Ill. Admin. Code tit. 35, § 604.600 – Filtration

a) Application of any one type of filter must be supported by water quality data representing a reasonable period of time to characterize the variations in water quality. The Agency may require pilot treatment studies to demonstrate the applicability of the method of filtration proposed.
b) Acceptable filters include the following types:
1) rapid rate gravity filters;
2) rapid rate pressure filters;
3) deep bed rapid rate gravity filters; and
4) biologically active filters.