Next to hardness, the presence of iron is probably the most common water problem faced by consumers and water treatment professionals. The secondary (aesthetic) maximum contaminant levels (MCL) for iron and manganese are 0.3 milligrams per liter (mg/l) and 0.05 mg/l, respectively. Iron and manganese in excess of the suggested maximum contaminant levels (MCL) usually results in discolored water, laundry, and plumbing fixtures.
Small amounts of iron are often found in water because of the large amount of iron present in the soil and because corrosive water will pick up iron from pipes. Clothing washed in water containing excessive iron may become stained a brownish color. The taste of beverages, such as tea and coffee, may also be affected by iron. Manganese produces a brownish color in laundered clothing, leaves black particles on fixtures and as with iron, affects the taste of beverages, including coffee and tea.
Well water from the faucet or tap is usually clear and colorless. However, when water containing colorless, dissolved iron is allowed to stand in a cooking container or comes in contact with a sink or bathtub, the iron combines with oxygen from the air to form reddish-brown particles (commonly called rust). Manganese forms brownish-black particles. These impurities can give a metallic taste to water or to food.
The rusty or brown stains on plumbing fixtures, fabrics, dishes, and utensils cannot be removed by soaps or detergents. Bleaches and alkaline builders (often sodium phosphate) can make the stains worse. Over time, iron deposits can build up in pressure tanks, water heaters, and pipelines, reducing the quantity and pressure of the water supply.
Unluckily, iron and manganese can often be quite difficult to treat. This is due primarily to the fact that iron can be present in several forms, and each form can potentially require a different method of removal.
Types of Iron
There are three main forms of iron and manganese. Other types are much rarer:
- Ferrous - This type of iron is often called "clear water iron" since it is not visible in poured water. It is found in water which contains no oxygen, such as water from deep wells or groundwater. Carbon dioxide reacts with iron in the ground to form water-soluble ferrous bicarbonate, which, in the water, produces ferrous ions (Fe++).
- Ferric - Ferric iron is also known as "red water iron". This type of iron is basically ferrous iron which has been exposed to oxygen (oxidized), usually from the air. As carbon dioxide leave the water, oxygen combines with the iron to form ferric ions (Fe+++). These oxidized particles are generally visible in poured water.
- Bacterial Iron - Slime depositing in toilet tanks or fouling water filters and softeners is a good indication of the presence of bacterial iron. Better described as iron biofouling, the iron bacteria problem is both complex and widespread. It attacks wells and water systems around the world in all sorts of aquifer environments, both contaminated and pristine. In some places, it causes great damage; in others, it is considered a minor nuisance.
Iron bacteria can be controlled by periodic well chlorination or it can be treated in the building. The treatment involves the following: Chlorination, retention, filtration. Activated carbon is usually used as the filter material so the excess chlorine can also be removed.
In theory, the elimination of ferric iron is simple - use a properly sized media filter to filter it from the water. In practice, however, there may be other issues:
- Some iron may be present in colloidal form. Unlike ferric iron, which will generally stick together to form large flakes, the tiny particles of colloidal iron do the opposite. Their large surface area and charge relative to their mass causes the individual particles to repel one another. As a result they will not coagulate. Their small size, then, makes them difficult to filter, and a coagulating agent is often required to obtain adequate filtration.
- Most water containing ferric iron also contains ferrous iron. This can add complexity to the process, since some of the methods for removing ferrous iron will also remove ferric iron.
There are a variety of ways for removing ferrous iron, each with its own strengths and limitations. These methods fall into two categories: Ion exchange and Oxidation / filtration
Ion Exchange (Water Softener)
Ion exchange should be considered only for the removal of small quantities of iron and manganese. For practical purposes in an everyday working softener, the upper limit is about 5 to 7 parts per million. Ion exchange involves the use of synthetic resins where a presaturant ion on the solid phase (the "adsorbent," usually sodium) is exchanged for the unwanted ions in the water. One of the major difficulties in using this method for controlling iron and manganese is that if any oxidation occurs during the process, the resulting precipitate can coat and foul the media. Cleaning would then be required using acid or sodium bisulfate
Where the concentration of iron is above 5 or 6 parts per million, or when there is both dissolved and precipitated iron or manganese in the water, a different approach is needed.
Oxidation / Filtration
Oxidation followed by filtration is a relatively simple process. The oxidant chemically oxidizes the iron or manganese (forming a particle), and kills iron bacteria and any other disease-causing bacteria that may be present. The filter then removes the iron or manganese particles.
Before iron and manganese can be filtered, they need to be oxidized to a state in which they can form insoluble complexes. Oxidation involves the transfer of electrons from the iron, manganese, or other chemicals being treated to the oxidizing agent. Oxidation methods fall into two groups: those using additives like chlorine, ozone or air; or those using an oxidizing filter media.
- Ozonation - An ozone generator is used to make ozone that is then fed by pump or by an air injector into the water stream to convert ferrous iron into ferric iron. Ozone has the greatest oxidizing potential of the common oxidizers. This is followed by a contact time tank and then by a catalytic medium or an inert multilayered filter for removal of the ferric iron.
- Chlorination - Chlorine can be introduced into water in one of several forms: a gas; as calcium hypochlorite; or commonly, as sodium hypochlorite. The treated water is then held in a retention tank where the iron precipitates out and is then removed by filtering with manganese greensand, anthracite/greensand or activated carbon. If applied this way, a dosage of one part of chlorine to each part of iron is used and 0.2 parts of potassium permanganate per part of iron is fed into the water downstream of the chlorine. The potassium permanganate and any chlorine residual serve to continuously regenerate the greensand.
- Aeration - Air is also used to convert dissolved iron into a form that can be filtered. This approach mimics what happens when untreated dissolved iron comes into contact with the air after leaving a faucet. Aeration methods can be of a two-tank or a single-tank variety. In a two-tank system, air is introduced into the first tank using a pump or other injection device. The dissolved iron precipitates in the first tank and is carried into the second tank where it is filtered in a Birm or multi-media filter. One drawback to this system is that water bearing the precipitated iron goes through the head of the first unit and the piping between the units. Particularly at lower flow rates, the sticky ferrous hydroxide tends to foul the valve on the first unit and may require cleaning every 6-24 months. A single-tank system essentially combines the two tanks of a single tank system into one. The iron is oxidized at the top of the tank before falling into the filter medium at the bottom. There is no potential fouling of the head. The iron is filtered before it goes through the outlet port of the valve. For very high levels of iron, chlorination with continuous regeneration is the only practical approach.
Oxidizing Filtration Media
- Manganese Greensand - the most common chemical oxidant used, it has a relatively high capacity for iron removal and can operate at high flow rates with moderate backwash requirements. Greensand is a processed material consisting of nodular grains of the zeolite mineral glauconite. The material is coated with manganese oxide. The ion exchange properties of the glauconite facilitates the bonding of the coating. This treatment gives the media a catalytic effect in the chemical oxidation reduction reactions necessary for iron and manganese removal. This coating is maintained through regeneration with potassium permanganate – about 1.5 to 2 oz. per cubic foot of greensand.
- Birm - acts as a catalyst to promote the reaction between the oxygen and iron dissolved in the water. It requires no regeneration but needs a relatively high level of dissolved oxygen and works best at a pH above 6.8.
- Pyrolox - a natural ore that oxidizes and then filters the resulting insoluble iron. It does not need to regenerate, therefore, it doesn’t need other chemicals. However, it needs, ideally, to backwash at 25 to 30 gallons per sq. ft.
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