A Maximum Critical Length

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 BOD 5 of 5 mg/l.  The amount of time required to reestablish the 5 mg/l BOD 5 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: t c ≤ -1 ln (5) K c E f ln = natural logarithm function E f = BOD 5 initially present in the stream K c = carbonaceous decay constant t c = maximum critical time of travel This relationship assumes that BOD 5 decays according to ordinary first order reaction kinetics.