The effects of leachate recirculation and the recirculation rate on the anaerobic treatment of domestic solid waste was investigated in three simulated landfill anaerobic bioreactors. A single pass reactor was operated without leachate recirculation while the other two reactors were operated with leachate recirculation. The leachate recirculation rate was 91/day (13% of the reactor volume) in Reactorg, while the recirculation rate was 21 1/day (30% of the reactor volume), in Reactor(21) . pH, chemical oxygen demand (COD), volatile fatty acids (VFA), ammonium-nitrogen (NH4-N) total and methane gas measurements in leachate samples were regularly monitored. After 220 days of anaerobic incubation, it was observed that the pH, COD, VFA concentrations, methane gas productions and methane percentages in Reactorg were better than the single pass reactor and Reactor(21). When the leachate recirculation rate was increased to three times a decrease in pH, and an increase in VFA and COD concentrations were observed in Reactor(21). The COD values were measured as 47 000, 39 000 and 52 000 mg/l while the VFA concentrations were 15 000, 13 000 and 21000 mg/l, respectively, in single pass, Reactorg and Reactor(21) after 220 days of anaerobic incubation. The values of pH were 5.89, 6.44 and 6.16, respectively, after anaerobic incubation. The mean methane percentages of single pass reactor, Reactorg and Reactor(21) were 30, 50 and 40%, respectively, after 50 days of incubation. Leachate recirculation reduced the waste stabilization time and was effective in enhancing methane gas production and improving leachate. However, leachate recirculation was not effective in removing ammonia from the leachate. The amounts of COD recovered by methane were 62.9, 162.3 and 94.6 g for single pass, Reactorg and Reactor(21), respectively, at the end of 220 days of anaerobic incubation. (C) 2003 Elsevier Ltd. All rights reserved.