NO2-N decrease and NO3-N increase while we lower the DO in the high chloride (~10,000 ppm) wastewater

the petrochemistry and chemical wastewater we deal , 1.the inflow : COD~1,000 ppm, NH3-N 20~30 ppm,TKN ~50 ppm; the process parameter,HRT ~12 hrs for one aerotion tank (DO 2~3 ppm) and HRT ~12 hrs for the other aerotion tank (DO 6~8 ppm) , 2.the effluents: COD ~200 ppm,NH3-N 2~20 ppm,and NO2-N 2~20 ppm, NO3-N 0~2 ppm.3.while we lower the DO of the second aerotion tank from 6~8 ppm to 3~4 ppm,then the NO2-N change from 2~20 ppm to 0~2 ppm, and NO3-N change from 0~2 ppm to  2~20 ppm. 4.what happen to the second aerotion tank ,when we change the DO from 6~8 ppm to 3~4 ppm.5.the  NO2-N (2~20 ppm) will negative impact the sludge of aerotion tank ,and make the reduction rate of NH3-N is unstable, and the NH3-N of effluents is 2~20 ppm.