The issue is, what form is ...

The issue is, what form is the iron in flowing from the borehole?  If the iron is in a soluble form then it will flow through any filter used.  It is possible to use air to mix with the borehole prior to it entering a filter, but this is not as good because not all the iron will be in an insoluble oxidised state.  Ozone is an oxidising agent, as is chlorine, permanganate etc.  These all need a reaction time prior to filtration.

The choice of filter is also important.  Sand is the least useful.  Traditionally green sand, Base Iron Removal Media (BIRM), Filox are all better than sand at removal of oxidised sand although AFM may be suitable.

If the iron is mainly in the form of iron bacteria then filtration on its own would be enough.  However, in most cases there will be both iron bacteria and soluble iron.  

It also depends on the level of iron present within the borehole.