Municipal landfill leachates are considered to be one of the types of wastewater with the greatest environmental impact because of the high concentrations of ammonium, organic matter and salts which characterize them. The most critical steps are usually the removal of recalcitrant pollutants and ammonia. A recently developed process based on ozone enhanced biological degradation, carried out in an aerobic granular biomass system (SBBGR - Sequencing Batch Biofilter Granular Reactor), was tested at lab-scale for treating a typical medium-age landfill leachate. The results have shown that ozonation greatly improves the biological treatment's effectiveness, allowing the current limits for discharging into the sewer system to be met by using a specific ozone dose of 600 mg O(3)/L(influent); an operating cost of about 4(sic)per m(3) of leachate was obtained. A strong synergy between chemical and biological oxidation was observed with an O(3) (consumed)/COD(removed) ratio as low as 0.24. Molecular analysis allowed the microbial composition of the granular biomass to be defined and showed the establishment of stable nitrogen-removing bacterial populations throughout the SBBGR operation. Moreover, the study further confirmed the SBBGR's ability to effectively overcome the problem of filamentous bacteria overgrowth, which, in contrast, dramatically affects the effluent quality in conventional systems. (C) 2011 Elsevier Ltd. All rights reserved.