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