In many places, your garbage becomes "public" the moment you relinquish custody of it. These tubes will be metered, and the mechanism to do that will enable the police to grab your canisters as they arrive at network hubs.
You jest, but I fear we're not far from chemical analyzers being put into the sewage system, so that advertisers can detect a sudden appearance of e.g. pregnancy hormones in someone's toilet output, and helpfully spam them with baby stuff ads.
I remember reading about work to build something like that in public toilets to detect potential infections spreading around a city, but I'll be voting against such projects up and until the adtech industry is burned down to the ground.