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IRON
Iron
(Fe) The Water Supply Regulations limit
is 0.200 mg/l or 200 µg/l.
Iron is a common metal that
is found as ore and is widely distributed.
It is corroded by water in the presence of
oxygen. With acid waters iron is soluble and
is leached from iron bearing rock.
Iron is found in surface
waters and commonly found dissolved in borehole
waters; at first glance, fresh from the pump,
the water may appear crystal clear. However,
after it has come in contact with oxygen in
the air it will oxidise into iron oxide (rust)
and appear as visible solid particles causing
the water to colour orange/brown. After a
time the particles will tend to agglomerate
and settle.
A simple test for iron is
to take two clean identical bottles, run the
borehole/supply pump for a while, quickly
fill one bottle to the brim and tighten the
lid. Fill the second bottle to half full and
secure the lid. Vigorously shake the half
full bottle, remove its lid, leave to stand
and compare the colour of the two bottles.
If you can notice a difference, it is very
likely there is iron dissolved in the water.
Iron has very similar characteristics
to manganese, but is more readily oxidised.
It will cause orange/brown staining on sanitary
ware, laundry and other places such as where
there is a leaking tap - large stains will
occur. Iron oxides will deposit on the internal
surfaces of water pipes producing a brown
deposit. It has a very low taste threshold,
if you can see it, you can probably sense
a dry metallic taste.
There are treatment methods
to remove dissolved iron from water supplies,
powerful oxidants such as chlorine will cause
oxidation and the production of particles
that may be filtered out. For private water
supplies chemical addition is not always practical
or recommended. As iron is readily oxidised,
aeration is effective.
Another well established
method of removal is by apparent adsorption
onto manganese dioxide. A manganese dioxide
mixture or similar is retained within a filtration
vessel, the water to be treated is passed
through a column of the media and iron levels
are reduced. After a period of operation the
column is automatically backwashed to flush
off the deposited iron oxide particles and
the process can continue.
pH level is important in
the process of iron oxidation and removal.
It is usual for borehole waters to contain
high levels of carbon dioxide and consequently
carbonic acid which may depress the pH to
below the optimum range for iron removal.
CO2 removal must be
addressed before iron removal can take place.
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