We indicated in our last note that we would start a series of postings on applications for ozone in water treatment. The first application we are going to consider is iron and manganese removal. Iron and manganese are primarily a nuisance in water treatment since water contaminated by these species can stain water fixtures, and clothing that is washed with this water. In some industrial applications the presence of these materials can affect the operation of a process or the quality of a product.
There are numerous methods available for treating iron in water including simple aeration. Aeration is an option assuming the Fe is not complexed with organic materials or the reaction has to take place under acidic conditions. Manganese, complexed or not, cannot be oxidized by aeration. Chlorine can also be used for oxidation of iron and manganese, but the use of chlorine can result in the formation of THM if organic material is present in the water.
In cases where standard oxidants will not work or will cause problems downstream, ozone is may be a good choice. Application of ozone for iron and manganese removal depends on a variety of factors. We provide some base line information on the conditions and amounts of ozone required below. Pilot testing will define the exact amount of ozone required and the type of ozone generator equipment needed.
Ozone oxidizes iron from Fe (II) to Fe (III). Fe (III) hydrolyzes to Fe (OH)3 which precipitates to a solid form which can be filtered. It should be noted in most treatment methods including those using air or chlorine, that filtration is an important step in the process. The oxidation reaction requires 0.43 mg of ozone per mg of Fe (II). Excess ozone can be used without negative effect. Fe oxidizes in the pH range of 6-9. As soon as ozone is applied to water bearing ozone, it will be immediately oxidized. The water will turn a cloudy brown color.
Ozone oxidizes Mn (II) to MnO2 (Mn IV) which is insoluble and can be filtered out of the water. The oxidation reaction requires 0.88 mg of ozone per mg of Mn (II). Excess ozone beyond this ratio will form soluble Mn (VII), permanganate. This will be clear as the water will turn pink. If oxidizable organic material is present in the water and there is sufficient contact time, permanganate will be reduced back to MnO2 (Mn (IV)). Manganese oxidation is most effective around a pH of 8.
In general, when organic materials are present in water, more ozone will be required than the amount shown above since ozone will at least partially oxidize these materials. As we noted filtration is an important part of the process. The nature of the precipitated iron or manganese will vary depending on the nature of the organics present, pH and other factors. So selection of the filter requires some analysis.
It is important to note that if ozone is replacing another oxidant that at start-up ozone there might be stripping of deposits of iron and manganese existing in the piping and on pieces of equipment found in the water treatment plant. During the break in period, therefore, iron and manganese may remain high until these deposits are removed.
Ozone is most often used in applications where some of its other benefits can be used. Besides removing iron and manganese ozone will act as a micro flocculent to improve filtration, it is a potent disinfectant that may reduce the amount of other disinfectants required downstream and it can remove taste, odor and color from the water.
Ozone use is not indicated in all situations. If more than 100 micrograms of bromide ion are present the formation of bromated might be possible. With water temperature above 105 degree F ozone will decompose prematurely. Pilot testing should be conducted before ozone is selected.