In this study, the effects of shredding on the anaerobic treatment of domestic solid waste and leachate characteristics were investigated in three simulated landfill anaerobic bioreactors. All of the reactors were operated with leachate recirculation. One of them was loaded with raw waste (control reactor); the second reactor was loaded with shredded waste having a diameter of 0.5-1 cm (shredded reactor); the third reactor was loaded with compacted waste (compacted reactor) in order to compare the effects of shredding and compaction of solid wastes. The leachate recirculation rate was 300 ml/day in all of the reactors. pH, chemical oxygen demand (COD), volatile fatty acids (VIA), ammonium nitrogen (NH4-N) concentrations; total and methane gas productions in the leachate samples were regularly monitored. After 57 days of anaerobic incubation, it was observed that the pH, COD, VFA concentrations, and BOD5/COD ratio in the leachate of shredded reactor were better than the control and compacted reactor. The COD values were measured as 6400, 7700 and 2300 mg/l while the VFA concentrations were 2750, 3000 and 354 mg/l, respectively, in the leachate samples of the control, compacted and shredded reactor after 57 days of anaerobic incubation. The TN, TP NH4-N concentrations in municipal solid waste (MSW) reduced to 0.3, 0.4 and 0.1 from 8, 6.5 and 0.56 mg/g in shredded reactor by day 57. The values of pH were 6.88, 6.76 and 7.25, respectively, after anaerobic incubation, respectively in the aforementioned reactors. It was observed that the waste shredding increased the methane percentage in the anaerobic simulated reactor. Methane percentage of the control, compaction and shredded reactors were 36, 46 and 60%, respectively, after 57 days of incubation. It was found that MSWs having small size exhibited fast biodegradation. A BOD5/COD ratio of 0.44 achieved in the shredded reactor indicated the better MSW stabilization resulting in a high rate than that of compacted and control reactors. It was observed that waste shredding reduced the waste quantity, the organic content of the solid waste and the biodegradation time. (C) 2004 Elsevier Inc. All rights reserved.