Certification Action Line: Oxidation and iron

Feb. 1, 2016

Certification Action Line features questions and answers typical of those appearing in Water Quality Association certification examinations. Some answers may not satisfy everyone or every condition.

1. Oxidizing filters are used mainly to remove what from water?

a. Bacteria
b. Iron
c. Arsenic
d. Magnesium

2. True or False: Oxidation means combining with oxygen.

3. If iron and manganese are not completely removed by oxidation using chemical injection and filtration treatment, the problem may be_____.

a. Too much water hardness
b. High pH
c. Low contact time
d. Warm water temperature

4. True or False: Manganese can be removed from water by adding or exposing other manganese to the water.

5. True or False: You must ensure that no oxidant other than potassium permanganate is fed ahead of a manganese-coated filter.

6. True or False: Iron will settle out of water.

7. True or False: Iron does not impart a taste to water.

8. True or False: Treatment to remove iron and manganese from the water supply will not remove existing stains from bathroom and kitchen fixtures.

9. True or False: Removing iron and manganese from water will help in clothes laundering.

10. True or False: An oxidizing filter cannot be used to remove the rotten-egg smell from water.


1. b.  The soluble forms of iron, manganese and hydrogen sulfide in water can each be oxidized to insoluble forms directly in an oxidizing filter, and the precipitates are also removed by the filters’ mechanical filtration action.

2. False. In earlier years, oxidation referred to combining a substance with oxygen, but as chemistry has advanced, oxidation has come to mean a loss of electrons and gain in positive valence. For example, ferrous iron (Fe++), manganous manganese (Mn++) and sulfide (S) can be oxidized respectively to ferric iron (Fe+++), manganic manganese (Mn++++) and elemental sulfur (Sº). They each become insoluble precipitates in an oxidized form.

3. c. Adequate oxidant contact and contact time are critical for oxidation and the removal of iron, manganese and hydrogen sulfide. Sufficient oxidant dosage and a minimum of 20 minutes is required for iron, manganese and hydrogen sulfide (H2S) to oxidize and come out of solution. This allows time for the reaction to complete and for the precipitate to grow to a filterable size. The kinetics, or rate of the reaction, are dependent on the pH level. The higher the pH, the faster the rate of reaction. The reaction rates are also affected by temperature; the presence of organic material such as tannins; a high level of a particulate, iron, manganese and hydrogen sulfide concentration; and the choice of the oxidant and oxidant concentration.

4. True. Manganese occurs in numerous oxidation states including Mn+2, Mn+4 and Mn+7. Manganeses in potassium permangate carries an oxidation state of +7, so it oxidizes soluble Mn+2 to insoluble Mn+4. Also, coatings of manganese dioxide on filter materials create a large adsorption capacity for both ferrous iron (Fe+2) and Mn+2.

5. False. No harmful effects to a manganese-coated filter exist from prefeeding chlorine or ozone, for example. This is particularly beneficial with high levels of iron and hydrogen sulfide. It allows an effective oxidization for the hydrogen sulfide and a portion of the iron, a decrease in load on the oxidizing media, and a reduction of the frequency of regeneration.

6. True. If you let a pitcher of water sit out overnight and a layer of red or brown precipitate forms at the bottom, the precipitate is probably iron settling out of the water. When the water was drawn from the tap, it was likely aerated through the screen on the faucet, and the iron precipitated as oxidized ferric hydroxide upon standing in the pitcher. The precipitate is not harmful to health; it is just ugly.

7. False. People who have been raised on water with a high iron content sometimes say that it makes the water taste sweet. Those who are more accustomed to iron-free water frequently say that iron makes water taste metallic.

8. True. Cleaning removes deposits already on fixtures, and they will not return if the staining characteristics of the water are removed. Products on the market may remove stains on the fixtures, but they are frequently very difficult to remove because the deposit has leached into the porcelain.

9. True. Iron can cause clothing to yellow, and manganese causes laundry to grey. Reducing these materials will eliminate these problems. Clothing that is already stained cannot be restored, but future problems will be prevented.

10. False. The rotten-egg smell is likely hydrogen sulfide. A proper oxidizing filter will reduce the odor, but you should also check for other potential issues. If the problem only occurs in the hot water line, it could be generated inside the home. Frequently, hot water heaters are provided with a magnesium sacrificial anode to prevent the water heater tank from corroding. Under the right conditions, this sacrificial anode forms hydrogen sulfide when combined with sulfates and sulfate that reduce bacteria in the water. If the water in the area is corrosive, an aluminum or zinc sacrificial anode may be substituted. If the odor only occurs in major appliances such as a washing machine, the problem may also be sulfur-reducing bacteria. In this case, try superchlorinating the house water system to control the bacteria through this process. However, if neither resolves the issue, you might need an oxidizing filter.

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