Iron removal: Troubleshooting a common problem

Iron removal: Troubleshooting a common problem

After hardness, iron is the most common water problem dealt with by water treatment professionals. Unfortunately, iron also has a reputation of being difficult to deal with.

This reputation springs largely from the fact that iron can be found in water in several forms, each of which requires a potentially different approach for removal.

Other factors, notably the pH level of the water, will also affect how successful an approach will be. However, once the water conditions are known, choosing the best iron removal method is fairly straightforward.

Iron types

Iron is present in water in three main forms. While there are other forms of iron, they are much less common.

Bacterial iron

Iron bacteria is usually identified by slime in places such as toilet reservoirs or by the presence of a slimy mass fouling softeners or filters. This article will not get into bacterial iron removal. See "Solving iron bacteria problems in wells" in the January 2002 edition of Water Technology for additional information.

Ferric iron

Also known as red water iron, ferric iron is essentially clear water iron that has been exposed to oxygen ?usually from the air. Carbon dioxide leaves the water and the oxygen combines with iron to form ferric ions (Fe+++).

Ferrous iron

Often called clearwater iron because it is clear when poured, this substance is found in water that contains no oxygen. Typically, it comes from deeper wells and groundwater sources. Carbon dioxide acts on iron in the ground to form soluble ferrous bicarbonate. In water this forms ferrous ions (Fe++).

Treatment methods

Iron bacteria

Iron bacteria can be controlled by periodic well chlorination or it can be treated in the building. The treatment occurs as follows: Chlorination, retention, filtration. Activated carbon is usually used as the filter material so the excess chlorine can also be removed.

Ferric iron

Conceptually, dealing with ferric iron is simple ?just filter it from the water using a properly sized filter.

In practice, however, there are two additional issues:

?Some iron can exist in colloidal form. While ferric iron will usually stick together to form large flakes, the small particles of colloidal iron do the opposite. Because they have large surface charges, the smaller they are, the larger their surface area and charge relative to their mass.

The charges in the different particles repel each other and will not coagulate.

Their small size makes them hard to filter. When this happens, it may be necessary to add a coagulant to the water to stick the particles together, making them easier to filter.

In most waters containing ferric iron there will also be iron in solution in the water, which needs removing. To check this, use a membrane filter, preferably .22 micron, to filter out the insoluble iron and then test the water.

This adds complexity to the removal of the ferric iron since some of the methods for removing ferrous iron will remove ferric iron as well.

Ferrous iron

There are a variety of ways for removing ferrous iron, each with its own unique set of strengths and limitations.

The methods fall into two types:

Ion exchange

Ion exchange relies on the ability of softening resin to attract iron ions as well as hardness ions like calcium and magnesium. The ions of ferrous iron are cations like calcium and magnesium ions that a standard water softener is designed to remove. As Table 1 shows, the strong acid cation resins actually select the ferrous ions over calcium and magnesium ions.

Removing ferrous iron in a softener can be an effective and economical way of treating iron problems. However, there are limitations:

The amount of iron that can be removed is limited. There are reports of up to 50 parts of iron being removed by ion exchange, but for practical purposes in an everyday working softener, the upper limit is about 5 to 7 parts per million.

The unit needs to be specially designed if more than a couple parts per million of iron are in the water. Because the resin so strongly selects for the iron, it is harder for the sodium regenerant to knock the iron off the resin.

It is important to have an effective backwash to clean the resin and prevent channeling. An underbed and perhaps even a turbulator will assist in this.

Any ferric iron in the water will foul the resin. Unlike iron oxidized by air that forms the familiar dry rust, ferrous iron oxidized in water first forms ferric ions (Fe+++). These in turn combine with free hydroxyl ions in the water to produce ferric hydroxide, which will pass straight through the softener and into service and cause staining.

Even worse, ferric hydroxide is a sticky gelatinous substance that will clog the resin and coat it when coagulated. Over time, the softener ceases to function effectively on either iron or hardness.

At higher pH levels the softener will be ineffective. At low pH levels it is hard to precipitate iron from water. In fact, with pH as low as 4, ferric iron will dissolve back into the water.

In contrast, when the pH is above neutral it is much harder to keep the iron in the water dissolved. When the water? pH rises above 7.2 to 7.3, the softener? ability to grab iron from the water becomes increasingly limited.

Despite these limitations, softeners perform well in removing small quantities of clear water iron. Using an agent to clean the resin ?whether as a separate product or formulated in the salt used for regeneration ?will dissolve any iron sticking to the resin.

Where the concentration of iron is above 5 or 6 parts per million, or when there is both dissolved and precipitated iron in the water, a different approach is needed.

Oxidation additives plus filtration

Oxidation methods convert soluble iron into insoluble iron and then filter the insoluble iron. In turn, these methods fall into two groups: those using additives like chlorine, ozone or air; or those using an oxidizing filter media.

All means of removing iron by oxidization work in the same way - they turn soluble ferrous iron into insoluble ferric iron and filter it out. The various chemical methods differ in the pH range in which they are effective. These differences are shown in Table 2.

The filter will also pick up any ferric iron that was originally in the water. The methods differ in the way they oxidize the ferrous iron.

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.

For very high levels of iron, chlorination with continuous regeneration is the only practical approach.

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.

Oxidizing filter media

Pyrolusite a natural ore that oxidizes and then filters the resulting insoluble iron. It does not need to regenerate, therefore, it doesn? need other chemicals. However, it needs to backwash at 25 to 30 gallons per sq. ft.

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. It regenerates with potassium permanganate ?about 1.5 to 2 oz. per cubic foot of greensand.

Proprietary products - best known as Birm, which acts as a catalyst to promote the reaction between the oxygen and iron dissolved in the water. It requires no regeneration but needs a higher level of dissolved oxygen than is found naturally in most water, and is often used in conjunction with methods that aerate the water.

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