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Targeting iron in well water
Appearances can be deceiving when it comes to your customer's well water. Trace amounts of dissolved iron can be invisible to the naked eye but still cause all sorts of problems in your customer's home plumbing and fixtures.
According to the US Environmental Protection Agency's (EPA) National Secondary Drinking Water Regulations, which are only federal recommendations but may be enforced in some states, iron is not harmful but may cause aesthetic problems, such as staining, odor and an undesirable taste.
The secondary regulations call for the dissolved iron in treated water to be maintained at or below 0.3 milligrams per liter (mg/L). See the sidebar for a review of the main types of iron found in water: ferrous iron, ferric iron and iron bacteria.
Ion exchange
There are a few different ways to treat ferrous iron (Fe +2 ). One of the most common is ion exchange. Ion exchange resin used in water softeners has the ability to remove low levels of ferrous iron through the exchange process.
This treatment method can have great results when other water quality parameters are just right. Some of those parameters include the absence of ferric iron and a low to moderate level of ferrous iron.
If the pH level is high enough to oxidize iron, the softener's ability to remove the iron is reduced. Resin-stirring devices and resin cleaners can help the softener bed retain its capabilities.
Oxidation methods
Oxidation additives and filtration also are used to treat ferrous iron. Oxidation methods convert soluble ferrous iron (Fe +2 ) into insoluble ferric iron (Fe +3 ), allowing the ferric iron to be filtered out. Oxidation methods use chlorine, ozone (oxygen), ambient air (containing oxygen), or some type of oxidizing filter media.
All oxidation principles work from the same basic principle: the oxidant is used to precipitate the iron from a soluble to an insoluble form (ferrous to ferric).
Chlorination: Chlorine for treating ferrous iron can be injected with a chemical feed pump between the well pressure tank and a retention tank. The treated water then is held in the retention tank, where the iron precipitates, some settles and the remainder can then be removed by filtering.
It's important to properly size the retention tank and filters for this application, because improper sizing is a common misstep.
Ozonation: In this method of treating ferrous iron, an ozone generator makes the ozone (an allotropic form of oxygen) and feeds it into the water stream via a pump or air injector.
Ozone (O 3 ) has a great oxidizing potential, and it requires a retention tank in which the ozone can properly oxidize ferrous iron. After the correct contact time, the water is ready to be filtered. Catalytic media filters or multi-layered filters are often used.
Aeration: Air (which contains about 21 percent oxygen by volume) also is used to convert ferrous iron to the insoluble ferric form. Aeration systems can be very effective when sized correctly. This all depends on how much iron is in the water and the flow rate.
These systems are designed with an air compressor or some type of venturi or injector set up. The system will use either the existing pressure tank (if there is air/water contact) or an additional retention tank, along with an air volume control.
In cases where high flow is required, two retention tanks may be needed to provide the required retention time. A good rule of thumb is to allow 20 minutes retention time to achieve complete oxidation.
Drawbacks of aeration can include the large amount of equipment space required, milky water, partially oxidized iron, collection of iron ahead of the filter, air volume control failures and periodic service requirements.
Some systems use a single-tank filter with air injection that uses the freeboard of the tank for the retention time. These systems usually are seen in low-flow and low-iron situations.
Oxidizing filter media: Manganese greensand is one of the most commonly used oxidizing media, has been widely used municipally and has had relatively good success in residential iron removal. This medium is regenerated with potassium permanganate (KMnO 4 ) at the rate of approximately 1.5 to 2 ounces of KMnO 4 per cubic foot of greensand.
Ferric iron and iron bacteria removal
Removal of insoluble ferric iron requires a properly sized filter and the right pH range. If any ferrous iron is in the water, some type of oxidation or oxidizing media will be required.
Iron bacteria can be controlled by periodic well chlorination or by chlorination, retention and filtration. This treatment should be followed by carbon filtration to alleviate any chlorine residual in the drinking water.
Dissolved oxygen generation
A recently developed technology provides total iron removal by electrolytically producing oxygen from the water itself, and can offer some advantages over conventional oxidation or chemical additives.
One device using this technology uses titanium electrodes, which create the oxygen used for iron, manganese and hydrogen sulfide removal.
The water flows through the inlet port of a bypass loop and a flow switch turns the unit on. The electronic side of the device then sends power to the oxygen generators, which produce microbubbles of pure oxygen. The oxygen microbubbles react with dissolved ferrous iron to oxidize it to ferric iron. The device works on a real-time basis, and no retention time is needed.
The oxygen bubbles in this technology are 400 times smaller than standard aeration bubbles. This allows them to carry contaminants through the piping with no iron buildup and eliminates milky water at the tap.
To filter out the ferric iron created in this process, such a system should be followed with any one of a variety of filtration systems, such as oxidizer-treated minerals, manganese greensand, sand or multi-media filters. One device using this technology has a flow rate up to 20 gallons per minute (gpm) and can treat high iron concentrations.
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