This process further optimizes the NOx-N removal in the A2/O process. It recycles nitrified mixed liquor from each oxic stage to an anoxic stage for denitrification. Compared to the A2/O process, this process maintains higher BOD concentrations in the first two anoxic stages and realizes higher denitrification rates. Furthermore, this process has an exhauster with 15 to 30 min detention time installed at the end of each oxic stage to deplete the DO in the mixed liquor prior to recycling to the anoxic stage. This scheme further improves the denitrification rate.
This process can achieve an effluent NOx-N of 3 to 6 mg/l. Part b in Figure 7.38.8 is a schematic diagram of this process.
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