Matthew Poole and continuators on Numbers 19:21

21 And it shall be a perpetual statute unto them that he that sprinkleth the water of separation shall wash his clothes : and he that toucheth the water of separation , shall be unclean until even.

Because he is unclean, as it here follows. It is strange, that the same water should cleanse one person, and defile another, but God would have it so, partly to teach us that it did not cleanse by any vertue in it or in the work done, but onely by vertue of Gods appointment: partly to mind the Jews of the imperfection of their Priesthood and their ritual purifications and expiations, and consequently of the necessity of a better Priest and sacrifice and way of purifying, which these outward Rites did point at: and partly to shew that the efficacy of Gods ordinances doth not depend upon the person or quality of his Ministers, because the same person who was polluted himself could and did cleanse others.

Either by sprinkling of it, or by being sprinkled with it, for even he that was cleansed by it, was not fully cleansed as soon as he was sprinkled, but onely at the even of that day, as is said here and above, ver. 19.