Hi Arun.  I have come across ...

Hi Arun.  I have come across this when the disinfecting chlorine somehow managed to get into the activated sludge bioreactor.  We were dosing the treated effluent stream with liquid chlorine and a leaking check valve allowed the chlorine to weep into the bioreactor.  It was a lesson never to be repeated.  At first we had no idea why the biomass was so black.  Aeration was good and at first we looked for a cause in the sewage.  This was in an SBR type treatment plant.