At least the one I went to on a school field trip the solids go to a landfill. There are two kinds of solids gross solids (trash) and sludge. Trash is strained out and removed from the incoming waste stream. Sludge is whatever settles out of the various treatment steps. It's anything that the bacteria can't digest.

Wet wipes would classify as trash but likely slip through the incoming strainer. Other things the waste treatment people hated were tampons and condoms.