If the sandy bits that shook out of the used filter were beige/tan/brown, then it is most likely built-up calcium that let go off the walls of pipes in the water system. Very common.
If the bits are all black, then it could be activated charcoal/carbon from inside the filter, if
the filter has an organic absorber in it. If the filter info says it removes chlorine, for example, then it has activated charcoal in it.
Activated carbon - Wikipedia, the free encyclopedia
Around here, all of our water is surface water from reservoirs. When the water temp gets up there in summer and fall, a lot of chlorine gets added. You can turn on a faucet and smell it. Using a water filter that absorbs chlorine and other organics is a good idea here. The addition of large amounts of chlorine creates trihalomethanes as a trace by-product, of which most are carcinogenic. Leaving water sit out unrefrigerated in an open wide-mouthed container for at least a day seems to let the evil organics evaporate out.
In summer we also have the algae blooms and water turn-over that adds the wonderful "ewww, stale pond water!" smell to the water. Both methods above seem to get rid of it.
We need to add the wonderful surface water to REW's Texas list.
When you drink from a reservoir, and your treated sewage goes down into streams that feed into the top end of the next reservoir down stream, and on and on, you don't want to think too much
about where your water
came from... or what/who it's been through! I know I know, TMI!
If you are ever in New Orleans, stand on the Moonwalk there and watch the river flow... and think about where all that water has been