Removals of 10 pharmaceutical compounds by microbial sludge in membrane bioreactors (MBR) operated under infinite and limited solid retention time (SRT) were investigated. High removal ( > 80%) of DCF, TMP, NPX, IBP, and TCS were achieved but CBZ removals were low ( < 20%). The residual pharmaceutical compounds leftover from the biodegradation in different sludge particle size fractions was quantified through physical separation and filtration in series. The results revealed that hydrophobic compounds were mainly adsorbed onto the coarse particles ( > 0.45 mu m) where majority of adsorption site was available. Meanwhile, hydrophilic and moderate hydrophobic compounds were less associated with particles and they were retained in fine particles and gel-like substances (1 kDa-0.45 mu m). Most of the studied pharmaceutical compounds associated with fine particles and gel-like substances was subsequently rejected by membrane filtration in the MBRs. The operation of the MBR at high mixed liquor suspended solids concentration under long sludge age conditions could enhance the removals of pharmaceutical compounds through increased adsorption site on the sludge particles.