If it is a biodegradable solution that states as such on the package then you can generally let it go down almost any drain. If it is not biodegradable then you should save it with used motor oil or the like and any auto parts store would gladly take it in their oil disposal tank in the store. The chemicals in a "green" degreaser will neutralize the old oil and make it inert and that is why it is safe to go down the drain. Unless you have too much of the stuff then you should take it to a recycle facility.