Applications of a mathematical simulation model, which is based on a condensed summary of hypotheses related to “Scumming due to Actinomycetes”, are presented. The most important observations from practice may be simulated with the model: Substantial concentrations of hydrophobic substrate and especially non biodegradable or only slowly biodegradable surfactants may provoke a dramatic increase in the Actinomycetes population of the activated sludge.