ABSTRACT: The thickening and dewatering of waste activated sludge (WAS) from a pilotscale submerged membrane bioreactor (SMBR) and two bench-scale complete-mix activated sludge (CMAS) reactors (high-shear and low-shear aeration) treating the same municipal primary effluent was investigated. Solids settling and compaction were measured using the diluted sludge volume index (DSVI) analysis and a batch centrifugation analysis, respectively.Elevated levels of filamentous microorganisms resulted in higher DSVI values and lower centrifuged pellet concentration. Elevated levels of nocardioform bacteria resulted in lower solids float concentrations and higher colloidal material reduced solids recovery in batch flotation experiments. Sludge filterability, measured as time to filter (TTF), was shown to be a function of extracelluar polymeric substances (EPS) and colloidal material, where higher levels of either reduced sludge filterability.