A Simple Guide to Ion Exchange Demineralisers

Deionised (DI) water is quite simply water that has had the dissolved ions within it removed.  The resultant water is purer tap water which can then be used for a wide range of industrial applications. An ion exchange demineraliser is a popular technology used by many manufacturing industries to produce deionised water.

If you are currently looking to invest in either a standard or bespoke deionisation system, or if you just want to know more about them, then please read on.

If, after reading our guide you still have some unanswered questions, our team at AllWater Technologies Ltd is happy to help.  We can be contacted on enquiries@allwatertech.co.uk or +44 (0) 1934 751333.

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Contents: 

What is an Ion Exchange Demineraliser (DI)?

A DI plant uses a combination of cation and anion exchange resins to remove inorganic salts from water.  When water comes into contact with the cation resin, positively charged ions such as Na+ and Ca++ are removed from the water and replaced with H+ ions.  The ‘decationised’ water then comes into contact with the anion resin where negatively charged ions such as Cl– and SO4— are removed and replaced with OH–ions.  The end result is deionised or demineralised water.

With constant use, ion exchange resins become exhausted and require regeneration to ensure that the plant continues to operate efficiently.    Regeneration is generally carried out by drawing Hydrochloric acid through the cation columns and caustic through the anion columns.   Resin exhaustion can be measured through a rise in conductivity.

What is the difference between a demineraliser and a deioniser?

The terms demineraliser and deioniser are generally interchangeable and both are often simply referred to as ‘DI’ plants.

Why would I choose an Ion Exchange Demineraliser rather than a Reverse Osmosis (RO) System?

You can often achieve a better quality of water with a DI system than with an RO system. However this is dependent upon the type of DI system you have.   If the incoming water is of low conductivity, a DI system is appreciably more cost effective to run and produces less waste water.  Furthermore, if you have a water recovery application then a DI system may also be more resilient to fouling.

How do I know if my company needs a DI plant?

Since there are no ions in deionised water, it leaves behind no residue, spots or stains on surfaces.  This makes a DI plant a very popular choice for manufacturing industries that require a high end finish to their products such as car manufacturing.   It is a technology used in many other industrial settings as well, including:

  • Aerospace and defence
  • Chemical processing
  • Food and beverage
  • Microelectronics
  • Pharmaceutical and biotechnology
  • Surface finishing
  • General manufacturing

If you are unsure if a DI plant is the right choice for you, then our team at AllWater can help.  With expertise in other industrial water treatment technologies, they may recommend an alternative solution that better suits your requirements.

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What type of demineralisers are there?

There are three types of DI plant: twin bed, mixed bed and multiple beds.

Twin bed

In a twin bed DI system, the cation and the anion resins are contained in separate vessels.  The advantage of this arrangement is that the plant is often easier to operate and you are less likely to encounter problems.    You may have heard twin bed DI plants being referred to as either co-current or counter-current.  These terms simply refer to the direction of the flow of the chemicals used during regeneration and this is explained in further detail later on.

Mixed bed

With a mixed bed DI system both the cation and anion resins are contained within a single vessel.   With this arrangement, following regeneration the two resins are uniformly mixed and, as a result, as the water passes down the resin bed, it is continuously ‘polished’.   The outcome is water of a very high quality.   The downside with this type of DI plant is that they can be difficult to operate because achieving good separation of the resins during regeneration is extremely important.

Multiple beds

In some situations there may be significant advantages to having more beds containing additional resin types to those generally seen in twin bed DI systems.  For example, an incoming water with high conductivity and low Bicarbonate level would be more efficient employing a weak base anion resin, with strong base anion as final polishing.   Another example is a metal finishing rinse water application, where a high level of chromates may be present, but there is still a requirement to produce low conductivity-treated water.   To remove the chromates, the weak base anion resin can be used prior to the strong base anion resin.  The use of the strong base resin ensures the treated water is of high quality but of a low conductivity.

What types of resins are there?

Resins fall broadly into five categories:

Strong acid cation

This is the most commonly used ion exchange resin as it can be used for a number of processes such as softening and demineralisation.  The ‘strong’ property of the resin means that it can remove both weakly and strongly dissociated cations. For example, not only can it remove cations such as Na and Ca associated with weak anions such as Bicarbonate, but it can also remove those associated with strong anions such as Sulphates and Chlorides.

Weak acid cation

The weak function means that this type of resin is only able to remove cations that are associated with weak anions, such as Bicarbonate.   The benefit of this resin is that it offers greater capacity and at lower stoichiometric regenerant levels than a strong base resin.

Strong base anion

This is the most commonly used anion resin in DI systems where only a single anion resin is required. Its strong base functionality means that it can remove all anions including Bicarbonates and Silica.

Weak base anion

A weak base anion offers greater capacity at lower stoichiometric levels than strong base resins, but on the downside is unable to remove more weakly charged anions, for example, Bicarbonates, carbon dioxide and Silica.  The result is, if used with a strong acid cation resin in a DI system, the resultant water is likely to be acidic and dependent upon the make-up of the water may have a conductivity greater than 30 Microsiemens.   Despite this, this type of resin is often beneficial in metal finishing and water recovery type applications where some strong anions, such as chromates, may be difficult to remove from a strong base anion resin and are more easily removed from a weak base anion resin.

Mixed base anion

These are a relatively new development and offer the flexibility of good capacity at lower regenerant levels whilst still achieving higher quality water.  By having both strong and weak functional groups on a single resin the additional cost associated with extra valves and vessels for containing weak and strong base resins separately can also be negated.

Resin Types and Functional Groups

Resins are usually made from polyacrylic or polystyrene monomers and are either gel type or macroporous.

Strong acid cation resins are always polystyrenic and have a sulphonic acid functional group. Weak acid cation resins are always polyacrylic and have a carboxylic functional group. The amount of divinylbenzene cross linking in a cation resin can be increased to provide better mechanical and thermal protection for use in difficult applications.

Anion resins can be polystyrenic or polyacrylic in nature with strong base anion (SBA) resins having a quaternary amine functional group and weak base anion (WBA) resin, tertiary amine. SBA, polystyrenic, gel type resins can be broken down into 2 further groups, Type 1 and Type 2.

Type 1 resins are characterised by relatively low capacity but low silica leakage and are also better suited for higher temperature applications. Acrylic, strong base type 1 resins usually offer the best resistance to organic fouling whilst still offering low silica leakage so they can be good to use on low conductivity, high organic content type surface waters.

Type 2 resins offer better capacity and low silica leakage but not as low as their Type 1 counterparts. Type 2 SBA resins are most often used on waters with higher ionic loads where organic content is low. Macroporous type 2 anion resins offer better resistance to organic fouling than gel type resins but have a lower working capacity.

Why is resin bead size important?

Standard ion exchange resin beads vary in size, typically between 0.3-1.2mm.  In counter-current DI systems, the resin is graded to a more uniform size and smaller beads are utilised. The benefit is reduced pressure losses at higher velocities as well as better regeneration efficiency including reduced water volumes required for rinsing.

How long does regeneration of a DI plant take?

This really depends upon the type of DI plant you have.  Where regeneration is carried out using a town’s water supply, it is necessary to regenerate the cation column first.  You then use the ‘decationised’ water to regenerate the anion column.  Failure to do this can result in calcium precipitation on the anion resin bed.   In this scenario, regeneration may take as long as two and a half hours depending upon the type of plant.

If the regeneration is carried out using treated water, it is possible to reduce the time for the regeneration process as the cation and anion columns can be regenerated simultaneously rather than sequentially.   In this scenario, you may see the time taken for regeneration decrease to one and a half hours.  With some short cycle, rapid regeneration DI systems, this time can be further reduced to as little as 30 minutes.

What does regeneration of a DI plant involve?  What are the stages of DI plant regeneration?

The first stage of the regeneration process is called ‘backwashing’ of the resin.  On co-current DI systems and particularly where high levels of fouling may occur, for example, with water recovery DI plants, this stage is particularly important.   This stage results not only in de-compaction of the resin bed but it also aids the removal of trapped solids.  On counter-current, packed bed DI units, the backwash stage may be very small or short as the main function of this stage is simply to fix the bed in position in order to hold it in place during the next phase: the chemical draw stage.

During the ‘chemical draw’ stage, acid is drawn through the cation bed and caustic (Sodium Hydroxide) is drawn through the anion bed.   Hydrochloric acid is generally the first choice of acid for regeneration of the cation resins particularly where Calcium and/or Magnesium are present in the incoming water.  Sulphuric acid can also be used but care must be taken in order to prevent Calcium and Magnesium precipitation.   When using Sulphuric acid, it is considered best practice to draw the acid first at low strength and then to increase the concentration once most of the Calcium and Magnesium have been removed.   Due to the potential complications of using Sulphuric  Acid, Hydrochloric acid is generally used but the downside of this is fuming so there are often significant benefits to using Sulphuric acid wherever it is possible to do so.   Another advantage is that Sulphuric acid is cheaper to buy.

The next phase of the regeneration process is the ‘slow rinse’.  The purpose of this is to displace the regenerant chemicals through the resin beds.   This is followed by a ‘fast rinse’ to remove most remaining traces of chemicals from the resins.  The final stage is normally a recirculation stage where water from the outlet of the DI plant is returned to the inlet in order to further ‘polish’ water quality before the plant goes back in to service.  This also minimises water wastage.  During this final stage conductivity is monitored in order to ensure that the desired water quality is achieved before the plant is put back on line.

For a mixed bed DI plant the regeneration sequence is far more complicated.  During the first stage, the resin is backwashed, mainly to separate the cation resin from the anion resin.   Once separated, there is a period of settlement.   During settlement, the cation resin drops to the bottom of the vessel first and the anion resin settles on top.    Separation of the two resins is extremely important. If any anion resin remains trapped within the cation resin, the residual anion will regenerate into the Chloride or Sulphate form.  Furthermore, any cation resin trapped within the anion resin regenerates into the Sodium form.  The final result is poor quality treated water.

During the chemical draw phase acid is introduced at the bottom of the bed and caustic at the top simultaneously. The chemicals pass through the relevant resin until they meet at a central distributor where they are drained from the plant.   As with the regeneration of a twin bed DI plant, there then follows the slow rinse stage.  Following slow rinse, the resins are mixed together using air and the vessel is drained down to ‘lock the bed’ to ensure the resins are homogeneously mixed.   The vessel is then slowly refilled and the fast rinse and recirculation stages of regeneration are completed.

How do chemicals get added to the DI plant during regeneration?

Most commonly an eductor is used to draw the chemicals for regeneration into the DI plant.  This is also sometimes referred to as a jet pump or an ejector.    Because they have no moving parts, eductors are a low cost option.   In the eductor the flow of motive water passes over an orifice creating a negative pressure which draws the chemical.  In some cases a pump, such as a dosing pump, may be used to inject the chemicals.

What is the difference between a co-current DI plant and a counter-current DI plant?

The terms co-current and counter-current simply refer to the direction of the flow of the chemicals used during regeneration of a twin bed DI plant.

With a co-current DI arrangement the chemicals used for regeneration are drawn in the same direction of flow as the incoming water, generally from top to bottom of the vessel or ‘column’.   As the chemicals reach the bottom of the vessel, having passed through the more highly exhausted resin sitting at the top of the vessel, they become less efficient at regenerating the resins sitting at the bottom. For this reason, the quality of water achieved with a co-current DI plant is generally not as good as the water produced by a counter-current arrangement.   Typically a co-current DI plant will achieve a water quality measure of less than 10 Microsiemens, although this is of course dependent to some degree on the quality of the incoming water.

With a counter-current DI plant, the chemicals used for regeneration are drawn in the opposite direction to the flow of the incoming water.  As a result, the regenerant chemicals are at their highest strength when they come into contact with the least exhausted resin.  During service, water passing through the bed comes into contact with the most highly regenerated resin immediately before leaving the vessel.   This flow ‘polishes’ the water resulting in water of a higher quality.    With a counter-current DI plant you can expect to achieve a water quality of less than two Microsiemens, although this is also dependent upon the quality of the incoming water.  To further improve water quality a further polishing column (usually a cation column) can be added and with this addition you can achieve a water quality of less than one Microsiemen.

With counter-current DI plants, it is important that the resin bed is fixed in place at all times which is something to bear in mind when designing a DI plant.   Many of the common problems that occur with counter-current plants are the result of the resin beds either being over or under packed.

While the more sophisticated counter-current DI plants, without doubt, produce better quality water, they are not without their problems.   They are generally more expensive to manufacture and require more maintenance than co-current systems.  This is due to the requirement to keep the resin bed fixed both when the plant is in use and also when it is undergoing regeneration.   Furthermore, individuals in charge of the smooth running of the DI plant must make sure that the strict operational parameters are adhered to at all times.

Why are DI plants often referred to as cation limited?

If a DI plant is allowed to completely exhaust before regeneration takes place, then the regeneration process will be triggered by the rising conductivity caused by a leakage of ions.  In this scenario, you need to consider which ions are leaking and causing this rise in conductivity.   If the anion column exhausts first, then the most weakly bound ions typically Silica or Bicarbonate will be released first, depending upon the feed water.   These both have very low associated conductivity and depending upon the type of DI plant, appreciable levels may have leaked before regeneration is triggered.   If the DI plant is feeding a high pressure boiler or metal finishing application, this leakage of anions could be catastrophic.

If, on the other hand, the cation column exhausts first, then the leakage of Sodium is usually the culprit in conductivity rise.  Sodium has a high associated conductivity so small amounts can be easily detected.   For this reason, a DI plant is typically sized with less cation resin than anion resin, resulting in the cation column exhausting first.  This is also referred to as being cation limited.

Does water temperature affect DI performance?

Water temperature can affect DI performance.  Most resins have a maximum temperature that they can operate at and some resins are more resistant to temperature than others.   High temperatures can lead to degradation of the resin resulting in reduced functional capacity causing the release of species into the water that affect conductivity. Anion resins are generally more affected by high temperatures than cation resins.

Is a DI system cost-effective when compared to an alternative technology such as reverse osmosis?

Where the incoming water supply is of low conductivity, ion exchange demineralisation is a more cost-effective technology but if the conductivity of the incoming water is high then an RO system tends to become more cost-effective.   If you are unsure our team at AWT can carry out a cost comparison to determine which technology is the most cost-effective for your specific requirements.

Are DI plants/systems environmentally friendly?

DI plants are normally considered to be environmentally unfriendly based mainly on the fact that chemicals are used for the regeneration process.    However, this does not tell the whole story.   Again depending upon the nature of the incoming water, a DI plant may use less energy and waste less water than an alternative technology such as Reverse Osmosis (RO).

Is pre-treatment of water necessary?

Some pre-treatment of water may be required.  This will depend upon the nature of the incoming water and the type of DI plant.  For example, a short cycle DI plant operated on a low conductivity town’s water where appreciable levels of organics are present may need an organic scavenger or carbon filter for pre-treatment.  Likewise, a rinse water recovery DI system may also need carbon pre-filtration if there is a high incidence of organics or the presence of free chlorine.  Multi-media filtration might also be used in this instance, where suspended solids (possibly resulting from Metal Hydroxide precipitation) could be present.

How do I know what DI plant/system to choose?

The two things you will need to consider are the end quality of water you require and whether or not you have sufficient space to store the amount of treated water you need for production to continue whilst regeneration is taking place.  If you only require water quality of less than 10 Microsiemens and you have adequate treated water storage capacity, then a simplex, co-current DI plant may be satisfactory.  However if you need your water quality to be less than two Microsiemens and you have limited treated water capacity, then a short cycle counter-current DI plant might be more suitable.

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Do DI plants need regular servicing?

We would always recommend that your DI plant is serviced by a water treatment specialist in order to ensure it stays in good working order.  Frequency of servicing depends upon the type of DI plant you have installed, back-up capacity and overall importance of the DI plant to production.   We would always advise that you choose your service provider with care, particularly if you have a complex DI system. Make sure that they are familiar and have experience in servicing the type of DI plant you have.

What spares should I keep?

We don’t believe in our customers keeping unnecessary spares.  The spares you actually need will depend upon how critical your DI plant is to production and whether you have a back-up unit.    Our team at AWT are happy to consider your personal situation and make recommendations.

Are there any disadvantages to a DI plant?

The main disadvantage of a DI plant is that they need chemicals for the regeneration cycle.  This, of course, creates the issue of safe disposal of hazardous chemicals. When using Hydrochloric Acid for regeneration there may also be problems associated with corrosive fumes. This can be mitigated to some degree by use of a fume scrubber. For smaller systems, a resin type fume scrubber might be sufficient whereas for a larger storage tank it might be necessary to employ a water scrubber. AWT are able to offer both these types of systems and can consult with you regarding your specific needs.

We hope that this simple guide answers most of the questions you might have in relation to DI plants for industrial settings.  We would always recommend that you seek professional advice before placing any order.  AllWater Technologies Ltd have installed numerous water recovery DI plants both at home and abroad,  and can advise you on what system would work best for your specific situation.