human waste vs excretion

human waste and excretion both are nouns.

Word	Noun	Adjective	Verb	Adverb
human waste	Yes	No	No	No
excretion	Yes	No	No	No

As nouns, excretion is a hypernym of human waste; that is, excretion is a word with a broader meaning than human waste:

human waste: the body wastes of human beings
excretion: waste matter (as urine or sweat but especially feces) discharged from the body

Other hypernyms of human waste include body waste, excrement, excreta, excretory product.

human waste (noun)	excretion (noun)
the body wastes of human beings	the bodily process of discharging waste matter
	waste matter (as urine or sweat but especially feces) discharged from the body