My mom, who spent 10+ years living with a succession of cats (and one husband) on a 40′ sail boat, knows a thing or two about cat litter efficacy.  Her choice: Swheat Scoop natural wheat litter.  It’s clay-free, chemical-free, fragrance-free, 100% biodegradable and compostable, not to mention flushable.  If you can’t find it at your local supermarket, Petco
or PetSmart, you can order it at a 20% discount from PetFoodDirect.com using coupon code PET3411.