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Filterability of activated sludge is an important factor for the economical operation ofmembrane bioreactors (MBR). In the literature mainly investigations on sludge dewaterabilityin respect to further disposal are published. In this study, a procedure to determine filterabilityin a crossflow test cell is introduced. Its features are: no increase in sludge concentrationduring batch trials, crossflow conditions, and little impact on the sludge structure. Sludgesamples of eight different MBR and one conventional wastewater treatment plant wereexamined and compared. Contrary to the literature, no impact of the solid fraction (MLSS),sludge viscosity, or extractable extracellular polymer substances (EPS) concentration on thefilterability was found. Instead, the composition of the liquid phase affected the filterability ofactivated sludge most. A comparison of supernatant of activated sludge taken directly beforethe submerged module and permeate revealed a fraction of polysaccharides in the supernatantthat was not present in the permeate. The concentration of polysaccharides was found toinfluence filterability: the higher the polysaccharide concentration in the supernatant, thelower the filtration index. It was assumed that polysaccharides adsorb to the membranesurface. This reduces the effective pore size and forms a dense layer on the membranesurface. Both mechanisms reduce the filtration performance of membrane bioreactors. Includes 21 references, tables, figures.

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Edition: Vol. - No. Published: 03/05/2003 Number of Pages: 10File Size: 1 file , 330 KB