Certainly, one of the most frequent problems with suspended-growth systems is excessive filament presence, which leads to a condition referred to as bulking. Healthy, well-structured floc usually contains a nominal-to-moderate level of filament presence (Figures 16.25 and 16.26). In fact, it has been proposed that filaments may serve as a "backbone," giving strength to floc and allowing them to grow larger. Recent advancements with confocal microscopy have helped to confirm this backbone role with the filaments found in floc particles. The image shown in Figure 16.27 was clipped from an animated video sequence whose sequential frames depict the progressive rotation of a floc particle with a loosely arrayed, and highly nonhomogeneous, structure bearing several backbone-type filament threads. However, these filamentous forms are sometimes present in excessive amounts, extending out from the floc into the bulk solution, thereby leading to bulking behavior. Under these conditions the filaments can impede sedimentation in the secondary settling tank by two mechanisms. First, they act as a sort of parachute, increasing viscous drag and reducing the settling velocity of the flocs. Second, the filaments act to fend the flocs off from each other, preventing them from compacting

Figure 16.25 Suspended-growth floc with nominal filaments: (a) light microscope view at 400 x; (b) SEM view at ~600x.

toward the bottom of the tank, such that the sludge then takes on a "bulky" condition, with SVI values in excess of 150 mL/g.

Taken to an extreme, filaments may replace almost completely any semblance of a normal floclike structure (Figures 16.28 to 16.30). For many years, one particular sheathed bacteria, Sphaerotilus natans, was blamed routinely as the responsible agent for filamentous bulking problems, but further study demonstrated that several other bacterial forms, and occasionally fungi, could be involved. During the early 1970s the work of D. H. Eikelboom in The Netherlands developed a systematic microscopic method which

Figure 16.26 Suspended-growth floc (400 x) with moderate filament growth.
Figure 16.27 Confocal microscopy image of suspended growth floc structure.

recognized more than two dozen different filament types that could contribute to bulking events (Eikelboom and van Buijsen, 1981). Eikelboom's classic work, which was then continued and expanded by a group working with David Jenkins in the United States, strongly suggested that several causes were possible, each as a result of the excessive growth of a different type of filamentous organism. Table 16.6 provides an overview of

Figure 16.28 Suspended-growth floc with heavy filament growth: (a) light microscope view at 400 x, (b) SEM view at ~800x.
Figure 16.29 Solely filamentous suspended-growth matrix (400 x).

the various filamentous forms they typically found, including some forms that have only been distinguished with numbers (as originally assigned by Eikelboom) since they have not been formally identified and given official genus and species names (Jenkins et al., 1993). Table 16.7 provides a synopsis of the correlation observed between the various microbial filaments and their contribution to bulking problems.

Figure 16.30 Magnified images of floc filaments.

TABLE 16.6 Microbial Factors with Filamentous Bulking and Foaming in Activated Sludges

Percentage of Facilities Experiencing Designated Problematic Bacterial Growth


Bacterial Agent



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