Electroregeneration Technology for Multi-bed Ion Exchange Resins — In Vitro Electroregeneration of Multi-bed Resins

Characteristics of External Electroregeneration of Compound Bed Resin

A compound bed refers to a system where cation and anion resins are placed in two separate devices, one a cation bed and the other an anion bed, to distinguish it from a mixed bed where both resins are mixed in one device. Because the compound bed is located earlier in the water treatment system flow and bears the majority of the desalination load, its electroregeneration has different characteristics compared to a mixed bed:

1) Asynchronous Regeneration of Cation and Anion Beds

In the regeneration practice of compound bed water treatment systems, the cation and anion beds are often regenerated asynchronously, requiring separate regeneration at different times. When mixed bed resin is fed into the aforementioned external electroregenerator for regeneration, both H+ and OH- ions generated by water dissociation are utilized, resulting in neutral concentrate effluent. During compound bed electroregeneration, if the exhausted cation bed resin is regenerated first, H+ ions are utilized, but OH- ions are not, resulting in slightly alkaline concentrate effluent. If the exhausted anion bed resin is regenerated at a different time, OH- ions are utilized, but H+ ions are not, resulting in slightly acidic concentrate effluent. If the slightly alkaline (or acidic) wastewater can be collected to regenerate the corresponding anion (or cation) bed, additional regeneration equipment and systems are required; if it is discharged directly, the power consumption of external electroregeneration will increase due to the separate regeneration of the cation and anion beds.

2) High regeneration intensity is required for the external electroregenerator.

Compared to mixed beds, multi-bed systems typically bear the majority of the desalination load. For example, in a series desalination system of a single-stage multi-bed and a single-stage mixed bed, the multi-bed system needs to handle 90% of the desalination load, meaning that most of the salt in the water must be removed by the multi-bed system. The time for disconnecting the multi-bed system for regeneration is typically 8–24 hours, so all operations of external electroregeneration should be completed within 8 hours. Due to the large desalination load of the multi-bed system, the electroregeneration intensity in a short time is also high; therefore, the external electroregenerator for multi-bed systems should be a high-intensity electroregeneration device.

3) The Influence of Hardness Ions on Membrane Scaling

Mixed bed cation exchange resins, used as fine treatment equipment in water treatment systems, are primarily used to remove residual NaCl salts from the water; therefore, the exhausted cation exchange resin is in the Na form. Combined bed cation exchange resins, used to remove most of the salts in the water, therefore, in addition to the Na form, the exhausted cation exchange resin also contains Ca and Mg forms. When the exhausted cation exchange resin from the combined bed enters the external electroregenator for regeneration, the presence of a large number of OH- ions in the regeneration chamber can cause the surface of the ion exchange membrane and its ion channels to be blocked by Ca(OH)2 and Mg(OH)2 precipitates, causing the ion exchange membrane to lose its selective migration ability for ions. Therefore, the external electroregenator used for mixed bed resin regeneration cannot be directly used for the electroregeneration of exhausted cation exchange resin from the combined bed.

4) The Influence of Inorganic and Organic Precipitates on the Resin Surface

Because the combined bed is located at the beginning of the water treatment system flow, if the pretreatment equipment for removing suspended solids and organic matter in the water is not functioning properly, inorganic precipitates and organic matter will form on the resin surface. During the electroregeneration of the multi-bed resin, these inorganic and organic precipitates are carried into the external electroregenerator along with the resin. This can seriously contaminate or clog the ion exchange membrane, affecting the regeneration effect and preventing the external electroregenerator from working properly. Therefore, a resin wiping process needs to be added before the resin electroregeneration to clean the resin before sending it into the external electroregenerator for regeneration.