Cation and Anion resins can be mixed together in a single vessel to produce a very high water quality by the many ion exchange reactions that continually occur as the water passes through the Mixed Bed of resins.

The Mixed Bed typically consists of a Strong Acid Cation and a Strong Base Anion resin mixed together, usually at a volumetric ratio of 40% Cation to 60% Anion resin.Other ratios can be used depending on the water analysis, the capacity desired, and the required quality of the treated water. \

• Purolite Mixed Beds for Ultra Pure Water Production include:
  Picopure 1200 - a regenerable Mixed Bed
  Picopure 56 - for Final Polisher applications providing the highest purity water.

This form of deionization is typically used when the need for a water quality of less than 1 micromho conductivity (or 1 megohm-cm specific resistance or up to 0.5 ppm TDS as CaCO₃) is required.
A Mixed Bed unit can also be used as a polisher following a Two Bed or a Multi-Bed system. This configuration will be used when a water quality of up to 0.05 micromho conductivity (or 18.2 megohm-cm) is required.

Typical design for general purpose Mixed Bed units requires that regeneration of the resins be carried out within the same vessel. In this case it is necessary to separate the Anion from the Cation resin before the acid and caustic regenerants are introduced. This is accomplished by backwashing of the resins, using an adequate velocity of water to fluidize the bed.
The Cation, being higher in specific gravity, will settle to the bottom of the bed, while the Anion, being lighter, will rise to the top of the bed (in some designs, brine is used to assist in this separation).
Each of the separated resin beds are then regenerated. After regenerant chemicals have been introduced, the resins are rinsed and then remixed intimately using oil-free compressed air.

An Inert Resin can be used, together with the Cation and Anion resins, to facilitate better physical and visual separation before regeneration. The inert resin has no functionality and will not affect the resulting water quality. During the backwash phase the inert resin, because of its designed intermediate density,will sit in a layer between the Cation and Anion.

• Typical Purolite^®Inert Resins utilised are:
  IP 3
  IP 7