The mixed bed delivers the purest water in a demineralization system, and it is also the equipment that suffers most from operating errors. The two facts are inseparable.
It is made up of strong cationic (H⁺) and strong anionic (OH⁻) resins mixed in the same vessel, typically at a volumetric ratio of 40% cationic / 60% anionic. This happens because strong anionic resin usually has lower operating capacity and slower kinetics, so it is common to use a larger volume of it to balance ion removal and achieve high water purity. The ratios can vary depending on the application.
This configuration creates a high number of theoretical exchange stages in series, allowing water quality close to the thermodynamic ideal, that is, the maximum purity limit possible according to the chemical equilibrium of the ion exchange reactions. Conductivity in the range of 0.05 to 0.2 µS/cm and silica below 10 µg/L. [Purolite, Mixed Bed Resin Technology, 2018]
However, this mixture of resins is also a critical point of attention. Regeneration requires complete separation of the two types before applying the regenerants. If the resins remain mixed during regeneration, each reagent contaminates the opposite resin.
Inlet Water Quality
The mixed bed should not receive raw water. It should receive water that has already been demineralized and has a low ionic load. Influent conductivity is typically below 10 to 20 µS/cm, iron below 0.05 mg/L, and zero residual chlorine.
Separation
The density difference between the resins allows separation to be performed during backwash. The anionic resin has lower density: it stays in the upper part of the vessel, while the cationic resin settles at the bottom. The controlled upward water flow makes the anionic resin float to the top. [Dow Water & Process Solutions, AMBERLITE Ion Exchange Resins Engineering Manual, 2013]
The most common mistakes at this stage:
- Inadequate backwash flow: too high expands the bed to the point of mixing the resins again; too low fails to separate them. The correct flow rate is determined experimentally for each resin and operating temperature.
- Backwash water temperature: Type I strong anionic resins operate up to about 60°C; Type II up to approximately 35–40°C. Hot water during backwash can cause irreversible damage. [DuPont Water Solutions, Resin Product Data Sheets, 2022]
- Interface contamination: even with a well-executed separation, there is always a transition layer between the two beds. Well-designed systems have a collection point at the interface to discharge or control this layer.
After regeneration, the resins must be remixed, using compressed air or water. Poor mixing causes preferential channels and worsens water quality.
Osmotic Shock
Rapidly adding concentrated regenerant solutions onto a dry or poorly hydrated bed causes a sudden change in osmotic pressure in the resin beads. The beads can crack (osmotic cracking), generating fine fragments that increase head loss, reduce the bed's usable volume, and escape into the effluent.
The correct protocol requires gradual dilution, pre-soaking with treated water, and regenerant temperature control.
Replacement
There is no fixed timeline for resin replacement. The degradation rate depends on influent quality, the regeneration protocol, and operating conditions. Well-maintained resins in systems with good-quality influent can operate for 8 to 12 years. Systems with aggressive influent, residual chlorine, or high iron can compromise the bed within 2 to 3 years.
Operational indicators that point to the need for evaluation:
- Progressive reduction in cycle capacity (less water produced between regenerations).
- Increased silica leakage in the mixed bed effluent.
- Rising permeate conductivity even after correct regeneration.
- Increased head loss in the bed (an indicator of bead breakage).
The decision should be based on laboratory analysis of the resin, not just visual observation or time in service. Analyses cover particle size distribution, broken resin beads, exchange capacity, and the presence of deposits, among other parameters.
The Cost of Not Acting
Replacing a complete demineralization train for mid-size industrial boilers can represent an investment in the range of BRL 80,000 to BRL 400,000 or more, depending on the resin volume and type. This does not include the cost of plant shutdown, off-specification steam production, or corrective maintenance on affected boilers and turbines.
Most of this expense can be significantly postponed with proper monitoring and adequate regeneration protocols. [Estimate based on Brazilian market quotes, 2023-2024; values should be updated according to current pricing and supplier.]
Conclusion
Demineralization systems behave very specifically depending on influent quality and operating history. Before deciding on resin replacement, a careful evaluation of the operational data is worthwhile.
References
- Purolite. Mixed Bed Resin Technology. 2018.
- Dow Water & Process Solutions. AMBERLITE Ion Exchange Resins — Engineering Manual. 2013.
- DuPont Water Solutions. Resin Product Data Sheets. 2022.