Behaviour of filamentous microorganisms under anaerobic, anoxic, and oxic conditions has been reviewed from the point of view of filamentous bulking in nutrient removal activated sludge systems. It was concluded that the growth of most filamentous microorganisms is considerably suppressed under anaerobic and anoxic conditions and that the filamentous bulking in nutrient removal systems is chiefly caused by the growth of filamentous microorganisms in oxic zones. The role of particulate substrate hydrolysis was also discussed and the necessity of compartmentalization of oxic zones was shown.

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