This is a filtration method in which iron contained in raw water is oxidized with oxygen or sodium hypochlorite, precipitated as insoluble iron hydroxide, and then captured with a filter medium. Anthracite, filtered sand, ceramics S, etc. are generally used as the filter material. In addition, the fiber filter material “Acty Fiber” can also be used as our original filter material.

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1. By air oxidation
Iron in groundwater is generally said to exist in the form of ferrous bicarbonate. The oxidation reaction of ferric bicarbonate by contact with dissolved oxygen in raw water　and oxygen in the air is as follows,　and the produced Fe (OH) 3　or Fe2O3 / 3H2O becomes insoluble SS.

2Fe(HCO3)2　　＋　1/2O2　＋　H2O　→　2Fe(OH)3　　　 ＋　4CO2
2Fe(HCO3)2　　＋　1/2O2　＋　H2O　→　Fe2O3・3H2O　＋　4CO2

2. When due to chlorine oxidation
The oxidation reaction of ferrous bicarbonate by sodium hypochlorite is as follows,　and the produced Fe(OH)₃　or Fe₂O₃・3H₂O is an insoluble SS.

2Fe(HCO₃)₂ ＋ NaClO ＋ H₂O　→　2Fe(OH)₃ ＋　4CO₂＋　NaCl
2Fe(HCO₃)₂ ＋ NaClO ＋ H₂O　→　Fe₂O₃・3H₂O＋　4CO₂＋　NaCl

Compared to the catalytic oxidation method, the physical filtration method enables more precise filtration　and can almost completely remove iron from the treated water. However, since Fe(OH)₃ generated by the above reactions 1　and 2 is physically removed by the voids in the filter medium, the filtration duration, that is, the reverse, depends on the iron concentration in the raw water. The wash cycle may be shorter. In that case, a multi-layer filtration method, which improves the amount of iron trapped by stacking multiple layers of filter media, is effective.
Also, in the case of air oxidation, a large area is required as a pretreatment tank in order to completely oxidize iron. Fe(OH)₃ generated at that time may become fine SS, so it is necessary to consider the processing conditions. Therefore, in the air oxidation method, it is recommended to use it together with the contact filtration method.

[Reference] by Yu Takai　and Hiroshi Nakanishi, “Iron removal　and manganese removal treatment for water”