Unfortunately air is not ...

Unfortunately air is not very effective in oxidising manganese and works better for elevated iron to oxidise the iron. There are a few options that you can look at. You can oxidise the manganese with potassium permangante. Manganese greensand (Maddox) is definitely also a possibility. Especially if you have existing sand filters. You will just have to regenerate the filter media and that can be done with potassium permanganate. Another possibility is biological manganese removal. For this to work you need to get your pH to a certain level while the dissolved oxygen levels are also important. It may be possible to simply add a pH step to your current plant and inject oxygen in the water (if the DO is not adequate) such that you can get the desired conditions on your filter media for the organisms to grow. We have recently commissioned a biological iron and manganese removal plant. It works very well but where you have iron and manganese, you need 2 steps in series due to the fact that for Mn removal you need to raise the pH and control the DO levels. The same is not true for Fe removal which is generally much simpler.