Foaming control in the activated sludge process using
metabolic control
Background:  Activated sludge is the most widely used secondary treatment process for both domestic and
industrial wastewaters.  Whilst the activated sludge process is very robust, it is not without operational
issues. Foaming is caused by a range of specific filamentous bacteria and actinomycetes, and while much is
known about the micro-organisms the mechanisms that trigger and sustain foaming remain poorly

Objectives: The aim of this project is to identify the key factors that trigger foaming and to minimize the risk
of reoccurrence through the development of an operations algorithm.

Project: Ten activated sludge plants with a range of configurations and population equivalents have been
studied to identify whether foaming is linked too either design or operational factors.  The level of foaming is
evaluated using a ‘scum index’. This scale-rating system is based on a range of foam characteristics such
as colour, depth, stability and foam. This has been supported by microscopic and physico-chemical
analyses including VFA, total lipids and  hydrophobicity.  

The next phase will be the use of continuous flow  pilot plants to more clearly understand the function of
selected parameters on foam development and stability.

The data from both the field and pilot work will be used to develop a foaming control algorithm incorporating
fuzzy reasoning techniques. This algorithm will form an essential component of a foam risk assessment
model which will then be used to select which operational procedures and control strategies provide the
best chance for foaming abatement and control. From this a better understanding will be obtained as to how
to prevent the on-set of foaming within the activated process by controlling metabolic pathways through
kinetic factors.

Survey:  Plant operators are invited to take part in a questionnaire survey to help develop a predictive and
management model for activated sludge foaming.  The questionnaire can be downloaded
here, or you can
Martin Fryer directly who is conducting the survey. For further information use this link.


  • Fryer, M. and Gray, N.F*. (2012) Foaming Scum Index (FSI) – A new tool for the assessment and characterization of
    biological mediated activated sludge foams. Journal of Environmental Management , 110, 8-19.
  • Fryer, M., O’Flaherty, E. and Gray, N.F. (2011) Evaluating the Measurement of Activated Sludge Foam Potential. Water ,
    3(1), 424-444.
Martin Fryer
Centre for the Environment, TCD


Funded: Science Foundation Ireland.
 Water Technology Research Group
 Trinity College Dublin
   1980-2010 thirty years of research, training and consultancy
Martin is currently in his
final year of the project
and is writing his thesis
Key processes (numbered) associated with
the establishment of bacterial- induced
foaming within activated sludge plants