Download Iron and Manganese Removal

CE 370
Iron and Manganese
Removal
Fe++ and Mn++
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Ferrous iron Fe++ and manganous manganese
Mn++ are
 soluble
 invisible
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When exposed to air, they are oxidized to
ferric iron Fe+++ and manganic manganese
Mn++++, which are:
 insoluble
 visible
Oxidation
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The oxidation rate depends on:
 pH
 alkalinity
 organic
content
 presence of oxidizing agents
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If not removed from water, iron and
manganese oxides create problem of brown
color formation.
Oxidation
Reduced iron in water promotes the growth of
autotrophic bacteria in distribution mains.
 Elimination of iron bacteria is difficult and
expensive.
 When decays, iron bacteria release foul taste
and odor.
 Heavy chlorination followed by flushing
proved to be successful in some case.
 Removal of iron and manganese from water is
the best solution to that problem.

Oxidation of Iron and Manganese with
Oxygen, Chlorine, and Chlorine
Dioxide
4 Fe 2  O2  10 H 2O  4 Fe(OH ) 3  8 H 
2 Fe 2  Cl2  6 H 2O  2 Fe(OH ) 3  6 H   2Cl 
Fe 2  ClO2  3H 2O  Fe(OH ) 3  ClO2  3H 
2 Mn 2  O2  2 H 2O  2 MnO2  4 H 
Mn 2  Cl2  2 H 2O  MnO2  4 H   2Cl 
Mn 2  2ClO2  2 H 2O  MnO2  2ClO2  4 H 
Treatment Alternatives
Aeration, Sedimentation, and
Filtration

Aeration (air oxidation) is the simplest treatment in
removing iron. The reaction that takes place is in the
form of:
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Fe++(ferrous) + oxygen  FeOx (ferric oxide)
Soluble iron
Insoluble iron
Air oxidation can not remove manganese effectively.
Increasing the pH to 8.5 can enhance the oxidation
process. If manganese was not effectively removed
from the water, it can cause problems with postchlorination. When oxidized:


it can clog the solution-feed chlorinator
it cause a staining water
Aeration, Chemical Oxidation,
Sedimentation, and Filtration
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This is a common method for removing iron and manganese from
well water without softening treatment.
Preliminary aeration strips out dissolved gases and adds oxygen.
Iron and manganese are oxidized by free chlorine residual:
 Fe++ + Mn++ + Oxygen  FeOx + MnO2
Soluble ions
Insoluble metal oxides
or by potassium permanganate
 Fe(HCO3)2 + KMnO4  Fe(OH)3 +
MnO2
Ferrous bic. Potas. Perm.
Ferric Hydrx. Mang. Diox.
 Mn(HCO3)2
+ KMnO4
 MnO2
Manganous bic Potas Perm
Mang. Diox.
Potassium permanganate oxidizes iron and manganese at rates faster
than dissolved oxygen and its reaction is relatively pH independent.
Since iron and manganese can not be completely removed by
sedimentation, effective filtration is required.
Manganese Zeolite Process
Manganese zeolite is a natural greensand coated
with manganese dioxide. It has the capability to
remove iron and manganese from solution.
When manganese zeolite becomes saturated with
metal ions, it can be generated using potassium
permanganate. A continuos flow diagram is
shown in the following Figure.
Reaction Equations
Oxidation
Z  MnO2  Fe2  Z  Mn2O3  Fe3
Z  MnO2  Mn 2  Z  Mn2O3  Mn 3
Re generation
Z  Mn2O3  KMnO4  Z  MnO2
Manganese Zeolite Process
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KMnO4 is applied prior to filtration.
The filter is a dual-media filter with anthracite is
placed on top of the manganese zeolite.
Iron and manganese are oxidized by KMnO4.
The upper layer will remove the insoluble metal ions.
Any iron and manganese ions not oxidized, it will be
captured by the lower layer of manganese zeolite.
Any surplus KMnO4 will regenerate the greensand.
When the bed becomes saturated, it is backwashed by
KMnO4 to remove particles from the upper layer and
regenerate the greensand.
Water Softening
Lime and soda ash, which are used in water
softening, can remove iron and manganese from
water.
Biological Treatment
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New process
In the form of filters that support:
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Two separate filters are used
pH and dissolved oxygen of the feed are critical
Filtration rates are:
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Iron-oxidizing bacteria
Manganese-oxidizing bacteria
25 to 40 m/h for iron
10 to 40 m/h for manganese
Coarse sand is used
Sequestering
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Sequestering agents are used
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Agents are added:
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Sodium silicates
Phosphates
polyphosphates
In the form of colloidal complexes
to bind iron and manganese and prevent them from forming color and
turbidity
In distribution system
Chlorine is added simultaneously to oxidize iron and
manganese
Normal doses of silicates are 5 to 25 mg/l as SiO2
Iron is sequestered more with silicates than manganese