An activated sludge, nitrifying plant reports that each time they curtail aeration to prevent mixed liquor wash-out at storm flows, the clarifier effluent becomes clearer than normal after they resume the aeration.  Two possible explanations involve:

- release of extracellular polymers by stressed bugs,

- die-off of free swimmers.

Any other suggestions?

Does anybody else have a similar experience?

Without any operational data, it's hard to be sure what's happening.

We run a nitrifying activated sludge process.  To insure nitrification, the staff ran with an old sludge age with fast settling activated sludge (SVI in the 40's to 50's.  Fine particulates remained suspended in the liquid layer.  The activated sludge was undergoing endogenous respiration.

In addition to the SVI, microscopic examination of the activated sludge under phase contrast @ 200 X (or with a regular microscope with the condensor lowered to enhance contrast) the floc was dense and clotty.  You could tell that, because as you focus up and down...the floc was dense...no wastes or oxygen get's to the center of the floc.

If these are consistent with your experience, you might want to consider slowly lowering the sludge age/MLSS to promote growth of the better activated sludge.  Since you have to nitrify...be careful.  I'd confirm that the F:M was very low...SVI is low...pounds BOD loading to the aeration basin is low.

Please let us know whatever you find!!!!!

Thanks

Dean