Cat litter, I keep all my sawdust and just mix all types/grades in one bag. I don't generate enough to have constant free litter but I buy the big bales of bedding sawdust too. It works out as a fraction of the cost of dedicated litter and works just as well. Issues around what wood etc are a non go as the cat uses it once quick and doesn't lie in it.
If you don't have one yourself just throw it up on FB or gumtree, worst case you end up giving it away but still saves you a trip to the council tip where you said they refuse it anyway.