AWWA MTC57673

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