415 ILCS 45/5.1
Except as otherwise provided by Agency rules adopted pursuant to subsection (2.5) of Section 1 of this Act:
"Class A community water supply" means (i) any surface water community water supply and (ii) any community water supply that includes coagulation, lime softening, ultraviolet disinfection, membrane filtration, or sedimentation as a part of its primary treatment.
"Class B community water supply" means any community water supply that includes filtration (other than membrane filtration), aeration and filtration (other than membrane filtration), or ion exchange equipment as a part of its primary treatment.
"Class C community water supply" means any community water supply that uses chemical feeding as its only form of treatment.
"Class D community water supply" means any community water supply that has only pumpage, storage, or distribution facilities.
415 ILCS 45/5.1