Here is the short answer. ...

Here is the short answer. suspended iron is a physical foulant. Dissolved iron (ferrous primarily) gets removed by exchange. The Ferrous then is oxidized to ferric while associated to the resin. It essentially plugs up the resin pores. To remove it, it needs to be reduced to ferrous. Enter sodium hydrosulfite. The worse the pore pluggage, the greater the damage to the resin. adding 1lb of citirc acid to every 100lbs of salt every 4th regeneration will help keep the resin cleaner and decrease iron fouling/damage.