Iron and manganese are usually removed by oxidation of the dissolved
forms into an insoluble form by aeration or by chemical oxidization followed
by sand filtration. The success of removal by oxidation depends not only on
the oxidant used and its concentration, but also on pH and on the presence
of natural organic matter. Oxidation takes place at a faster rate at higher pH
values and the presence of organic matter makes removal more difficult. Both
iron and manganese tend to form bonds with humic acids and other natural
organic matter compounds present in water.When air is used as oxidant
this causes removal difficulties and in this case the oxidation with ozone is
recommended. In general, the removal of iron is normally easier than that of
manganese, but a high content of iron requires treatment with several tanks
in series. It has been stated that the oxidation of iron with ozone is rapid, but tends to form colloidal particles difficult to remove by sand or anthracite
filtration [37]. The ozone dose required for oxidation can be estimated stoichiometrically
as 0.43 mg/mg iron and 0.88 mg/mg manganese, the latter for 8.0 < pH < 8.5, from the following reactions