Waste that's been flushed

SEWAGE

This liquid byproduct often travels through underground pipes to treatment plants before being released into rivers or oceans. You might avoid it if you see a "no swimming" sign nearby.

The clue "Waste that's been flushed" points directly to "sewage," which is the term for liquid waste carried away through sewers after being flushed down toilets or drains. It’s a straightforward definition with no hidden wordplay—just a clear reference to the kind of waste that disappears when you pull the handle. If you’ve ever thought about where toilet water goes, this is the word for it.