The Levites were purified by washing and sprinkling and sacrifices. See Leviticus 15:13-15 Numbers 19:11, &c.
Related Commentaries of Numbers 8:21
Numbers 8:21
21 And the Levites were purified, and they washed their clothes; and Aaron offered them as an offering before the LORD; and Aaron made an atonement for them to cleanse them.