Iron in Drinking Water

AEX-323
Agriculture and Natural Resources
Date: 
05/21/2021
Karen Mancl, Professor, Dept. of Food, Agricultural & Biological Engineering, The Ohio State University

A drinking glass sits on a surface, with water from an unseen source flowing into it. The angle and flow suggest the water is coming from a pitcher.Iron occurs naturally in rocks and soil, which easily dissolves in water. Iron is therefore commonly found in well water. While not a health issue, iron gives water a metallic taste and will stain fixtures and clothing. The amount of iron and the nature of its source point to the most effective treatment approach.  

Iron Bacteria

One source of iron in well water is iron bacteria. These naturally occurring bacteria live on iron found in water or in plumbing components. The bacteria form a slime layer—called a biofilm—on the inside of pipes and water tanks. It only takes a small amount of iron to support the growth of iron bacteria.

To control iron bacteria, periodically shock the well and plumbing with chlorine. This process is described in Ohioline’s “Shock Chlorination of Wells” AEX-318 (Mancl 2019). Iron bacteria are difficult to kill, as the slime layer offers a level of protection from the chlorine. Also, iron bacteria always grow back, so it will be necessary to shock chlorinate the water system on a regular basis. Check the back of the toilet tank for any slime build-up to help develop a shock chlorination schedule. Treatments may be necessary every six to 24 months. Long periods of no water use will contribute to the slime buildup. Vacation homes or even extra bathrooms that see little use may thus require more frequent treatments.

Iron in Well Water

The other source of iron in wells is iron that has dissolved in the water. Deep in the ground, groundwater has no oxygen. Under these anaerobic conditions, iron in rock, sand or gravel is colorless and easily dissolves. Once the groundwater is pumped to the surface and exposed to air, small, rusty, red particles slowly form. This change from clear, dissolved iron to red iron particles is called oxidation and is the same process that occurs when iron or steel starts to rust.

The amount of iron dissolved in water helps determine the most effective type of treatment. A small amount of colorless iron can be sequestered with polyphosphate or removed with a water softener. But once the iron has turned red, or if it is a large amount, separate iron removal filters should be used.  

Polyphosphate

If a water test reveals a small amount of iron has dissolved in water, it can be captured before it has a chance to oxidize and turn red. One way to achieve this is by adding polyphosphate to the water. Polyphosphate surrounds the iron and sequesters it, so it cannot oxidize and turn red. Phosphates are sometimes added to detergents to sequester minerals that interfere with cleaning. Phosphate feeders can inject a small amount of this sequestering agent into the water, tying up the iron so it cannot oxide into red particles.

Water Softener

Another way to capture small amounts of colorless iron is by using a water softener. In this process, dissolved, colorless iron is exchanged for sodium in the same way hardness minerals are exchanged. When using a water softener to remove small amounts of iron, special “iron-out” salt should be used to regenerate the softener and protect it from iron buildup.

Iron (greensand) Filter

For larger amounts of iron, or iron that has turned red, an iron filter is recommended. Iron filters do two things—first, a chemical is used to quickly oxidize the iron to particles, and then, the particles are filtered out. An iron filter is often called a greensand filter and is designed to do the two steps in one tank (Figure 1; Wagenet, Mancl and Sailus 1995; page 55). The greensand is first treated with a dark purple solution of potassium permanganate. Potassium permanganate is a strong oxidizing agent, that will quickly turn dissolved iron into large, feathery, black particles that are easily trapped in the filter. To maintain the system, the filter is backwashed to remove the particles, then more potassium permanganate is added.  

Potassium permanganate is a corrosive chemical, so care must be taken when handling it. Wear gloves, goggles, and an apron to protect clothing when refilling the feed tank. Also, thoroughly wash the iron particles out of the filter to ensure proper filter operation. Each filter has minimum backwash requirements of flow rate and water pressure. If your water system has a low flow rate, consider using an iron filter cleaning service. A water company will bring in a clean, recharged iron filter on a pre-determined schedule and take away the used one. Backwashing and recharging with potassium permanganate are done in a shop with adequate water and chemical-handling equipment.

Water flows down into a tube-shaped filter and mixes with Potassium permanganate. Iron is removed and the treated water is pumped out of the filter.

Greensand filter regenerated with potassium permanganate. (Wagenet, Mancl and Sailus—Northeast Regional Agricultural Services)

Chlorine Followed by Filtration

For high levels of iron, a two-tank system is recommended. In the first tank, chlorine—a strong oxidizing agent—is mixed with the water to form red iron particles. It takes 20 to 30 minutes for the iron particles to form, so the tank must be large enough to hold on to the water before it goes out for use in the home. The second tank is filled with filter sand that traps the iron particles. The filter has minimum backwash requirements of flow rate and water pressure. Thoroughly washing the iron particles out of the filter ensures proper filter operation.

A chlorine bleach solution is pumped into the first tank to form the iron particles. Chlorine is a corrosive chemical, so care must be taken when handling it. Wear gloves, goggles, and an apron to protect clothing when refilling tanks.

Iron Removal Systems

Treatment for best results Iron concentration in water
Polyphosphate Less than 2 mg/l
Water softener Less than 5 mg/l
Iron (greensand) filter Less than 10 mg/l
Chlorine followed by filtration More than 10 mg/l

Remember, iron in water is not a health hazard, but makes the water unpleasant to use. Be sure to test the water at an independent lab before shopping for treatment equipment. To find a testing lab, see Ohioline’s AEX-315 “Where to Have Your Water Tested” (Mancl and Bradbury 2021). 

Visit The Ohio State University Soil Environment Learning Lab at setll.osu.edu for more information on water testing and treatment.

References

Mancl, Karen. “Shock Chlorination of Wells.” Ohioline. The Ohio State University, June 10, 2019. ohioline.osu.edu/factsheet/aex-318

Mancl, Karen and Jamie Bradbury. “Where to Have Your Water Tested.” Ohioline. The Ohio State University, February 19, 2021. ohioline.osu.edu/factsheet/AEX-315

Wagenet, Linda; Mancl, Karen; and Martin Sailus. “Home Water Treatment.” Northeast Regional Agricultural Services—NRAES, January 1995. hdl.handle.net/1813/67139