Heavy Metal Retention & Ion Exchange Disadvantages

Ion exchange is a method of water treatment used to remove contaminants, including heavy metals. It may be done by exposing water to water softener before using reverse osmosis to purify it. It may also be be done by demineralisation, in which the water is exposed to resin beads that exchange hydrogen ions for cations and hydroxyl ions for anions, resulting in pure water. The resin beads are often reused after being treated with a process called "regeneration." Both ion exchange methods remove inorganic contaminants, although they have disadvantages.

Bacterial Problems

• Ion exchange does not remove or kill microorganisms, such as bacteria, that are in the water. If ion exchange is the first type of treatment used on the water, the ion exchange beds can act as a bacterial culture medium. This leads to higher contamination with bacterial waste products and lowered purification effectiveness.

Bead Breakdown

• The beads used in ion exchange beds may have particles that break off, especially if exposed to heat or after several years of use. These particles travel with the water being treated. If this happens after the water has gone through particle filtration, the bead particles may never be removed from the water supply. Over time, it is possible for the particles to release contaminants into the water.

Chlorine Damage

• Ion exchange beads break down easily when exposed to oxidizing disinfectants like chlorine. This means they cannot be exposed to water supplies that contain even a small amount of chlorine. Also, water sent through treatment beds cannot be treated with chlorine to kill bacteria.

Iron Fouling

• Resin beads that interact with water that contains a form of dissolved iron, Fe3+, cause ferric hydroxide to precipitate out of the water. This material prevents further ion exchange and essentially clogs the treatment bed. This type of iron most often shows up in water that has been piped from anaerobic underground supplies, containing Fe2+, which subsequently comes into contact with air and oxidizes to Fe3+.

Fouling During Regeneration

• Ion exchange beads that are reused are usually regenerated every 12 to 48 hours, and regeneration is done in the same tank or treatment bed used for water treatment. If sulfuric acid is used to regenerate cation resin beads in a demineraliser -- and the water treated has high calcium content -- it can cause calcium sulphate to precipitate out of the regeneration solution. This causes scale buildup on the beads, tanks and pipes in the facility. Over time, this clogs the treatment bed.