In this study, we evaluated the performance of an anaerobic membrane bioreactor in terms of digestion and thickening of excess sludge from an aerobic membrane bioreactor. A digestion reactor equipped with an external polytetrafluoroethylene tubular microfiltration membrane module was operated in semi-batch mode. Solids were concentrated by repeated membrane filtration and sludge feeding, and their concentration reached 25,400 mg/L after 92 d. A high chemical oxygen demand (COD) removal efficiency, i.e., 98%, was achieved during operation. A hydraulic retention time of 34 d and a pulse organic loading rate of 2200 mg-COD/(L-reactor) gave a biogas production rate and biogas yield of 1.33 L/(reactor d) and 0.08 L/g-CODinput, respectively. The external membrane unit worked well without membrane cleaning for 90 d. The transmembrane pressure reached 25 kPa and the filtration flux decreased by 80% because of membrane fouling after operation for 90 d. (C) 2016 Elsevier Ltd. All rights reserved.