A pre-anoxic MLE activated sludge process has the following influent wastewater characteristics and is operated with an

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A pre-anoxic MLE activated sludge process has the following
influent wastewater characteristics and is operated with an anoxic
volume of 500 m3 and MLVSS of 2000 mg/L at
20°C. Given that influent flow rate = 12,000 m3 /d,
influent soluble BOD (rbCOD) is 60 mg/L, and the nitrate production
based on the influent flow rate (i.e., NOx) is 30 mg/L.
a) For an return activated sludge (RAS) of 0.7, determine the IR
that should be used to meet an effluent NO3-N
concentration limit of 9 mg/L, assuming that the nitrate removal is
limited only by the anoxic tank capacity.
b) According to the SDNR vs. F/M curves, the influent BOD has to
be above what level in order to avoid using external carbon
sources?