Preservative
Antiseptic
noun
A prevent rot dose is a dose of something you put on your cut or wound that prevents it from rotting (or you put it on something else to prevent it from rotting). This is an antiseptic or preservative.
ぼうふざい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
古い防腐剤はどこに捨てればいいですか。
Where should I throw away old antiseptics?