Illinois Administrative Code
Title 35 - ENVIRONMENTAL PROTECTION
Part 375 - COMBINED SEWER OVERFLOW EXCEPTION CRITERIA AND FIRST FLUSH DETERMINATION
Subpart A - INTRODUCTION
Section 375.102 - Definitions

All definitions in 35 Ill. Adm. Code 301 shall apply here. No incorporation by reference in this Part contains any later amendment or addition.

"Combined Sewer" means a sewer designed and constructed to receive both wastewater and land runoff.

"Dry Weather Flow" means the flowrate consisting of wastewater and non excessive sewer infiltration in the absence of inflow sources attributable to direct precipitation or snowmelt and is not restricted to meteorologic drought conditions. Nonexcessive sewer infiltration is defined at 40 CFR 35.2005(1984).

"First Flush" is defined as that volume of water and associated flow rate needed to carry solids or BOD concentrations is excess of the dry weather flow. This volume is shown on the "cross hatched" area in Appendix A.

35 Ill. Adm. Code 306.305(a) uses the term "first flush to describe those flows requiring complete treatment. When sewage is flowing through a sewer system at fairly uniform lower velocities, which occur during dry weather conditions, some of the solids present in the waste are deposited in the pipes. In addition, solids and some organic material that may be washed into the combined sewer systems tend to build up on streets, parking lots, etc.

When a storm event occurs, the volume of wastewater carried by the sewer increases and the velocities in the sewer also increase. The high volume of storm water causes material to be washed from the streets into the sewer and the sludge deposited in the sewers to be scoured from the pipes. This material is carried to the sewage treatment plant where it exerts an additional organic load on the biological system and an additional solids load on the physical systems. The maximum impact of this phenonmenon tends to occur at the beginning of a storm event and taper off as the storm continues and there are no additional solids to remove.

"Human Contact Activities" means wading, swimming or any other activity involving partial or total immersion of the human body in the subject waterway whether for recreational or other purpose.

"Wet Weather Flow" means a flowrate condition, elevated above dry weather flow, attributable to land runoff as the result of snowmelt or a precipitation event.