The author concedes that the town wouldn’t have to spend as much sorting trash under a mandatory system if all residents sorted their garbage. He's not disputing the truth of this claim. Instead, he points out that not all residents actually will sort their garbage under a mandatory system.

The author agrees that the town wouldn’t have to spend as much under a mandatory system if all residents sorted their garbage; in other words, the author grants this fact (meaning, accepts that it's true). This fact lends some support for a mandatory system. Still, the editorialist defends the current voluntary system as a better option. So the fact lends some support to an alternative (mandatory system) to the practice the editorial defends as preferable (voluntary system).

The author isn’t trying refute any claim. Although we can say the author prefers the voluntary system over the mandatory system, there is no claim that "the mandatory system is preferable," or "we should use the mandatory system," so we can't say that the author is trying to refute any claim. Also, the referenced text isn't offered as an example of a broader idea.

The fact that sorting garbage would cost less if all residents sorted their garbage doesn’t support the editorialist’s argument. It’s a fact that one might think supports the mandatory sorting system. But the author argues that we should prefer the current voluntary system.

The author doesn’t try to support the claim that sorting garbage would cost less if everyone sorted their own garbage. He argues instead for retaining a voluntary system.