Iron and Manganese Removal CE 370 Iron and Manganese Removal

Fe++ and Mn++ Ferrous iron Fe++ and manganous manganese Mn++ are soluble invisible When exposed to air, they are oxidized to ferric iron Fe+++ and manganic manganese Mn++++, which are: insoluble visible

Oxidation The oxidation rate depends on: pH alkalinity organic content presence of oxidizing agents 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

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: 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 post-chlorination. When oxidized: it can clog the solution-feed chlorinator it cause a staining water

Aeration, Chemical Oxidation, Sedimentation, and Filtration 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

Manganese Zeolite Process 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 New process In the form of filters that support: Iron-oxidizing bacteria Manganese-oxidizing bacteria Two separate filters are used pH and dissolved oxygen of the feed are critical Filtration rates are: 25 to 40 m/h for iron 10 to 40 m/h for manganese Coarse sand is used

Sequestering Sequestering agents are used Agents are added: Sodium silicates Phosphates polyphosphates Agents are added: 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