Problem: An activated sludge system with the MLE process (i.e., pre- anoxic system with internal recycle) has the following influent wastewater characteristics and is operated with an aerobic volume of 4000 m3 and an aerobic SRT of 10 days at 20 oC. Given that the nitrate production based on the influent flow rate is 50 mg/L and that the anoxic hydraulic retention time is 3 hours, (a) what is the effluent NO3-N concentration assuming that the nitrate removal is limited only by the anoxic tank capacity; (b) Given that the return activated sludge ratio is 0.5, what is the internal recycle ratio to achieve the calculated effluent NO3-N concentration?
- Influent: flow rate=12,000 m3/day
- Influent soluble BOD=120 mg/L; total BOD = 240 mg/L; effluent soluble BOD is negligible
- Activated sludge: Y=0.40 g VSS/ g BOD; kd=0.03 g VSS/g VSSd
- (Use the SDNR vs. F/M curve and ignore correction for internal recycle for this particular problem)