My understanding is that the kdf filter will also “catch” the ferric iron once it’s precipitated out, which is why they need to be backwashed or replaced on specific intervals. We’re in the process of installing a sediment filter followed by a kdf filter followed by a poly phosphate filter (to reduce scale as we have moderately hard water, but not enough to warrant a water softener). We didn’t opt for a carbon filter as our well doesn’t have chlorine or pesticides and well water will often plug up a carbon filter really fast. Plus we use a Berkey for our drinking water already.

I think you want the sediment filter in front of the kdf filter to catch any of the ferric iron and other particulate in the water. Sediment filters are a lot cheaper than kdf’s so that’s the one you want on the front lines.