The TDS on the softened side will always be higher than on the untreated side. Remember, a softener is NOT a filter, but an exchange unit. It requires enough sodium to replace the hardness minerals and in doing so, over compensates, thus leaving TDS marginally higher.

If your TDS is astronomically higher, then the softener is not working well and too much salt is passing through. Perhaps not backwashing properly.

Andy Christensen, CWS-II