As the owner of two cats in a small apartment, the issue of the litter box is actually an important issue to consider in our household for sanitary and odor issues. We've been using silica gel litter (aka crystals) for several years now after trying a wide assortment of environmentally friendly options made out of corn, pine wood pellets, recycled newspaper, clumping sawdust, barley, and even dried orange peel (how about Soylent Green litter?). Some worked better than others, none worked better at absorbing odors (guests have remarked our apartment does not smell like the usual small space, multi-pet abode), and most importantly the cats seem to like the sodium silicate best...
Blue Naturally Fresh Pellet Non-Clumping Cat Litter, Walnut Shells
This is a non-clumping litter, but it still must have twice daily maintenance. All non-litter contents of the box should be removed and disposed of in a sanitary fashion. The cat’s urine turns the wood pellets to sawdust and is absorbed; this sawdust filters to the bottom of the box. Obvious wet sawdust should be removed as best as possible. At the end of the week, the box should be completely emptied of all contents. Once empty, give the inside a quick wipe-down with white vinegar and then dry. The box is then ready again for a new fill of fresh wood pellets.
An alternative would be wood stove pellets.
Weruva masks the pee-poop odor with its own strong, woody fragrance and clumps as well as any non-clay based litter. These flushable cat litter pellets are a good choice if you are looking for any kind of natural cat litter and not just ones that can safely go down the toilet.
Training Your Cat to Wood Pellet Litter | PetHelpful