Ill. Admin. Code tit. 35, § 168.250
a) Regarding any matter not privileged under Section 7 and 7.1 of the Environmental Protection Act (Ill. Rev. Stat. 1983, ch. 111½, pars. 1007 and 1007.1), the Hearing Officer shall order discovery upon written request of any party when the parties cannot agree on the legitimate scope of discovery, if the Hearing Officer determines that the disputed matters are relevant to the subject matter involved in the case. Subject to the requirements of this section, the Hearing Officer may order:
d) All depositions and interrogatories taken pursuant to this rule shall be for purposes of discovery only, except as herein provided.