---
title: "Removal of iron and manganese"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:iron-manganese-removal"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/iron-manganese-removal
site: BCT Engineering
---

# Removal of iron and manganese

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2081 Baisakh, 3 marks; 2075 Asoj, 4 marks; 2067 Asar (old course) (asked 3 times)
- Video: [How to Remove Iron & Manganese from Water (Complete Guide)](https://www.youtube.com/watch?v=IwXINW7F9qc) — WATERMAN
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/iron-manganese-removal
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Explain how iron (and manganese) is removed from water (by aeration and other methods).

## Short answer

Iron and manganese are removed by aeration, which oxidises Fe2+ to Fe(OH)3 floc, settled and filtered. Mn2+ requires higher pH (about 9.5) or strong oxidants like chlorine or potassium permanganate, followed by sedimentation and filtration.

## Answer

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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the basic principle of removing iron and manganese?
   A. Dissolving them further
   B. Oxidising soluble Fe2+ and Mn2+ to insoluble ferric hydroxide and MnO2 that can be settled and filtered
   C. Boiling the water
   D. Adding more CO2
   Answer: B. The soluble forms must be converted to insoluble precipitates before sedimentation and filtration remove them. (Key point: Principle)

2. Which statement about oxidation by aeration is correct?
   A. Mn oxidises faster than Fe
   B. Both oxidise equally at pH 5
   C. Fe oxidises easily above pH 7.5, while Mn needs pH about 9.5 so lime may be added
   D. Neither is affected by pH
   Answer: C. Manganese oxidises slowly and needs a high pH, so lime is added to raise it. (Key point: pH for oxidation)

3. Which is a chemical oxidation method for Fe and Mn?
   A. Slow sand filtration only
   B. Chlorine, potassium permanganate or ozone
   C. Plain sedimentation
   D. Screening
   Answer: B. Chlorine, KMnO4 and ozone oxidise Fe and Mn quickly; the precipitate is then removed by coagulation and filtration. (Key point: Alternative methods)

4. How is manganese greensand regenerated?
   A. With KMnO4
   B. With brine
   C. With lime
   D. With sulphur dioxide
   Answer: A. Greensand coated with MnO2 oxidises and filters Fe and Mn, and is regenerated with potassium permanganate. (Key point: Greensand)

## Related in Water Supply Engineering

- [Iron and manganese: removal methods, aeration](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Airon-manganese-objectionable-aeration) — Water Supply Engineering, chapter 6
- [Aeration purposes; Fe/Mn removal; residual chlorine](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aaeration-iron-residual-chlorine) — Water Supply Engineering, chapter 6