Fe0. The suitability of 
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Fe0/sand filters for water treatment arises from the fact that iron corrosion products act as 
collectors [71,75] for all classes of contaminants [62,66,76]. 
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The suitability of Fe0-amended BSF for the developing world arises from the fact that the 
quality of available water is rarely assessed. A perfect illustration is the arsenic crisis in South 
East Asia where people have consumed As-contaminated waters for decades [41,77]. On the 
other hand, success of conventional BSF and other water treatment technology are not 
currently validated by biological and/or chemical analysis but by the decrease of the 
frequency of water born diseases [4,8,13,21,44]. For all these reasons, the need of a 
technology able at removing all classes of contaminants for water is obvious. 
3 Fe0-amended BSF on an historical perspective 
The present section will consider only stand-alone Fe0 filters for save drinking water 
provision.  
3.1 First generation Fe0-amended BSF: removal of chemicals 
The efficiency of iron-oxide-coated sand for contaminant removal is well-documented and 
has been used for water treatment in household filters for decades [20,42,78-80]. 
Conventional BSF have been proven efficient to remove dissolved iron from the aqueous 
phase. Moreover, BSF has been proven more efficient in removing inorganic contaminants 
(e.g. As) when the inflowing water was rich in dissolved Fe. Accordingly, Khan et al. [37] 
amended conventional 3-Kolshi filters with a layer of Fe0 to achieve better As removal by a 
BSF-like filter. It is very important to notice that Fe0 is added by Khan et al. [37] to produce 
iron oxides for As removal in filters. Accordingly, the Fe0-amended 3-Kolshi filter can be 
regarded as first generation Fe0-amended BSF.  
The first generation Fe0-amended BSF was proven very efficient but not sustainable because 
of too rapid loss of hydraulic conductivity [40,41,77]. As demonstrated in previous works 
[12,47-51,54], “diluting” Fe0 with (inert and) non-expansive materials is a pre-requisite for 
sustainability. In other words, the first generation Fe0-amended BSF was wrongly designed. 
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3.2 Second generation Fe0-amended BSF: removal of pathogens 155 
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The second generation Fe0-amended BSF was born with the work of You et al. [81]. These 
authors demonstrated that adding Fe0 to a sand filter, efficiently removed viruses. The 
removal was ascribed to pathogen adsorption onto iron (hydr)oxides formed via (anaerobic) 
iron corrosion. Virus removal by Fe0 was rapid and most of the removed viruses were 
irreversibly bound. The irreversible bounding of viruses by Fe0 corresponds to co-
precipitation as presented two years later by Noubactep [64-70]. Strictly, the filter of You et 
al. [81] is not necessarily a biosand filter but a Fe0 filter for pathogen removal. In fact, the 
formation a Schmutzdecke layer is not explicitly intended. Rather, Fe0 itself is the pathogen 
removing agent. However, recent research based on You et al. [81] are testing Fe0-amended 
BSF as alternative to conventional BSF [33,45,46,63,72]. 
3.3 Third generation Fe0-amended BSF: removal of chemicals and pathogens 
The third generation Fe0-amended BSF was born with the work of Ngai et al. [38]. The filter 
designed by Ngai et al. [38,39] combines the concept of a BSF with the innovation of a 
diffuser basin containing reactive Fe0. In this design, pathogens are removed mostly by 
physical straining provided by the fine sand layer (sand bed) [23]. Chemical contamination 
(e.g. As) is removed by adsorption onto iron oxides and hydroxides generated from rusted 
Fe0.  
A fundamental mistake was recently discovered in this design [12]. In fact, molecular O2 
necessary for the formation of the Schmutzdecke is quantitatively consumed by the over-
laying Fe0 layer. In other words, a reactive over-laying Fe0 makes the BSF inoperative for 
pathogen removal as no Schmutzdecke is formed. Thus, the obtained filter is at best a first 
generation Fe0 filter and strictly not a Fe0-amended BSF because the ‘Fe0 unit’ and the ‘BSF 
unit’ are two different components of the filter. On the other hand a pure Fe0 layer (100 % 
Fe0) at the entrance of the filter will rapidly clog due to the formation of voluminous 
corrosion products (η > 2.1) [52,53]. 
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3.4 Fourth generation Fe0-amended BSF 181 
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The fourth generation Fe0-amended BSF was born with the work of Gottinger [43]. It is 
characterized by a reactive zone containing a mixture of Fe0 and an inert material (e.g. sand). 
Fe0 is purposefully mixed with sand to prevent clogging [12]. It should be noticed that mixing 
Fe0 and sand is current in the research on Fe0 for groundwater remediation [74,82-86]. The 
present communication is limited to household and small community Fe0 filters. However, the 
results are up-scalable to larger Fe0/H2O systems.  
The filter designed by Leupin et al. [87,88] fulfilled these criteria but is classed here as the 
chronology is not an absolute factor. Furthermore this filter was designed mainly  for As 
removal. Gottinger [43] was the first researcher to mix Fe0 and sand on a volumetric basis and 
test several Fe0/sand ratios (e.g. 50/50 and 40/60 and 0/100) in triplicate columns. Rangsivek 
and Jekel [89-91] tested 10/90, 20/80 and 30/70 Fe/pumice volumetric mixtures for metal 
removal but the tests were not systematic. They mostly worked with the 10/90 mixture 
without specifying the rational [89,91]. Independent theoretical calculations [47,54] have 
shown that the threshold value of the volumetric proportion of Fe0 for which the column 
reactive zone is clogged at Fe0 depletion is 52 %. The calculations were based on the 
assumption of ideally packed spherical Fe0 particles for which an initial porosity of 36 % is 
relevant. Natural sand is never perfectly spherical. Kubare and Haarhoff [32] reported an 
average porosity of 45 % for sand beds. However, the theoretical threshold value for Fe0 
depletion of 52 % (corresponding to 36 % porosity) will be considered in discussing the 
design of Fe0-amended BSF. The meaning of this value is that any filter containing more than 
52 % Fe0 (volumetric proportion of the solid phase) will clog before Fe0 completely depletes. 
The excess Fe0 amount relative to 52 % should be regarded are pure material wastage with the 
additional curse of shortening the filter service life [54]. 
4 Designing Fe0-amended BSF 
4.1 Strategic planning 
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The presentation above showed that a Fe0-amended BSF should be sequenced as follows: (i) 
supernatant water (e.g. 5 cm), (ii) fine sand (e.g. 40 to 50 cm), (iii) Fe
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0/sand (x cm), (iv) fine 
sand (y cm), (v) sand (e.g. 5 cm)  and (vi) gravel (e.g. 5 cm). The thickness of individual 
layers is from Lea [56].  In the design of Jenkins et al. [72] sand and gravel are replaced by 
gravel and rock respectively. The major feature for these layers is the difference in particle 
size. The above sequence suggests that the depths of the Fe0/sand layer and that of the 
underlying fine sand are yet to be determined (x and y values). However, the paramount 
question is which Fe0 material should be used? 
Characterizing the intrinsic reactivity of Fe0 materials [43,92-98] is a key issue for designing 
Fe0-amended BSF. The ideal material should be able to efficiently provide clean water for at 
least 12 months (one year). Therefore, all tested materials should be characterized for their 
intrinsic chemical reactivity [94,95,98] and their sphericity in order to enable results 
comparability. Next to the intrinsic reactivity, used devices should be characterized by 
[32,61]: (i) their dimensions, e.g. internal diameter and depth of reactive layers for cylindrical 
columns, (ii) the particle size and the surface state of used materials (e.g. Fe0, gravel, pumice, 
sand), (iii) the mass of Fe0, and (iv) the volumetric proportion of Fe0 in the reactive layer. The 
next section will discuss the thickness of the reactive layer. 
4.2 Thickness of the Fe0/sand layer 
A cylindrical bed is considered; H is the height and D is the internal diameter. The cylinder 
contains a reactive zone with the height Hrz (x value from section 4.1) and the volume Vrz. Hrz 
is necessarily lesser than H (x < H or Hrz < H). Beds are supposed to be filled by spherical 
granular materials. The compactness (or packing density) C (-) is defined as the ratio of the 
volume of the particles to the total packing volume (Vrz). Considering the granular material as 
composed of mono-dispersed spheres subjected to soft vibrations, the compactness C is 
generally considered to be equal to 0.64 for a random close packing [51]. It is assumed that 
the particles are non porous.  
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The initial porosity Φ0 (-) of the reactive zone and the thickness Hrz of the reactive zone are 
respectively then given by: 
Φ0 = 1 – C      (1) 
V
D
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H rz2rz .π=      (2) 236 
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The filling of the bed porosity  by iron corrosion products determines the filter service life and 
has been extensively discussed in previous works [51,54]. 
To completely define the x value (Hrz), Vrz has to be correlated to the mass of Fe0. Vrz is the 
volume occupied ideally by Fe0 and sand of similar grain size and shape (Eq. 3). For 
simplification VFe will be expressed as a fraction of Vrz (Eq. 4). 
Vrz = Vpore + VFe + Vsand    (3) 
αpore + αFe + αsand = 1     (3a) 
Vpore = αpore * Vrz     (4) 
VFe + Vsand = (1 - αpore) * Vrz    (4a) 
VFe = τ * (VFe + Vsand) =  τ * (1 - αpore) * Vrz (5) 
Where αi is the volumetric fraction of each phase, τ (≤ 0.52) is the volumetric proportion of 
Fe0 relative to the solid phase (Fe0 + sand) in the reactive zone [54]. Per definition, αpore = 
Φ = 1 - C and C = 1 - Φ. Thus Eq. 5 reads:  
VFe = τ * C * Vrz     (5a) 
The mass of Fe0 necessary to fill the volume VFe is given by Eq. 6: 
mFe = ρFe * VFe     (6) 
Where ρFe is the specific weight of Fe (7800 kg/m3). 
Combining Eq. 2, 4, 5 and 6 give the following relationship between Hrz (x value) and mFe 
(Eq. 7): 
Hrz = 4 * mFe / [(πD2) * τ * C * ρFe]  (7) 
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Eq. 7 is the equation of the reactive zone. Three major issues should be considered: (i) τ ≤ 
0.52 (52 % Fe
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0 (v/v)), (ii) Hrz ≥ 5 cm, and (iii) Hrz < H. The threshold value Hrz = 5 cm is 
considered the minimum height for the realization of a homogeneous well-mixed Fe0/sand 
reactive zone [99]. Ideally, Hrz is only a fraction of H (Hrz << H) for example, Noubactep et 
al. [12] considered that Hrz should fulfil the condition Hrz ≤ 0.1 H. 
The mass of sand to be used is deduced from Eq. 8. 
msand = ρsand * Vsand =  ρsand * (1 - τ) * C * Vrz (8) 
Where ρsand is the specific weight of sand (2650 kg/m3). 
4.2.1 Illustration 
To discuss the applicability of the established equations, the biosand filter of Lea [56] with a 
diameter of 30 cm a height of 90 cm is used. The bed volume of this filter is 63.6 L. 
Accordingly if the reactive layer must occupy at most 1/10 of the bed, its volume should not 
be larger than 6.4 L. 
For comparison, at τ = 0.52, 1 kg of Fe0 is mixed to 0.16 kg of sand and the mixture occupies 
a volume (Vrz) of 0.2 L. When mixed to 8.32 kg of sand, the same mass of Fe0 (1 kg) 
corresponds to a τ value of 0.02 (about 6 % w/w) and the mixture occupies a volume Vrz = 
5.01 L. 
Given that water treatment by Fe0 is a deep bed filtration, the present example illustrates the 
necessity of using lower values of τ with the additional advantage of reducing the clogging 
probability. The discussion in the next section will be mostly based on the threshold τ value of 
0.52. 
4.3 Discussion 
This work attempts to sustain research on Fe0-amended BSF by optimizing filter design in the 
perspective to rationalize experimental conditions and enable/ease results comparison. 
Accordingly, the most important issue regards sizing a filtration bed. Both for laboratory and 
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field works, one of the first task is to decide which column(s) to use. In some cases, available 
columns are simply used. But even in these cases, they should be properly filled to achieve 
reliable results. Sizing a filter bed will be discussed here in two different perspectives: (i) 
selecting the appropriate bed size, and (ii) selecting the appropriate thickness of the reactive 
layer (H
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rz). 
4.3.1 Rationale selection of the column diameter 
Eq. 7 gives the thickness of the reactive zone Hrz as a function of mFe, D, τ and C. From these 
4 parameters, C can be considered a constant. Accordingly,  Hrz is a function of the used mass 
of Fe0 (mFe), the volumetric proportion of Fe0 in the solid phase (τ) and the internal diameter 
of the cylinder (D). Accordingly, Eq. 7 can allow the calculation of the diameter of the 
column to be used to achieve a certain Hrz at a given τ value. 
The results of such calculations are summarized in Tab. 1 for τ = 0.52, C = 0.64 cm and the 
following values of  mFe: 1.0, 2.5, 5.0, 10.0 and 25.0 kg. From Tab. 1, it is obvious that the 
sole mass of Fe0 that is applicable for lab experiment (D = 3 cm) is 1 kg. The corresponding 
Hrz value is 55 cm which represents 61 % of the filter height (90 cm). Rigorously, the 
resulting system is not a BSF but rather a SONO-like filter. Nevertheless, the following 
sequence could be tested: 20 cm sand bed, 55 cm reactive zone (x = 55 cm) and 15 cm sand 
bed (y = 15 cm). Whether this design is satisfactory or not should be tested in laboratory 
investigation. In the case y = 15 cm sand bed is not sufficient to capture dissolved Fe from the 
reactive zone, the possibility of adding a second sand bed should be tested (Fig. 1). On the 
other hand, for Hrz > 0.33 H the perspective of sandwiching an Fe0/filter between two BSF 
must be tested (Fig. 2). The first BSF removes dissolved O2 among others and the second BSF 
removes dissolved Fe. 
Fig. 3 summarizes the results of the variation of Hrz as a function of D (τ = 0.52). It is clearly 
shown that the thickness of the reactive layer increases with decreasing diameter. It is shown 
(Fig. 3a) that Hrz values of up to more than 1300 cm (13 m) are obtained for larger mFe values. 
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However, relevant values must fulfil the condition Hrz ≤ H (H = 90 cm). Fig. 3b is limited to 
H
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rz = 0.67 H (60 cm) for a better visualization. Table 1 shows that D = 50 cm is the largest 
value which may enable a 5 cm Hrz while using 25 kg Fe0. this situation is likely to occur in a 
field small-community-scale water plant. However, because Fe0 beds are deep bed filtration 
systems, it is advantageous to use a lower τ value (τ ≤ 0.52) to achieve a thicker Hrz with the 
same Fe0 mass. 
4.3.2 Rationale for the thickness of the reactive zone 
The thickness of the reactive zone is necessarily correlated to the intrinsic reactivity of used 
Fe0. It is intuitive to assume that for each Fe0 material, a range of τ values (τ ≤ 0.52) may exist 
for which filter operation is optimal. A survey of the experimental conditions of available 
works suggests that they are highly qualitative. For example, Tellen et al. [45] introduced a 5 
cm reactive zone containing 0.5 kg Fe0 in 44 cm sand bed without specifying the thickness of 
the sand layer under-laying the reactive zone.  
The design of Tellen et al. [45] was slightly modified by Pachocka [33]. She introduced a 3.2 
cm reactive zone containing a Fe0 volumetric proportion of 15 % (τ ≤ 0.15). The Fe0 mass is 
not specified but calculations using Eq. 7 while considering the geometry and the dimensions 
of her device showed that about 2.2 kg Fe0 was used. Discounting the fact that building a 
homogenous 3.2 cm Fe0/sand layer is difficult, it is certain that such a thin layer is not suitable 
for a deep bed filtration process. Accordingly, guidelines are urgently needed to assist the 
laborious work of amending conventional BSF with reactive Fe0. 
The suitability of (τ, Hrz) values for several Fe0 materials and material amounts should be 
tested in laboratory studies before a trend is identified for generalization and scaling up to 
household and small-community Fe0-amended BFS.  
For further illustration, calculations are made using Eq. 7 for D = 30 cm and various Fe0 
masses. The results are summarized in Fig. 4 and Tab. 2. Values in Tab. 2 are obtained for τ = 
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0.52. Fig. 4 shows clearly, that Hrz decreases with increasing τ values. For example, 25 kg of 
Fe
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0 occupy a 13.6 cm at τ = 0.52, 26.2 cm at τ = 0.27 and  101.2 cm at τ = 0.07. While 
limiting the scale to Hrz = 0.67 H (Hrz = 60 cm), it is shown that 5.0, 10.0 and 25.0 kg are not 
suitable for τ < 7.0 % ( or τ < 0.07). These examples show clearly that there is an infinite 
number of possible Fe0-amended BSF. A systematic approach is therefore essential to identify 
and characterize useful combinations. The next section will discuss the design of a BSF 
containing 3 kg of a reactive Fe0. 
4.4 Constructing a Filter with 3 kg Fe0 
A conventional concrete intermittent BSF is used. An approach is suggested to purposefully 
amend it with a reactive zone containing 3 kg Fe0. First, Eq. 7 is modified to account for the 
filter geometry. The cross-section (S) is a square with a length (L) of 30 cm (0.3 m). The 
volume of the filter is V = S*H = L2*H. The modified Eq. 7 reads as (Eq. 7a): 
Hrz = mFe / [(L2) * τ * C * ρFe]   (7a) 
The results (Tab. 3) show that Hrz takes values from 1.28 to 33.39 cm when the volumetric 
proportion of Fe varies from 0.52 to 0.02 %. The corresponding admixed mass of sand varies 
between 0.94 and 49.94 kg. Taking 50 cm as the maximal thickness of fine sand (sand bed) 
the resulting beds represent 2.6 to 66.8 % of the sand bed volume. 
Considering the practical constrain that Hrz ≤ 5 cm [99], only τ values less than 0.25 are 
applicable. That is, 3.0 kg Fe0 should be mixed to at least 6.46 kg sand to obtain a Fe0-
amended BSF. The efficiency of selected possible filters (τ ≤ 0.25) should be tested. It is 
important to notice in this regard that the first permeable reactive barrier (demonstration pilot 
scale in Borden, Ontario/Canada) contained only 8 % volumetric ratio of Fe0 (τ = 0.08) and 
has been properly working for more than five years [82,100]. Accordingly, smaller τ values 
should be tested in parallel experiments. In this effort the suitability of the thickness of the 
under-laying sand bed should be tested (y value section 4.1 – Fig. 1).  
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For Prz > 10 % (τ ≤ 0.12 – Tab. 3) the alternative of a three-column-system should be used 
(Fig. 2). In the perspective of a Fe
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0/sand column in sandwich between two conventional BSF 
(section 4.3.1) the possibility of using thin columns to save Fe0 must be tested. For example, 
while using 1 kg of Fe0, halving the internal diameter from 9.0 to 4.5 increases the Hrz value 
from 6.1 to 24.2 (τ = 0.52 – Fig. 3). This is a 4 times thicker layer for deep bed filtration in 
comparison to only 5.6 cm Hrz in a conventional BSF (L = 30 cm – Tab. 3). In other words, 
using thinner columns enhanced efficiency and save Fe0. 
The mathematical relation for the variation of Hrz values at constant volume (Vrz1 = Vrz2) is 
easy to establish and is given as follows: 
Hrz1*D12 = Hrz2*D22     (9) 
For D2 = D1/2, Hrz2 = 4* Hrz1. This corresponds to the results obtained while working with 
masses. 
4.5 Ways to efficient Fe0-amended BSF 
The presented concept of Fe0-amended BSF is based on the profound understanding of the 
complex chemical and physical processes involved in the 20-years-old Fe0 remediation 
technology [64-70,82,100-105]. To mimic the subsurface Fe0 bed as closely as possible, 
anoxic conditions must be created before the Fe0/sand layer (reactive zone). This condition is 
satisfied the best by a conventional BSF. On the other hand, to avoid dissolved  Fe in the 
effluent, water from the reactive zone must migrate through a thick fine sand bed. Ideally this 
is another conventional BSF (Fig. 2). Accordingly, the simplest way to efficient Fe0-amended 
BSF seems to go through an experimental design with a reactive zone (a small column – Eq. 
9) sandwiched between two conventional BSF. After such conclusive principle experiments, 
the next advantageous step could be to test three compartments in the same column (Fig. 1). 
In this manner, the important requirement of compact systems for less skilled populations 
(including illiterates) is properly addressed [19]. Progressively, it is conceivable to 
 15
miniaturize the system down to Brita-type filters for a limited volume of tap water. However 
the focus of this communication is on household and small-community Fe
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0-amended BSF. 
Provided that a relevant reactive Fe0 is characterized, selected, and used, the effectiveness of 
Fe0-amended BSF does not need to be demonstrated, except some technology verification in 
the field (monitoring). Beside proper system design, the two sole tasks are: (i) selecting and 
processing the appropriate Fe0 materials (including composites), and (ii) avoiding the use of 
Fe0-amended BSF for waters of pH < 4.5 [12]. In fact, the Fe0 remediation technology is 
based on the anodic dissolution of iron in neutral and close to neutral aqueous systems. In this 
pH range, primary iron dissolution is followed by a continuous build up and transformation of 
a corrosion product layer in the vicinity of Fe0 [106-108]. 
There is strong evidence that the Fe0-amended BSF will be efficient. For example, Westerhoff 
and James [109] performed field continuous-flow experiments at τ = 0.25 (PFe = 50 %. w/w) 
for almost one year for efficient removal of nitrate. During this period, up to 1500 bed 
volumes of water were treated. Assuming that two bed volumes of a BSF correspond to the 
daily need of a rural family, the 1500 bed volumes corresponds to the water demand for 2 
years. However, the τ value (0.25) of Westerhoff and James [109] was not the result of any 
systematic preliminary work. Considering that a system at τ = 0.08 was efficient for more 
than 5 years under anoxic conditions [82,100], it is likely that the system of Westerhoff and 
James [109] was not optimal with regard to long-term permeability. 
5 Concluding remarks 
Save drinking water provision using Fe0-amended BSF is already proven an efficient 
technology [33,37,41,45,46,81]. The estimated huge number of people (884 million) still 
living without access to improved drinking water [110] is an urgent appeal to the scientific 
community for (i) a rapid improvement, and (ii) a generalized implementation of this efficient 
technology. This technology primarily has a single serious limitation: available water must 
have a pH value ≥ 4.5. Fortunately, natural water with pH ≤ 4.5 is unusual. Therefore, Fe0-
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amended BSF should be regarded as a universal technology that can be used at household and 
small community levels, but in general for decentralized safe drinking water provision. 
Using a mathematical modelling, the present work has presented several tools to optimise the 
efficiency of conventional BSF by adding a reactive zone containing a layer of Fe0 admixed to 
a non expansive material (e.g. MnO2, pumice, sand) [47,51,54]. A systematic procedure is 
introduced to sustain further research on Fe0-amended BSF. In particular the reactive zone 
must have a minimal thickness of 5 cm (Hrz ≤ 5 cm), and the volumetric proportion of Fe0 in 
the reactive zone must not be larger than 52 % (τ ≤ 0.52). As concerning the sequential design 
of the Fe0-amended BSF, it is essential that dissolved O2 is quantitatively removed from water 
before its reaches the reactive zone. On the other hand, a find sand layer must be placed 
downwards from the reactive zone and its thickness should be sufficient for scavenging 
dissolved Fe from the reactive zone. Like for other filters [4,40] layers of other treatment 
materials (e.g. activated carbon, wooden charcoal, Zeolite) [111,112] can be purposefully 
added before and/or after the reactive zone to optimise the treatment operation. It is essential 
to notice in this regard that efficient and low-cost traditional methods for aqueous iron 
removal in packed columns have been described in the literature [112-116]. 
Further research at several fronts is needed to develop approaches for the proper design of the 
Fe0-amended BSF. Relevant research fields include the intrinsic reactivity of Fe0, the size, 
surface state and geometry of Fe0 and non reactive particles, the size of the filter or the 
column, the configuration the Fe0/admixture layer, the thickness of individual layers, the 
nature of the admixing agent, the relative particle size of materials in the reactive layer 
(uniformity), the size of the resulting devices, and the water flow velocity. Further 
mathematical and numerical modelling should be applied to design and validate experimental 
results. Pilot scale installations are needed to validate the practicality of experimental and 
modelling results. 
 17
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Thoughtful comments provided by Emmanuel Chimi (University of Douala, Cameroon) on 
the draft manuscript are gratefully acknowledged. Thanks are due to Sabine Caré (Université 
Paris-Est/France) for fruitful collaboration on the evolution of the porosity of iron filtration 
systems. The manuscript was improved by the insightful comments of anonymous reviewers 
from CLEAN Soil, Air, Water. 
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 29
Figure captions 737 
738 
739 
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757 
 
Figure 1: Schematic diagram of a Fe0-amended biosand filter (BSF). The first column is a 
conventional BSF. The thickness of the reactive zone (x value) and the aptitude of the under-
laying sand bed (y value) to free water from the reactive zone from iron will be tested for each 
material. For larger x values or very reactive Fe0, a third column could be essential. 
 
Figure 2: Schematic diagram of a three compartments Fe0-amended biosand filter (BSF). The 
first and the third columns are conventional BSF. The thickness of column 2 (reactive zone) 
depends on the intrinsic reactivity of used Fe0 and can be optimised using Eq. 7 and 9. The 
dimension of column 3 (second BSF) and also be optimise using Eq. 8 and 9. 
 
Figure 3: Variation of the height of the reactive layer (Hrz) as function of the diameter (D 100 
≤ cm) of used filter: (a) all values, and (b) Hrz ≤ 60 cm. The lines are not fitting functions; 
they simply connect points to facilitate visualization. 
 
Figure 4: Variation of the height of the reactive layer (Hrz) as function of the initial Fe0 
volumetric proportion for different used masses of Fe0. The lines are not fitting functions; 
they simply connect points to facilitate visualization. 
 
 30
Table 1: Variation of the height of the Fe0/sand reactive layer (Hrz) as a function of the 
internal diameter (3.0 ≤ D (cm) ≤ 100) of used columns for a τ value of 0.52. The 
corresponding masses of sand and Fe
757 
758 
759 
760 
761 
762 
763 
764 
0 are given. Given that τ = 0.52 is the threshold value for 
sustainable Fe0 beds, these calculations show clearly that for laboratory studies (D ≤ 12 cm), a 
maximum of 2.5 kg sand should be used to keep the reactive layer lesser than 10 cm. On the 
other hand, if a field column has an internal diameter of 50 cm 25 kg Fe0 will build a 5 cm 
reactive layer. 
 
D Fe0 mass 
(cm) 1.0 kg 2.5 kg 5.0 kg 10.0 kg 25.0 kg 
3 55.0 136.2 272.4 544.8 1362.0 
6 13.6 34.1 68.1 136.2 340.5 
12 3.4 8.5 17.0 34.1 85.1 
30 0.54 1.36 2.72 5.45 13.62 
50 0.20 0.50 1.00 2.00 5.00 
75 0.09 0.22 0.44 0.87 2.18 
100 0.05 0.12 0.24 0.49 1.21 
765 
766 
 
 31
Table 2: Variation of the height of the Fe0/sand reactive layer (Hrz) as a function of the mass 
of Fe
766 
767 
768 
769 
770 
771 
772 
0 for a τ value of 0.52. The corresponding mass of sand is given. Calculations 
are made for a cylindrical bed with a internal diameter of 30 cm. Given that τ = 0.52 
is the threshold value for sustainable Fe0 beds, such a bed should not contain less 
than 10 kg of Fe0 to fulfil the condition Hrz ≤ 5 cm. The corresponding Vrz value is 
3.85 L. 
 
Fe0 (kg) 0.5 1.0 2.5 5.0 7.5 10.0 20.0 
Sand (kg) 0.157 0.314 0.785 1.568 2.352 3.136 6.272 
Hrz (cm) 0.27 0.54 1.36 2.72 4.09 5.45 10.90 
Vrz (L) 0.19 0.39 0.96 1.93 2.89 3.85 7.71 
773 
774 
775 
 
 
 32
Table 3: Data for the amendment of a conventional BSF with  a reactive layer containing 3 kg 
Fe
775 
776 
777 
778 
779 
780 
0 mixed with several masses of sand (msand) to yield various proportion of Fe0 (PFe). Hrz is 
the resulting thickness of the reactive layer, Vrz its volume and Prz its volumetric ratio relative 
to a 50 cm reactive layer. 
 
 
τ Hrz Vrz msand PFe (%) Prz
(-) (cm) (L) (kg) (w/w) (v/v) (%) 
0.52 1.28 1.16 0.94 76.13 52.0 2.6 
0.47 1.42 1.28 1.15 72.30 47.0 2.8 
0.42 1.59 1.43 1.41 68.07 42.0 3.2 
0.37 1.80 1.62 1.74 63.35 37.0 3.6 
0.32 2.09 1.88 2.17 58.07 32.0 4.2 
0.27 2.47 2.23 2.76 52.12 27.0 4.9 
0.22 3.04 2.73 3.61 45.36 22.0 6.1 
0.17 3.93 3.54 4.98 37.61 17.0 7.9 
0.12 5.56 5.01 7.47 28.64 12.0 11.1 
0.07 9.54 8.59 13.54 18.14 7.0 19.1 
0.02 33.39 30.05 49.94 5.67 2.0 66.8 
781   
 33