Running water, i.e, waters flowing from a spring or river which are the purest. These manifestly signify God's Spirit, which is oft compared to water, 1 Thessalonians 7:38,39, and by which alone true purification is obtained. In a vessel, where they were to be mixed, and then the water was to be strained out and kept for this use.
Related Commentaries of Numbers 19:17
Numbers 19:17
17 And for an unclean person they shall take of the ashesb of the burnt heifer of purification for sin, and running water shall be put thereto in a vessel: