I am sure you are familiar with the saying - I cannot remember the exact words used, but the gist of it is - correct or "scold" in private but praise in public. You may want to approach him directly and tell him that you admire his active participation during the meeting and tell him that he might not be aware that he has some garbage that are not segregated. You never know, he might just be taking out the garbage, but someone else in the household put them in the wrong bag? I don't know ... benefit of the doubt.
Another scenario is during the next meeting, bring up another neighbor that you saw was really segregating, so that you can make people aware that you have your eyes open or that someone is watching. A word of caution though, if you have neighbors that are not too warmed up to the idea of segregating, they might see you as a party-pooper, so get an idea first of what types of neighbors you have.