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Ion Exchange

1. Executive Summary

Ion exchange is a water treatment method where one or more undesirable ionic contaminants are removed from water
by exchange with another non-objectionable, or less objectionable ionic substance.
Both the contaminant and the exchanged substance must be dissolved and have the same type of electrical charge (positive or negative).
A typical example of ion exchange is a process called “water softening” aiming to reduce calcium and magnesium content.
Nevertheless, ion exchange is also efficient in removing toxic metals from water.

2. Applicability

The most common applications of ion exchangers are water softening (remove calcium and magnesium ions),
water demineralisation (removal of all ions), and de-alkalisation (removal of bicarbonates).

Cation exchange resins can also remove most positively charged ions in water such as iron, lead, radium, barium, aluminium and copper among others.

Anionic exchange units can remove nitrate, sulfate, and other negatively charged atoms (called anions).

Researchers are developing resins to selectively remove nitrate more efficiently than can currently be done.

Ion exchangers are also used to remove or recover metal ions from wastewater in the chemical industry.
Some contaminants (such as arsenic, fluoride, lithium ions) are difficult to remove with ion exchange due to a poor selectivity of the resins.

Ion exchangers are also used to remove or recover metal ions from wastewater in the chemical industry.
Some contaminants (such as arsenic, fluoride, lithium ions) are difficult to remove with ion exchange due to a poor selectivity of the resins

3. Regeneration wastewater

Most ion exchange systems are containers of ion exchange resin operated on a cyclic basis.
Water flows through the resin container until the resin is considered exhausted
when water leaving the exchanger contains more than the desired maximum concentration of the ions being removed.
Resin is then regenerated by sequentially backwashing the resin bed to remove accumulated solids,
flushing removed ions from the resin with a concentrated solution of replacement ions,
and rinsing the flushing solution from the resin. Production of backwash, flushing,
and rinsing wastewater during regeneration of ion exchange media limits the usefulness of ion exchange for wastewater treatment.

Water softeners are regenerated with brine containing ten percent sodium chloride.
Aside from the soluble chloride salts of divalent cations removed from the softened water,
softener regeneration wastewater contains the unused fifty to seventy percent of the sodium chloride
regeneration flushing brine required to reverse ion exchange resin equilibria.
Deionizing resin regeneration with sulfuric acid and sodium hydroxide is approximately twenty to forty percent efficient.
Neutralized deionizer regeneration wastewater contains all of the removed
ions plus 2.5 to five times their equivalent concentration as sodium sulfate.

4. Ion Exchange Resins

Ion exchange resins contains many fine pores that fill with water.
Synthetic and industrially produced ion exchange resins consist of small, microporous beads that are insoluble in water and organic solvents.
The most widely used base-materials are polystyrene and polyacrylate. The diameter
of the beads is in the range of 0.3 to 1.3 mm. The beads are composed of around 50% water,
which is dispersed in the gel-structured compartments of the material.

5. Operation and Maintenance

Maintenance of water softening equipment is somewhat dependent on the type of softener.
Some degree of monitoring or managing the regeneration process is generally required.
Adequate backwashing of the resin bed is important to ensure the regeneration of the unit. However, regeneration creates wastewater.

6. Costs

The costs for ion exchange systems are very variable depending on scale and region.
Moreover, costs depend on pretreatment requirements, discharge requirements and utilisation.

7. Health Aspects

People on restricted sodium diets due to health reasons should account for increased intake through softened water.
Drinking and cooking with softened water is often avoided by having a cold water line to the kitchen tap that bypasses the water softener.
This provides hard water for drinking cooking and other uses. It is not recommended to repeatedly use softened water for plants,
lawns or gardens due to the sodium content.

8. Advantages / Disadvantages

8-1 Advantages

·  One of the most appropriate technologies to removes dissolved inorganic ions effectively
·  Possibility to regenerate resin
·  Relatively inexpensive initial capital investmentr

8-2 Disadvantages

·  Does not remove effectively bacteria
·  High operation costs over long-term
·  The process of regenerating the ion exchange beds dumps salt water into the environment (regeneration)