Lots of debate over whether or not softeners affect sewage disposal fields. May be just the extra water involved overloads the system. I would rent from a water conditioning company a iron removal tank(s). Saves you the trouble of messing with KMnO4, which stains everthing a bright purple. Won't soften,but then you didn't mention hardness as a problem.
The newer softeners use less water than a load of clothes and the amount of salt that enters the leach field is negligible. They're nothing like the old ones.
