Nocardia occurs preferably at a high sludge retention times, but sometimes (rarely) at a low sludge retention time (spec. sludge loading 0.1-0.7 kg/kg d). Fat promotes the growth of Nocardia.
Nocardia is hydrophobic, which together with the tangled arrangement of the filaments in the sludge allows the formation of massive and extremely stable foam, which can hardly be eliminated even by spraying. Skimming the foam on the surface alleviates the current situation but does not solve the underlying problem.
Nocardia and the resulting foam in the activated sludge plant also leads in most cases to massive foaming in the digester and the corresponding operational problems.


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