The anaerobic digestion of primary sludge (PS) and thickened waste activated sludge (TWAS) using an anaerobic fluidized bed bioreactor (AnFBR) employing zeolite particles as the carrier media was investigated at different organic loading rates (OLRs). PS was tested at OLRs from 4.2 to 39 kg COD/m(3)-d corresponding to hydraulic retention times (HRTs) from 1.0 to 8.9 days. The highest COD removal and VSS destruction efficiencies for primary sludge of 85% and 88%, respectively, were achieved at an HRT of 8.9 days and OLR of 4.2 kg COD/m(3)-d. For TWAS, VSS destruction efficiencies varied from 42% at an HRT of 2.6 days and OLR of 13.1 kg COD/m(3)-d to 69% at an HRT of 8.8 days and an OLR of 4.2 kg COD/m(3)-d. The first-order COD biodegradation rates in the AnFBR for PS and TWAS were 0.4 d(-1) and 0.1 d(-1), respectively, almost double the rates in conventional high-rate digesters. (C) 2014 Elsevier Ltd. All rights reserved.