Tailings at mines are often mined for other minerals once their values rises enough.

https://en.wikipedia.org/wiki/Tailings#Reprocessing

I wonder if concentrated sludge can be collected and processed in a specialized plant with higher efficiency, reaching market efficiencies.

Platinum and palladium from catalytic converters accumulates in roadside dust:

https://www.forbes.com/sites/timworstall/2013/07/15/the-stre...

I've often wondered about the economic viability of 'mining' landfills for recoverable materials.

But I suspect it's less expensive to focus on 'simpler' extraction methods from known deposits of materials rather than the wider range commingled garbage.

As they say oop north, where there's muck there's money. Although I don't think its meant quite so literally.

pecunia non olet am i right?