Leachate from a municipal landfill was combined with domestic wastewater and treated in batch, semi-continuously fed-batch (SCFB) and continuous-flow (CF) activated sludge systems with and without powdered activated carbon (PAC) addition. In the absence of PAC, nitrification was severely inhibited and nitrite accumulated to about 85-100% of the total NOx-N. Addition of PAC to activated sludge reactors enhanced nitrification. In continuous-flow operation, nitrite accumulation could be completely prevented by PAC addition.