Scumming due to actinomycetes: Towards a better understanding by modelling

Abstract

The ACTISCUM model, which represents a condensed summary of hypotheses related to “scumming due to Actinomycetes”, is introduced. In order to simulate the competition between Actinomycetes and heterotrophic floc-forming microorganisms in activated sludge systems hydrophilic and hydrophobic (lipidic) substrates must be differentiated. The most important observations from practice may be simulated with the ACTISCUM model: small concentrations of surfactants and substantial concentrations of hydrophobic substrate may lead to the proliferation of Actinomycetes. Excessive growth of Actinomycetes may be suppressed by aerobic selectors at low solids retention times and by anoxic selectors at all solids retention times. For improved accuracy of model predictions the maximum specific growth rates, the lysis coefficients and the substrate half-saturation coefficients of heterotrophic floc-forming microorganisms and Actinomycetes and the hydrolysis constants should be subjected to further research. It is demonstrated with the ACTISCUM model that especially non-biodegradable, or only slowly biodegradable surfactants, may provoke a dramatic increase of the Actinomycetes population in the activated sludge. It is shown that inoculation of Actinomycetes favours their proliferation, a fact well known in practice. For certain wastewater compositions the anoxic selector has to be significantly enlarged in order to successfully suppress the Actinomycetes. Actinomycetes may be washed out at low solids retention times, but their washout always corresponds with a loss of the nitrifiers.