Low F/M bulking and scumming in activated sludge plants is a phenomenon which is not completely understood. Casey et al. hypothesised that low F/M bulking and scumming is caused by the denitrification intermediate nitric oxide. Based on this hypothesis, a mathematical simulation model is formulated which predicts the most important observations related to low F/M bulking and scumming: surfactants and hydrophobic substrates may lead to the proliferation of low F/M filaments, and low F/M bulking and scumming is normally not suppressed by aerobic or small anoxic selectors. Beside these well established facts, the model predicts that incomplete nitrification can cause serious low F/M bulking and scumming problems, that large anoxic selectors with complete denitrification prevent low F/M bulking and scumming and extended sludge blankets in the secondary clarifiers influence the proliferation of low F/M filaments: in completely aerobic systems in a negative way, in systems with predenitrification zones in a positive way.