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The bioreactor environment for the mechanically sorted organic fraction (MSOF) of residual municipal solid waste was simulated for a period of 300 days. A closed-loop system was implemented for analysing the leachate purification process due to its recirculation through MSOF. Maximum concentrations of Cu, Zn and Ni in the leachate were about 60, 20 and 15 mg L(-1), whereas Pb and Cr were about 5.5 and 7 mg L(-1). About 60 days from the start of recirculation these concentrations dropped to < 1 mg L(-1). Chemical (COD) and biological oxygen demand (BOD5) dropped, respectively, by about 50 and 80%, achieving a COD-to-BOD5 ratio > 7. Volatile fatty acids, which were about 10 g L(-1), fell to about 3.5 g L(-1) whereas biomethane production was about 34 NL kg TS(-1). As expected, recirculation under strictly anaerobic conditions gave minor benefits in reducing nitrogen and ammonium which reached final concentrations of about 4 and 3.8 g L(-1), respectively.