Quote Originally Posted by gordyjamz View Post
What did they do in the old days before CYA?
Good question. Historically, anhydrous ammonia had been used to stabilize chlorine since monochloramine is more stable to UV than hypochlorous acid. Sulfamic acid and sulfamates have also been used and, if I am not mistaken, so have hydantoins and other ammonia and urea like chemicals (which include cyanuric acid and cyanurates!)

It seems that chlorine/nitrogen bond that occurs has the main stabilizing effect against loss from UV light..