deionisation Continued...

Feed water then continues to the second vessel in series where it comes in to contact with anion resin. The anion resin is operated in the hydroxyl form and negatively charged anions such as chloride (Cl-) and sulphate (SO4- -) are removed by the resin and replaced with hydroxyl ions (OH-). Together the hydrogen ions and hydroxyl ions combine to form pure water (H2O).

The resins have a finite capacity for removal before they become exhausted. To return them to service they are regenerated, cation resin typically with hydrochloric acid and anion resin with sodium hydroxide (caustic).

In co-current demineralisers the regenerant chemicals are introduced in the same direction as the flow to service. This means that during service the incoming water first comes in to contact with the most highly regenerated resin. Typical quality from this type of plant is < 25 µS/cm (dependant upon influent feed quality). In this type of demineraliser a freeboard is normally left above the bed enabling it to be backwashed.

If a higher quality of water is required then counter current demineralisers are generally employed. In this type of demineraliser the regenerants chemicals are introduced in the opposite direction to the service flow rate. This means that during service the incoming water comes in to contact with the most regenerated resin last, leading to higher performance. Typical quality from this type of plant is < 5 µS/cm (dependant upon influent feed quality). In this type of demineraliser the resin beds are normally packed so no backwashing is possible. Because of this, care must be taken when considering any additional pre-treatment to prevent solids fouling of the resin beds.

Where even higher quality water is required mixed bed demineralisers are often employed to purify the water even further. Mixed bed demineralisers utilise a single vessel containing a mixture of both cation and anion resins, which are thoroughly mixed prior to service to produce ultrapure water up to 18.2 megOhm quality. On regeneration the resins are backwashed within the vessel and the differing densities of the resins cause them to separate allowing them to be regenerated individually.