Iron, even in small quantities, can be one of the most troublesome elements found in water. As little as 0.3 ppm (parts per million) of iron can cause staining of fixtures, sinks, flooring, and also most anything else it comes into contact with. Concentrations below 0.3 ppm can still have profound adverse effects in manufacturing processes.
Iron affects the tastes of foods and beverages, can contribute to the block-age of pipes, and can cause many other unwanted problems. Since it has so many undesirable properties, iron removal is an important phase of water treatment.
Elemental, metallic iron Fe
Ferrous iron Fe + +
Ferric iron Fe + + +
In underground strata, far from the oxidizing effects of oxygen in air, conditions usually favor the reduction of the natural ferric iron deposits to the ferrous state. Since the ferrous salts are highly soluble, ground water supplies frequently carry significant concentrations, and as this ferrous iron is in true solution, the water may be perfectly clear and colorless, with no visible evidence of the iron present. However, when ferrous iron is exposed to the atmosphere, oxygen from the air readily converts it to the ferric state. Ferric iron then reacts with the alkalinity in the water to form ferric hydroxide, the insoluble brown gelatinous matter which causes so much staining.
The corrosion of iron or steel water lines may also add iron to the water. Metal corrosion, an electro-chemical process, converts the elemental metallic iron to the soluble ferrous state. In the absence of oxygen and other oxidizing agents, the ferrous iron may be simply carried away with the water. Where oxidizing materials are present, the insoluble ferric hydroxide forms readily. This, too, may be carried along with the water, or since it is quite insoluble and gelatinous in nature, may deposit and stick inside the water lines. This is true even when natural ferrous iron is oxidized inside of pipes.
Ferric hydroxide which is deposited inside a water line has a tendency to lose water, particularly in hot water lines, according to the reaction:
2 Fe (OH)3 --------------> Fe2 O3 + H2O
This ferric oxide is the same rust which forms when an iron or steel structure is exposed to both air and moisture. During periods of high water flow, these rust particles may break free to cause rust stains on materials which comes into contact with water.
Iron may also be present in water in combination with organic matter. Many natural and man-made organic compounds will react, particularly with ferrous iron, to form heavily colored compounds which can cause severe staining. These compounds are usually very stable, and tie up the iron so that it is not free to react as are other forms. The iron bounds into such compounds called “chelated” or “organic” and clearly present problems in water treatment.
“Iron bacteria” is a term applied to a group of small organisms which appear to convert ferrous iron to the ferric state as part of their metabolism. It is suspected that these organisms may even attack steel pipe to obtain iron, thus causing a form of corrosion. As the iron bacteria grow, they develop masses of gelatinous and filamentous organic matter, which physically trap the ferric hydroxide produced. Heavy growths of these organisms have been known to plug pipes completely, but it is more common for clumps to break away during periods of high flow to produce “slugs” of iron laden water, which can cause all of the previously described staining problems.
Iron bacteria can be identified by a microscopic examination of the turbidity they produce, but the necessary laboratory facilities are not always readily available. However, the presence of a brown, slime-like growth at the surface of the water in a toilet flush tank is a good indication of the presence of iron bacteria in the system.
Iron Removal with Water Softeners
More water softeners are used to remove iron from household water supplies than any other devices or systems. Many of these installations are successful and consistently remove both hardness and iron. At other installations, intermittent leakage of iron through the softener occurs, but the total water quality improvement is so great that the users are reasonably satisfied. In still other cases, softeners fail to do a satisfactory job, passing iron continuously or in “slugs”, or gradually losing capacity due to fouling of the softener bed. Thus, we have several degrees of success with softeners in iron removal.
On some iron waters, it is immediately clear that a softener should not be used. Dissolved organic-iron compounds may simply pass through a softener, unaffected by either ion exchange or the filtering action of the softener bed. Solid organic matter particles frequently contain quantities of precipitated iron, and the combined solids can lead to rapid fouling of the bed. Iron bacteria in water can rapidly foul softener beds, and worse, can grow and pass through the beds in slugs. When such materials are found in the water, softeners should not be applied.
Technically, industrial water softeners can remove dissolved ferrous iron by ion exchange, just as they remove calcium and magnesium. Further, precipitated iron can be removed by filtration. Problems arise, however, in getting the iron out of the softener bed during regeneration. Precipitated iron (ferric hydroxide), formed when soluble ferrous iron is oxidized by oxygen in air, is a gelatinous, sticky material which tends to adhere to the beads or particles in a softener bed. These strictly physical properties make it difficult to rid a softener bed of this material even with a thorough backwash, and it may gradually accumulate in the bed. After a period of this accumulation, slugs of iron may appear in the softened water immediately after regeneration or after abrupt increases in water flow through the softener. Since the materials may also clog the pores of the ion exchange material in the softener bed, it can gradually reduce the hardness removal capacity. Both of these effects create obvious problems for the user of the water.
Ideally, ferrous iron removed by ion exchange should be discharged with the hardness during the usual brine regeneration. In practice, however, it is usual for at least some of this iron to be converted to the ferric, insoluble state by oxygen in the regeneration water or brine. Thus, some of this iron is retained in the softener, and in time, may produce the slugs or fouling previously described above.
Once a softener is badly fouled, it is difficult to clean it except by drastic methods, most of which are not practical. It is frequently more practical to replace the bed with new ion exchange resin rather than to attempt cleaning. Lightly or moderately fouled resin may often be cleaned with a number of proprietary formulations on the market today. These include several mild acids, reducing agents, sequestering or dispersing agents, and blends of several of these materials.
In some installations, large doses of these cleaning materials may be used at extended intervals to rejuvenate fouled beds. Alternately, relatively small doses may be applied with each regeneration in a preventative maintenance program. Some water softener manufacturers have developed automatic dispensers for cleaning agents and have them available either as standard components or as optional accessories.
A number of elements of water softener design can further minimize iron fouling. These include sufficient freeboard above the softener bed to permit full expansion of the bed during backwash, adequate backwash time in the regeneration cycle, and large backwash outlet openings to permit the iron to pass from the softener tank easily. The use of more frequent regenerations with fully automatic softener valves helps to remove precipitated iron before it “sets”, and a fast downflow final rinse packs the bed and reduces slugs of iron into the softened water.
Many factors affect the ability of softeners to remove iron successfully; the form of iron and its concentration, softener design, the presence or absence of organic matter and dissolved oxygen, cleaning procedures, regeneration frequency, pH, temperature, and usage characteristics.
Traditional Iron Removal Methods
As discussed earlier, ion exchange may be used for iron removal, but the method works best when the iron concentration is low and when all or most of the iron is in the soluble state. When the iron in a water supply is largely precipitated, when the concentration is high or substantial organic matter is also present, when the iron is chelated or when iron bacteria are encountered, one of the following methods should be applied.
Relatively high concentrations of inorganic iron, whether ferrous or ferric (dissolved or precipitated), may be removed with iron filters. They are similar in appearance and size to conventional water softeners but contain beds of media which have mild oxidizing power. As the iron-bearing water is passed through the bed, any soluble ferrous iron is converted to the insoluble ferric state and then filtered from the water. Any previously precipitated iron is removed by simple mechanical filtration.
Several different filter media may be used in these iron filters, including manganese greensand, Birm, MTM, multi-media, sand, and other synthetic materials. In most cases, the higher oxides of manganese produce the desired oxidizing action.
Periodic backwashing is necessary to remove the precipitated iron from the bed, and less frequently, regeneration may be necessary to restore the oxidizing power of the filter media. With most media, this is accomplished by passing a solution of potassium permanganate through the bed and then rinsing, a process mechanically similar to the regeneration of a water softener. Usually about four ounces of potassium permanganate is used for each cubic foot of filter media.
A special case exists when sufficient dissolved oxygen can be added to the water and the filter bed then simply serves as a catalyst to speed the reaction between the oxygen and iron. In such installations, the bed must still be backwashed periodically, but no chemical regeneration of the filter media is necessary.
As with water softeners, iron filters do have limitations. Since the oxidizing action is relatively mild, it will not work well when organic matter, either combined with the iron or completely separate, is present in the water and iron bacteria will not be killed. Extremely high iron concentrations may require inconvenient frequent backwashing and/or regeneration. Finally, iron filter media requires high flow rates for proper backwashing and such water flows are not always available.
In those cases where neither ion exchange nor iron filters are applicable, chemical feed pumps and filters may be used in combination with great effectiveness. In such cases, a chemical feed pump may be used to introduce a solution of an oxidizing agent such as sodium or calcium hypochlorite or potassium permanganate, into the feed water. The oxidizing agent will then not only oxidize soluble iron to the insoluble ferric state, but will also attach any organic matter present. When either of the hypochlorites are used, the water will be disinfected at the same time.
These oxidizing solutions should be fed into the water line ahead of a mixing and contact tank to ensure complete reaction with the iron and organic matter and to allow coagulation of small particles into filterable sizes. In most cases, the pressure tank of a private water system fills this need, but occasionally, slowly acting forms of organic matter require additional contact time. In such cases, additional tanks or contact vessels must be provided. Following the mixing and contact, a filter is used to remove the precipitated iron from the water.