"Dilution Ratio" means the ratio of the seven-day once in ten year low flow of the receiving stream or the lowest flow of the receiving stream when effluent discharge is expected to occur, whichever is greater, to the average flow of the treatment works for the design year.
Ill. Admin. Code tit. 35, § 301.270
Amended at 3 Ill. Reg. 25, page 190, effective June 21, 1979