In my experience biological ...

In my experience biological foaming in most cases is due to nocardioforms or Microthrix parvicella. M. parvicella tend to disappear after dosing polyaluminum chloride, but this may not happen is foaming is due to nocardiform organisms. Also it may take at least three times the SRT (sludge retention time). One other way is to selectively discharge foaming from the top of the oxidation basin by an adjustable overflow baffle. This flow should be accounted for as additional waste sludge. A reduction of the exraction from the secondary settling may be necessary to avoid the washout of nitrifiers, especially at low temperatures. A second option is to apply ozone to a fraction of the sludge recycle flow (see Gardoni D., Ficara E., Vergine P., Canziani R. (2015) A full-scale plug-flow reactor for biological sludge ozonation, Water Science and Technology, 74(4), 560-565. doi: 10.2166/wst.2014.432, attached). I hope this info may be useful.