Wastewater: Activated Sludge – Biological Foaming

Biological foam is due to operating conditions in the wastewater treatment plant.

  • It accumulates on the surface of the aeration basin and may carry over into the final clarifier and effluent discharge.

The use of detergents also generates foam but the comments below relate solely to biological (bacterial) foams.

Three types of biological foaming may be observed:

  • Foam type 1: white, frothy, not particularly stable foam
  • Foam type 2: white/brown foam, stable and containing fine particles of mixed liquor solids
  • Foam type 3: dark, stable, heavy, ‘chocolate mousse’, foam


  • Foam type 1: few or no filaments (microscopic examination); high F/M loading or plant start-up situation
  • Foam type 2: significant – but not necessarily predominant – presence of filaments (microscopic examination)
  • Foam type 3: likely significant presence of higher life forms such as rotifers (microscopic examination); low F/M, long sludge age

Principal causes:

  • Foam type 1: protein materials not biodegraded due to excessively high F/M or, in a plant start-up situation, the optimum MLSS concentration not yet having been reached
  • Foam type 2: certain filamentous bacteria produce extracellular polymer (ECP) substances which are surface active and cause foaming
  • Foam type 3: long sludge age systems operating on high nitrogen raw wastewaters generate high mixed liquor nitrate concentrations due to biological nitrification.  In the absence of a dedicated denitrification facility the nitrate may denitrify in the aeration basin and final clarifier.  The  resulting nitrogen gas causes the MLSS  to  float  in  the  final clarifier and aeration basin, forming the characteristic dark brown foam or floating sludge

Correction strategy:

  • Foam type 1: Reduce the F/M (e.g. by increasing the MLSS).  Check that detergent use is not excessive as detergent foam is similar to this type of protein foam.
  • Foam type 2: Use the same strategy as for sludge bulking.
  • Foam type 3: Install a dedicated anoxic tank for denitrification.  This is a specialised item and expert assistance should be obtained for its design.

ECOS Environmental Consultants Limited