The secretary shall classify all water supply systems and wastewater treatment facilities actually used or intended for use by the public, with due regard to the size, type, character and quantity of water or wastewater to be treated and other physical, chemical and biological conditions affecting such treatment facilities and supply systems, and according to the skill, knowledge and experience that the operator must have to supervise successfully the operation of the facilities so as to protect the public health and to protect the water resources of the state.