SCUMMING DUE TO ACTINOMYCETES - TOWARDS A BETTER UNDERSTANDING BY MODELING

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 f loc-forming microorganisms in activated sludge systems hydrophilic and hydrophobic (lipidic) substrates must be differentiated. The most imp ortant observations from practice may be simulated with the ACTISCUM m odel: small concentrations of surfactants and substantial concentratio ns of hydrophobic substrate may lead to the proliferation of Actinomyc etes. Excessive growth of Actinomycetes may be suppressed by aerobic s electors 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 substra te half-saturation coefficients of heterotrophic floc-forming microorg anisms and Actinomycetes and the hydrolysis constants should be subjec ted to further research. It is demonstrated with the ACTISCUM model th at especially non-biodegradable, or only slowly biodegradable surfacta nts, may provoke a dramatic increase of the Actinomycetes population i n the activated sludge. It is shown that inoculation of Actinomycetes favours their proliferation, a fact well known in practice. For certai n wastewater compositions the anoxic selector has to be significantly enlarged in order to successfully suppress the Actinomycetes. Actinomy cetes may be washed out at low solids retention times, but their washo ut always corresponds with a loss of the nitrifiers.