Four simulated landfill anaerobic bioreactors were performed to investigate the influence of alkalinity on the anaerobic treatment of municipal solid waste (MSW). Leachate was recirculated in all the four reactors. One reactor was operated without alkalinization. The other three were operated under alkaline conditions. Na2CO3, NaHCO3 and NaOH were added to leachate in the second, third and fourth reactor, respectively. Experimental results showed that CO32 and HCO3 addition had a more pronounced effect on MSW stabilization while the effect of addition of OH was weak. The concentration of COD, BOD5, total nitrogen (TN), ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N), etc. in leachate significantly reduced in four reactors. The removal efficiencies were 90.56%,92.21%, 92.74% and 90.29% for COD, 66.45%,72.38%, 68.62% and 68.44% for NO3--N, and 96.5%, 98.75%, 97.75% and 98% for NO2--N in the control, Na2CO3, NaHCO3 and OH- added reactors, respectively. The final BOD5/COD was 0.262, 0.104, 0.124, and 0.143, and pH was 7.13, 7.28, 7.42, and 7.24 for control, Na2CO3 added, NaHCO3 added, and OH- added reactor, respectively. Therefore, alkalinity addition had positive effect on the stabilization of MSW. (C) 2008 Elsevier B.V. All rights reserved.