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Four simulated landfill bioreactors operating under different experimental conditions were evaluated in this study. The reactors were filled with the organic fraction of municipal solid wastes (MSW) and operated as: anaerobic, anaerobic with pH adjustment, semi-aerobic and intermittent aeration bioreactors. The parameters studied in the leachate included pH, redox potential, BOD5, COD, DOC, SO4(2-), NH4(+)-N, NO3(-)-N, NO2(-)-N, Cl- and electrical conductivity. Also, the MSW mass settlement was measured at certain intervals. Leachate recirculation took place in all bioreactors. The results indicated that the intermittent aerobic reactor had higher organic and ammonia removal efficiencies than the anaerobic versions. Furthermore, the necessary stabilization time was reduced under aerobic conditions and the leachate toxicity decreased more rapidly. The pH adjustment in the anaerobic bioreactor had positive results in the decomposition of the organic matter enhancing the development of microbial activity.

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