The problem is not what to fill it with, but how to dispose of the, erm, clumps, on a daily basis.  I want it out of my apartment, and I usually depend on plastic bags for that.  And it is not just the smell, which can be easily masked, but the fact that, well, if I were a cat, I'd want my litter cleaned regularly too.  Cats don't want to walk around in that.  I don't think the plumbing in my vintage walk-up can handle flushable litter, and I don't want to experiment to find out.

I assume people with dogs they have to walk face a similar problem...

"Talking nonsense is the sole privilege mankind possesses over the other organisms." -Dostoevsky