The municipal landfill leachate was treated in a hydrolysis-acidification reactor (HAR)/aerobic bio-contact oxidation reactor (ABOR) following a pretreatment with ultrafiltration (UF) membrane. Experiments were conducted continuously for 44 days at a constant flow rate of 20 ld(-1) and organic loading rates (OLRs) from 0.75 to 1.5 kgCOD m(-3) per day The results showed that COD of the leachate steadily decreased from 20,015 mg l(-1) to less than 3000 mg l(-1), and NH(4)-N decreased from 368.6 mg l(-1) to 259.3 mg l(-1) in the UF process. The COD and NH(4)-N removal efficiency of HAR was 56.7% and 27.7%, and that of ABOR was 94.6% and 86.7%, respectively The total COD and NH(4)-N removal efficiency reached 99.6% and 93.2%, respectively. UF and HAR played a critical role in raising the biodegradability of the landfill leachate, while ABOR had an important function on removing the dissolved NH(4)-N in leachate. (c) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V All rights reserved.