The Nitration and Denitration reaction alternately in a activated sludge system

Dear all,

The stripping sour water is treated by activated sludge to be the make-up water for cooling tower, the NH3-N reduction rate is improved in the past 1 month, we think the loading of COD is reduced maybe, and that make the nitrifying bacteria to be the dominant strains. The strange is the NH3-N reduction reaction , the nitration and denitration reaction alternately in a activated sludge system (data as the attachment) .

1.Is the loading of COD reduced ( from about 600 ppm reduce to 300 ppm), and to trigger the nitration?
   (a) the activated sludge system is 8,000 M3, the MLSS is about 2,500~3,000 ppm.
   (b) the inflow is about 4,000 M3/D.

2.The NH3-N react to be NO3-N (nitrate is produce), and alternately to denitration (nitrate is disappear )in a few days?

Thanks.