Mont. Code Ann. § 37-42-104

The department shall classify all water treatment plants, water distribution systems, and wastewater treatment plants with due regard to the size, type, physical conditions affecting such water treatment plants and distribution systems, character of wastewaters to be treated and other physical conditions affecting such waste treatment plants, and according to the skill, knowledge, and experience that the operator in responsible charge must have to successfully supervise the operation of such water treatment plants and water distribution systems and such wastewater treatment facilities so as to protect the public health and prevent unlawful pollution.