Generally Cation exchange resins regenerated with Hydro Chloric Acid does the thing. What level of removal you require? As 1600 ppm is quite high, why can't you opt for RO?
Amberyst 15 (wet or dry) Rohm-Haas! It's a Sulfonic Acid resin. Pretty cheap, too, if bought in bulk direct from them. Plus they can be regenerated with a strong acid. Ask for a sample and run some ion exchange lab tests. It's been working well for me to remove ~ 100 ppm levels of Na, K, and Ca. Quark is right, though. At those levels, it won't take long to exhaust your ion exchange beads. Depending on the level of Na you will allow in your effluent, look at some pre-filtration types or an all-out RO system. Make sure your stream is free of solids and fines. The fines might account for some of the Na detection, but they are not actually dissolved, but rather colloidal. Flocculants help to remove colloids from water systems.
