Depending on the pore size and age of the resin sodium (and other ions) can be trapped within the pores and then slowly elutes.

Sometimes an polymeric anion exchangers, depending on its age, use and operation, can build up some "oxidized" spots, acting as cation exchanger. We saw things like this in our laboratories, when ion exchange resins, used for DI water systems, were regenerated "too often".

As I do not know exactly what your application is, may I just comment, that adding a cation exchanger after the anion exchanger will trap the sodium.

My two cents.

Thanks for your help ,,yes anion resin is old ,, we have cation and anion resin followed with mixed bed with flow capacity 150m3/hr used for production Demi- water for boilers ,, the feed water is from R.O UNIT WITH conductivity around 50mm/s,,, lately we faced this propelm as sodium ion bleading from anion resin and effluent water have high PH around 9 ,, mixed bed after it helps in this case but we need solution for anion exchanger to be in normal condition again ,, any ideas ?

It might sound strange, but my suggestion is to play safe and get in touch with the producer of the resin or the constructor of the facility, and discuss it directly with them.

It might be - the "ultima ratio" - that an exchange of the resin might be needed. However, this is only guess work from my side.

Please get in touch with the resin producer or the company that constructed and built the DI water facility.