The DI resins are typically discarded after a single use, but you can recharge them (as long as they're separate anion and cation resins rather than a mixed bed) - to quote one of Randy's articles:
"Recharging DI Resins
When DI resins are present as mixed bed filters, they are essentially one-time use devices that must be thrown away when depleted. These are the types supplied with many commercial RO/DI systems, and are what I use. Separate bed DI resins have certain advantages, however. In particular, if they are kept in different cartridges, they can be recharged. Trendy deals of the
Makro Specials and exclusive price range.
The recharging process is essentially the reverse of the deionizing process (Figures 6-8). A strong acid (usually hydrochloric acid, HCl) is used to swap H+ for all of the positively charged ions on the cation-binding resin (Na+, K+, Ca++, Mg++, etc.). Likewise, a strong base (usually sodium hydroxide, NaOH) is used to swap OH- for all of the negatively charged ions on the anion binding resin. Both hydrochloric acid and sodium hydroxide are readily available and inexpensive. They are, however, potentially dangerous to work with. The detailed procedures and safety precautions are beyond the scope of this article, but can be found
elsewhere online."*
*Source: