Agency rules in 35 Ill. Adm. Code 380 (Procedure for the Certification of Operators of Wastewater Treatment Works) classify treatment works by type and size based on the volume and nature of wastewater influent to the treatment works and the complexity of the treatment works. Those rules also contain corresponding standards defining the class or classes of operators competent to operate a particular type or size of treatment works.
Ill. Admin. Code tit. 35, § 312.102