Historical development, as well as the main principles of a kinetic selection theory (KST) of non-filamentous microorganisms is reviewed, and the original definition of a selector for filamentous bulking sludge control is given. The idea of a selector based on KST originated from experiments with a compartmentalized system at the beginning of seventies. Since that time, KST has been completed and confirmed by additional experiments with both intermittent substrate feeding and influence of degree of longitudinal mixing on sludge bulking. Nevertheless, numerous examples of practical use of a selector showed both positive and negative results on filamentous bulking control. Consequently, it is concluded that due to both a large diversity of filamentous organism's physiology and a complexity of bulking phenomena, there is no universal method to control the growth of all filamentous species. For each case, a specific analysis is necessary before choosing the specific solution. In the case of low F/M bulking with filaments like Microthrix p., types 0041 or 0092, the application of KST using contact zone has given good results for several full scale treatment plants.

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