After a certain operation time the anion resin may need a special regeneration to remove colour substances, hardness etc. Therefore provision are provided to regenerate the anion resin with hydrochloric acid as well.The anion resin is then completely converted into the Cl- form, which has the smallest bead volume, which will remove additionally colour substances and precipitates from the resins. After the special regeneration the resin needs then the normal regeneration with the double amount of caustic soda to convert the resin in OH- form again.