The Top 4 Oxidation Methods For Iron & Manganese
By Gerry Bulfin, CEO, Clean Water Systems & Stores Inc.
Iron is one of the earth's most plentiful resources, making up at least five percent of the earth's crust. In well water iron is usually found as ferrous iron, which is in a dissolved state and may appear clear when first drawn from the tap.
When the level of iron in water exceeds the 0.3 mg/l limit, the water may have a red, brown, or yellow color and stain laundry and fixtures. The water may also have a metallic taste and an offensive odor. Water system distribution piping can become restricted or clogged, and for homeowners, appliances such as water heaters, dishwashers and washing machines plugged with rust and sediment.
In some applications ion-exchange resin can be used to remove ferrous iron in exchange for sodium, but the most popular and common method is oxidize the iron to a ferric state and then filter out the precipitated particles.
One way to accomplish this is to use one of many types of oxidizing manganese-based filter media, sold under various brand names. To work properly this media requires one of four oxidants:
- Air
- Chlorine
- Potassium Permanganate
- Ozone
After injection with air, chlorine, permanganate or ozone, the water flows through the iron filter media and oxidation takes place on the manganese dioxide filter media. Periodic backwash and rinses clean the iron filter media of the trapped media.
Manganese Dioxide Coated Iron Filter Media
Coated filter media such as manganese greensand typically contain 1% - 10% manganese oxide in a coating over a mineral substrate such as silica or dolomite.
To provide the oxidizing power to precipitate iron and manganese, small greensand iron filters are automatically cleaned and restored with potassium permanganate during each backwash cycle. This is known as ‘intermittent regeneration’.
As an alternative to using potassium permanganate, chlorine is injected ahead of the greensand filter to regenerate the filter media continuously. This is known as Continuous Regeneration.
In some applications where the water has hydrogen sulfide “rotten-egg” odor in it, and/or iron bacteria, pre-chlorination or ozone gas injection also aids in oxidizing the hydrogen sulfide and disinfecting the water. Ozone has the added benefit of not increasing the formation of THM’s if organic matter is present as is the case with chlorine.
Manganese Dioxide Solid Core Iron Filter Media
Unlike Manganese Greensand which is coated with a manganese oxide coating, manganese dioxide iron filters use a natural mined solid manganese oxide ore in a relatively pure form.
Most manganese ore is found in lower concentrations of less than 50% manganese, and is widely used to in the manufacture of batteries and many other important industrial uses. In a few areas in the world, there are mines that contain manganese with a very high concentration of manganese oxide. This mineral is selected and carefully graded especially for water treatment.
Manganese dioxide solid core iron filter media utilize an oxidation-reduction reaction and filtration process like greensand, but at a much higher level of performance. This type of filter media contains greater than 85% manganese dioxide.
This type of solid manganese dioxide media is heavier and requires a strong backwash flow rate to lift and clean the oxidized particles from the filter bed. Because of the higher performance, faster throughput can be realized, and higher flow rates with smaller filter vessels can be used, which compensates somewhat for the higher backwash flow required.
This type of solid manganese dioxide is very long lasting and easily lasts 10 to 15 years.
These filters can use either Intermittent or Continuous Regeneration.
In many applications where the ORP can be raised above 200 millivolts with aeration alone, no chemical regeneration is required. A typical treatment plant will include air injection under pressure where the water flows through a tank containing dissolved air. This increases the dissolved oxygen in the water and raises the ORP so the manganese dioxide media can be regenerated with simple backwashing and rinsing.
Gerry Bulfin CWS-VI is CEO of Clean Water Systems & Stores Inc. Bulfin can be reached at gb@cleanwaterstore.com or 831.462.8500.