Numbers 19:19

And the clean person shall sprinkle upon the unclean
The clean priest shall sprinkle upon the unclean man, as the Targum of Jonathan; that is, he shall sprinkle the water of purification upon him that is unclean in any of the above ways:

on the third day, and on the seventh day; (See Gill on Numbers 19:12),

and on the seventh day he shall purify himself;
either the unclean person, who shall perfect his purification, as Jarchi interprets it, that is, by doing what follows; or else the clean person, who becomes in some measure unclean, by sprinkling and touching the water of separation, as appears from ( Numbers 19:21 ) as the priest that sprinkled the blood of the heifer, and the man that burnt it and gathered its ashes, ( Numbers 19:7-9 )

and wash his clothes, and bathe himself in water, and shall be clean at
even;
in like manner as the man that let go the goat into the wilderness, ( Leviticus 16:26 ) .