Iron Manganese Removal with Ozone
Iron and manganese removal is one of the more common uses for ozone in drinking water systems. Iron and manganese are easily oxidized by ozone. This document will serve to help understand the fundamentals of iron and manganese oxidation with ozone. We will also cover the practical application of ozone in this application while offering helpful tips learned over the years.
Ozone oxidation of iron and manganese is an extremely fast reaction. In many ozone applications, elevated levels of iron and manganese can cause nuisance issues due to soluble iron and manganese inadvertently oxidizing by ozone and dropping out of solution in less than ideal locations. If those concerns are what brought you here, keep reading, we will offer helpful tips to mitigate these issues as best as possible.
Iron and manganese in water cause no health-related issues, the main purpose for iron and manganese removal is aesthetics due to the discolouration of water. Removal also may be necessary due to the build-up of iron and manganese on pipes, fixtures and other surfaces.
Both Iron Fe(II) and Manganese Mn(II) are soluble (non-removable) in water causing them to flow directly through conventional filtrations without some form of oxidation to transform them into particulates (removable).
Soluble Iron Fe(II) is called ferrous iron. Ferrous Iron Fe(II) is oxidized to Ferric Iron Fe(III) by ozone. This Ferric Iron Fe(III) will then hydrolyze to form Fe(OH)3 which is particulate and can be removed by standard filtration. The reaction of Ferrous Iron Fe(II) to Ferric Iron Fe(II) consumes 0.43 mg of ozone per mg of Fe(II). Iron can also be oxidized by oxygen. Due to the oxidation of iron by oxygen, an ozone system for iron removal may be more efficient than the calculated ozone demand of 0.43 mg ozone per mg iron.
The oxidation of ferrous iron requires only an electron exchange and therefore is a fast reaction. The speed of this reaction will typically consume almost all ozone in iron oxidation reaction before any manganese oxidation.
Soluble Manganese Mn(II) is oxidized by ozone to form manganese dioxide MnO2 which is particulate and can be easily removed by standard filtration. This process consumes 0.88 mg of ozone per mg of Manganese Mn(II). However, over oxidation of manganese will form soluble permanganate MnO4. While permanganate will normally return to manganese dioxide MnO2 over time (20-30 minutes), it is best to design a manganese removal system with the proper ozone dosages and integrated controls to prevent over oxidation.
In general, if organic substances are present in the water, ozone will also oxidize these substances, so more ozone is needed than the above amounts. The type of precipitation depends on temperature and water chemistry.
It is important to note that ozone can remove iron and manganese deposits in treatment plants during start-up. Therefore, iron and manganese can remain high during the run-in period until these deposits are removed. Ozone not only removes iron and manganese but also offers other benefits such as micro flocculant for water treatment which leads to improved filtration.