Iron water filters work by oxidizing dissolved iron in water and then filtering out the resulting solid particles. This process typically involves introducing air or an oxidizing agent, followed by a filtration media that traps the precipitated iron.
The Oxidation Process
The first step is oxidizing the dissolved iron (ferrous iron, Fe2+) into insoluble iron (ferric iron, Fe3+), which forms a solid precipitate. This is generally achieved through one of two methods:
- Air Injection (Oxidation): Some systems draw air into the media tank, creating an "air cap." As water flows through this air cap, the oxygen in the air oxidizes the dissolved iron. This is a chemical reaction: 4Fe2+ + O2 + 10H2O → 4Fe(OH)3 + 8H+. The Fe(OH)3 is the solid precipitate.
- Oxidizing Filter Media: Some filters use a media coated with manganese dioxide or similar oxidizing agents. These media promote the oxidation of the iron as the water passes through.
Filtration Media
Once the iron has been oxidized and turned into solid particles, the next step is to physically remove these particles from the water. This is done using a filtration media. Common filter media include:
- Birm: A common and effective media for iron removal. It acts as an insoluble catalyst to enhance the reaction between dissolved oxygen and iron. It doesn't get consumed in the process. Birm media requires no chemical regeneration.
- Manganese Greensand: Coated with manganese oxides, it both oxidizes and filters iron. It requires periodic regeneration with potassium permanganate to replenish the oxidizing coating.
- Multi-Media Filters: These filters use layers of different sized media (e.g., garnet, sand, anthracite) to trap particulate matter, including oxidized iron.
- Calcite: Calcite is used to neutralize acidity to raise the pH levels and assist the filtration process.
The Process in Detail: An Example with Birm
- Water enters the filter tank.
- The water passes through an air cap or directly through the Birm media.
- Dissolved iron is oxidized: Either by air in the air cap or aided by the catalytic action of the Birm. Ferrous iron (Fe2+) converts to ferric iron (Fe3+), forming solid iron particles.
- The solid iron particles are trapped by the Birm media. The media acts as a physical filter, capturing the iron precipitates.
- Clean water exits the filter tank.
Backwashing
Over time, the filter media becomes clogged with the accumulated iron particles. To maintain the filter's effectiveness, a process called backwashing is necessary. Backwashing involves reversing the flow of water through the filter, flushing out the trapped iron particles. The frequency of backwashing depends on the iron concentration in the water and the filter's capacity.
Maintenance
Regular backwashing is essential to the proper function of the filter. Some filters also require periodic regeneration of the filter media (e.g., manganese greensand filters require regeneration with potassium permanganate). Replacing the filter media every few years is also often recommended.
