The combined biological treatability of landfill leachate and domestic wastewater was investigated in both semi-continuously fed batch (SCFB) and continuous-flow (CF) activated sludges with recycle. Powdered activated carbon (PAC) was added in order to investigate the improvement in organic carbon removal and nitrification. The results obtained in SCFB and CF operations were compared to each other. In both types of operations, COD and ammonia removal efficiencies decreased with an increase in the leachate ratio of the total wastewater. As the leachate ratio increased. the positive effect of PAC on COD removal and nitrification became more apparent. In SCFB-type operations, nitrification was more inhibited than in CF operations. Additionally, the enhancement of nitrification was more apparent in CF operations than in SCFB operations where there was PAC addition. In CF operations, sufficient PAC addition could completely prevent nitrification inhibition and nitrite accumulation was avoided. With regard to nitrification, the positive impact of PAC was observed after some time since inhibition of nitrifiers was more severe than heterotrophs.