I'm new to developing sheet film and I've just noticed that a fine black silt is accumulating in the bottom of my developing tray. My only guess is that it may be residue from the antihalation layer. I'm reusing the developer for several sheets, but within recommended usage and it seems to be working just fine. The silt stays in the very bottom and rinses out easily after I pour the developer back into its bottle.
This is HP5+ and Delta 100 in Ilford's ID-11.
Update: I decided to experiment a little and put some developer and water on separate areas of the back of a sheet of film. The water loosens the antihalation coating which rubs off on a rag, showing a black residue. The developer completely dissolves/transforms the antihalation layer, leaving no visible residue when I wipe off the excess. From this, I assume that the silt is not from the antihalation coating, but from some other reaction.
From the informative responses from Alan Marcus, David Berry and Jindra Lacko, my current guess is that it is some other precipitate, possibly from the tap water I'm using. Though I think my tap water is pretty good as such and I've never had a noticeable problem in the past, I think I'll go with distilled from now on.