We have problems with calcium sulfate(s) scaling a wide variety of equipment. I found some "Gypsum Dissolver" produced by JACAM chemicals that works good for removing "gypsum" scaling on small parts, but at $13 per gallon, the stuff is kinda pricey. You can dilute the stuff...
Ultrapure water will also take on a small amount of carbon dioxide from the air, which will cause the pH of the water to drop via hydrolysis. Our RO membrane permeate typically will run pH 5.0-5.5, and can be raised to ~6.8 by sparging with nitrogen.
You may want to investigate the possibility of using activiated carbon impregnated with zero valent iron, then treating the effluent via traditional lime neutralization with polymer settling agents. The zero valent iron will reduce Cr(IV) to Cr(III), apparently, and then the trivalent chromium...