What would be the best way of reducing COD levels prior to a Bioreactor?

Hi,

We run an AMBR system at our site. The COD levels have risen substantially lately and the blowers are struggling to reach D.O.(Dissolved Oxygen) set points. All process water is collected in a sump, transferred to a balance tank which undergoes DAF before going into the Aerobic - bio tank.

Please suggest some methods for reducing COD levels prior to a Bioreactor.

Thanks.