While iron (Fe) and manganese (Mn) don’t pose health
problems, water contaminated by these species can stain water fixtures and
clothing that is washed with this water.
A typical treatment process involves oxidation, which makes the metals
insoluble, followed by filtration.
In general, Fe is fairly easily oxidized by oxygen or
chlorine. Mn is
more difficult to oxidize, but can still be oxidized by chlorine at the proper
pH levels. For both Fe and Mn there are
conditions involving pH or Fe/Mn complexes with organics that make removal less
efficient. In these cases ozone can be
an effective oxidizing agent that is not as sensitive to pH and organics.
Another issue with the use of chlorine based compounds such
as bleach is the potential to form trihalomethanes (THM) from organics in
water. Control of THM has become a
public health issue and the EPA regulates these compounds in water to less than
100 ppb. Some public water systems have
switched to ozone from chlorine due to this issue.
Application of ozone for iron and manganese removal depends
on a variety of factors. The following
discussion provides some base line information on the conditions and amounts of
ozone required. Pilot testing will
define the exact amount of ozone required and the type of ozone generator
equipment required.
Ozone oxidizes iron from Fe (II) to Fe (III). Fe (III) hydrolyzes to Fe (OH)3 which
precipitates to a solid form which can be filtered. The oxidation reaction requires 0.43 mg of
ozone per mg of Fe (II). Excess ozone
can be used without negative effect.
Typically, a dose of 0.50 mg/l is used.
Note that we are referring to the transferred dose of ozone since there
is some loss of ozone in the dissolution process. Fe oxidizes in the pH range of 6-9.
In general, when organic materials are present in water,
more ozone may be required than the amount shown above since ozone will also
oxidize these materials. The nature of
the precipitate will depend on temperature and water chemistry.
Ozone oxidizes Mn (II) to MnO2 (Mn IV) which is insoluble
and can be filtered out of the water.
The oxidation reaction requires 0.88 mg of ozone per mg of Mn (II). Excess ozone beyond this ratio will form
soluble Mn (VII), permanganate, turning the water pink. If oxidizable organic material is present in
the water and there is sufficient contact time, permanganate will be reduced
back to MnO2 (Mn (IV)). Manganese
oxidation is most effective around a pH of 8.
Ozone generator output can be controlled via an ORP monitor
automatically. This prevent over or
under dosing of ozone into the water. It is important to note that at start-up ozone might strip
deposits of iron and manganese in the treatment plant. During the break in period, therefore, iron
and manganese may remain high until these deposits are removed.
To learn more about ozone in drinking water treatment visit:
Post is part of the efforts of the Water Research Center and Carbon County Groundwater Guardians to education private well owners and water suppliers in our area. If you need any further information, please visit our Drinking Water Helpguide Page, Water Quality Library, or search our site.
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