Removal of manganese is ...

Removal of manganese is generally a two step process -- first an oxidation step followed by physcial removal. The chemistry of manganese is complex. Oxygen will not oxidize Mn(II) to Mn(IV). To do that, you need a stronger oxidant -- HOCl, ClO2, MnO4, O3, ferrate, etc. The oxidation step is generally pretty fast, on the order of minutes. However, getting the resultant particualtes to grow so that they are easily removaed by clarification is often more dependent on pH, especially with chlorine. It is less dependent on ph with stronger oxidants like permanganate and ozone. There are many available resources, including some excellent ones by Dr. Bill Knocke of Virginia Tech (see the Water Research Foundation website for these reports)