Iron is a water contaminant that takes two major forms. The water-soluble form is known as the ferrous state and has a + 2 valence state. In non-aerated well waters ferrous iron behaves much like calcium or magnesium hardness in that it can be removed by softeners or its precipitation in the back end of the RO system can be controlled by the use of a dispersant chemical in an RO feed water. The water-insoluble form is known as the ferric state and has a + 3 valence state. Typically, RO manufacturers will recommend that combined iron levels be less than 0.05 ppm in the RO feed.

         If all iron is in the soluble ferrous form, iron levels up to 0.5 ppm in the feed can be tolerated if the pH is less than 7.0 (though an iron dispersant is recommended). The introduction of air into water with soluble ferrous iron will result in the oxidation to insoluble ferric iron.

          Soluble iron can be found in deep wells, but can be converted into the more troublesome insoluble iron by the introduction of air by being placed in tanks or by leaky pump seals. Soluble iron can be treated with dispersants or can be removed by iron filters, softeners or lime softening.

          Insoluble ferric iron oxides or ferric hydroxides, being colloidal in nature, will foul the front end of the RO system. Sources of insoluble iron are aerated well waters, surface sources, and iron scale from unlined pipe and tanks. Insoluble iron can be removed by iron filters, lime softening, softeners (with limits), ultrafiltration (with limits) and multimedia filtration with polyelectrolyte feed (with limits). Precautions are required with the use of potassium permanganate in manganese greensand iron filters in that potassium permanganate is an oxidant that could damage any polyamide membrane. Precautions are also required with a cationic polyelectrolyte in that they can irreversibly foul a negatively charged polyamide membrane. Corrosion proof vessels and piping (e.g. FRP, PVC or stainless steels) are recommended for all RO systems, RO pretreatment, and distribution piping coming to the RO system.

          Iron as foulant will quickly increase RO feed pressure requirements and increase permeate TDS. In some cases, the presence of iron can create a bio-fouling problem by being the energy source for iron-reducing bacteria. Iron-reducing bacteria can cause the formation of a slimy biofilm that can plug the RO feed path.

           Many RO systems operate with ferric chloride used as a coagulant in pretreatment. The ferric ions complex with the colloidal material in the feed and grow into floc which should be removed by pretreatment. When ferric chloride is added, the chloride will add extra TDS to the feed. Problems with the introduction of ferric chloride occur when excess iron is not removed by the pretreatment. As a result significant iron precipitates may be found on the membrane surface. The presence trace amounts of iron on the membrane is not a problem by itself. However, if free chlorine is passed to the RO, then the iron can catalyze and accelerate the chlorine oxidation of the membrane.

            There does not exist clear information on the catalytic effect of iron when chloramines are present, but it is possible that the reaction rate will be higher.