Control of Bulking Sludge ...
Published by Paul Fleming, Fleming Consulting - Owner
Control of Bulking Sludge
Another issue in the activated sludge process is sludge age. If it is too old, filamentous bacteria develop, and prevent sludge from settling properly. You should be able to find plenty of reference material about testing sludge age and interpreting the results.
Some people add hydrogen peroxide to control the filamentous bacteria, but this is strictly a stop-gap measure.
Dissolved oxygen could be a factor. If a septic condition occurs, the bacteria go to the nitrate and nitrite compounds for oxygen, releasing gas bubbles that can buoy the sludge.
Good luck. let us know what you come up with.