Comments on “Effect of groundwater iron and phosphate on the efficacy of arsenic 
removal by iron-amended BioSand Filters” 
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C. Noubactep 
Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, 
Germany. 
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e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax. +49 551 399379 
 
In a recent study, Chiew et al. (1) reported on the performance of Kanchan arsenic filter 
(KAF) for arsenic and pathogen removal in rural Cambodia. As-contaminated groundwater 
sources were spiked with lab cultured E. coli and MS2 and filtered through KAF devices. The 
KAF, designed and distributed in Nepal by Ngai et al. (2), is rigorously a conventional 
BioSand filter (BSF) amended with a Fe-oxide-producing unit for arsenic removal (Fe0 unit). 
The results of Chiew et al. (1) partly revealed no significant difference between the KAF and 
the BSF as shown by a reference system without Fe0 unit. Therefore, the discussion on KAF 
efficiency based on Fe/P ratio is surprising for two reasons: (i) iron can not be expected to 
quantitatively dissolve at pH > 5 (3), and (ii) Fe-oxides are a well known PO43--removing 
agent (4). 
Upon proper designing, KAF should combine pathogen removal in the BSF and arsenic 
removal in the Fe0 unit (2). Furthermore, beside As, nitrate and pathogen should also be 
removed or inactivated in the Fe0 unit (5). The reported results contradict this theoretical 
prediction and the results achieved in Nepal (2). This discrepancy suggests the existence of 
experimental biases. A possible bias consisted in flushing influent water for 10 min. During 
this time, interactions of O2 (air) and dissolved FeII species may have afforded precipitation of 
iron hydroxides, possibly lowering the As concentration of the influent. In addition, 
introducing colloidal iron hydroxides in the Fe0 unit could impair Fe0 reactivity by covering 
its surface or filling the pore space. The conclusions of Chiew et al. (1) support the view of 
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Schmidt and Cairncross (6) that widespread promotion of household water treatment is 
premature. 
On the other hand, the argument that added Fe0 (5 kg) were inefficient due to insufficient 
contact time with the water is not acceptable. In fact, only 1980 litres of water was filtered 
during the whole experiment (22 weeks). This volume corresponds at the most to 737 g As, 
yielding a molar ratio Fe/As ≥ 8,364. Therefore, submerging the Fe0 bed could enable a better 
As removal efficiency provided the used material is of adequate reactivity. Accordingly, even 
though Chiew et al. (1) have not exactly reproduced the original KAF design (2), the reported 
discrepancy in As removal may be mostly attributed to the difference in the intrinsic reactivity 
of used iron nails (Fe0). 
Despite large variability in microbial and chemical contaminant levels, natural waters used as 
drinking water could be regarded as low-level contaminated waters. In fact, contaminant 
concentrations are larger than accepted drinking water standards but still relatively low (here,  
[As]0 ≤ 372 μg/L). Dissolved species will certainly interact with forming and transforming 
iron oxides and will be removed from the aqueous solution by several mechanisms including; 
adsorption and co-precipitation (5). This conclusion, based on the state-of-art knowledge on 
contaminant removal in Fe0/H2O systems, shows that a well-designed iron filter can properly 
produce safe drinking water. The unique challenge is to find out efficient ways to characterize 
Fe0 reactivity and proper selected material for domestic use.  
The ability of Fe0 filters to produce safe drinking water has been already demonstrated in the 
framework of SONO filter development towards 3-Kolshi filters (7). In fact, the 3-Kolshi 
filters containing only 3 kg Fe0 were very efficient for arsenic removal but were abandoned 
because of rapid decrease of water flow rate (porosity loss). Because the porosity loss of the 
filter is due to the expansive nature of corrosion products formation, the 100 % Fe0 bed can be 
replaced by a bed containing an optimal proportion of Fe0 for efficient contaminant removal 
and an inert material as filling material. The theoretical ratio between the volume of corrosion 
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products and the volume of iron consumed during the corrosion process varies between 2.0 
for Fe
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3O4 and 6.40 for Fe(OH)3.2H2O (8). Lowering the proportion of Fe0 in the filter will 
certainly extend its service live. Fe0 can be mixed with sand, gravel or pumice. 
In conclusion the reported failure of KAF in Cambodia is mainly due to the paucity of 
scientific understanding of the complex chemical and physical processes involved in the 
process of aqueous contaminant removal by Fe0. It is expected that immersing the Fe0 unit 
will increase the KAF treatment efficiency. However, the universal use of KAF filters 
depends on the ability of researchers to develop reliable strategies to accurately test the long-
term reactivity of Fe0 material for these devices.  
References 
(1) Chiew, H.; Sampson, M.L.; Huch, S.; Ken, S.; Bostick, B.C. Effect of Groundwater Iron 
and Phosphate on the Efficacy of Arsenic Removal by Iron-Amended BioSand Filters. 
Environ. Sci. Technol. 2009, 43, 6295-6300. 
(2) Ngai, T.K.K.; Shrestha, R.R.; Dangol, B.; Maharjan, M.; Murcott, S.E. Design for 
sustainable development – Household drinking water filter for arsenic and pathogen 
treatment in Nepal. J. Environ. Sci. Health 2007, A42, 1879-1888. 
(3) Aleksanyan, A.Y.; Podobaev, A.N.; Reformatskaya, I.I. Steady-state anodic dissolution of 
iron in neutral and close-to-neutral media. Protect. Met. 2007, 43, 66-69. 
(4) Zeng, L.; Li, X.; Liu, J. Adsorptive removal of phosphate from aqueous solutions using 
iron oxide tailings. Water Res. 2004, 38, 1318-1326. 
(5) Noubactep, C. A critical review on the mechanism of contaminant removal in Fe0–H2O 
systems. Environ. Technol. 2008, 29, 909-920. 
(6) Schmidt, W.-P.; Cairncross, S. Household water treatment in poor populations: Is there 
enough evidence for scaling up now? Environ. Sci. Technol. 2009, 43, 986-992. 
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(7) Hussan, A.; Munir, A.K.M. A simple and effective arsenic filter based on composite iron 
matrix: Development and deployment studies for groundwater of Bangladesh. J. 
Environ. Sci. Health 2007, A42, 1869–1878. 
(8) Caré, S.; Nguyen, Q.T.; L'Hostis, V.; Berthaud, Y. Mechanical properties of the rust layer 
induced by impressed current method in reinforced mortar. Cement Concrete Res. 2008, 
38, 1079-1091. 
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