Black pellet casings found in bath water

Hoping someone can shed some light on what these bad boys are please?

Recently came rushing out of bathroom taps (hot and cold), lasting for around a week.

Explanation given by estate agency (without inspection); mains had been turned off, and tank water level dropped due to useage. When mains were turned on again, dregs of crap at the bottom of the water tank became sucked into water supply.

That explains the how, but not the what.

Any ideas anyone?

Cheers