To remove manganese you need ...
Published by Yahia Elbasha, Water Treatment Specialist at Global Water Intelligence
To remove manganese you need to make an oxidation from Mn+2 to Mn+4, but Mn not easily oxidized in natural water pH range (7-8), & need much more contact time, but at pH not less than 9 it easly & rapidly oxidizied, so that firstly you need to raise water pH using forexample (sodium carbonate), then using an oxidizing agent like Cl2, ClO2, Kmno4 and the precipitation will be MnO2 and you can remove this with sand filters.