Waste

Waste products were recycled as food by chemical toilets. (TV: Gridlock)
Flies were known to consume waste matter. As such, the Tenth Doctor deemed it "perfect natural" that the Tritovores, a race of fly-like humanoids, fed on what others "[left] behind from their behind. (TV: Planet of the Dead)
While running away from a horde of new humans, Chip jumped down a tube marked "WASTE" in green text. (TV: New Earth)