I work for a municipal water dept.
When the raw groundwater enters the plant, it is whacked with sodium hypochlorite to oxidize the iron and manganese. Obviously, this also serves to disinfect the water. The water is also treated with potassium hydroxide, and sodium hexameta phosphate then goes over the sand filters, and finds it's way to the clearwell before being pumped into the distribution system.
There are 3 sand filters, which backwash every 75 hours, 25 hours apart.
The backwash water travels to 2 sludge basins. The basins are cleaned each month on a rotating schedule.
So none of that really matters, just a little background info.
My question is this...
Can I use the precipitated manganese as substrate?
I have no idea what properties the manganese has, in terms of what may leech from it. It is Blacker than black, and resembles a medium blast media in size and shape.