Bulking in activated sludge systems due to proliferation of Sphaerotilus natans is very common in laboratory-scale but rare in full-scale systems. From two laboratory-scale studies it is concluded that a cause for proliferation was attached growth of S.natans on the walls of the feed lines and reactor surfaces continuously seeding the mixed liquor. It is suggested that S.natans bulking in laboratory-scale systems is common compared to full-scale systems because the surface area/volume ratio of the former is orders of magnitude higher than that of the latter so that the potential for seeding from attached growths in laboratory-scale plants is correspondingly higher. It would appear that in laboratory-scale activated sludge systems regular cleaning of the feed lines and daily scrubbing of the reactor and other wetted surfaces will eliminate S.natans bulking due to seeding of the mixed liquor from these surfaces.

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