Four trials with different durations (1, 2, 3 and 4 days) of neutralized leachate recirculation and a control with distilled water recirculation for 4 days were set up to investigate the effect of recirculation duration on MSW biopretreatment by a combined hydrolytic-aerobic process. Results showed that recirculation could enhance water removal and organics degradation rates and thus lead to a lower water content of biopretreated materials, when compared with the control. Recirculation with duration of 4 days had the highest rates of water removal (83.7%) and organics degradation (62.1%) and lowest final water content (43.1%) due to the elimination of acid inhibition. As a result, recirculation with duration of 4 days had the highest sorting efficiency (75%) and heating value (10570 kJ/kg) and lowest respiration activity (98.6 mgO(2)/g). Thereby, neutralized leachate recirculation with duration of 4 days was suggested to pretreat municipal solid waste with high water content. (C) 2009 Elsevier Ltd. All rights reserved.