Hello Everyone!
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.
WHEW!!
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.

Welcome to the forums! I have no idea at all what would happen if you used precipitated manganese as a substrate, nor what it may contain by this stage. But if I had this on hand I'd be willing to set up a 10 gallon with it and test water and see.

That is a great idea!!!
I never do that kind of thing. My thought process always starts with assuming someone must have tried this before.
It is 2019 after all. I then attempt to find any info they may have shared about the experience. I wouldn't even consider using something like this normally, but I'm trying to find a cheap (free) alternative to the commercially available black substrates. Especially as I'm in the planning stage of my forever tank. Many years of saving $ (then spending it on more responsible things, most recently a big hit last week for new roof) has me pricing out a 400 gallon with 24 square feet of floor space.