You should never use the substrate ever again.
Its not even natural for them as they dont live on sand in the wild, the closest thing to really replicate that type of texture is a somewhat hard surface which would resemble hard packed dirt and clay in their home range of Australia.
If you use it again your risking impaction, fungal infections, bacteria breeding grounds, prolapses, eye infections, and multiple other things.
In the wild if their in an area with more looser substrates they can easily escape all the bacteria if needed, however in a small tank they cant.
Some people see their beardie take a few licks of the substrate and think nothing of it, until eventually in a few years the bearded dragon gets so much sand in their digestive system of licking it up occasionally over the years it gets impacted and dies if untreated.
Stay with non risky substrates like paper towels, non adhesive shelf liner, tile or news paper.