Any of the new. alternative source litters are sustainable. There is recycled paper, wheat, corn and more. I have used the Swheat Scoop for 5 years with good results. Seems like there are new materials coming out all the time.

I would recommend going to a big pet website like Petsmart or Petco and browsing the selection. Just beware - there are as many different opinions on cat litter as there are on politics and religion. You really need to try it with your cat(s).

As far as "sustainable" which I assume means environmentally friendly, clay litter is certainly that. Obviously it has its drawbacks, dust being the primary drawback.

spedigrees - Sorry, but clay litter is NOT environmentally friendly. It is strip-mined, leaving huge bare, open pits. Recently the companies have been pressured to repair the damage but it is still damaging. There is lots of info on this online.

It depends on where you live ... I use pellet fuel made from the leftovers from lumber mill operations. It's ground up, compressed bark and mill scraps.

It takes less transportation to get it to me than if I bought wheat or corn based products from the Midwest.

And we compost the used litter (minus cat poop) for less waste and better garden.

Pellets sounded good to me but my cat didn't think so. Some cats that are raised on small-particle cat litter and like the way it feels when they dig in it won't move to the pellets. And my cat sure likes to dig.

Blue has a litter made out of walnut shells. I like the thought of it but having tried it now I can say it tracks around quite a bit and stepping on small pieces of walnut shells in bare feet is not a pleasant experience. It isn't bad as a litter so if you have your box(s) in a place where tracking a bit wont matter much then its worth a try.