Densification may be an alternative method for controlling sludge bulking; however, little information is available on the method. Various methods including different oxygen concentrations, surface velocities of aeration, calcium concentrations and extended starvation period were employed for triggering densification of filamentous sludge in this study. It was found that high surface velocity of aeration could effectively improve the settleability of filamentous sludge in the short term, which resulted in filamentous granulation, but could not avoid filamentous bulking even after granulation. Increase of calcium concentration could also improve the settleability by forming pigtail filaments colony in the short term, but could not trigger granulation. Extension of starvation period seems effective neither for further densification nor for granulation. Additionally, increased ovality of meshes formed by filaments was frequently associated with a dense structure of filamentous sludge. It was suggested that pigtail type of filamentous colony that thus increased the resistance to elevated shear force may encourage the densification of sludge, and two kind of densification growth patterns, i.e., granular or cluster growth pattern of filamentous sludge, were identified.