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Ion exchange
 

Ion exchange usage in wastewater treatment

About ion exchange

Ion exchange (IX) is a technology used in a variety of applications from household dishwashers to complex industrial uses in for instance the pharmaceutical, chemical, petrochemical, metal finishing and semiconductor industries. The concept behind it is – as the name indicates – an exchange of ions by means of a special resin. The purpose of ion exchange is to purify, decontaminate, or soften water and wastewater. In an industrial context ion exchange is used both within process and utility.
Ion exchange is a relatively inexpensive treatment process and often requires a limited amount of space. It is simple to operate and maintain. When the resin is saturated it is regenerated and can be reused. The operating hours between regenerating can be prolonged by choosing the right resin with respect to the composition of the wastewater and the unwanted ions.

Treatment of industrial wastewater

At Aquarden we use ion exchange in relation to utility – more precisely for wastewater treatment – both as part of a total solution or as a treatment method itself. Some examples include:

Example 1: Ion exchange as a pre-treatment

An example of how ion exchange is used as pre-treatment is cleaning of wastewater containing organic solvents with Aquarden’s Waterox system.

Ion exchange is used to exchange salts in the wastewater with hydrogen and hydroxide which forms water. This results in a higher capacity to a lower cost for the Waterox system as the capacity is higher when the cleaning frequency is lower.

Read more in our case with Alfa Laval.

Example 2: Recycling of rinse water

Ion exchange is also applicable as a stand-alone solution. For instance, in the metal finishing and surface treatment industry rinse water is recycled in order to reduce the amount of wastewater. The rinse bath often consists of three baths of which the last can use recycled water by purification with ion exchange.

Example 3: Polishing of wastewater after chemical treatment

Ion exchange can also be applied for polishing of wastewater after chemical treatment e.g. for removal of heavy metals. With the right choice of ion exchange resin, it is often possible to reduce the concentration of metals to a level that complies with the general discharge permit for municipal wastewater.

Tech talk!

An ion exchange column contains a resin in which the exchange of the ions occurs. The resin can either be cation, anion, or a mix of both (mixed bed). A cation resin exchanges cations, and an anion resin exchanges anions. A mixed bed ion exchanger consists of both cation and anion resins and hence exchanges both.

The resin is placed in a column where the water is running upwards, which will generate a fluidized bed. In the illustration to the right the wastewater is pumped through the column containing a cation resin. When the cations of the wastewater pass through the cation resin the ion exchange occurs.

Resins

The resin consists of small particles which are made of a porous material with ion binding sites. The resin in the illustration is a cation resin with the cation A+ bound at the ionic binding sites. When the wastewater containing the cation B+ passes through the resin the cations are exchanged due to either:

  • Equilibrium: The difference in concentration of cation A+ in the resin and the wastewater is evened out, hence cation A+ is exchanged with cation B+, until  equilibrium occurs.
  • Affinity: The affinity of the resin is higher for cation B+ than for cation A+, hence cation A+ in the resin is exchanged by the cation B+ from the wastewater. This process continues until all A+ is replaced.

The resin is constructed of an inert carrier material coated with an active chemical component with ion exchange properties. The inert carrier is normally cross-linked polystyrene. For standard ion exchange applications, it is sufficient to distinguish between strong and weak cation exchange resins as well as strong and weak anion exchange resins. However, other more special applications require resins with special properties and require expert consulting.

Standard resins have different affinity to ions and differ in which ions they exchange with. Therefore, the choice of resin depends on the specific case.

For instance, you can remove Ba+2, selectively in water with Ca+2, Mg+2 and Na+ by using a strongly acidic cation resin. This is not possible with a weak acidic cation resin. However, a strong acidic/basic resin uses more chemicals for regeneration therefore a weak resin should be used whenever possible.

Guideline for selecting the most suitable resins for a specific case

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