3. A pre-anoxic MLE activated sludge process has the following influent wastewater characteristics and is operated with an anoxic volume of 500m3 and MLVSS of 2000mg/L at 20C. Given that influent flow rate =12,000m3/d, influent soluble BOD (rbCOD) is 60mg/L, and the nitrate production based on the influent flow rate (i.e., NOx ) is 30mg/L. a) For an return activated sludge (RAS) of 0.9, determine the IR that should be used to meet an effluent NO3N concentration limit of 5mg/L, assuming that the nitrate removal is limited only by the anoxic tank capacity. b) According to the SDNR vs. F/M curves in the Lecture 17 notes, the influent BOD has to be above what level in order to avoid using external carbon sources