Subsurface Iron and Arsenic Removal

Subsurface iron and arsenic removal for drinking water treatment in Bangladesh
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An introduction to subsurface
iron removal
Iron removal in the Netherlands
In the Netherlands there has been a long history of
groundwater treatment, either above-ground or below
the surface. Since the start of drinking water supply in
the Netherlands, the preferred source has always been
microbiologically safe groundwater (in the Netherlands
this source can be found in confined sandy aquifers
in most parts of the country; Smeets et al., 2009).
Conventional above-ground treatment consists in
general of the combination of aeration, e.g., cascade
or plate aeration, and subsequent sand filtration. Prechlorination
of the groundwater is not applied, as
iron removal can also be achieved with the (slower)
oxidation kinetics as obtained by aeration. Iron removal
processes can be approached from a chemical and
biological perspective, with the latter being considered
only since several decades (Mouchet, 1992; Czekalla et
al., 1985). Chemical iron removal can be subdivided
into oxidation-flocculation and adsorptive-oxidation.
The formation of flocs results in faster clogging of the
filter bed, whereas adsorptive iron removal produces a
neatly ordered coating onto the sand grains (Sharma
et al., 2001). Biological iron removal relies on the
presence of iron oxidizing bacteria, such as Gallionella
spp., to immobilize iron in the filter bed. An additional
advantage of the biosand filters is that nitrification of
ammonium-containing anaerobic groundwater can
be obtained as well. Apart from aeration-filtration,
also nanofiltration, reverse osmosis, activated carbon
filtration and pellet softening reactors are sometimes
practised to treat groundwater in the Netherlands.
After treatment, the groundwater is usually biologically
stable and, therefore, post-chlorination of the water to
prevent bacterial growth is not needed (Smeets et al.,
2009).
The principle of SIR
For groundwater treatment a new iron removal
technology was introduced in the 1970s, based on
similar oxidation-adsorption principles as sand
filtration, but utilizing the natural treatment capacity of
the aquifer. The principle of Subsurface Iron Removal
(SIR), or in-situ iron removal, is that aerated water is
A
B
1.0
C
C 0
Injection Oxygen front
C
C 0
0.5
0
1.0
0.5
0
Injection
front
distance from the well
Fe 2+ front Fe 2+ front
Abstraction
t 2
t 1
distance from the well
Figure 2.1 Behaviour of injection water, oxygen and iron front
at a distance from the well (obtained from van Beek,
1983)
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