Md. Code, Environment § 12–401

(a) The Secretary shall classify all waterworks, wastewater works, and industrial wastewater works in this State.

(b) In determining the classification of a waterworks, wastewater works, or industrial wastewater works, the Secretary shall consider:

(1) The size or capacity of the facility;

(2) The type of treatment used;

(3) The character of the water or wastes treated; and

(4) Any other physical conditions that affect the facility.