Section 373.APPENDIX A   Maximum Critical Length

The maximum critical length of a receiving stream is the distance (downstream from the wastewater source) required at 7-day 10-year low flow to reestablish an instream BOD₅ of 5 mg/l.  The amount of time required to reestablish the 5 mg/l BOD₅ is termed the critical time of travel.  The maximum critical length can be estimated by computing the maximum critical time of travel, which can be approximated using the following equation:

This relationship assumes that BOD₅ decays according to ordinary first order reaction kinetics.