If the iron and manganese is ...

If the iron and manganese is in the reduced form in ground water, then it needs to be aerated to oxidise it.  The other alternative is to dose sodium hypochlorite to oxidise the metals.  There are better materials to remove the metals than sand filtration.  Base Iron Removal Media (BIRM), Green Sand or Fylox are all better than sand itself.  If the iron or manganese is of bacterial origin, then a better material to use is AFM (Active Filtration Media) which is based on recycled glass and has a coating on the surface.  Sand itself can be used but bacteria will start coating the surface of the grain, which will increase backwash time, whereas AFM does not suffer from the same problem because bacteria don't grow on the surfaces the same.