What is the black sludge at the bottom of my old coolant bottles?8

[The motor guy]

Stopped by advanced auto to drop off my old coolant. The last little bit at the bottom of each bottle had some grayish black sludge in it. The rest was clean as could be and I wondered at first if I was pouring new coolant in until the last bit, then I was like okay this stuff must be used. I think whatever rust and other solids settled out at the bottom of each container. I also noticed the funnel on the used coolant barrel had a bunch of black residue on it too, so I think this is a common occurrence.

 

[Escape08]

Could be dirt in the engine, but my guess the black debri is rubber coming from a deterioting coolant/radiator hose.