Iron (Fe) and manganese (Mn) in their soluble form are colorless in water. They react with dissolved oxygen or chemical oxidizers in water to form insoluble compounds. These insoluble compounds give water a red or black color causing many customer complaints. Higher concentrations are generally associated with deep groundwater wells or when using the bottom intake.
Iron and manganese can cause staining of laundry and plumbing fixtures, discoloration of water (rust to black), turbidity formation and is a food source for iron-reducing bacteria. Iron-reducing bacteria can form thick slime layers on the walls of pipe, causing a restriction to flow. When this slime breaks away, rust-colored water (iron) and/or black particulates (manganese) develop. Flushing of the distribution can also cause discoloration, so the public should be informed of all planned flushing activities. Iron-reducing bacteria can also cause taste and odor as a by-product of their reproduction. Chlorine can control the growth of iron bacteria.
The normal mode of treatment for iron and manganese is through oxidation. Oxidation is the introduction of oxygen atoms, which chemically converts soluble iron and manganese compounds to insoluble compounds. Dissolved oxygen or chemical oxidants may be used to complete the chemical reaction. A retention period and then sedimentation and/or filtration to remove the precipitates must follow the process. This can be accomplished in different ways.
Oxidation by aeration brings air and water into physical contact with one another. A higher pH results in a faster reaction: effective manganese precipitation requires a pH greater than 10. In addition, this process is expensive to operate. The reaction is too slow for effective manganese removal and has reduced effectiveness in colder temperatures.
Oxidation with chlorine will precipitate Fe and Mn. A higher chlorine residual results in a faster reaction. Superchlorination (5-10 ppm) followed by dechlorination allows quicker precipitation and a less retention time requirement. This process is effective and inexpensive, but it requires retention time and filtration to complete the process. The proper dosage and reaction time are also very important. If you dose too little, you will not treat the iron and manganese. If you overdose, this creates more disinfection by-products (DBPs), which you will have to deal with later in the water system.
Another chemical used for oxidation of Fe and Mn is potassium permanganate. It works like chlorine, and an exact dosage is required. Too little results in no precipitation; too much results in a carry over of pink-colored water.
Another process that simultaneously oxidizes and filters out iron and manganese is called greensand filtration. Greensand filter media requires periodic regeneration with potassium permanganate. The permanganate is generally fed prior to the greensand filter to aid in oxidation and provide continuous regeneration. This is an effective alternative to chlorine when high chlorine dosages are a concern.
The removal of iron and manganese is sometimes necessary to meet the secondary standards required for drinking water. The secondary standards are aesthetic in nature and not health related.