Removal of iron and manganese

Iron and manganese occur in ground water (and deep reservoirs) as soluble ferrous (Fe$^{2+}$) and manganous (Mn$^{2+}$) bicarbonates. They have to be oxidised to insoluble ferric hydroxide and manganese dioxide, which can then be settled and filtered.

Methods

1. Aeration, sedimentation and filtration (most common method) Water is aerated (cascade, spray or tray aerator) so it takes oxygen and loses CO$_2$, raising the pH:

$$4Fe(HCO_3)_2 + O_2 + 2H_2O \rightarrow 4Fe(OH)_3\downarrow + 8CO_2$$

The ferric hydroxide floc is settled and then filtered through a rapid sand filter. Fe oxidises easily above pH 7.5; Mn oxidises slowly and needs a pH of about 9.5, so lime may be added.

2. Chemical oxidation Chlorine, potassium permanganate or ozone oxidise Fe and Mn quickly:

$$3Fe^{2+} + MnO_4^- + 7H_2O \rightarrow 3Fe(OH)_3\downarrow + MnO_2\downarrow + 5H^+$$ $$3Mn^{2+} + 2MnO_4^- + 2H_2O \rightarrow 5MnO_2\downarrow + 4H^+$$

Chlorine dose is about 0.64 mg/l per mg/l Fe and 1.3 mg/l per mg/l Mn. The precipitate is removed by coagulation and filtration.

3. Manganese greensand filter: the glauconite sand coated with MnO$_2$ oxidises and filters Fe and Mn, and is regenerated with KMnO$_4$.

4. Lime softening: at pH above 10 Fe and Mn precipitate as hydroxides along with hardness.

5. Ion exchange (zeolite) softener: removes small amounts of Fe and Mn, but the resin gets fouled when oxidised iron is present.

6. Others: coagulation with alum at pH above 8, and in-situ (underground) treatment.

Treated water should have Fe below 0.3 mg/l and Mn below about 0.05 to 0.1 mg/l, the levels at which taste and staining problems are avoided (Nepal's NDWQS limits are Fe 0.3 mg/l and Mn 0.2 mg/l).